FFmpeg/libavformat/matroskaenc.c

/*
 * Matroska muxer
 * Copyright (c) 2007 David Conrad
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <stdint.h>

#include "config_components.h"

#include "av1.h"
#include "avc.h"
#include "hevc.h"
#include "avformat.h"
#include "avio_internal.h"
#include "avlanguage.h"
#include "dovi_isom.h"
#include "flacenc.h"
#include "internal.h"
#include "isom.h"
#include "matroska.h"
#include "mux.h"
#include "riff.h"
#include "version.h"
#include "vorbiscomment.h"
#include "wv.h"

#include "libavutil/avstring.h"
#include "libavutil/channel_layout.h"
#include "libavutil/crc.h"
#include "libavutil/dict.h"
#include "libavutil/hdr_dynamic_metadata.h"
#include "libavutil/intfloat.h"
#include "libavutil/intreadwrite.h"
#include "libavutil/lfg.h"
#include "libavutil/mastering_display_metadata.h"
#include "libavutil/mathematics.h"
#include "libavutil/opt.h"
#include "libavutil/parseutils.h"
#include "libavutil/pixdesc.h"
#include "libavutil/random_seed.h"
#include "libavutil/rational.h"
#include "libavutil/samplefmt.h"
#include "libavutil/stereo3d.h"

#include "libavcodec/av1.h"
#include "libavcodec/bytestream.h"
#include "libavcodec/codec_desc.h"
#include "libavcodec/codec_par.h"
#include "libavcodec/defs.h"
#include "libavcodec/itut35.h"
#include "libavcodec/xiph.h"
#include "libavcodec/mpeg4audio.h"

/* Level 1 elements we create a SeekHead entry for:
 * Info, Tracks, Chapters, Attachments, Tags (potentially twice) and Cues */
#define MAX_SEEKHEAD_ENTRIES 7

/* Largest known-length EBML length */
#define MAX_EBML_LENGTH ((1ULL << 56) - 2)
/* The dynamic buffer API we rely upon has a limit of INT_MAX;
 * and so has avio_write(). */
#define MAX_SUPPORTED_EBML_LENGTH FFMIN(MAX_EBML_LENGTH, INT_MAX)

#define MODE_MATROSKAv2 0x01
#define MODE_WEBM       0x02

#define IS_WEBM(mkv) (CONFIG_WEBM_MUXER && CONFIG_MATROSKA_MUXER ? \
                      ((mkv)->mode == MODE_WEBM) : CONFIG_WEBM_MUXER)
#define IS_SEEKABLE(pb, mkv) (((pb)->seekable & AVIO_SEEKABLE_NORMAL) && \
                              !(mkv)->is_live)

enum {
    DEFAULT_MODE_INFER,
    DEFAULT_MODE_INFER_NO_SUBS,
    DEFAULT_MODE_PASSTHROUGH,
};

typedef struct ebml_master {
    int64_t         pos;                ///< absolute offset in the containing AVIOContext where the master's elements start
    int             sizebytes;          ///< how many bytes were reserved for the size
} ebml_master;

typedef struct ebml_stored_master {
    AVIOContext    *bc;
    int64_t         pos;
} ebml_stored_master;

typedef enum EbmlType {
    EBML_UINT,
    EBML_SINT,
    EBML_FLOAT,
    EBML_UID,
    EBML_STR,
    EBML_UTF8 = EBML_STR,
    EBML_BIN,
    EBML_BLOCK,   ///< pseudo-type for writing (Simple)Blocks
    EBML_MASTER,
} EbmlType;

typedef struct BlockContext {
    struct mkv_track *track;
    const AVPacket   *pkt;
    int16_t           rel_ts;
    uint8_t           flags;
    NALUList          h2645_nalu_list;
} BlockContext;

typedef struct EbmlMaster {
    int nb_elements;               ///< -1 if not finished
    int containing_master;         ///< -1 if no parent exists
} EbmlMaster;

typedef struct EbmlElement {
    uint32_t id;
    EbmlType type;
    unsigned length_size;
    uint64_t size;                 ///< excluding id and length field
    union {
        uint64_t uint;
        int64_t  sint;
        double   f;
        const char    *str;
        const uint8_t *bin;
        struct MatroskaMuxContext *mkv; ///< used by EBML_BLOCK
        EbmlMaster master;
    } priv;
} EbmlElement;

typedef struct EbmlWriter {
    unsigned nb_elements;
    int      current_master_element;
    EbmlElement *elements;
} EbmlWriter;

#define EBML_WRITER(max_nb_elems) \
     EbmlElement elements[max_nb_elems]; \
     EbmlWriter writer = (EbmlWriter){ .elements = elements, \
                                       .current_master_element = -1 }

typedef struct mkv_seekhead_entry {
    uint32_t        elementid;
    uint64_t        segmentpos;
} mkv_seekhead_entry;

typedef struct mkv_seekhead {
    int64_t                 filepos;
    mkv_seekhead_entry      entries[MAX_SEEKHEAD_ENTRIES];
    int                     num_entries;
    int                     reserved_size;
} mkv_seekhead;

typedef struct mkv_cuepoint {
    uint64_t        pts;
    int             stream_idx;
    int64_t         cluster_pos;        ///< offset of the cluster containing the block relative to the segment
    int64_t         relative_pos;       ///< relative offset from the position of the cluster containing the block
    int64_t         duration;           ///< duration of the block according to time base
} mkv_cuepoint;

typedef struct mkv_cues {
    mkv_cuepoint   *entries;
    int             num_entries;
} mkv_cues;

struct MatroskaMuxContext;

typedef struct mkv_track {
    int             write_dts;
    int             has_cue;
    uint64_t        uid;
    unsigned        track_num;
    int             track_num_size;
    int             sample_rate;
    unsigned        offset;
    int64_t         sample_rate_offset;
    int64_t         last_timestamp;
    int64_t         duration;
    int64_t         duration_offset;
    uint64_t        max_blockaddid;
    int64_t         blockadditionmapping_offset;
    int             codecpriv_offset;
    unsigned        codecpriv_size;     ///< size reserved for CodecPrivate excluding header+length field
    int64_t         ts_offset;
    uint64_t        default_duration_low;
    uint64_t        default_duration_high;
    /* This callback will be called twice: First with a NULL AVIOContext
     * to return the size of the (Simple)Block's data via size
     * and a second time with the AVIOContext set when the data
     * shall be written.
     * The callback shall not return an error on the second call. */
    int             (*reformat)(struct MatroskaMuxContext *, AVIOContext *,
                                const AVPacket *, int *size);
} mkv_track;

typedef struct MatroskaMuxContext {
    const AVClass      *class;
    AVFormatContext    *ctx;

    int                 mode;
    ebml_stored_master  info;
    ebml_stored_master  track;
    ebml_stored_master  tags;
    int64_t             segment_offset;
    AVIOContext        *cluster_bc;
    int64_t             cluster_pos;    ///< file offset of the current Cluster
    int64_t             cluster_pts;
    int64_t             duration_offset;
    int64_t             duration;
    mkv_track          *tracks;
    mkv_seekhead        seekhead;
    mkv_cues            cues;
    int64_t             cues_pos;

    BlockContext        cur_block;

    /* Used as temporary buffer to use the minimal amount of bytes
     * to write the length field of EBML Masters.
     * Every user has to reset the buffer after using it and
     * different uses may not overlap. It is currently used in
     * mkv_write_tag(), in mkv_assemble_cues() as well as in
     * mkv_update_codecprivate() and mkv_write_track(). */
    AVIOContext        *tmp_bc;

    AVPacket           *cur_audio_pkt;

    unsigned            nb_attachments;
    int                 have_video;

    int                 wrote_chapters;
    int                 wrote_tags;

    int                 reserve_cues_space;
    int                 cluster_size_limit;
    int64_t             cluster_time_limit;
    int                 write_crc;
    int                 is_live;

    int                 is_dash;
    int                 dash_track_number;
    int                 allow_raw_vfw;
    int                 flipped_raw_rgb;
    int                 default_mode;
    int                 move_cues_to_front;

    uint32_t            segment_uid[4];
} MatroskaMuxContext;

/** 2 bytes * 3 for EBML IDs, 3 1-byte EBML lengths, 8 bytes for 64 bit
 * offset, 4 bytes for target EBML ID */
#define MAX_SEEKENTRY_SIZE 21

/** 4 * (1-byte EBML ID, 1-byte EBML size, 8-byte uint max) */
#define MAX_CUETRACKPOS_SIZE 40

/** DURATION_STRING_LENGTH must be <= 112 or the containing
 * simpletag will need more than one byte for its length field. */
#define DURATION_STRING_LENGTH 19

/** 2 + 1 Simpletag header, 2 + 1 + 8 Name "DURATION", rest for TagString */
#define DURATION_SIMPLETAG_SIZE (2 + 1 + (2 + 1 + 8) + (2 + 1 + DURATION_STRING_LENGTH))

/** Seek preroll value for opus */
#define OPUS_SEEK_PREROLL 80000000

static int ebml_id_size(uint32_t id)
{
    return (av_log2(id) + 7U) / 8;
}

static void put_ebml_id(AVIOContext *pb, uint32_t id)
{
    int i = ebml_id_size(id);
    while (i--)
        avio_w8(pb, (uint8_t)(id >> (i * 8)));
}

/**
 * Write an EBML size meaning "unknown size".
 *
 * @param bytes The number of bytes the size should occupy (maximum: 8).
 */
static void put_ebml_size_unknown(AVIOContext *pb, int bytes)
{
    av_assert0(bytes <= 8);
    avio_w8(pb, 0x1ff >> bytes);
    if (av_builtin_constant_p(bytes) && bytes == 1)
        return;
    ffio_fill(pb, 0xff, bytes - 1);
}

/**
 * Returns how many bytes are needed to represent a number
 * as EBML variable length integer.
 */
static int ebml_num_size(uint64_t num)
{
    int bytes = 0;
    do {
        bytes++;
    } while (num >>= 7);
    return bytes;
}

/**
 * Calculate how many bytes are needed to represent the length field
 * of an EBML element whose payload has a given length.
 */
static int ebml_length_size(uint64_t length)
{
    return ebml_num_size(length + 1);
}

/**
 * Write a number as EBML variable length integer on `bytes` bytes.
 * `bytes` is taken literally without checking.
 */
static void put_ebml_num(AVIOContext *pb, uint64_t num, int bytes)
{
    num |= 1ULL << bytes * 7;
    for (int i = bytes - 1; i >= 0; i--)
        avio_w8(pb, (uint8_t)(num >> i * 8));
}

/**
 * Write a length as EBML variable length integer.
 *
 * @param bytes The number of bytes that need to be used to write the number.
 *              If zero, the minimal number of bytes will be used.
 */
static void put_ebml_length(AVIOContext *pb, uint64_t length, int bytes)
{
    int needed_bytes = ebml_length_size(length);

    // sizes larger than this are currently undefined in EBML
    av_assert0(length < (1ULL << 56) - 1);

    if (bytes == 0)
        bytes = needed_bytes;
    // The bytes needed to write the given size must not exceed
    // the bytes that we ought to use.
    av_assert0(bytes >= needed_bytes);
    put_ebml_num(pb, length, bytes);
}

/**
 * Write a (random) UID with fixed size to make the output more deterministic
 */
static void put_ebml_uid(AVIOContext *pb, uint32_t elementid, uint64_t uid)
{
    put_ebml_id(pb, elementid);
    put_ebml_length(pb, 8, 0);
    avio_wb64(pb, uid);
}

static void put_ebml_uint(AVIOContext *pb, uint32_t elementid, uint64_t val)
{
    int i, bytes = 1;
    uint64_t tmp = val;
    while (tmp >>= 8)
        bytes++;

    put_ebml_id(pb, elementid);
    put_ebml_length(pb, bytes, 0);
    for (i = bytes - 1; i >= 0; i--)
        avio_w8(pb, (uint8_t)(val >> i * 8));
}

static void put_ebml_float(AVIOContext *pb, uint32_t elementid, double val)
{
    put_ebml_id(pb, elementid);
    put_ebml_length(pb, 8, 0);
    avio_wb64(pb, av_double2int(val));
}

static void put_ebml_binary(AVIOContext *pb, uint32_t elementid,
                            const void *buf, int size)
{
    put_ebml_id(pb, elementid);
    put_ebml_length(pb, size, 0);
    avio_write(pb, buf, size);
}

static void put_ebml_string(AVIOContext *pb, uint32_t elementid,
                            const char *str)
{
    put_ebml_binary(pb, elementid, str, strlen(str));
}

/**
 * Write a void element of a given size. Useful for reserving space in
 * the file to be written to later.
 *
 * @param size The number of bytes to reserve, which must be at least 2.
 */
static void put_ebml_void(AVIOContext *pb, int size)
{
    av_assert0(size >= 2);

    put_ebml_id(pb, EBML_ID_VOID);
    // we need to subtract the length needed to store the size from the
    // size we need to reserve so 2 cases, we use 8 bytes to store the
    // size if possible, 1 byte otherwise
    if (size < 10) {
        size -= 2;
        put_ebml_length(pb, size, 0);
    } else {
        size -= 9;
        put_ebml_length(pb, size, 8);
    }
    ffio_fill(pb, 0, size);
}

static ebml_master start_ebml_master(AVIOContext *pb, uint32_t elementid,
                                     uint64_t expectedsize)
{
    int bytes = expectedsize ? ebml_length_size(expectedsize) : 8;

    put_ebml_id(pb, elementid);
    put_ebml_size_unknown(pb, bytes);
    return (ebml_master) { avio_tell(pb), bytes };
}

static void end_ebml_master(AVIOContext *pb, ebml_master master)
{
    int64_t pos = avio_tell(pb);

    if (avio_seek(pb, master.pos - master.sizebytes, SEEK_SET) < 0)
        return;
    put_ebml_length(pb, pos - master.pos, master.sizebytes);
    avio_seek(pb, pos, SEEK_SET);
}

static EbmlElement *ebml_writer_add(EbmlWriter *writer,
                                    uint32_t id, EbmlType type)
{
    writer->elements[writer->nb_elements].id   = id;
    writer->elements[writer->nb_elements].type = type;
    return &writer->elements[writer->nb_elements++];
}

static void ebml_writer_open_master(EbmlWriter *writer, uint32_t id)
{
    EbmlElement *const elem = ebml_writer_add(writer, id, EBML_MASTER);
    EbmlMaster *const master = &elem->priv.master;

    master->containing_master = writer->current_master_element;
    master->nb_elements = -1;

    writer->current_master_element = writer->nb_elements - 1;
}

static void ebml_writer_close_master(EbmlWriter *writer)
{
    EbmlElement *elem;
    av_assert2(writer->current_master_element >= 0);
    av_assert2(writer->current_master_element < writer->nb_elements);
    elem = &writer->elements[writer->current_master_element];
    av_assert2(elem->type == EBML_MASTER);
    av_assert2(elem->priv.master.nb_elements < 0); /* means unset */
    elem->priv.master.nb_elements = writer->nb_elements - writer->current_master_element - 1;
    av_assert2(elem->priv.master.containing_master < 0 ||
               elem->priv.master.containing_master < writer->current_master_element);
    writer->current_master_element = elem->priv.master.containing_master;
}

static void ebml_writer_close_or_discard_master(EbmlWriter *writer)
{
    av_assert2(writer->nb_elements > 0);
    av_assert2(0 <= writer->current_master_element);
    av_assert2(writer->current_master_element < writer->nb_elements);
    if (writer->current_master_element == writer->nb_elements - 1) {
        const EbmlElement *const elem = &writer->elements[writer->nb_elements - 1];
        /* The master element has no children. Discard it. */
        av_assert2(elem->type == EBML_MASTER);
        av_assert2(elem->priv.master.containing_master < 0 ||
                   elem->priv.master.containing_master < writer->current_master_element);
        writer->current_master_element = elem->priv.master.containing_master;
        writer->nb_elements--;
        return;
    }
    ebml_writer_close_master(writer);
}

static void ebml_writer_add_string(EbmlWriter *writer, uint32_t id,
                                   const char *str)
{
    EbmlElement *const elem = ebml_writer_add(writer, id, EBML_STR);

    elem->priv.str = str;
}

static void ebml_writer_add_bin(EbmlWriter *writer, uint32_t id,
                                const uint8_t *data, size_t size)
{
    EbmlElement *const elem = ebml_writer_add(writer, id, EBML_BIN);

#if SIZE_MAX > UINT64_MAX
    size = FFMIN(size, UINT64_MAX);
#endif
    elem->size = size;
    elem->priv.bin = data;
}

static void ebml_writer_add_float(EbmlWriter *writer, uint32_t id,
                                  double val)
{
    EbmlElement *const elem = ebml_writer_add(writer, id, EBML_FLOAT);

    elem->priv.f = val;
}

static void ebml_writer_add_uid(EbmlWriter *writer, uint32_t id,
                                uint64_t val)
{
    EbmlElement *const elem = ebml_writer_add(writer, id, EBML_UID);
    elem->priv.uint = val;
}

static void ebml_writer_add_uint(EbmlWriter *writer, uint32_t id,
                                 uint64_t val)
{
    EbmlElement *elem = ebml_writer_add(writer, id, EBML_UINT);
    elem->priv.uint = val;
}

static void ebml_writer_add_sint(EbmlWriter *writer, uint32_t id,
                                 int64_t val)
{
    EbmlElement *elem = ebml_writer_add(writer, id, EBML_SINT);
    elem->priv.sint = val;
}

static void ebml_writer_add_block(EbmlWriter *writer, MatroskaMuxContext *mkv)
{
    EbmlElement *elem = ebml_writer_add(writer, MATROSKA_ID_BLOCK, EBML_BLOCK);
    elem->priv.mkv = mkv;
}

static int ebml_writer_str_len(EbmlElement *elem)
{
    size_t len = strlen(elem->priv.str);
#if SIZE_MAX > UINT64_MAX
    len = FF_MIN(len, UINT64_MAX);
#endif
    elem->size = len;
    return 0;
}

static av_const int uint_size(uint64_t val)
{
    int bytes = 0;
    do {
        bytes++;
    } while (val >>= 8);
    return bytes;
}

static int ebml_writer_uint_len(EbmlElement *elem)
{
    elem->size = uint_size(elem->priv.uint);
    return 0;
}

static av_const int sint_size(int64_t val)
{
    uint64_t tmp = 2 * (uint64_t)(val < 0 ? val^-1 : val);
    return uint_size(tmp);
}

static int ebml_writer_sint_len(EbmlElement *elem)
{
    elem->size = sint_size(elem->priv.sint);
    return 0;
}

static int ebml_writer_elem_len(EbmlWriter *writer, EbmlElement *elem,
                                int remaining_elems);

static int ebml_writer_master_len(EbmlWriter *writer, EbmlElement *elem,
                                  int remaining_elems)
{
    int nb_elems = elem->priv.master.nb_elements >= 0 ? elem->priv.master.nb_elements : remaining_elems - 1;
    EbmlElement *const master = elem;
    uint64_t total_size = 0;

    master->priv.master.nb_elements = nb_elems;
    for (; elem++, nb_elems > 0;) {
        int ret = ebml_writer_elem_len(writer, elem, nb_elems);
        if (ret < 0)
            return ret;
        av_assert2(ret < nb_elems);
        /* No overflow is possible here, as both total_size and elem->size
         * are bounded by MAX_SUPPORTED_EBML_LENGTH. */
        total_size += ebml_id_size(elem->id) + elem->length_size + elem->size;
        if (total_size > MAX_SUPPORTED_EBML_LENGTH)
            return AVERROR(ERANGE);
        nb_elems--;      /* consume elem */
        elem += ret, nb_elems -= ret; /* and elem's children */
    }
    master->size = total_size;

    return master->priv.master.nb_elements;
}

static int ebml_writer_block_len(EbmlElement *elem)
{
    MatroskaMuxContext *const mkv = elem->priv.mkv;
    BlockContext *const block = &mkv->cur_block;
    mkv_track *const track = block->track;
    const AVPacket *const pkt = block->pkt;
    int err, size;

    if (track->reformat) {
        err = track->reformat(mkv, NULL, pkt, &size);
        if (err < 0) {
            av_log(mkv->ctx, AV_LOG_ERROR, "Error when reformatting data of "
                   "a packet from stream %d.\n", pkt->stream_index);
            return err;
        }
    } else {
        size = pkt->size;
        if (track->offset <= size)
            size -= track->offset;
    }
    elem->size = track->track_num_size + 3U + size;

    return 0;
}

static void ebml_writer_write_block(const EbmlElement *elem, AVIOContext *pb)
{
    MatroskaMuxContext *const mkv = elem->priv.mkv;
    BlockContext *const block = &mkv->cur_block;
    mkv_track *const track = block->track;
    const AVPacket *const pkt = block->pkt;

    put_ebml_num(pb, track->track_num, track->track_num_size);
    avio_wb16(pb, block->rel_ts);
    avio_w8(pb, block->flags);

    if (track->reformat) {
        int size;
        track->reformat(mkv, pb, pkt, &size);
    } else {
        const uint8_t *data = pkt->data;
        unsigned offset = track->offset <= pkt->size ? track->offset : 0;
        avio_write(pb, data + offset, pkt->size - offset);
    }
}

static int ebml_writer_elem_len(EbmlWriter *writer, EbmlElement *elem,
                                int remaining_elems)
{
    int ret = 0;

    switch (elem->type) {
        case EBML_FLOAT:
        case EBML_UID:
            elem->size = 8;
            break;
        case EBML_STR:
            ret = ebml_writer_str_len(elem);
            break;
        case EBML_UINT:
            ret = ebml_writer_uint_len(elem);
            break;
        case EBML_SINT:
            ret = ebml_writer_sint_len(elem);
            break;
        case EBML_BLOCK:
            ret = ebml_writer_block_len(elem);
            break;
        case EBML_MASTER:
            ret = ebml_writer_master_len(writer, elem, remaining_elems);
            break;
    }
    if (ret < 0)
        return ret;
    if (elem->size > MAX_SUPPORTED_EBML_LENGTH)
        return AVERROR(ERANGE);
    elem->length_size = ebml_length_size(elem->size);
    return ret; /* number of elements consumed excluding elem itself */
}

static int ebml_writer_elem_write(const EbmlElement *elem, AVIOContext *pb)
{
    put_ebml_id(pb, elem->id);
    put_ebml_num(pb, elem->size, elem->length_size);
    switch (elem->type) {
        case EBML_UID:
        case EBML_FLOAT: {
            uint64_t val = elem->type == EBML_UID ? elem->priv.uint
                                                  : av_double2int(elem->priv.f);
            avio_wb64(pb, val);
            break;
        }
        case EBML_UINT:
        case EBML_SINT: {
            uint64_t val = elem->type == EBML_UINT ? elem->priv.uint
                                                   : elem->priv.sint;
            for (int i = elem->size; --i >= 0; )
                avio_w8(pb, (uint8_t)(val >> i * 8));
            break;
        }
        case EBML_STR:
        case EBML_BIN: {
            const uint8_t *data = elem->type == EBML_BIN ? elem->priv.bin
                                         : (const uint8_t*)elem->priv.str;
            avio_write(pb, data, elem->size);
            break;
        }
        case EBML_BLOCK:
            ebml_writer_write_block(elem, pb);
            break;
        case EBML_MASTER: {
            int nb_elems = elem->priv.master.nb_elements;

            elem++;
            for (int i = 0; i < nb_elems; i++)
                i += ebml_writer_elem_write(elem + i, pb);

            return nb_elems;
        }
    }
    return 0;
}

static int ebml_writer_write(EbmlWriter *writer, AVIOContext *pb)
{
    int ret = ebml_writer_elem_len(writer, writer->elements,
                                   writer->nb_elements);
    if (ret < 0)
        return ret;
    ebml_writer_elem_write(writer->elements, pb);
    return 0;
}

static void mkv_add_seekhead_entry(MatroskaMuxContext *mkv, uint32_t elementid,
                                   uint64_t filepos)
{
    mkv_seekhead *seekhead = &mkv->seekhead;

    av_assert1(seekhead->num_entries < MAX_SEEKHEAD_ENTRIES);

    seekhead->entries[seekhead->num_entries].elementid    = elementid;
    seekhead->entries[seekhead->num_entries++].segmentpos = filepos - mkv->segment_offset;
}

static int start_ebml_master_crc32(AVIOContext **dyn_cp, MatroskaMuxContext *mkv)
{
    int ret;

    if (!*dyn_cp && (ret = avio_open_dyn_buf(dyn_cp)) < 0)
        return ret;

    if (mkv->write_crc)
        put_ebml_void(*dyn_cp, 6); /* Reserve space for CRC32 so position/size calculations using avio_tell() take it into account */

    return 0;
}

static int end_ebml_master_crc32(AVIOContext *pb, AVIOContext **dyn_cp,
                                 MatroskaMuxContext *mkv, uint32_t id,
                                 int length_size, int keep_buffer,
                                 int add_seekentry)
{
    uint8_t *buf, crc[4];
    int ret, size, skip = 0;

    size = avio_get_dyn_buf(*dyn_cp, &buf);
    if ((ret = (*dyn_cp)->error) < 0)
        goto fail;

    if (add_seekentry)
        mkv_add_seekhead_entry(mkv, id, avio_tell(pb));

    put_ebml_id(pb, id);
    put_ebml_length(pb, size, length_size);
    if (mkv->write_crc) {
        skip = 6; /* Skip reserved 6-byte long void element from the dynamic buffer. */
        AV_WL32(crc, av_crc(av_crc_get_table(AV_CRC_32_IEEE_LE), UINT32_MAX, buf + skip, size - skip) ^ UINT32_MAX);
        put_ebml_binary(pb, EBML_ID_CRC32, crc, sizeof(crc));
    }
    avio_write(pb, buf + skip, size - skip);

fail:
    if (keep_buffer) {
        ffio_reset_dyn_buf(*dyn_cp);
    } else {
        ffio_free_dyn_buf(dyn_cp);
    }
    return ret;
}

/**
 * Output EBML master. Keep the buffer if seekable, allowing for later updates.
 * Furthermore always add a SeekHead Entry for this element.
 */
static int end_ebml_master_crc32_tentatively(AVIOContext *pb,
                                             ebml_stored_master *elem,
                                             MatroskaMuxContext *mkv, uint32_t id)
{
    if (IS_SEEKABLE(pb, mkv)) {
        uint8_t *buf;
        int size = avio_get_dyn_buf(elem->bc, &buf);

        if (elem->bc->error < 0)
            return elem->bc->error;

        elem->pos = avio_tell(pb);
        mkv_add_seekhead_entry(mkv, id, elem->pos);

        put_ebml_id(pb, id);
        put_ebml_length(pb, size, 0);
        avio_write(pb, buf, size);

        return 0;
    } else
        return end_ebml_master_crc32(pb, &elem->bc, mkv, id, 0, 0, 1);
}

static void put_xiph_size(AVIOContext *pb, int size)
{
    ffio_fill(pb, 255, size / 255);
    avio_w8(pb, size % 255);
}

/**
 * Free the members allocated in the mux context.
 */
static void mkv_deinit(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;

    ffio_free_dyn_buf(&mkv->cluster_bc);
    ffio_free_dyn_buf(&mkv->info.bc);
    ffio_free_dyn_buf(&mkv->track.bc);
    ffio_free_dyn_buf(&mkv->tags.bc);
    ffio_free_dyn_buf(&mkv->tmp_bc);

    av_freep(&mkv->cur_block.h2645_nalu_list.nalus);
    av_freep(&mkv->cues.entries);
    av_freep(&mkv->tracks);
}

/**
 * Initialize the SeekHead element to be ready to index level 1 Matroska
 * elements. Enough space to write MAX_SEEKHEAD_ENTRIES SeekHead entries
 * will be reserved at the current file location.
 */
static void mkv_start_seekhead(MatroskaMuxContext *mkv, AVIOContext *pb)
{
    mkv->seekhead.filepos = avio_tell(pb);
    // 21 bytes max for a Seek entry, 6 bytes max for the SeekHead ID
    // and size, 6 bytes for a CRC32 element, and 2 bytes to guarantee
    // that an EBML void element will fit afterwards
    mkv->seekhead.reserved_size = MAX_SEEKHEAD_ENTRIES * MAX_SEEKENTRY_SIZE + 14;
    put_ebml_void(pb, mkv->seekhead.reserved_size);
}

/**
 * Write the SeekHead to the file at the location reserved for it
 * and seek to destpos afterwards. When error_on_seek_failure
 * is not set, failure to seek to the position designated for the
 * SeekHead is not considered an error and it is presumed that
 * destpos is the current position; failure to seek to destpos
 * afterwards is always an error.
 *
 * @return 0 on success, < 0 on error.
 */
static int mkv_write_seekhead(AVIOContext *pb, MatroskaMuxContext *mkv,
                              int error_on_seek_failure, int64_t destpos)
{
    AVIOContext *dyn_cp = NULL;
    mkv_seekhead *seekhead = &mkv->seekhead;
    int64_t remaining, ret64;
    int i, ret;

    if ((ret64 = avio_seek(pb, seekhead->filepos, SEEK_SET)) < 0)
        return error_on_seek_failure ? ret64 : 0;

    ret = start_ebml_master_crc32(&dyn_cp, mkv);
    if (ret < 0)
        return ret;

    for (i = 0; i < seekhead->num_entries; i++) {
        mkv_seekhead_entry *entry = &seekhead->entries[i];
        ebml_master seekentry = start_ebml_master(dyn_cp, MATROSKA_ID_SEEKENTRY,
                                                  MAX_SEEKENTRY_SIZE);

        put_ebml_id(dyn_cp, MATROSKA_ID_SEEKID);
        put_ebml_length(dyn_cp, ebml_id_size(entry->elementid), 0);
        put_ebml_id(dyn_cp, entry->elementid);

        put_ebml_uint(dyn_cp, MATROSKA_ID_SEEKPOSITION, entry->segmentpos);
        end_ebml_master(dyn_cp, seekentry);
    }
    ret = end_ebml_master_crc32(pb, &dyn_cp, mkv,
                                MATROSKA_ID_SEEKHEAD, 0, 0, 0);
    if (ret < 0)
        return ret;

    remaining = seekhead->filepos + seekhead->reserved_size - avio_tell(pb);
    put_ebml_void(pb, remaining);

    if ((ret64 = avio_seek(pb, destpos, SEEK_SET)) < 0)
        return ret64;

    return 0;
}

static int mkv_add_cuepoint(MatroskaMuxContext *mkv, int stream, int64_t ts,
                            int64_t cluster_pos, int64_t relative_pos, int64_t duration)
{
    mkv_cues *cues = &mkv->cues;
    mkv_cuepoint *entries = cues->entries;
    unsigned idx = cues->num_entries;

    if (ts < 0)
        return 0;

    entries = av_realloc_array(entries, cues->num_entries + 1, sizeof(mkv_cuepoint));
    if (!entries)
        return AVERROR(ENOMEM);
    cues->entries = entries;

    /* Make sure the cues entries are sorted by pts. */
    while (idx > 0 && entries[idx - 1].pts > ts)
        idx--;
    memmove(&entries[idx + 1], &entries[idx],
            (cues->num_entries - idx) * sizeof(entries[0]));

    entries[idx].pts           = ts;
    entries[idx].stream_idx    = stream;
    entries[idx].cluster_pos   = cluster_pos - mkv->segment_offset;
    entries[idx].relative_pos  = relative_pos;
    entries[idx].duration      = duration;

    cues->num_entries++;

    return 0;
}

static int mkv_assemble_cues(AVStream **streams, AVIOContext *dyn_cp, AVIOContext *cuepoint,
                             const mkv_cues *cues, mkv_track *tracks, int num_tracks,
                             uint64_t offset)
{
    for (mkv_cuepoint *entry = cues->entries, *end = entry + cues->num_entries;
         entry < end;) {
        uint64_t pts = entry->pts;
        uint8_t *buf;
        int size;

        put_ebml_uint(cuepoint, MATROSKA_ID_CUETIME, pts);

        // put all the entries from different tracks that have the exact same
        // timestamp into the same CuePoint
        for (int j = 0; j < num_tracks; j++)
            tracks[j].has_cue = 0;
        do {
            ebml_master track_positions;
            int idx = entry->stream_idx;

            av_assert0(idx >= 0 && idx < num_tracks);
            if (tracks[idx].has_cue && streams[idx]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
                continue;
            tracks[idx].has_cue = 1;
            track_positions = start_ebml_master(cuepoint, MATROSKA_ID_CUETRACKPOSITION, MAX_CUETRACKPOS_SIZE);
            put_ebml_uint(cuepoint, MATROSKA_ID_CUETRACK           , tracks[idx].track_num);
            put_ebml_uint(cuepoint, MATROSKA_ID_CUECLUSTERPOSITION , entry->cluster_pos + offset);
            put_ebml_uint(cuepoint, MATROSKA_ID_CUERELATIVEPOSITION, entry->relative_pos);
            if (entry->duration > 0)
                put_ebml_uint(cuepoint, MATROSKA_ID_CUEDURATION    , entry->duration);
            end_ebml_master(cuepoint, track_positions);
        } while (++entry < end && entry->pts == pts);
        size = avio_get_dyn_buf(cuepoint, &buf);
        if (cuepoint->error < 0)
            return cuepoint->error;
        put_ebml_binary(dyn_cp, MATROSKA_ID_POINTENTRY, buf, size);
        ffio_reset_dyn_buf(cuepoint);
    }

    return 0;
}

static int put_xiph_codecpriv(AVFormatContext *s, AVIOContext *pb,
                              const AVCodecParameters *par,
                              const uint8_t *extradata, int extradata_size)
{
    const uint8_t *header_start[3];
    int header_len[3];
    int first_header_size;
    int err, j;

    if (par->codec_id == AV_CODEC_ID_VORBIS)
        first_header_size = 30;
    else
        first_header_size = 42;

    err = avpriv_split_xiph_headers(extradata, extradata_size,
                                    first_header_size, header_start, header_len);
    if (err < 0) {
        av_log(s, AV_LOG_ERROR, "Extradata corrupt.\n");
        return err;
    }

    avio_w8(pb, 2);                    // number packets - 1
    for (j = 0; j < 2; j++) {
        put_xiph_size(pb, header_len[j]);
    }
    for (j = 0; j < 3; j++)
        avio_write(pb, header_start[j], header_len[j]);

    return 0;
}

#if CONFIG_MATROSKA_MUXER
static int put_wv_codecpriv(AVIOContext *pb, const uint8_t *extradata, int extradata_size)
{
    if (extradata && extradata_size == 2)
        avio_write(pb, extradata, 2);
    else
        avio_wl16(pb, 0x410); // fallback to the most recent version
    return 0;
}

static int put_flac_codecpriv(AVFormatContext *s, AVIOContext *pb,
                              const AVCodecParameters *par,
                              const uint8_t *extradata, int extradata_size)
{
    int write_comment = (par->ch_layout.order == AV_CHANNEL_ORDER_NATIVE &&
                         !(par->ch_layout.u.mask & ~0x3ffffULL) &&
                         !ff_flac_is_native_layout(par->ch_layout.u.mask));
    int ret = ff_flac_write_header(pb, extradata, extradata_size,
                                   !write_comment);

    if (ret < 0)
        return ret;

    if (write_comment) {
        const char *vendor = (s->flags & AVFMT_FLAG_BITEXACT) ?
                             "Lavf" : LIBAVFORMAT_IDENT;
        AVDictionary *dict = NULL;
        uint8_t buf[32];
        int64_t len;

        snprintf(buf, sizeof(buf), "0x%"PRIx64, par->ch_layout.u.mask);
        av_dict_set(&dict, "WAVEFORMATEXTENSIBLE_CHANNEL_MASK", buf, 0);

        len = ff_vorbiscomment_length(dict, vendor, NULL, 0);
        av_assert1(len < (1 << 24) - 4);

        avio_w8(pb, 0x84);
        avio_wb24(pb, len);

        ff_vorbiscomment_write(pb, dict, vendor, NULL, 0);

        av_dict_free(&dict);
    }

    return 0;
}

static int get_aac_sample_rates(AVFormatContext *s, MatroskaMuxContext *mkv,
                                const uint8_t *extradata, int extradata_size,
                                int *sample_rate, int *output_sample_rate)
{
    MPEG4AudioConfig mp4ac;
    int ret;

    ret = avpriv_mpeg4audio_get_config2(&mp4ac, extradata, extradata_size, 1, s);
    /* Don't abort if the failure is because of missing extradata. Assume in that
     * case a bitstream filter will provide the muxer with the extradata in the
     * first packet.
     * Abort however if s->pb is not seekable, as we would not be able to seek back
     * to write the sample rate elements once the extradata shows up, anyway. */
    if (ret < 0 && (extradata_size || !IS_SEEKABLE(s->pb, mkv))) {
        av_log(s, AV_LOG_ERROR,
               "Error parsing AAC extradata, unable to determine samplerate.\n");
        return AVERROR(EINVAL);
    }

    if (ret < 0) {
        /* This will only happen when this function is called while writing the
         * header and no extradata is available. The space for this element has
         * to be reserved for when this function is called again after the
         * extradata shows up in the first packet, as there's no way to know if
         * output_sample_rate will be different than sample_rate or not. */
        *output_sample_rate = *sample_rate;
    } else {
        *sample_rate        = mp4ac.sample_rate;
        *output_sample_rate = mp4ac.ext_sample_rate;
    }
    return 0;
}
#endif

static int mkv_assemble_native_codecprivate(AVFormatContext *s, AVIOContext *dyn_cp,
                                            const AVCodecParameters *par,
                                            const uint8_t *extradata,
                                            int extradata_size,
                                            unsigned *size_to_reserve)
{
    switch (par->codec_id) {
    case AV_CODEC_ID_VORBIS:
    case AV_CODEC_ID_THEORA:
        return put_xiph_codecpriv(s, dyn_cp, par, extradata, extradata_size);
    case AV_CODEC_ID_AV1:
        if (extradata_size)
            return ff_isom_write_av1c(dyn_cp, extradata,
                                      extradata_size, 1);
        else
            *size_to_reserve = (AV1_SANE_SEQUENCE_HEADER_MAX_BITS + 7) / 8 + 100;
        break;
#if CONFIG_MATROSKA_MUXER
    case AV_CODEC_ID_FLAC:
        return put_flac_codecpriv(s, dyn_cp, par, extradata, extradata_size);
    case AV_CODEC_ID_WAVPACK:
        return put_wv_codecpriv(dyn_cp, extradata, extradata_size);
    case AV_CODEC_ID_H264:
        return ff_isom_write_avcc(dyn_cp, extradata,
                                  extradata_size);
    case AV_CODEC_ID_HEVC:
        return ff_isom_write_hvcc(dyn_cp, extradata,
                                  extradata_size, 0);
    case AV_CODEC_ID_ALAC:
        if (extradata_size < 36) {
            av_log(s, AV_LOG_ERROR,
                   "Invalid extradata found, ALAC expects a 36-byte "
                   "QuickTime atom.");
            return AVERROR_INVALIDDATA;
        } else
            avio_write(dyn_cp, extradata + 12,
                       extradata_size - 12);
        break;
    case AV_CODEC_ID_AAC:
        if (extradata_size)
            avio_write(dyn_cp, extradata, extradata_size);
        else
            *size_to_reserve = MAX_PCE_SIZE;
        break;
    case AV_CODEC_ID_ARIB_CAPTION: {
        unsigned stream_identifier, data_component_id;
        switch (par->profile) {
        case AV_PROFILE_ARIB_PROFILE_A:
            stream_identifier = 0x30;
            data_component_id = 0x0008;
            break;
        case AV_PROFILE_ARIB_PROFILE_C:
            stream_identifier = 0x87;
            data_component_id = 0x0012;
            break;
        default:
            av_log(s, AV_LOG_ERROR,
                   "Unset/unknown ARIB caption profile %d utilized!\n",
                   par->profile);
            return AVERROR_INVALIDDATA;
        }
        avio_w8(dyn_cp, stream_identifier);
        avio_wb16(dyn_cp, data_component_id);
        break;
    }
#endif
    default:
        if (CONFIG_MATROSKA_MUXER && par->codec_id == AV_CODEC_ID_PRORES &&
            ff_codec_get_id(ff_codec_movvideo_tags, par->codec_tag) == AV_CODEC_ID_PRORES) {
            avio_wl32(dyn_cp, par->codec_tag);
        } else if (extradata_size && par->codec_id != AV_CODEC_ID_TTA)
            avio_write(dyn_cp, extradata, extradata_size);
    }

    return 0;
}

static int mkv_assemble_codecprivate(AVFormatContext *s, AVIOContext *dyn_cp,
                                     AVCodecParameters *par,
                                     const uint8_t *extradata, int extradata_size,
                                     int native_id, int qt_id,
                                     uint8_t **codecpriv, int *codecpriv_size,
                                     unsigned *max_payload_size)
{
    MatroskaMuxContext av_unused *const mkv = s->priv_data;
    unsigned size_to_reserve = 0;
    int ret;

    if (native_id) {
        ret = mkv_assemble_native_codecprivate(s, dyn_cp, par,
                                               extradata, extradata_size,
                                               &size_to_reserve);
        if (ret < 0)
            return ret;
#if CONFIG_MATROSKA_MUXER
    } else if (par->codec_type == AVMEDIA_TYPE_VIDEO) {
        if (qt_id) {
            if (!par->codec_tag)
                par->codec_tag = ff_codec_get_tag(ff_codec_movvideo_tags,
                                                    par->codec_id);
            if (   ff_codec_get_id(ff_codec_movvideo_tags, par->codec_tag) == par->codec_id
                && (!extradata_size || ff_codec_get_id(ff_codec_movvideo_tags, AV_RL32(extradata + 4)) != par->codec_id)
            ) {
                avio_wb32(dyn_cp, 0x5a + extradata_size);
                avio_wl32(dyn_cp, par->codec_tag);
                ffio_fill(dyn_cp, 0, 0x5a - 8);
            }
            avio_write(dyn_cp, extradata, extradata_size);
        } else {
            if (!ff_codec_get_tag(ff_codec_bmp_tags, par->codec_id))
                av_log(s, AV_LOG_WARNING, "codec %s is not supported by this format\n",
                       avcodec_get_name(par->codec_id));

            if (!par->codec_tag)
                par->codec_tag = ff_codec_get_tag(ff_codec_bmp_tags,
                                                  par->codec_id);
            if (!par->codec_tag && par->codec_id != AV_CODEC_ID_RAWVIDEO) {
                av_log(s, AV_LOG_ERROR, "No bmp codec tag found for codec %s\n",
                       avcodec_get_name(par->codec_id));
                return AVERROR(EINVAL);
            }

            /* If vfw extradata updates are supported, this will have
             * to be updated to pass extradata(_size) explicitly. */
            ff_put_bmp_header(dyn_cp, par, 0, 0, mkv->flipped_raw_rgb);
        }
    } else if (par->codec_type == AVMEDIA_TYPE_AUDIO) {
        unsigned int tag;
        tag = ff_codec_get_tag(ff_codec_wav_tags, par->codec_id);
        if (!tag) {
            av_log(s, AV_LOG_ERROR, "No wav codec tag found for codec %s\n",
                   avcodec_get_name(par->codec_id));
            return AVERROR(EINVAL);
        }
        if (!par->codec_tag)
            par->codec_tag = tag;

        /* Same comment as for ff_put_bmp_header applies here. */
        ff_put_wav_header(s, dyn_cp, par, FF_PUT_WAV_HEADER_FORCE_WAVEFORMATEX);
#endif
    }

    *codecpriv_size = avio_get_dyn_buf(dyn_cp, codecpriv);
    if (dyn_cp->error < 0)
        return dyn_cp->error;
    *max_payload_size = *codecpriv_size + size_to_reserve;

    return 0;
}

static void mkv_put_codecprivate(AVIOContext *pb, unsigned max_payload_size,
                                 const uint8_t *codecpriv, unsigned codecpriv_size)
{
    unsigned total_codecpriv_size = 0, total_size;

    av_assert1(codecpriv_size <= max_payload_size);

    if (!max_payload_size)
        return;

    total_size = 2 + ebml_length_size(max_payload_size) + max_payload_size;

    if (codecpriv_size) {
        unsigned length_size = ebml_length_size(codecpriv_size);

        total_codecpriv_size = 2U + length_size + codecpriv_size;
        if (total_codecpriv_size + 1 == total_size) {
            /* It is impossible to add one byte of padding via an EBML Void. */
            length_size++;
            total_codecpriv_size++;
        }
        put_ebml_id(pb, MATROSKA_ID_CODECPRIVATE);
        put_ebml_length(pb, codecpriv_size, length_size);
        avio_write(pb, codecpriv, codecpriv_size);
    }
    if (total_codecpriv_size < total_size)
        put_ebml_void(pb, total_size - total_codecpriv_size);
}

static int mkv_update_codecprivate(AVFormatContext *s, MatroskaMuxContext *mkv,
                                   uint8_t *side_data, int side_data_size,
                                   AVCodecParameters *par, AVIOContext *pb,
                                   mkv_track *track, unsigned alternative_size)
{
    AVIOContext *const dyn_bc = mkv->tmp_bc;
    uint8_t *codecpriv;
    unsigned max_payload_size;
    int ret, codecpriv_size;

    ret = mkv_assemble_codecprivate(s, dyn_bc, par,
                                    side_data, side_data_size, 1, 0,
                                    &codecpriv, &codecpriv_size, &max_payload_size);
    if (ret < 0)
        goto fail;
    if (codecpriv_size > track->codecpriv_size && !alternative_size) {
        ret = AVERROR(ENOSPC);
        goto fail;
    } else if (codecpriv_size > track->codecpriv_size) {
        av_assert1(alternative_size < track->codecpriv_size);
        codecpriv_size = alternative_size;
    }
    avio_seek(pb, track->codecpriv_offset, SEEK_SET);
    mkv_put_codecprivate(pb, track->codecpriv_size,
                         codecpriv, codecpriv_size);

    if (!par->extradata_size) {
        ret = ff_alloc_extradata(par, side_data_size);
        if (ret < 0)
            goto fail;
        memcpy(par->extradata, side_data, side_data_size);
    }
fail:
    ffio_reset_dyn_buf(dyn_bc);
    return ret;
}

#define MAX_VIDEO_COLOR_ELEMS 20
static void mkv_write_video_color(EbmlWriter *writer, const AVStream *st,
                                  const AVCodecParameters *par)
{
    const AVPacketSideData *side_data;

    ebml_writer_open_master(writer, MATROSKA_ID_VIDEOCOLOR);

    if (par->color_trc != AVCOL_TRC_UNSPECIFIED &&
        par->color_trc < AVCOL_TRC_NB) {
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORTRANSFERCHARACTERISTICS,
                             par->color_trc);
    }
    if (par->color_space != AVCOL_SPC_UNSPECIFIED &&
        par->color_space < AVCOL_SPC_NB) {
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORMATRIXCOEFF,
                             par->color_space);
    }
    if (par->color_primaries != AVCOL_PRI_UNSPECIFIED &&
        par->color_primaries < AVCOL_PRI_NB) {
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORPRIMARIES,
                             par->color_primaries);
    }
    if (par->color_range != AVCOL_RANGE_UNSPECIFIED &&
        par->color_range < AVCOL_RANGE_NB) {
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORRANGE, par->color_range);
    }
    if (par->chroma_location != AVCHROMA_LOC_UNSPECIFIED &&
        par->chroma_location <= AVCHROMA_LOC_TOP) {
        int xpos, ypos;

        av_chroma_location_enum_to_pos(&xpos, &ypos, par->chroma_location);
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORCHROMASITINGHORZ,
                             (xpos >> 7) + 1);
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORCHROMASITINGVERT,
                             (ypos >> 7) + 1);
    }

    side_data = av_packet_side_data_get(st->codecpar->coded_side_data, st->codecpar->nb_coded_side_data,
                                        AV_PKT_DATA_CONTENT_LIGHT_LEVEL);
    if (side_data) {
        const AVContentLightMetadata *metadata = (AVContentLightMetadata *)side_data->data;
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORMAXCLL,
                             metadata->MaxCLL);
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOCOLORMAXFALL,
                             metadata->MaxFALL);
    }

    side_data = av_packet_side_data_get(st->codecpar->coded_side_data, st->codecpar->nb_coded_side_data,
                                        AV_PKT_DATA_MASTERING_DISPLAY_METADATA);
    if (side_data) {
        const AVMasteringDisplayMetadata *metadata = (AVMasteringDisplayMetadata *)side_data->data;
        ebml_writer_open_master(writer, MATROSKA_ID_VIDEOCOLORMASTERINGMETA);
        if (metadata->has_primaries) {
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_RX,
                                  av_q2d(metadata->display_primaries[0][0]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_RY,
                                  av_q2d(metadata->display_primaries[0][1]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_GX,
                                  av_q2d(metadata->display_primaries[1][0]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_GY,
                                  av_q2d(metadata->display_primaries[1][1]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_BX,
                                  av_q2d(metadata->display_primaries[2][0]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_BY,
                                  av_q2d(metadata->display_primaries[2][1]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_WHITEX,
                                  av_q2d(metadata->white_point[0]));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_WHITEY,
                                  av_q2d(metadata->white_point[1]));
        }
        if (metadata->has_luminance) {
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_LUMINANCEMAX,
                                  av_q2d(metadata->max_luminance));
            ebml_writer_add_float(writer, MATROSKA_ID_VIDEOCOLOR_LUMINANCEMIN,
                                  av_q2d(metadata->min_luminance));
        }
        ebml_writer_close_or_discard_master(writer);
    }

    ebml_writer_close_or_discard_master(writer);
}

#define MAX_VIDEO_PROJECTION_ELEMS 6
static void mkv_handle_rotation(void *logctx, const AVStream *st,
                                double *yaw, double *roll)
{
    const int32_t *matrix;
    const AVPacketSideData *side_data =
        av_packet_side_data_get(st->codecpar->coded_side_data, st->codecpar->nb_coded_side_data,
                                AV_PKT_DATA_DISPLAYMATRIX);

    if (!side_data)
        return;

    matrix = (int32_t *)side_data->data;

    /* Check whether this is an affine transformation */
    if (matrix[2] || matrix[5])
        goto ignore;

    /* This together with the checks below test whether
     * the upper-left 2x2 matrix is nonsingular. */
    if (!matrix[0] && !matrix[1])
        goto ignore;

    /* We ignore the translation part of the matrix (matrix[6] and matrix[7])
     * as well as any scaling, i.e. we only look at the upper left 2x2 matrix.
     * We only accept matrices that are an exact multiple of an orthogonal one.
     * Apart from the multiple, every such matrix can be obtained by
     * potentially flipping in the x-direction (corresponding to yaw = 180)
     * followed by a rotation of (say) an angle phi in the counterclockwise
     * direction. The upper-left 2x2 matrix then looks like this:
     *         | (+/-)cos(phi) (-/+)sin(phi) |
     * scale * |                             |
     *         |      sin(phi)      cos(phi) |
     * The first set of signs in the first row apply in case of no flipping,
     * the second set applies in case of flipping. */

    /* The casts to int64_t are needed because -INT32_MIN doesn't fit
     * in an int32_t. */
    if (matrix[0] == matrix[4] && -(int64_t)matrix[1] == matrix[3]) {
        /* No flipping case */
        *yaw = 0;
    } else if (-(int64_t)matrix[0] == matrix[4] && matrix[1] == matrix[3]) {
        /* Horizontal flip */
        *yaw = 180;
    } else {
ignore:
        av_log(logctx, AV_LOG_INFO, "Ignoring display matrix indicating "
               "non-orthogonal transformation.\n");
        return;
    }
    *roll = 180 / M_PI * atan2(matrix[3], matrix[4]);

    /* We do not write a ProjectionType element indicating "rectangular",
     * because this is the default value. */
}

static int mkv_handle_spherical(void *logctx, EbmlWriter *writer,
                                const AVStream *st, uint8_t private[],
                                double *yaw, double *pitch, double *roll)
{
    const AVPacketSideData *sd = av_packet_side_data_get(st->codecpar->coded_side_data,
                                                         st->codecpar->nb_coded_side_data,
                                                         AV_PKT_DATA_SPHERICAL);
    const AVSphericalMapping *spherical;

    if (!sd)
        return 0;

    spherical = (const AVSphericalMapping *)sd->data;
    if (spherical->projection != AV_SPHERICAL_EQUIRECTANGULAR      &&
        spherical->projection != AV_SPHERICAL_EQUIRECTANGULAR_TILE &&
        spherical->projection != AV_SPHERICAL_CUBEMAP) {
        av_log(logctx, AV_LOG_WARNING, "Unknown projection type\n");
        return 0;
    }

    switch (spherical->projection) {
    case AV_SPHERICAL_EQUIRECTANGULAR:
    case AV_SPHERICAL_EQUIRECTANGULAR_TILE:
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOPROJECTIONTYPE,
                             MATROSKA_VIDEO_PROJECTION_TYPE_EQUIRECTANGULAR);
        AV_WB32(private,      0); // version + flags
        if (spherical->projection == AV_SPHERICAL_EQUIRECTANGULAR) {
            AV_WB32(private +  4, 0);
            AV_WB32(private +  8, 0);
            AV_WB32(private + 12, 0);
            AV_WB32(private + 16, 0);
        } else {
            AV_WB32(private +  4, spherical->bound_top);
            AV_WB32(private +  8, spherical->bound_bottom);
            AV_WB32(private + 12, spherical->bound_left);
            AV_WB32(private + 16, spherical->bound_right);
        }
        ebml_writer_add_bin(writer, MATROSKA_ID_VIDEOPROJECTIONPRIVATE,
                            private, 20);
        break;
    case AV_SPHERICAL_CUBEMAP:
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOPROJECTIONTYPE,
                             MATROSKA_VIDEO_PROJECTION_TYPE_CUBEMAP);
        AV_WB32(private,     0); // version + flags
        AV_WB32(private + 4, 0); // layout
        AV_WB32(private + 8, spherical->padding);
        ebml_writer_add_bin(writer, MATROSKA_ID_VIDEOPROJECTIONPRIVATE,
                            private, 12);
        break;
    default:
        av_assert0(0);
    }

    *yaw   = (double) spherical->yaw   / (1 << 16);
    *pitch = (double) spherical->pitch / (1 << 16);
    *roll  = (double) spherical->roll  / (1 << 16);

    return 1; /* Projection included */
}

static void mkv_write_video_projection(void *logctx, EbmlWriter *wr,
                                       const AVStream *st, uint8_t private[])
{
    double yaw = 0, pitch = 0, roll = 0;
    int ret;

    ebml_writer_open_master(wr, MATROSKA_ID_VIDEOPROJECTION);

    ret = mkv_handle_spherical(logctx, wr, st, private, &yaw, &pitch, &roll);
    if (!ret)
        mkv_handle_rotation(logctx, st, &yaw, &roll);

    if (yaw)
        ebml_writer_add_float(wr, MATROSKA_ID_VIDEOPROJECTIONPOSEYAW, yaw);
    if (pitch)
        ebml_writer_add_float(wr, MATROSKA_ID_VIDEOPROJECTIONPOSEPITCH, pitch);
    if (roll)
        ebml_writer_add_float(wr, MATROSKA_ID_VIDEOPROJECTIONPOSEROLL, roll);

    ebml_writer_close_or_discard_master(wr);
}

#define MAX_FIELD_ORDER_ELEMS 2
static void mkv_write_field_order(EbmlWriter *writer, int is_webm,
                                  enum AVFieldOrder field_order)
{
    switch (field_order) {
    case AV_FIELD_UNKNOWN:
        break;
    case AV_FIELD_PROGRESSIVE:
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOFLAGINTERLACED,
                             MATROSKA_VIDEO_INTERLACE_FLAG_PROGRESSIVE);
        break;
    case AV_FIELD_TT:
    case AV_FIELD_BB:
    case AV_FIELD_TB:
    case AV_FIELD_BT:
        ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOFLAGINTERLACED,
                             MATROSKA_VIDEO_INTERLACE_FLAG_INTERLACED);
        if (!is_webm) {
            switch (field_order) {
            case AV_FIELD_TT:
                ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOFIELDORDER,
                                     MATROSKA_VIDEO_FIELDORDER_TT);
                break;
            case AV_FIELD_BB:
                ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOFIELDORDER,
                                     MATROSKA_VIDEO_FIELDORDER_BB);
                break;
            case AV_FIELD_TB:
                ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOFIELDORDER,
                                     MATROSKA_VIDEO_FIELDORDER_TB);
                break;
            case AV_FIELD_BT:
                ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOFIELDORDER,
                                     MATROSKA_VIDEO_FIELDORDER_BT);
                break;
            }
        }
    }
}

#define MAX_STEREO_MODE_ELEMS 1
static int mkv_write_stereo_mode(AVFormatContext *s, EbmlWriter *writer,
                                 const AVStream *st, int is_webm,
                                 int *h_width, int *h_height)
{
    const char *error_message_addendum = "";
    const AVDictionaryEntry *tag;
    MatroskaVideoStereoModeType format = MATROSKA_VIDEO_STEREOMODE_TYPE_NB;

    /* The following macros create bitfields where the ith bit
     * indicates whether the MatroskaVideoStereoModeType with that value
     * uses double width/height or is WebM compatible. */
#define FLAG(STEREOMODETYPE, BOOL) | (BOOL) << (STEREOMODETYPE)
#define WDIV1(STEREOMODETYPE, STEREO3DTYPE, FLAGS, WDIV, HDIV, WEBM) \
    FLAG(STEREOMODETYPE, WDIV)
#define WDIV2(STEREOMODETYPE, WDIV, HDIV, WEBM) \
    FLAG(STEREOMODETYPE, WDIV)
    // The zero in the following line consumes the first '|'.
    const unsigned width_bitfield = 0 STEREOMODE_STEREO3D_MAPPING(WDIV1, WDIV2);

#define HDIV1(STEREOMODETYPE, STEREO3DTYPE, FLAGS, WDIV, HDIV, WEBM) \
    FLAG(STEREOMODETYPE, HDIV)
#define HDIV2(STEREOMODETYPE, WDIV, HDIV, WEBM) \
    FLAG(STEREOMODETYPE, HDIV)
    const unsigned height_bitfield = 0 STEREOMODE_STEREO3D_MAPPING(HDIV1, HDIV2);

#define WEBM1(STEREOMODETYPE, STEREO3DTYPE, FLAGS, WDIV, HDIV, WEBM) \
    FLAG(STEREOMODETYPE, WEBM)
#define WEBM2(STEREOMODETYPE, WDIV, HDIV, WEBM) \
    FLAG(STEREOMODETYPE, WEBM)
    const unsigned webm_bitfield = 0 STEREOMODE_STEREO3D_MAPPING(WEBM1, WEBM2);

    *h_width = 1;
    *h_height = 1;

    if ((tag = av_dict_get(st->metadata, "stereo_mode", NULL, 0)) ||
        (tag = av_dict_get( s->metadata, "stereo_mode", NULL, 0))) {

        for (int i = 0; i < MATROSKA_VIDEO_STEREOMODE_TYPE_NB; i++)
            if (!strcmp(tag->value, ff_matroska_video_stereo_mode[i])){
                format = i;
                break;
            }
        if (format == MATROSKA_VIDEO_STEREOMODE_TYPE_NB) {
            long stereo_mode = strtol(tag->value, NULL, 0);
            if ((unsigned long)stereo_mode >= MATROSKA_VIDEO_STEREOMODE_TYPE_NB)
                goto fail;
            format = stereo_mode;
        }
    } else {
        const AVPacketSideData *sd;
        const AVStereo3D *stereo;
        /* The following macro presumes all MATROSKA_VIDEO_STEREOMODE_TYPE_*
         * values to be in the range 0..254. */
#define STEREOMODE(STEREOMODETYPE, STEREO3DTYPE, FLAGS, WDIV, HDIV, WEBM) \
    [(STEREO3DTYPE)][!!((FLAGS) & AV_STEREO3D_FLAG_INVERT)] = (STEREOMODETYPE) + 1,
#define NOTHING(STEREOMODETYPE, WDIV, HDIV, WEBM)
        static const unsigned char conversion_table[][2] = {
            STEREOMODE_STEREO3D_MAPPING(STEREOMODE, NOTHING)
        };
        int fmt;

        sd = av_packet_side_data_get(st->codecpar->coded_side_data, st->codecpar->nb_coded_side_data,
                                     AV_PKT_DATA_STEREO3D);
        if (!sd)
            return 0;

        stereo = (const AVStereo3D*)sd->data;

        /* A garbage AVStereo3D or something with no Matroska analogon. */
        if ((unsigned)stereo->type >= FF_ARRAY_ELEMS(conversion_table))
            return 0;

        fmt = conversion_table[stereo->type][!!(stereo->flags & AV_STEREO3D_FLAG_INVERT)];
        /* Format with no Matroska analogon; ignore it */
        if (!fmt)
            return 0;
        format = fmt - 1;
    }

    // if webm, do not write unsupported modes
    if (is_webm && !(webm_bitfield >> format)) {
        error_message_addendum = " for WebM";
        goto fail;
    }

    *h_width  = 1 << ((width_bitfield  >> format) & 1);
    *h_height = 1 << ((height_bitfield >> format) & 1);

    // write StereoMode if format is valid
    ebml_writer_add_uint(writer, MATROSKA_ID_VIDEOSTEREOMODE, format);

    return 0;
fail:
    av_log(s, AV_LOG_ERROR,
            "The specified stereo mode is not valid%s.\n",
            error_message_addendum);
    return AVERROR(EINVAL);
}

static void mkv_write_blockadditionmapping(AVFormatContext *s, const MatroskaMuxContext *mkv,
                                           const AVCodecParameters *par, AVIOContext *pb,
                                           mkv_track *track, const AVStream *st)
{
#if CONFIG_MATROSKA_MUXER
    const AVDOVIDecoderConfigurationRecord *dovi;
    const AVPacketSideData *sd;

    if (IS_SEEKABLE(s->pb, mkv)) {
        track->blockadditionmapping_offset = avio_tell(pb);
        // We can't know at this point if there will be a block with BlockAdditions, so
        // we either write the default value here, or a void element. Either of them will
        // be overwritten when finishing the track.
        put_ebml_uint(pb, MATROSKA_ID_TRACKMAXBLKADDID, 0);
        if (par->codec_type == AVMEDIA_TYPE_VIDEO) {
            // Similarly, reserve space for an eventual
            // HDR10+ ITU T.35 metadata BlockAdditionMapping.
            put_ebml_void(pb, 3 /* BlockAdditionMapping */
                            + 4 /* BlockAddIDValue */
                            + 4 /* BlockAddIDType */);
        }
    }

    sd = av_packet_side_data_get(st->codecpar->coded_side_data, st->codecpar->nb_coded_side_data,
                                 AV_PKT_DATA_DOVI_CONF);

    if (!sd)
        return;

    dovi = (const AVDOVIDecoderConfigurationRecord *)sd->data;
    if (dovi->dv_profile <= 10) {
        ebml_master mapping;
        uint8_t buf[ISOM_DVCC_DVVC_SIZE];
        uint32_t type;

        uint64_t expected_size = (2 + 1 + (sizeof(DVCC_DVVC_BLOCK_TYPE_NAME) - 1))
                                + (2 + 1 + 4) + (2 + 1 + ISOM_DVCC_DVVC_SIZE);

        if (dovi->dv_profile > 7) {
            type = MATROSKA_BLOCK_ADD_ID_TYPE_DVVC;
        } else {
            type = MATROSKA_BLOCK_ADD_ID_TYPE_DVCC;
        }

        ff_isom_put_dvcc_dvvc(s, buf, dovi);

        mapping = start_ebml_master(pb, MATROSKA_ID_TRACKBLKADDMAPPING, expected_size);

        put_ebml_string(pb, MATROSKA_ID_BLKADDIDNAME, DVCC_DVVC_BLOCK_TYPE_NAME);
        put_ebml_uint(pb, MATROSKA_ID_BLKADDIDTYPE, type);
        put_ebml_binary(pb, MATROSKA_ID_BLKADDIDEXTRADATA, buf, sizeof(buf));

        end_ebml_master(pb, mapping);
    }
#endif
}

static int mkv_write_track_video(AVFormatContext *s, MatroskaMuxContext *mkv,
                                 const AVStream *st, const AVCodecParameters *par,
                                 AVIOContext *pb)
{
    const AVDictionaryEntry *tag;
    int display_width_div = 1, display_height_div = 1;
    uint8_t color_space[4], projection_private[20];
    EBML_WRITER(MAX_FIELD_ORDER_ELEMS + MAX_STEREO_MODE_ELEMS      +
                MAX_VIDEO_COLOR_ELEMS + MAX_VIDEO_PROJECTION_ELEMS + 8);
    int ret;

    ebml_writer_open_master(&writer, MATROSKA_ID_TRACKVIDEO);

    ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEOPIXELWIDTH , par->width);
    ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEOPIXELHEIGHT, par->height);

    mkv_write_field_order(&writer, IS_WEBM(mkv), par->field_order);

    // check both side data and metadata for stereo information,
    // write the result to the bitstream if any is found
    ret = mkv_write_stereo_mode(s, &writer, st, IS_WEBM(mkv),
                                &display_width_div,
                                &display_height_div);
    if (ret < 0)
        return ret;

    if (par->format == AV_PIX_FMT_YUVA420P ||
        ((tag = av_dict_get(st->metadata, "alpha_mode", NULL, 0)) ||
         (tag = av_dict_get( s->metadata, "alpha_mode", NULL, 0))) && strtol(tag->value, NULL, 0))
        ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEOALPHAMODE, 1);

    // write DisplayWidth and DisplayHeight, they contain the size of
    // a single source view and/or the display aspect ratio
    if (st->sample_aspect_ratio.num) {
        int64_t d_width = av_rescale(par->width, st->sample_aspect_ratio.num, st->sample_aspect_ratio.den);
        if (d_width > INT_MAX) {
            av_log(s, AV_LOG_ERROR, "Overflow in display width\n");
            return AVERROR(EINVAL);
        }
        if (d_width != par->width || display_width_div != 1 || display_height_div != 1) {
            if (IS_WEBM(mkv) || display_width_div != 1 || display_height_div != 1) {
                ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYWIDTH,
                                     d_width / display_width_div);
                ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYHEIGHT,
                                     par->height / display_height_div);
            } else {
                AVRational display_aspect_ratio;
                av_reduce(&display_aspect_ratio.num, &display_aspect_ratio.den,
                            par->width  * (int64_t)st->sample_aspect_ratio.num,
                            par->height * (int64_t)st->sample_aspect_ratio.den,
                            1024 * 1024);
                ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYWIDTH,
                                     display_aspect_ratio.num);
                ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYHEIGHT,
                                     display_aspect_ratio.den);
                ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYUNIT,
                                     MATROSKA_VIDEO_DISPLAYUNIT_DAR);
            }
        }
    } else if (display_width_div != 1 || display_height_div != 1) {
        ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYWIDTH,
                             par->width / display_width_div);
        ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYHEIGHT,
                             par->height / display_height_div);
    } else if (!IS_WEBM(mkv))
        ebml_writer_add_uint(&writer, MATROSKA_ID_VIDEODISPLAYUNIT,
                             MATROSKA_VIDEO_DISPLAYUNIT_UNKNOWN);

    if (par->codec_id == AV_CODEC_ID_RAWVIDEO) {
        AV_WL32(color_space, par->codec_tag);
        ebml_writer_add_bin(&writer, MATROSKA_ID_VIDEOCOLORSPACE,
                            color_space, sizeof(color_space));
    }
    mkv_write_video_color(&writer, st, par);
    mkv_write_video_projection(s, &writer, st, projection_private);

    return ebml_writer_write(&writer, pb);
}

static void mkv_write_default_duration(mkv_track *track, AVIOContext *pb,
                                       AVRational duration)
{
    put_ebml_uint(pb, MATROSKA_ID_TRACKDEFAULTDURATION,
                  1000000000LL * duration.num / duration.den);
    track->default_duration_low  = 1000LL * duration.num / duration.den;
    track->default_duration_high = track->default_duration_low +
                                    !!(1000LL * duration.num % duration.den);
}

static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
                           AVStream *st, mkv_track *track, AVIOContext *pb,
                           int is_default)
{
    AVCodecParameters *par = st->codecpar;
    ebml_master subinfo, track_master;
    int native_id = 0;
    int qt_id = 0;
    int bit_depth;
    int sample_rate = par->sample_rate;
    int output_sample_rate = 0;
    int j, ret;
    const AVDictionaryEntry *tag;

    if (par->codec_type == AVMEDIA_TYPE_ATTACHMENT)
        return 0;

    track_master = start_ebml_master(pb, MATROSKA_ID_TRACKENTRY, 0);
    put_ebml_uint(pb, MATROSKA_ID_TRACKNUMBER, track->track_num);
    put_ebml_uid (pb, MATROSKA_ID_TRACKUID,    track->uid);
    put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGLACING, 0);    // no lacing (yet)

    if ((tag = av_dict_get(st->metadata, "title", NULL, 0)))
        put_ebml_string(pb, MATROSKA_ID_TRACKNAME, tag->value);
    tag = av_dict_get(st->metadata, "language", NULL, 0);
    put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE,
                    tag && tag->value[0] ? tag->value : "und");

    // The default value for TRACKFLAGDEFAULT is 1, so add element
    // if we need to clear it.
    if (!is_default)
        put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGDEFAULT, 0);

    if (st->disposition & AV_DISPOSITION_FORCED)
        put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGFORCED, 1);

    if (IS_WEBM(mkv)) {
        const char *codec_id;
        if (par->codec_id != AV_CODEC_ID_WEBVTT) {
            for (j = 0; ff_webm_codec_tags[j].id != AV_CODEC_ID_NONE; j++) {
                if (ff_webm_codec_tags[j].id == par->codec_id) {
                    codec_id = ff_webm_codec_tags[j].str;
                    native_id = 1;
                    break;
                }
            }
        } else {
            if (st->disposition & AV_DISPOSITION_CAPTIONS) {
                codec_id = "D_WEBVTT/CAPTIONS";
                native_id = MATROSKA_TRACK_TYPE_SUBTITLE;
            } else if (st->disposition & AV_DISPOSITION_DESCRIPTIONS) {
                codec_id = "D_WEBVTT/DESCRIPTIONS";
                native_id = MATROSKA_TRACK_TYPE_METADATA;
            } else if (st->disposition & AV_DISPOSITION_METADATA) {
                codec_id = "D_WEBVTT/METADATA";
                native_id = MATROSKA_TRACK_TYPE_METADATA;
            } else {
                codec_id = "D_WEBVTT/SUBTITLES";
                native_id = MATROSKA_TRACK_TYPE_SUBTITLE;
            }
        }

        if (!native_id) {
            av_log(s, AV_LOG_ERROR,
                   "Only VP8 or VP9 or AV1 video and Vorbis or Opus audio and WebVTT subtitles are supported for WebM.\n");
            return AVERROR(EINVAL);
        }

        put_ebml_string(pb, MATROSKA_ID_CODECID, codec_id);
    } else {
        if (st->disposition & AV_DISPOSITION_COMMENT)
            put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGCOMMENTARY, 1);
        if (st->disposition & AV_DISPOSITION_HEARING_IMPAIRED)
            put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGHEARINGIMPAIRED, 1);
        if (st->disposition & AV_DISPOSITION_VISUAL_IMPAIRED)
            put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGVISUALIMPAIRED,  1);
        if (st->disposition & (AV_DISPOSITION_ORIGINAL | AV_DISPOSITION_DUB) &&
            (st->disposition & (AV_DISPOSITION_ORIGINAL | AV_DISPOSITION_DUB))
                            != (AV_DISPOSITION_ORIGINAL | AV_DISPOSITION_DUB))
            put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGORIGINAL,
                          !!(st->disposition & AV_DISPOSITION_ORIGINAL));

        // look for a codec ID string specific to mkv to use,
        // if none are found, use AVI codes
        if (par->codec_id == AV_CODEC_ID_FFV1) {
            /* FFV1 is actually supported natively in Matroska,
             * yet we use the VfW way to mux it for compatibility
             * with old demuxers. (FIXME: Are they really important?) */
        } else if (par->codec_id != AV_CODEC_ID_RAWVIDEO || par->codec_tag) {
            for (j = 0; ff_mkv_codec_tags[j].id != AV_CODEC_ID_NONE; j++) {
                if (ff_mkv_codec_tags[j].id == par->codec_id) {
                    put_ebml_string(pb, MATROSKA_ID_CODECID, ff_mkv_codec_tags[j].str);
                    native_id = 1;
                    break;
                }
            }
        } else {
            if (mkv->allow_raw_vfw) {
                native_id = 0;
            } else {
                av_log(s, AV_LOG_ERROR, "Raw RGB is not supported Natively in Matroska, you can use AVI or NUT or\n"
                                        "If you would like to store it anyway using VFW mode, enable allow_raw_vfw (-allow_raw_vfw 1)\n");
                return AVERROR(EINVAL);
            }
        }
    }

    switch (par->codec_type) {
        AVRational frame_rate;
        int audio_frame_samples;

    case AVMEDIA_TYPE_VIDEO:
        mkv->have_video = 1;
        put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, MATROSKA_TRACK_TYPE_VIDEO);

        frame_rate = (AVRational){ 0, 1 };
        if (st->avg_frame_rate.num > 0 && st->avg_frame_rate.den > 0)
            frame_rate = st->avg_frame_rate;
        else if (st->r_frame_rate.num > 0 && st->r_frame_rate.den > 0)
            frame_rate = st->r_frame_rate;

        if (frame_rate.num > 0)
            mkv_write_default_duration(track, pb, av_inv_q(frame_rate));

        if (CONFIG_MATROSKA_MUXER && !native_id &&
            ff_codec_get_tag(ff_codec_movvideo_tags, par->codec_id) &&
            ((!ff_codec_get_tag(ff_codec_bmp_tags,   par->codec_id) && par->codec_id != AV_CODEC_ID_RAWVIDEO) ||
             par->codec_id == AV_CODEC_ID_SVQ1 ||
             par->codec_id == AV_CODEC_ID_SVQ3 ||
             par->codec_id == AV_CODEC_ID_CINEPAK))
            qt_id = 1;

        if (qt_id)
            put_ebml_string(pb, MATROSKA_ID_CODECID, "V_QUICKTIME");
        else if (!native_id) {
            // if there is no mkv-specific codec ID, use VFW mode
            put_ebml_string(pb, MATROSKA_ID_CODECID, "V_MS/VFW/FOURCC");
            track->write_dts = 1;
            ffformatcontext(s)->avoid_negative_ts_use_pts = 0;
        }

        ret = mkv_write_track_video(s, mkv, st, par, pb);
        if (ret < 0)
            return ret;

        break;

    case AVMEDIA_TYPE_AUDIO:
        if (par->initial_padding) {
            int64_t codecdelay = av_rescale_q(par->initial_padding,
                                              (AVRational){ 1, par->sample_rate },
                                              (AVRational){ 1, 1000000000 });
            if (codecdelay < 0) {
                av_log(s, AV_LOG_ERROR, "Initial padding is invalid\n");
                return AVERROR(EINVAL);
            }
            put_ebml_uint(pb, MATROSKA_ID_CODECDELAY, codecdelay);

            track->ts_offset = av_rescale_q(par->initial_padding,
                                            (AVRational){ 1, par->sample_rate },
                                            st->time_base);
            ffstream(st)->lowest_ts_allowed = -track->ts_offset;
        }
        if (par->codec_id == AV_CODEC_ID_OPUS)
            put_ebml_uint(pb, MATROSKA_ID_SEEKPREROLL, OPUS_SEEK_PREROLL);
#if CONFIG_MATROSKA_MUXER
        else if (par->codec_id == AV_CODEC_ID_AAC) {
            ret = get_aac_sample_rates(s, mkv, par->extradata, par->extradata_size,
                                       &sample_rate, &output_sample_rate);
            if (ret < 0)
                return ret;
        }
#endif

        put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, MATROSKA_TRACK_TYPE_AUDIO);

        audio_frame_samples = av_get_audio_frame_duration2(par, 0);
        if (audio_frame_samples)
            mkv_write_default_duration(track, pb, (AVRational){ audio_frame_samples,
                                                                par->sample_rate });
        if (!native_id)
            // no mkv-specific ID, use ACM mode
            put_ebml_string(pb, MATROSKA_ID_CODECID, "A_MS/ACM");

        subinfo = start_ebml_master(pb, MATROSKA_ID_TRACKAUDIO, 6 + 4 * 9);
        put_ebml_uint(pb, MATROSKA_ID_AUDIOCHANNELS, par->ch_layout.nb_channels);

        track->sample_rate_offset = avio_tell(pb);
        put_ebml_float (pb, MATROSKA_ID_AUDIOSAMPLINGFREQ, sample_rate);
        if (output_sample_rate)
            put_ebml_float(pb, MATROSKA_ID_AUDIOOUTSAMPLINGFREQ, output_sample_rate);

        bit_depth = av_get_bits_per_sample(par->codec_id);
        if (!bit_depth && par->codec_id != AV_CODEC_ID_ADPCM_G726) {
            if (par->bits_per_raw_sample)
                bit_depth = par->bits_per_raw_sample;
            else
                bit_depth = av_get_bytes_per_sample(par->format) << 3;
        }
        if (!bit_depth)
            bit_depth = par->bits_per_coded_sample;
        if (bit_depth)
            put_ebml_uint(pb, MATROSKA_ID_AUDIOBITDEPTH, bit_depth);
        end_ebml_master(pb, subinfo);
        break;

    case AVMEDIA_TYPE_SUBTITLE:
        if (!native_id) {
            av_log(s, AV_LOG_ERROR, "Subtitle codec %d is not supported.\n", par->codec_id);
            return AVERROR(ENOSYS);
        }
        if (!IS_WEBM(mkv) && st->disposition & AV_DISPOSITION_DESCRIPTIONS)
            put_ebml_uint(pb, MATROSKA_ID_TRACKFLAGTEXTDESCRIPTIONS, 1);

        if (!IS_WEBM(mkv) || par->codec_id != AV_CODEC_ID_WEBVTT)
            native_id = MATROSKA_TRACK_TYPE_SUBTITLE;

        put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, native_id);
        break;
    default:
        av_log(s, AV_LOG_ERROR, "Only audio, video, and subtitles are supported for Matroska.\n");
        return AVERROR(EINVAL);
    }

    if (!IS_WEBM(mkv))
        mkv_write_blockadditionmapping(s, mkv, par, pb, track, st);

    if (!IS_WEBM(mkv) || par->codec_id != AV_CODEC_ID_WEBVTT) {
        uint8_t *codecpriv;
        int codecpriv_size, max_payload_size;
        track->codecpriv_offset = avio_tell(pb);
        ret = mkv_assemble_codecprivate(s, mkv->tmp_bc, par,
                                        par->extradata, par->extradata_size,
                                        native_id, qt_id,
                                        &codecpriv, &codecpriv_size, &max_payload_size);
        if (ret < 0)
            goto fail;
        mkv_put_codecprivate(pb, max_payload_size, codecpriv, codecpriv_size);
        track->codecpriv_size = max_payload_size;
    }

    end_ebml_master(pb, track_master);
    ret = 0;
fail:
    ffio_reset_dyn_buf(mkv->tmp_bc);

    return ret;
}

static int mkv_write_tracks(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    AVIOContext *pb = s->pb;
    int video_default_idx = -1, audio_default_idx = -1, subtitle_default_idx = -1;
    int i, ret;

    if (mkv->nb_attachments == s->nb_streams)
        return 0;

    ret = start_ebml_master_crc32(&mkv->track.bc, mkv);
    if (ret < 0)
        return ret;

    if (mkv->default_mode != DEFAULT_MODE_PASSTHROUGH) {
        int video_idx = -1, audio_idx = -1, subtitle_idx = -1;

        for (i = s->nb_streams - 1; i >= 0; i--) {
            AVStream *st = s->streams[i];

            switch (st->codecpar->codec_type) {
#define CASE(type, variable)                                  \
            case AVMEDIA_TYPE_ ## type:                       \
                variable ## _idx = i;                         \
                if (st->disposition & AV_DISPOSITION_DEFAULT) \
                    variable ## _default_idx = i;             \
                break;
            CASE(VIDEO,    video)
            CASE(AUDIO,    audio)
            CASE(SUBTITLE, subtitle)
#undef CASE
            }
        }

        video_default_idx = FFMAX(video_default_idx, video_idx);
        audio_default_idx = FFMAX(audio_default_idx, audio_idx);
        if (mkv->default_mode != DEFAULT_MODE_INFER_NO_SUBS)
            subtitle_default_idx = FFMAX(subtitle_default_idx, subtitle_idx);
    }
    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];
        int is_default = st->disposition & AV_DISPOSITION_DEFAULT ||
                             i == video_default_idx || i == audio_default_idx ||
                             i == subtitle_default_idx;
        ret = mkv_write_track(s, mkv, st, &mkv->tracks[i],
                              mkv->track.bc, is_default);
        if (ret < 0)
            return ret;
    }

    return end_ebml_master_crc32_tentatively(pb, &mkv->track, mkv,
                                             MATROSKA_ID_TRACKS);
}

static int mkv_write_simpletag(AVIOContext *pb, const AVDictionaryEntry *t)
{
    EBML_WRITER(4);
    uint8_t *key = av_strdup(t->key);
    uint8_t *p   = key;
    const uint8_t *lang = NULL;
    int ret;

    if (!key)
        return AVERROR(ENOMEM);

    if ((p = strrchr(p, '-')) &&
        (lang = ff_convert_lang_to(p + 1, AV_LANG_ISO639_2_BIBL)))
        *p = 0;

    p = key;
    while (*p) {
        if (*p == ' ')
            *p = '_';
        else if (*p >= 'a' && *p <= 'z')
            *p -= 'a' - 'A';
        p++;
    }

    ebml_writer_open_master(&writer, MATROSKA_ID_SIMPLETAG);
    ebml_writer_add_string(&writer, MATROSKA_ID_TAGNAME, key);
    if (lang)
        ebml_writer_add_string(&writer, MATROSKA_ID_TAGLANG, lang);
    ebml_writer_add_string(&writer, MATROSKA_ID_TAGSTRING, t->value);
    ret = ebml_writer_write(&writer, pb);

    av_freep(&key);
    return ret;
}

static void mkv_write_tag_targets(MatroskaMuxContext *mkv, AVIOContext *pb,
                                  uint32_t elementid, uint64_t uid)
{
    ebml_master targets = start_ebml_master(pb, MATROSKA_ID_TAGTARGETS,
                                            4 + 1 + 8);
    if (elementid)
        put_ebml_uid(pb, elementid, uid);
    end_ebml_master(pb, targets);
}

static int mkv_check_tag_name(const char *name, uint32_t elementid)
{
    return av_strcasecmp(name, "title") &&
           av_strcasecmp(name, "stereo_mode") &&
           av_strcasecmp(name, "creation_time") &&
           av_strcasecmp(name, "encoding_tool") &&
           av_strcasecmp(name, "duration") &&
           (elementid != MATROSKA_ID_TAGTARGETS_TRACKUID ||
            av_strcasecmp(name, "language")) &&
           (elementid != MATROSKA_ID_TAGTARGETS_ATTACHUID ||
            (av_strcasecmp(name, "filename") &&
             av_strcasecmp(name, "mimetype")));
}

static int mkv_write_tag(MatroskaMuxContext *mkv, const AVDictionary *m,
                         AVIOContext **pb, unsigned reserved_size,
                         uint32_t elementid, uint64_t uid)
{
    const AVDictionaryEntry *t = NULL;
    AVIOContext *const tmp_bc = mkv->tmp_bc;
    uint8_t *buf;
    int ret = 0, size, tag_written = 0;

    mkv_write_tag_targets(mkv, tmp_bc, elementid, uid);

    while ((t = av_dict_iterate(m, t))) {
        if (mkv_check_tag_name(t->key, elementid)) {
            ret = mkv_write_simpletag(tmp_bc, t);
            if (ret < 0)
                goto end;
            tag_written = 1;
        }
    }
    if (reserved_size)
        put_ebml_void(tmp_bc, reserved_size);
    else if (!tag_written)
        goto end;

    size = avio_get_dyn_buf(tmp_bc, &buf);
    if (tmp_bc->error) {
        ret = tmp_bc->error;
        goto end;
    }
    if (!*pb) {
        ret = start_ebml_master_crc32(pb, mkv);
        if (ret < 0)
            goto end;
    }
    put_ebml_binary(*pb, MATROSKA_ID_TAG, buf, size);

end:
    ffio_reset_dyn_buf(tmp_bc);
    return ret;
}

static int mkv_write_tags(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    int i, ret, seekable = IS_SEEKABLE(s->pb, mkv);

    mkv->wrote_tags = 1;

    ff_metadata_conv_ctx(s, ff_mkv_metadata_conv, NULL);

    ret = mkv_write_tag(mkv, s->metadata, &mkv->tags.bc, 0, 0, 0);
    if (ret < 0)
        return ret;

    for (i = 0; i < s->nb_streams; i++) {
        const AVStream *st = s->streams[i];
        mkv_track *track = &mkv->tracks[i];

        if (st->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT)
            continue;

        ret = mkv_write_tag(mkv, st->metadata, &mkv->tags.bc,
                            seekable ? DURATION_SIMPLETAG_SIZE : 0,
                            MATROSKA_ID_TAGTARGETS_TRACKUID, track->uid);
        if (ret < 0)
            return ret;
        if (seekable)
            track->duration_offset = avio_tell(mkv->tags.bc) - DURATION_SIMPLETAG_SIZE;
    }

    if (mkv->nb_attachments && !IS_WEBM(mkv)) {
        for (i = 0; i < s->nb_streams; i++) {
            const mkv_track *track = &mkv->tracks[i];
            const AVStream     *st = s->streams[i];

            if (st->codecpar->codec_type != AVMEDIA_TYPE_ATTACHMENT)
                continue;

            ret = mkv_write_tag(mkv, st->metadata, &mkv->tags.bc, 0,
                                MATROSKA_ID_TAGTARGETS_ATTACHUID, track->uid);
            if (ret < 0)
                return ret;
        }
    }

    if (mkv->tags.bc) {
        return end_ebml_master_crc32_tentatively(s->pb, &mkv->tags, mkv,
                                                 MATROSKA_ID_TAGS);
    }
    return 0;
}

static int mkv_new_chapter_ids_needed(const AVFormatContext *s)
{
    for (unsigned i = 0; i < s->nb_chapters; i++) {
        if (!s->chapters[i]->id)
            return 1;
        for (unsigned j = 0; j < i; j++)
            if (s->chapters[j]->id == s->chapters[i]->id)
                return 1;
    }
    return 0;
}

static int mkv_write_chapters(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    AVIOContext *dyn_cp = NULL, *dyn_tags = NULL, **tags, *pb = s->pb;
    ebml_master editionentry;
    AVRational scale = {1, 1E9};
    int ret, create_new_ids;

    if (!s->nb_chapters || mkv->wrote_chapters)
        return 0;

    ret = start_ebml_master_crc32(&dyn_cp, mkv);
    if (ret < 0)
        return ret;

    editionentry = start_ebml_master(dyn_cp, MATROSKA_ID_EDITIONENTRY, 0);
    if (!IS_WEBM(mkv)) {
        put_ebml_uint(dyn_cp, MATROSKA_ID_EDITIONFLAGDEFAULT, 1);
        /* If mkv_write_tags() has already been called, then any tags
         * corresponding to chapters will be put into a new Tags element. */
        tags = mkv->wrote_tags ? &dyn_tags : &mkv->tags.bc;
    } else
        tags = NULL;

    create_new_ids = mkv_new_chapter_ids_needed(s);

    for (unsigned i = 0; i < s->nb_chapters; i++) {
        AVChapter *const c   = s->chapters[i];
        int64_t chapterstart = av_rescale_q(c->start, c->time_base, scale);
        int64_t chapterend   = av_rescale_q(c->end,   c->time_base, scale);
        const AVDictionaryEntry *t;
        uint64_t uid = create_new_ids ? i + 1ULL : c->id;
        EBML_WRITER(7);

        if (chapterstart < 0 || chapterstart > chapterend || chapterend < 0) {
            av_log(s, AV_LOG_ERROR,
                   "Invalid chapter start (%"PRId64") or end (%"PRId64").\n",
                   chapterstart, chapterend);
            ret = AVERROR_INVALIDDATA;
            goto fail;
        }

        ebml_writer_open_master(&writer, MATROSKA_ID_CHAPTERATOM);
        ebml_writer_add_uint(&writer, MATROSKA_ID_CHAPTERUID, uid);
        ebml_writer_add_uint(&writer, MATROSKA_ID_CHAPTERTIMESTART, chapterstart);
        ebml_writer_add_uint(&writer, MATROSKA_ID_CHAPTERTIMEEND, chapterend);
        if ((t = av_dict_get(c->metadata, "title", NULL, 0))) {
            ebml_writer_open_master(&writer, MATROSKA_ID_CHAPTERDISPLAY);
            ebml_writer_add_string(&writer, MATROSKA_ID_CHAPSTRING, t->value);
            ebml_writer_add_string(&writer, MATROSKA_ID_CHAPLANG  , "und");
        }
        ret = ebml_writer_write(&writer, dyn_cp);
        if (ret < 0)
            goto fail;

        if (tags) {
            ff_metadata_conv(&c->metadata, ff_mkv_metadata_conv, NULL);

            ret = mkv_write_tag(mkv, c->metadata, tags, 0,
                                MATROSKA_ID_TAGTARGETS_CHAPTERUID, uid);
            if (ret < 0)
                goto fail;
        }
    }
    end_ebml_master(dyn_cp, editionentry);
    mkv->wrote_chapters = 1;

    ret = end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_CHAPTERS, 0, 0, 1);
    if (ret < 0)
        goto fail;
    if (dyn_tags)
        return end_ebml_master_crc32(pb, &dyn_tags, mkv,
                                     MATROSKA_ID_TAGS, 0, 0, 1);
    return 0;

fail:
    if (tags) {
        /* tags == &mkv->tags.bc can only happen if mkv->tags.bc was
         * initially NULL, so we never free older tags. */
        ffio_free_dyn_buf(tags);
    }
    ffio_free_dyn_buf(&dyn_cp);
    return ret;
}

static const char *get_mimetype(const AVStream *st)
{
    const AVDictionaryEntry *t;

    if (t = av_dict_get(st->metadata, "mimetype", NULL, 0))
        return t->value;
    if (st->codecpar->codec_id != AV_CODEC_ID_NONE) {
        const AVCodecDescriptor *desc = avcodec_descriptor_get(st->codecpar->codec_id);
        if (desc && desc->mime_types) {
            return desc->mime_types[0];
        } else if (st->codecpar->codec_id == AV_CODEC_ID_TEXT)
            return "text/plain";
    }

    return NULL;
}

static int mkv_write_attachments(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    AVIOContext *dyn_cp = NULL, *pb = s->pb;
    int i, ret;

    if (!mkv->nb_attachments)
        return 0;

    ret = start_ebml_master_crc32(&dyn_cp, mkv);
    if (ret < 0)
        return ret;

    for (i = 0; i < s->nb_streams; i++) {
        const AVStream *st = s->streams[i];
        mkv_track *track = &mkv->tracks[i];
        EBML_WRITER(6);
        const AVDictionaryEntry *t;
        const char *mimetype;

        if (st->codecpar->codec_type != AVMEDIA_TYPE_ATTACHMENT)
            continue;

        ebml_writer_open_master(&writer, MATROSKA_ID_ATTACHEDFILE);

        if (t = av_dict_get(st->metadata, "title", NULL, 0))
            ebml_writer_add_string(&writer, MATROSKA_ID_FILEDESC, t->value);
        if (!(t = av_dict_get(st->metadata, "filename", NULL, 0))) {
            av_log(s, AV_LOG_ERROR, "Attachment stream %d has no filename tag.\n", i);
            ffio_free_dyn_buf(&dyn_cp);
            return AVERROR(EINVAL);
        }
        ebml_writer_add_string(&writer, MATROSKA_ID_FILENAME, t->value);

        mimetype = get_mimetype(st);
        av_assert0(mimetype);
        ebml_writer_add_string(&writer, MATROSKA_ID_FILEMIMETYPE, mimetype);
        ebml_writer_add_bin(&writer, MATROSKA_ID_FILEDATA,
                            st->codecpar->extradata, st->codecpar->extradata_size);
        ebml_writer_add_uid(&writer, MATROSKA_ID_FILEUID, track->uid);
        ret = ebml_writer_write(&writer, dyn_cp);
        if (ret < 0) {
            ffio_free_dyn_buf(&dyn_cp);
            return ret;
        }
    }
    return end_ebml_master_crc32(pb, &dyn_cp, mkv,
                                 MATROSKA_ID_ATTACHMENTS, 0, 0, 1);
}

static int64_t get_metadata_duration(AVFormatContext *s)
{
    const AVDictionaryEntry *duration = av_dict_get(s->metadata, "DURATION",
                                                    NULL, 0);
    int64_t max = 0;
    int64_t us;

    if (duration && (av_parse_time(&us, duration->value, 1) == 0) && us > 0) {
        av_log(s, AV_LOG_DEBUG, "get_metadata_duration found duration in context metadata: %" PRId64 "\n", us);
        return us;
    }

    for (unsigned i = 0; i < s->nb_streams; i++) {
        int64_t us;
        duration = av_dict_get(s->streams[i]->metadata, "DURATION", NULL, 0);

        if (duration && (av_parse_time(&us, duration->value, 1) == 0))
            max = FFMAX(max, us);
    }

    av_log(s, AV_LOG_DEBUG, "get_metadata_duration returned: %" PRId64 "\n", max);
    return max;
}

static void ebml_write_header(AVIOContext *pb,
                              const char *doctype, int version)
{
    EBML_WRITER(8);
    ebml_writer_open_master(&writer, EBML_ID_HEADER);
    ebml_writer_add_uint  (&writer, EBML_ID_EBMLVERSION,              1);
    ebml_writer_add_uint  (&writer, EBML_ID_EBMLREADVERSION,          1);
    ebml_writer_add_uint  (&writer, EBML_ID_EBMLMAXIDLENGTH,          4);
    ebml_writer_add_uint  (&writer, EBML_ID_EBMLMAXSIZELENGTH,        8);
    ebml_writer_add_string(&writer, EBML_ID_DOCTYPE,            doctype);
    ebml_writer_add_uint  (&writer, EBML_ID_DOCTYPEVERSION,     version);
    ebml_writer_add_uint  (&writer, EBML_ID_DOCTYPEREADVERSION,       2);
    /* The size is bounded, so no need to check this. */
    ebml_writer_write(&writer, pb);
}

static int mkv_write_info(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    const AVDictionaryEntry *tag;
    int64_t creation_time;
    AVIOContext *pb;
    int ret = start_ebml_master_crc32(&mkv->info.bc, mkv);
    if (ret < 0)
        return ret;
    pb = mkv->info.bc;

    put_ebml_uint(pb, MATROSKA_ID_TIMECODESCALE, 1000000);
    if ((tag = av_dict_get(s->metadata, "title", NULL, 0)))
        put_ebml_string(pb, MATROSKA_ID_TITLE, tag->value);
    if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
        put_ebml_string(pb, MATROSKA_ID_MUXINGAPP, LIBAVFORMAT_IDENT);
        if ((tag = av_dict_get(s->metadata, "encoding_tool", NULL, 0)))
            put_ebml_string(pb, MATROSKA_ID_WRITINGAPP, tag->value);
        else
            put_ebml_string(pb, MATROSKA_ID_WRITINGAPP, LIBAVFORMAT_IDENT);

        if (!IS_WEBM(mkv))
            put_ebml_binary(pb, MATROSKA_ID_SEGMENTUID, mkv->segment_uid, 16);
    } else {
        const char *ident = "Lavf";
        put_ebml_string(pb, MATROSKA_ID_MUXINGAPP , ident);
        put_ebml_string(pb, MATROSKA_ID_WRITINGAPP, ident);
    }

    if (ff_parse_creation_time_metadata(s, &creation_time, 0) > 0) {
        // Adjust time so it's relative to 2001-01-01 and convert to nanoseconds.
        int64_t date_utc = (creation_time - 978307200000000LL) * 1000;
        uint8_t date_utc_buf[8];
        AV_WB64(date_utc_buf, date_utc);
        put_ebml_binary(pb, MATROSKA_ID_DATEUTC, date_utc_buf, 8);
    }

    // reserve space for the duration
    mkv->duration = 0;
    mkv->duration_offset = avio_tell(pb);
    if (!mkv->is_live) {
        int64_t metadata_duration = get_metadata_duration(s);

        if (s->duration > 0) {
            int64_t scaledDuration = av_rescale(s->duration, 1000, AV_TIME_BASE);
            put_ebml_float(pb, MATROSKA_ID_DURATION, scaledDuration);
            av_log(s, AV_LOG_DEBUG, "Write early duration from recording time = %" PRIu64 "\n", scaledDuration);
        } else if (metadata_duration > 0) {
            int64_t scaledDuration = av_rescale(metadata_duration, 1000, AV_TIME_BASE);
            put_ebml_float(pb, MATROSKA_ID_DURATION, scaledDuration);
            av_log(s, AV_LOG_DEBUG, "Write early duration from metadata = %" PRIu64 "\n", scaledDuration);
        } else if (s->pb->seekable & AVIO_SEEKABLE_NORMAL) {
            put_ebml_void(pb, 11);              // assumes double-precision float to be written
        }
    }
    return end_ebml_master_crc32_tentatively(s->pb, &mkv->info,
                                             mkv, MATROSKA_ID_INFO);
}

static int mkv_write_header(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    AVIOContext *pb = s->pb;
    int ret, version = 2;

    ret = avio_open_dyn_buf(&mkv->tmp_bc);
    if (ret < 0)
        return ret;

    if (!IS_WEBM(mkv) ||
        av_dict_get(s->metadata, "stereo_mode", NULL, 0) ||
        av_dict_get(s->metadata, "alpha_mode", NULL, 0))
        version = 4;

    for (unsigned i = 0; i < s->nb_streams; i++) {
        if (s->streams[i]->codecpar->codec_id == AV_CODEC_ID_OPUS ||
            av_dict_get(s->streams[i]->metadata, "stereo_mode", NULL, 0) ||
            av_dict_get(s->streams[i]->metadata, "alpha_mode", NULL, 0))
            version = 4;
    }

    ebml_write_header(pb, s->oformat->name, version);
    put_ebml_id(pb, MATROSKA_ID_SEGMENT);
    put_ebml_size_unknown(pb, 8);
    mkv->segment_offset = avio_tell(pb);

    // We write a SeekHead at the beginning to point to all other level
    // one elements (except Clusters).
    mkv_start_seekhead(mkv, pb);

    ret = mkv_write_info(s);
    if (ret < 0)
        return ret;

    ret = mkv_write_tracks(s);
    if (ret < 0)
        return ret;

    ret = mkv_write_chapters(s);
    if (ret < 0)
        return ret;

    if (!IS_WEBM(mkv)) {
        ret = mkv_write_attachments(s);
        if (ret < 0)
            return ret;
    }

    /* Must come after mkv_write_chapters() to write chapter tags
     * into the same Tags element as the other tags. */
    ret = mkv_write_tags(s);
    if (ret < 0)
        return ret;

    if (!IS_SEEKABLE(pb, mkv)) {
        ret = mkv_write_seekhead(pb, mkv, 0, avio_tell(pb));
        if (ret < 0)
            return ret;
    }

    if (s->metadata_header_padding > 0) {
        if (s->metadata_header_padding == 1)
            s->metadata_header_padding++;
        put_ebml_void(pb, s->metadata_header_padding);
    }

    if (mkv->reserve_cues_space || mkv->move_cues_to_front) {
        if (IS_SEEKABLE(pb, mkv)) {
            mkv->cues_pos = avio_tell(pb);
            if (mkv->reserve_cues_space >= 1) {
                if (mkv->reserve_cues_space == 1)
                    mkv->reserve_cues_space++;
                put_ebml_void(pb, mkv->reserve_cues_space);
            }
        } else
            mkv->reserve_cues_space = -1;
    }

    mkv->cluster_pos = -1;

    // start a new cluster every 5 MB or 5 sec, or 32k / 1 sec for streaming or
    // after 4k and on a keyframe
    if (IS_SEEKABLE(pb, mkv)) {
        if (mkv->cluster_time_limit < 0)
            mkv->cluster_time_limit = 5000;
        if (mkv->cluster_size_limit < 0)
            mkv->cluster_size_limit = 5 * 1024 * 1024;
    } else {
        if (mkv->cluster_time_limit < 0)
            mkv->cluster_time_limit = 1000;
        if (mkv->cluster_size_limit < 0)
            mkv->cluster_size_limit = 32 * 1024;
    }

    return 0;
}

#if CONFIG_MATROSKA_MUXER
static int mkv_reformat_h2645(MatroskaMuxContext *mkv, AVIOContext *pb,
                              const AVPacket *pkt, int *size)
{
    int ret;
    if (pb) {
        ff_nal_units_write_list(&mkv->cur_block.h2645_nalu_list, pb, pkt->data);
    } else {
        ret = ff_nal_units_create_list(&mkv->cur_block.h2645_nalu_list, pkt->data, pkt->size);
        if (ret < 0)
            return ret;
        *size = ret;
    }
    return 0;
}

static int mkv_reformat_wavpack(MatroskaMuxContext *mkv, AVIOContext *pb,
                                const AVPacket *pkt, int *size)
{
    const uint8_t *src = pkt->data;
    int srclen = pkt->size;
    int offset = 0;
    int ret;

    while (srclen >= WV_HEADER_SIZE) {
        WvHeader header;

        ret = ff_wv_parse_header(&header, src);
        if (ret < 0)
            return ret;
        src    += WV_HEADER_SIZE;
        srclen -= WV_HEADER_SIZE;

        if (srclen < header.blocksize)
            return AVERROR_INVALIDDATA;

        offset += 4 * !!header.initial + 8 + 4 * !(header.initial && header.final);
        if (pb) {
            if (header.initial)
                avio_wl32(pb, header.samples);
            avio_wl32(pb, header.flags);
            avio_wl32(pb, header.crc);

            if (!(header.initial && header.final))
                avio_wl32(pb, header.blocksize);

            avio_write(pb, src, header.blocksize);
        }
        src    += header.blocksize;
        srclen -= header.blocksize;
        offset += header.blocksize;
    }
    *size = offset;

    return 0;
}
#endif

static int mkv_reformat_av1(MatroskaMuxContext *mkv, AVIOContext *pb,
                            const AVPacket *pkt, int *size)
{
    int ret = ff_av1_filter_obus(pb, pkt->data, pkt->size);
    if (ret < 0)
        return ret;
    *size = ret;
    return 0;
}

static int webm_reformat_vtt(MatroskaMuxContext *mkv, AVIOContext *pb,
                             const AVPacket *pkt, int *size)
{
    const uint8_t *id, *settings;
    size_t id_size, settings_size;
    unsigned total = pkt->size + 2U;

    if (total > INT_MAX)
        return AVERROR(ERANGE);

    id       = av_packet_get_side_data(pkt, AV_PKT_DATA_WEBVTT_IDENTIFIER,
                                       &id_size);
    settings = av_packet_get_side_data(pkt, AV_PKT_DATA_WEBVTT_SETTINGS,
                                       &settings_size);
    if (id_size > INT_MAX - total || settings_size > INT_MAX - (total += id_size))
        return AVERROR(ERANGE);
    *size = total += settings_size;
    if (pb) {
        avio_write(pb, id, id_size);
        avio_w8(pb, '\n');
        avio_write(pb, settings, settings_size);
        avio_w8(pb, '\n');
        avio_write(pb, pkt->data, pkt->size);
    }
    return 0;
}

static void mkv_write_blockadditional(EbmlWriter *writer, const uint8_t *buf,
                                      size_t size, uint64_t additional_id)
{
    ebml_writer_open_master(writer, MATROSKA_ID_BLOCKMORE);
    ebml_writer_add_uint(writer, MATROSKA_ID_BLOCKADDID, additional_id);
    ebml_writer_add_bin (writer, MATROSKA_ID_BLOCKADDITIONAL, buf, size);
    ebml_writer_close_master(writer);
}

static int mkv_write_block(void *logctx, MatroskaMuxContext *mkv,
                           AVIOContext *pb, const AVCodecParameters *par,
                           mkv_track *track, const AVPacket *pkt,
                           int keyframe, int64_t ts, uint64_t duration,
                           int force_blockgroup, int64_t relative_packet_pos)
{
    uint8_t t35_buf[6 + AV_HDR_PLUS_MAX_PAYLOAD_SIZE];
    uint8_t *side_data;
    size_t side_data_size;
    uint64_t additional_id;
    unsigned track_number = track->track_num;
    EBML_WRITER(12);
    int ret;

    mkv->cur_block.track  = track;
    mkv->cur_block.pkt    = pkt;
    mkv->cur_block.rel_ts = ts - mkv->cluster_pts;
    mkv->cur_block.flags  = 0;

    /* Open a BlockGroup with a Block now; it will later be converted
     * to a SimpleBlock if possible. */
    ebml_writer_open_master(&writer, MATROSKA_ID_BLOCKGROUP);
    ebml_writer_add_block(&writer, mkv);

    if (duration > 0 && (par->codec_type == AVMEDIA_TYPE_SUBTITLE ||
        /* If the packet's duration is inconsistent with the default duration,
         * add an explicit duration element. */
        track->default_duration_high > 0 &&
        duration != track->default_duration_high &&
        duration != track->default_duration_low))
        ebml_writer_add_uint(&writer, MATROSKA_ID_BLOCKDURATION, duration);

    av_log(logctx, AV_LOG_DEBUG,
           "Writing block of size %d with pts %" PRId64 ", dts %" PRId64 ", "
           "duration %" PRId64 " at relative offset %" PRId64 " in cluster "
           "at offset %" PRId64 ". TrackNumber %u, keyframe %d\n",
           pkt->size, pkt->pts, pkt->dts, pkt->duration, relative_packet_pos,
           mkv->cluster_pos, track_number, keyframe != 0);

    side_data = av_packet_get_side_data(pkt,
                                        AV_PKT_DATA_SKIP_SAMPLES,
                                        &side_data_size);
    if (side_data && side_data_size >= 10) {
        int64_t discard_padding = AV_RL32(side_data + 4);
        if (discard_padding) {
            discard_padding = av_rescale_q(discard_padding,
                                           (AVRational){1, par->sample_rate},
                                           (AVRational){1, 1000000000});
            ebml_writer_add_sint(&writer, MATROSKA_ID_DISCARDPADDING, discard_padding);
        }
    }

    ebml_writer_open_master(&writer, MATROSKA_ID_BLOCKADDITIONS);
    side_data = av_packet_get_side_data(pkt,
                                        AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL,
                                        &side_data_size);
    if (side_data && side_data_size >= 8 &&
        // Only the Codec-specific BlockMore (id == 1) is currently supported.
        (additional_id = AV_RB64(side_data)) == MATROSKA_BLOCK_ADD_ID_TYPE_OPAQUE) {
        mkv_write_blockadditional(&writer, side_data + 8, side_data_size - 8,
                                  additional_id);
        track->max_blockaddid = FFMAX(track->max_blockaddid, additional_id);
    }

    if (par->codec_type == AVMEDIA_TYPE_VIDEO) {
        side_data = av_packet_get_side_data(pkt,
                                            AV_PKT_DATA_DYNAMIC_HDR10_PLUS,
                                            &side_data_size);
        if (side_data && side_data_size) {
            uint8_t *payload = t35_buf;
            size_t payload_size = sizeof(t35_buf) - 6;

            bytestream_put_byte(&payload, ITU_T_T35_COUNTRY_CODE_US);
            bytestream_put_be16(&payload, ITU_T_T35_PROVIDER_CODE_SMTPE);
            bytestream_put_be16(&payload, 0x01); // provider_oriented_code
            bytestream_put_byte(&payload, 0x04); // application_identifier

            ret = av_dynamic_hdr_plus_to_t35((AVDynamicHDRPlus *)side_data, &payload,
                                            &payload_size);
            if (ret < 0)
                return ret;

            mkv_write_blockadditional(&writer, t35_buf, payload_size + 6,
                                      MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
            track->max_blockaddid = FFMAX(track->max_blockaddid,
                                          MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
        }
    }

    ebml_writer_close_or_discard_master(&writer);

    if (!force_blockgroup && writer.nb_elements == 2) {
        /* Nothing except the BlockGroup + Block. Can use a SimpleBlock. */
        writer.elements++;    // Skip the BlockGroup.
        writer.nb_elements--;
        av_assert2(writer.elements[0].id == MATROSKA_ID_BLOCK);
        writer.elements[0].id = MATROSKA_ID_SIMPLEBLOCK;
        if (keyframe)
            mkv->cur_block.flags |= 1 << 7;
    } else if (!keyframe)
        ebml_writer_add_sint(&writer, MATROSKA_ID_BLOCKREFERENCE,
                             track->last_timestamp - ts);

    return ebml_writer_write(&writer, pb);
}

static int mkv_end_cluster(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    int ret;

    if (!mkv->have_video) {
        for (unsigned i = 0; i < s->nb_streams; i++)
            mkv->tracks[i].has_cue = 0;
    }
    mkv->cluster_pos = -1;
    ret = end_ebml_master_crc32(s->pb, &mkv->cluster_bc, mkv,
                                MATROSKA_ID_CLUSTER, 0, 1, 0);
    if (ret < 0)
        return ret;

    avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_FLUSH_POINT);
    return 0;
}

static int mkv_check_new_extra_data(AVFormatContext *s, const AVPacket *pkt)
{
    MatroskaMuxContext *mkv = s->priv_data;
    mkv_track *track        = &mkv->tracks[pkt->stream_index];
    AVCodecParameters *par  = s->streams[pkt->stream_index]->codecpar;
    uint8_t *side_data;
    size_t side_data_size;
    int ret;

    side_data = av_packet_get_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA,
                                        &side_data_size);

    switch (par->codec_id) {
#if CONFIG_MATROSKA_MUXER
    case AV_CODEC_ID_AAC:
        if (side_data_size && mkv->track.bc) {
            int output_sample_rate = 0;
            ret = get_aac_sample_rates(s, mkv, side_data, side_data_size,
                                       &track->sample_rate, &output_sample_rate);
            if (ret < 0)
                return ret;
            if (!output_sample_rate)
                output_sample_rate = track->sample_rate; // Space is already reserved, so it's this or a void element.
            ret = mkv_update_codecprivate(s, mkv, side_data, side_data_size,
                                          par, mkv->track.bc, track, 0);
            if (ret < 0)
                return ret;
            avio_seek(mkv->track.bc, track->sample_rate_offset, SEEK_SET);
            put_ebml_float(mkv->track.bc, MATROSKA_ID_AUDIOSAMPLINGFREQ, track->sample_rate);
            put_ebml_float(mkv->track.bc, MATROSKA_ID_AUDIOOUTSAMPLINGFREQ, output_sample_rate);
        } else if (!par->extradata_size && !track->sample_rate) {
            // No extradata (codecpar or packet side data).
            av_log(s, AV_LOG_ERROR, "Error parsing AAC extradata, unable to determine samplerate.\n");
            return AVERROR(EINVAL);
        }
        break;
    case AV_CODEC_ID_FLAC:
        if (side_data_size && mkv->track.bc) {
            if (side_data_size != par->extradata_size) {
                av_log(s, AV_LOG_ERROR, "Invalid FLAC STREAMINFO metadata for output stream %d\n",
                       pkt->stream_index);
                return AVERROR(EINVAL);
            }
            ret = mkv_update_codecprivate(s, mkv, side_data, side_data_size,
                                          par, mkv->track.bc, track, 0);
            if (ret < 0)
                return ret;
        }
        break;
#endif
    // FIXME: Remove the following once libaom starts propagating proper extradata during init()
    //        See https://bugs.chromium.org/p/aomedia/issues/detail?id=2208
    case AV_CODEC_ID_AV1:
        if (side_data_size && mkv->track.bc && !par->extradata_size) {
            // If the reserved space doesn't suffice, only write
            // the first four bytes of the av1C.
            ret = mkv_update_codecprivate(s, mkv, side_data, side_data_size,
                                          par, mkv->track.bc, track, 4);
            if (ret < 0)
                return ret;
        } else if (!par->extradata_size)
            return AVERROR_INVALIDDATA;
        break;
    default:
        if (side_data_size)
            av_log(s, AV_LOG_DEBUG, "Ignoring new extradata in a packet for stream %d.\n", pkt->stream_index);
        break;
    }

    return 0;
}

static int mkv_write_packet_internal(AVFormatContext *s, const AVPacket *pkt)
{
    MatroskaMuxContext *mkv = s->priv_data;
    AVIOContext *pb;
    AVCodecParameters *par  = s->streams[pkt->stream_index]->codecpar;
    mkv_track *track        = &mkv->tracks[pkt->stream_index];
    int is_sub              = par->codec_type == AVMEDIA_TYPE_SUBTITLE;
    /* All subtitle blocks are considered to be keyframes. */
    int keyframe            = is_sub || !!(pkt->flags & AV_PKT_FLAG_KEY);
    int64_t duration        = FFMAX(pkt->duration, 0);
    int64_t cue_duration    = is_sub ? duration : 0;
    int ret;
    int64_t ts = track->write_dts ? pkt->dts : pkt->pts;
    int64_t relative_packet_pos;

    if (ts == AV_NOPTS_VALUE) {
        av_log(s, AV_LOG_ERROR, "Can't write packet with unknown timestamp\n");
        return AVERROR(EINVAL);
    }
    ts += track->ts_offset;

    if (mkv->cluster_pos != -1) {
        int64_t cluster_time = ts - mkv->cluster_pts;
        if ((int16_t)cluster_time != cluster_time) {
            ret = mkv_end_cluster(s);
            if (ret < 0)
                return ret;
            av_log(s, AV_LOG_WARNING, "Starting new cluster due to timestamp\n");
        }
    }

    if (mkv->cluster_pos == -1) {
        ret = start_ebml_master_crc32(&mkv->cluster_bc, mkv);
        if (ret < 0)
            return ret;
        mkv->cluster_bc->direct = 1;
        mkv->cluster_pos = avio_tell(s->pb);
        put_ebml_uint(mkv->cluster_bc, MATROSKA_ID_CLUSTERTIMECODE, FFMAX(0, ts));
        mkv->cluster_pts = FFMAX(0, ts);
        av_log(s, AV_LOG_DEBUG,
               "Starting new cluster with timestamp "
               "%" PRId64 " at offset %" PRId64 " bytes\n",
               mkv->cluster_pts, mkv->cluster_pos);
    }
    pb = mkv->cluster_bc;

    relative_packet_pos = avio_tell(pb);

    /* The WebM spec requires WebVTT to be muxed in BlockGroups;
     * so we force it even for packets without duration. */
    ret = mkv_write_block(s, mkv, pb, par, track, pkt,
                          keyframe, ts, duration,
                          par->codec_id == AV_CODEC_ID_WEBVTT,
                          relative_packet_pos);
    if (ret < 0)
        return ret;
    if (keyframe && IS_SEEKABLE(s->pb, mkv) &&
        (par->codec_type == AVMEDIA_TYPE_VIDEO    ||
         par->codec_type == AVMEDIA_TYPE_SUBTITLE ||
         !mkv->have_video && !track->has_cue)) {
        ret = mkv_add_cuepoint(mkv, pkt->stream_index, ts,
                               mkv->cluster_pos, relative_packet_pos,
                               cue_duration);
        if (ret < 0)
            return ret;
        track->has_cue = 1;
    }

    track->last_timestamp = ts;
    mkv->duration   = FFMAX(mkv->duration,   ts + duration);
    track->duration = FFMAX(track->duration, ts + duration);

    return 0;
}

static int mkv_write_packet(AVFormatContext *s, const AVPacket *pkt)
{
    MatroskaMuxContext *mkv = s->priv_data;
    int codec_type          = s->streams[pkt->stream_index]->codecpar->codec_type;
    int keyframe            = !!(pkt->flags & AV_PKT_FLAG_KEY);
    int cluster_size;
    int64_t cluster_time;
    int ret;
    int start_new_cluster;

    ret = mkv_check_new_extra_data(s, pkt);
    if (ret < 0)
        return ret;

    if (mkv->cluster_pos != -1) {
        if (mkv->tracks[pkt->stream_index].write_dts)
            cluster_time = pkt->dts - mkv->cluster_pts;
        else
            cluster_time = pkt->pts - mkv->cluster_pts;
        cluster_time += mkv->tracks[pkt->stream_index].ts_offset;

        cluster_size  = avio_tell(mkv->cluster_bc);

        if (mkv->is_dash && codec_type == AVMEDIA_TYPE_VIDEO) {
            // WebM DASH specification states that the first block of
            // every Cluster has to be a key frame. So for DASH video,
            // we only create a Cluster on seeing key frames.
            start_new_cluster = keyframe;
        } else if (mkv->is_dash && codec_type == AVMEDIA_TYPE_AUDIO &&
                   cluster_time > mkv->cluster_time_limit) {
            // For DASH audio, we create a Cluster based on cluster_time_limit.
            start_new_cluster = 1;
        } else if (!mkv->is_dash &&
                   (cluster_size > mkv->cluster_size_limit ||
                    cluster_time > mkv->cluster_time_limit ||
                    (codec_type == AVMEDIA_TYPE_VIDEO && keyframe &&
                     cluster_size > 4 * 1024))) {
            start_new_cluster = 1;
        } else
            start_new_cluster = 0;

        if (start_new_cluster) {
            ret = mkv_end_cluster(s);
            if (ret < 0)
                return ret;
        }
    }

    if (mkv->cluster_pos == -1)
        avio_write_marker(s->pb,
                          av_rescale_q(pkt->dts, s->streams[pkt->stream_index]->time_base, AV_TIME_BASE_Q),
                          keyframe && (mkv->have_video ? codec_type == AVMEDIA_TYPE_VIDEO : 1) ? AVIO_DATA_MARKER_SYNC_POINT : AVIO_DATA_MARKER_BOUNDARY_POINT);

    // check if we have an audio packet cached
    if (mkv->cur_audio_pkt->size > 0) {
        ret = mkv_write_packet_internal(s, mkv->cur_audio_pkt);
        av_packet_unref(mkv->cur_audio_pkt);
        if (ret < 0) {
            av_log(s, AV_LOG_ERROR,
                   "Could not write cached audio packet ret:%d\n", ret);
            return ret;
        }
    }

    // buffer an audio packet to ensure the packet containing the video
    // keyframe's timecode is contained in the same cluster for WebM
    if (codec_type == AVMEDIA_TYPE_AUDIO) {
        if (pkt->size > 0)
            ret = av_packet_ref(mkv->cur_audio_pkt, pkt);
    } else
        ret = mkv_write_packet_internal(s, pkt);
    return ret;
}

static int mkv_write_flush_packet(AVFormatContext *s, AVPacket *pkt)
{
    MatroskaMuxContext *mkv = s->priv_data;

    if (!pkt) {
        if (mkv->cluster_pos != -1) {
            int ret = mkv_end_cluster(s);
            if (ret < 0)
                return ret;
            av_log(s, AV_LOG_DEBUG,
                   "Flushing cluster at offset %" PRIu64 " bytes\n",
                   avio_tell(s->pb));
        }
        return 1;
    }
    return mkv_write_packet(s, pkt);
}

static int mkv_write_trailer(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    AVIOContext *pb = s->pb;
    int64_t endpos, ret64;
    int ret, ret2 = 0;

    // check if we have an audio packet cached
    if (mkv->cur_audio_pkt->size > 0) {
        ret = mkv_write_packet_internal(s, mkv->cur_audio_pkt);
        if (ret < 0) {
            av_log(s, AV_LOG_ERROR,
                   "Could not write cached audio packet ret:%d\n", ret);
            return ret;
        }
    }

    if (mkv->cluster_pos != -1) {
        ret = end_ebml_master_crc32(pb, &mkv->cluster_bc, mkv,
                                    MATROSKA_ID_CLUSTER, 0, 0, 0);
        if (ret < 0)
            return ret;
    }

    ret = mkv_write_chapters(s);
    if (ret < 0)
        return ret;

    if (!IS_SEEKABLE(pb, mkv))
        return 0;

    endpos = avio_tell(pb);

    if (mkv->cues.num_entries && mkv->reserve_cues_space >= 0) {
        AVIOContext *cues = NULL;
        uint64_t size, offset = 0;
        int length_size = 0;

redo_cues:
        ret = start_ebml_master_crc32(&cues, mkv);
        if (ret < 0)
            return ret;

        ret = mkv_assemble_cues(s->streams, cues, mkv->tmp_bc, &mkv->cues,
                                mkv->tracks, s->nb_streams, offset);
        if (ret < 0) {
            ffio_free_dyn_buf(&cues);
            return ret;
        }

        if (mkv->reserve_cues_space || mkv->move_cues_to_front) {
            size  = avio_tell(cues);
            length_size = ebml_length_size(size);
            size += 4 + length_size;
            if (offset + mkv->reserve_cues_space < size) {
                if (mkv->move_cues_to_front) {
                    offset = size - mkv->reserve_cues_space;
                    ffio_reset_dyn_buf(cues);
                    goto redo_cues;
                }
                av_log(s, AV_LOG_WARNING,
                       "Insufficient space reserved for Cues: "
                       "%d < %"PRIu64". No Cues will be output.\n",
                       mkv->reserve_cues_space, size);
                ret2 = AVERROR(EINVAL);
                goto after_cues;
            } else {
                if (offset) {
                    ret = ff_format_shift_data(s, mkv->cues_pos + mkv->reserve_cues_space,
                                               offset);
                    if (ret < 0) {
                        ffio_free_dyn_buf(&cues);
                        return ret;
                    }
                    endpos += offset;
                }
                if ((ret64 = avio_seek(pb, mkv->cues_pos, SEEK_SET)) < 0) {
                    ffio_free_dyn_buf(&cues);
                    return ret64;
                }
                if (mkv->reserve_cues_space == size + 1) {
                    /* There is no way to reserve a single byte because
                     * the minimal size of an EBML Void element is 2
                     * (1 byte ID, 1 byte length field). This problem
                     * is solved by writing the Cues' length field on
                     * one byte more than necessary. */
                    length_size++;
                    size++;
                }
            }
        }
        ret = end_ebml_master_crc32(pb, &cues, mkv, MATROSKA_ID_CUES,
                                    length_size, 0, 1);
        if (ret < 0)
            return ret;
        if (mkv->reserve_cues_space) {
            if (size < mkv->reserve_cues_space)
                put_ebml_void(pb, mkv->reserve_cues_space - size);
        } else if (!mkv->move_cues_to_front)
            endpos = avio_tell(pb);
    }

after_cues:
    /* Lengths greater than (1ULL << 56) - 1 can't be represented
     * via an EBML number, so leave the unknown length field. */
    if (endpos - mkv->segment_offset < (1ULL << 56) - 1) {
        if ((ret64 = avio_seek(pb, mkv->segment_offset - 8, SEEK_SET)) < 0)
            return ret64;
        put_ebml_length(pb, endpos - mkv->segment_offset, 8);
    }

    ret = mkv_write_seekhead(pb, mkv, 1, mkv->info.pos);
    if (ret < 0)
        return ret;

    if (mkv->info.bc) {
        // update the duration
        av_log(s, AV_LOG_DEBUG, "end duration = %" PRIu64 "\n", mkv->duration);
        avio_seek(mkv->info.bc, mkv->duration_offset, SEEK_SET);
        put_ebml_float(mkv->info.bc, MATROSKA_ID_DURATION, mkv->duration);
        ret = end_ebml_master_crc32(pb, &mkv->info.bc, mkv,
                                    MATROSKA_ID_INFO, 0, 0, 0);
        if (ret < 0)
            return ret;
    }

    if (mkv->track.bc) {
        // write Tracks master
        if (!IS_WEBM(mkv)) {
            AVIOContext *track_bc = mkv->track.bc;

            for (unsigned i = 0; i < s->nb_streams; i++) {
                const mkv_track *track = &mkv->tracks[i];

                if (!track->max_blockaddid)
                    continue;

                // We reserved a single byte to write this value.
                av_assert0(track->max_blockaddid <= 0xFF);

                avio_seek(track_bc, track->blockadditionmapping_offset, SEEK_SET);

                put_ebml_uint(track_bc, MATROSKA_ID_TRACKMAXBLKADDID,
                              track->max_blockaddid);
                if (track->max_blockaddid == MATROSKA_BLOCK_ADD_ID_ITU_T_T35) {
                    ebml_master mapping_master = start_ebml_master(track_bc, MATROSKA_ID_TRACKBLKADDMAPPING, 8);
                    put_ebml_uint(track_bc, MATROSKA_ID_BLKADDIDTYPE,
                                  MATROSKA_BLOCK_ADD_ID_TYPE_ITU_T_T35);
                    put_ebml_uint(track_bc, MATROSKA_ID_BLKADDIDVALUE,
                                  MATROSKA_BLOCK_ADD_ID_ITU_T_T35);
                    end_ebml_master(track_bc, mapping_master);
                }
            }
        }

        avio_seek(pb, mkv->track.pos, SEEK_SET);
        ret = end_ebml_master_crc32(pb, &mkv->track.bc, mkv,
                                    MATROSKA_ID_TRACKS, 0, 0, 0);
        if (ret < 0)
            return ret;
    }

    // update stream durations
    if (mkv->tags.bc) {
        AVIOContext *tags_bc = mkv->tags.bc;
        int i;
        for (i = 0; i < s->nb_streams; ++i) {
            const AVStream     *st = s->streams[i];
            const mkv_track *track = &mkv->tracks[i];

            if (track->duration_offset > 0) {
                double duration_sec = track->duration * av_q2d(st->time_base);
                char duration_string[DURATION_STRING_LENGTH + 1] = "";
                ebml_master simpletag;

                av_log(s, AV_LOG_DEBUG, "stream %d end duration = %" PRIu64 "\n", i,
                       track->duration);

                avio_seek(tags_bc, track->duration_offset, SEEK_SET);
                simpletag = start_ebml_master(tags_bc, MATROSKA_ID_SIMPLETAG,
                                              2 + 1 + 8 + 23);
                put_ebml_string(tags_bc, MATROSKA_ID_TAGNAME, "DURATION");

                snprintf(duration_string, sizeof(duration_string), "%02d:%02d:%012.9f",
                         (int) duration_sec / 3600, ((int) duration_sec / 60) % 60,
                         fmod(duration_sec, 60));

                put_ebml_binary(tags_bc, MATROSKA_ID_TAGSTRING,
                                duration_string, DURATION_STRING_LENGTH);
                end_ebml_master(tags_bc, simpletag);
            }
        }

        avio_seek(pb, mkv->tags.pos, SEEK_SET);
        ret = end_ebml_master_crc32(pb, &mkv->tags.bc, mkv,
                                    MATROSKA_ID_TAGS, 0, 0, 0);
        if (ret < 0)
            return ret;
    }

    avio_seek(pb, endpos, SEEK_SET);

    return ret2;
}

static uint64_t mkv_get_uid(const mkv_track *tracks, int i, AVLFG *c)
{
    while (1) {
        uint64_t uid;
        int k;
        uid  = (uint64_t)av_lfg_get(c) << 32;
        uid |= av_lfg_get(c);
        if (!uid)
            continue;
        for (k = 0; k < i; k++) {
            if (tracks[k].uid == uid)
                break;
        }
        if (k == i)
            return uid;
    }
}

static int mkv_init(struct AVFormatContext *s)
{
    FFFormatContext *const si = ffformatcontext(s);
    MatroskaMuxContext *mkv = s->priv_data;
    AVLFG c;
    unsigned nb_tracks = 0;
    int i;

    mkv->ctx = s;

    for (i = 0; i < s->nb_streams; i++) {
        if (s->streams[i]->codecpar->codec_id == AV_CODEC_ID_ATRAC3 ||
            s->streams[i]->codecpar->codec_id == AV_CODEC_ID_COOK ||
            s->streams[i]->codecpar->codec_id == AV_CODEC_ID_RA_288 ||
            s->streams[i]->codecpar->codec_id == AV_CODEC_ID_SIPR ||
            s->streams[i]->codecpar->codec_id == AV_CODEC_ID_RV10 ||
            s->streams[i]->codecpar->codec_id == AV_CODEC_ID_RV20 ||
            s->streams[i]->codecpar->codec_id == AV_CODEC_ID_RV30) {
            av_log(s, AV_LOG_ERROR,
                   "The Matroska muxer does not yet support muxing %s\n",
                   avcodec_get_name(s->streams[i]->codecpar->codec_id));
            return AVERROR_PATCHWELCOME;
        }
    }

    if (s->avoid_negative_ts < 0) {
        s->avoid_negative_ts = 1;
        si->avoid_negative_ts_use_pts = 1;
    }

    if (!CONFIG_MATROSKA_MUXER ||
        (CONFIG_WEBM_MUXER && !strcmp(s->oformat->name, "webm"))) {
        mkv->mode      = MODE_WEBM;
        mkv->write_crc = 0;
    } else
        mkv->mode = MODE_MATROSKAv2;

    mkv->cur_audio_pkt = ffformatcontext(s)->pkt;

    mkv->tracks = av_calloc(s->nb_streams, sizeof(*mkv->tracks));
    if (!mkv->tracks)
        return AVERROR(ENOMEM);

    if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
        av_lfg_init(&c, av_get_random_seed());

        // Calculate the SegmentUID now in order not to waste our random seed.
        for (i = 0; i < 4; i++)
            mkv->segment_uid[i] = av_lfg_get(&c);
    }

    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];
        const AVCodecParameters *const par = st->codecpar;
        mkv_track *track = &mkv->tracks[i];

        switch (par->codec_id) {
#if CONFIG_MATROSKA_MUXER
        case AV_CODEC_ID_WAVPACK:
            track->reformat = mkv_reformat_wavpack;
            break;
        case AV_CODEC_ID_H264:
        case AV_CODEC_ID_HEVC:
            if ((par->codec_id == AV_CODEC_ID_H264 && par->extradata_size > 0 ||
                 par->codec_id == AV_CODEC_ID_HEVC && par->extradata_size > 6) &&
                (AV_RB24(par->extradata) == 1 || AV_RB32(par->extradata) == 1))
                track->reformat = mkv_reformat_h2645;
            break;
        case AV_CODEC_ID_PRORES:
            /* Matroska specification requires to remove
             * the first QuickTime atom. */
            track->offset = 8;
            break;
#endif
        case AV_CODEC_ID_AV1:
            track->reformat = mkv_reformat_av1;
            break;
        case AV_CODEC_ID_WEBVTT:
            track->reformat = webm_reformat_vtt;
            break;
        }

        if (s->flags & AVFMT_FLAG_BITEXACT) {
            track->uid = i + 1;
        } else {
            track->uid = mkv_get_uid(mkv->tracks, i, &c);
        }

        // ms precision is the de-facto standard timescale for mkv files
        avpriv_set_pts_info(st, 64, 1, 1000);

        if (st->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT) {
            if (IS_WEBM(mkv)) {
                av_log(s, AV_LOG_WARNING, "Stream %d will be ignored "
                       "as WebM doesn't support attachments.\n", i);
            } else if (!get_mimetype(st)) {
                av_log(s, AV_LOG_ERROR, "Attachment stream %d has no mimetype "
                       "tag and it cannot be deduced from the codec id.\n", i);
                return AVERROR(EINVAL);
            }
            mkv->nb_attachments++;
            continue;
        }

        nb_tracks++;
        track->track_num = mkv->is_dash ? mkv->dash_track_number : nb_tracks;
        track->track_num_size = ebml_num_size(track->track_num);
    }

    if (mkv->is_dash && nb_tracks != 1)
        return AVERROR(EINVAL);

    return 0;
}

static int mkv_check_bitstream(AVFormatContext *s, AVStream *st,
                               const AVPacket *pkt)
{
    int ret = 1;

    if (CONFIG_MATROSKA_MUXER && st->codecpar->codec_id == AV_CODEC_ID_AAC) {
        if (pkt->size > 2 && (AV_RB16(pkt->data) & 0xfff0) == 0xfff0)
            ret = ff_stream_add_bitstream_filter(st, "aac_adtstoasc", NULL);
    } else if (st->codecpar->codec_id == AV_CODEC_ID_VP9) {
        ret = ff_stream_add_bitstream_filter(st, "vp9_superframe", NULL);
    } else if (CONFIG_MATROSKA_MUXER &&
               st->codecpar->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE) {
        ret = ff_stream_add_bitstream_filter(st, "pgs_frame_merge", NULL);
    }

    return ret;
}

static const AVCodecTag additional_audio_tags[] = {
    { AV_CODEC_ID_ALAC,      0XFFFFFFFF },
    { AV_CODEC_ID_ATRAC1,    0xFFFFFFFF },
    { AV_CODEC_ID_MLP,       0xFFFFFFFF },
    { AV_CODEC_ID_OPUS,      0xFFFFFFFF },
    { AV_CODEC_ID_PCM_S16BE, 0xFFFFFFFF },
    { AV_CODEC_ID_PCM_S24BE, 0xFFFFFFFF },
    { AV_CODEC_ID_PCM_S32BE, 0xFFFFFFFF },
    { AV_CODEC_ID_QDMC,      0xFFFFFFFF },
    { AV_CODEC_ID_QDM2,      0xFFFFFFFF },
    { AV_CODEC_ID_RA_144,    0xFFFFFFFF },
    { AV_CODEC_ID_TRUEHD,    0xFFFFFFFF },
    { AV_CODEC_ID_NONE,      0xFFFFFFFF }
};

static const AVCodecTag additional_subtitle_tags[] = {
    { AV_CODEC_ID_DVB_SUBTITLE,      0xFFFFFFFF },
    { AV_CODEC_ID_DVD_SUBTITLE,      0xFFFFFFFF },
    { AV_CODEC_ID_HDMV_PGS_SUBTITLE, 0xFFFFFFFF },
    { AV_CODEC_ID_ARIB_CAPTION,      0xFFFFFFFF },
    { AV_CODEC_ID_NONE,              0xFFFFFFFF }
};

#define OFFSET(x) offsetof(MatroskaMuxContext, x)
#define FLAGS AV_OPT_FLAG_ENCODING_PARAM
static const AVOption options[] = {
    { "reserve_index_space", "Reserve a given amount of space (in bytes) at the beginning of the file for the index (cues).", OFFSET(reserve_cues_space), AV_OPT_TYPE_INT,   { .i64 = 0 },   0, INT_MAX,   FLAGS },
    { "cues_to_front", "Move Cues (the index) to the front by shifting data if necessary", OFFSET(move_cues_to_front), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, FLAGS },
    { "cluster_size_limit",  "Store at most the provided amount of bytes in a cluster. ",                                     OFFSET(cluster_size_limit), AV_OPT_TYPE_INT  , { .i64 = -1 }, -1, INT_MAX,   FLAGS },
    { "cluster_time_limit",  "Store at most the provided number of milliseconds in a cluster.",                               OFFSET(cluster_time_limit), AV_OPT_TYPE_INT64, { .i64 = -1 }, -1, INT64_MAX, FLAGS },
    { "dash", "Create a WebM file conforming to WebM DASH specification", OFFSET(is_dash), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, FLAGS },
    { "dash_track_number", "Track number for the DASH stream", OFFSET(dash_track_number), AV_OPT_TYPE_INT, { .i64 = 1 }, 1, INT_MAX, FLAGS },
    { "live", "Write files assuming it is a live stream.", OFFSET(is_live), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, FLAGS },
    { "allow_raw_vfw", "allow RAW VFW mode", OFFSET(allow_raw_vfw), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, FLAGS },
    { "flipped_raw_rgb", "Raw RGB bitmaps in VFW mode are stored bottom-up", OFFSET(flipped_raw_rgb), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, FLAGS },
    { "write_crc32", "write a CRC32 element inside every Level 1 element", OFFSET(write_crc), AV_OPT_TYPE_BOOL, { .i64 = 1 }, 0, 1, FLAGS },
    { "default_mode", "Controls how a track's FlagDefault is inferred", OFFSET(default_mode), AV_OPT_TYPE_INT, { .i64 = DEFAULT_MODE_PASSTHROUGH }, DEFAULT_MODE_INFER, DEFAULT_MODE_PASSTHROUGH, FLAGS, .unit = "default_mode" },
    { "infer", "For each track type, mark each track of disposition default as default; if none exists, mark the first track as default.", 0, AV_OPT_TYPE_CONST, { .i64 = DEFAULT_MODE_INFER }, 0, 0, FLAGS, .unit = "default_mode" },
    { "infer_no_subs", "For each track type, mark each track of disposition default as default; for audio and video: if none exists, mark the first track as default.", 0, AV_OPT_TYPE_CONST, { .i64 = DEFAULT_MODE_INFER_NO_SUBS }, 0, 0, FLAGS, .unit = "default_mode" },
    { "passthrough", "Use the disposition flag as-is", 0, AV_OPT_TYPE_CONST, { .i64 = DEFAULT_MODE_PASSTHROUGH }, 0, 0, FLAGS, .unit = "default_mode" },
    { NULL },
};

static const AVClass matroska_webm_class = {
    .class_name = "matroska/webm muxer",
    .item_name  = av_default_item_name,
    .option     = options,
    .version    = LIBAVUTIL_VERSION_INT,
};

#if CONFIG_MATROSKA_MUXER
static int mkv_query_codec(enum AVCodecID codec_id, int std_compliance)
{
    for (int i = 0; ff_mkv_codec_tags[i].id != AV_CODEC_ID_NONE; i++)
        if (ff_mkv_codec_tags[i].id == codec_id)
            return 1;

    if (std_compliance < FF_COMPLIANCE_NORMAL) {
        enum AVMediaType type = avcodec_get_type(codec_id);
        // mkv theoretically supports any video/audio through VFW/ACM
        if (type == AVMEDIA_TYPE_VIDEO || type == AVMEDIA_TYPE_AUDIO)
            return 1;
    }

    return 0;
}

const FFOutputFormat ff_matroska_muxer = {
    .p.name            = "matroska",
    .p.long_name       = NULL_IF_CONFIG_SMALL("Matroska"),
    .p.mime_type       = "video/x-matroska",
    .p.extensions      = "mkv",
    .priv_data_size    = sizeof(MatroskaMuxContext),
    .p.audio_codec     = CONFIG_LIBVORBIS_ENCODER ?
                         AV_CODEC_ID_VORBIS : AV_CODEC_ID_AC3,
    .p.video_codec     = CONFIG_LIBX264_ENCODER ?
                         AV_CODEC_ID_H264 : AV_CODEC_ID_MPEG4,
    .init              = mkv_init,
    .deinit            = mkv_deinit,
    .write_header      = mkv_write_header,
    .write_packet      = mkv_write_flush_packet,
    .write_trailer     = mkv_write_trailer,
    .p.flags           = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |
#if FF_API_ALLOW_FLUSH
                         AVFMT_TS_NONSTRICT | AVFMT_ALLOW_FLUSH,
#else
                         AVFMT_TS_NONSTRICT,
#endif
    .p.codec_tag       = (const AVCodecTag* const []){
         ff_codec_bmp_tags, ff_codec_wav_tags,
         additional_audio_tags, additional_subtitle_tags, 0
    },
    .p.subtitle_codec  = AV_CODEC_ID_ASS,
    .query_codec       = mkv_query_codec,
    .check_bitstream   = mkv_check_bitstream,
    .p.priv_class      = &matroska_webm_class,
    .flags_internal    = FF_OFMT_FLAG_ALLOW_FLUSH,
};
#endif

#if CONFIG_WEBM_MUXER
static int webm_query_codec(enum AVCodecID codec_id, int std_compliance)
{
    for (int i = 0; ff_webm_codec_tags[i].id != AV_CODEC_ID_NONE; i++)
        if (ff_webm_codec_tags[i].id == codec_id)
            return 1;

    return 0;
}

const FFOutputFormat ff_webm_muxer = {
    .p.name            = "webm",
    .p.long_name       = NULL_IF_CONFIG_SMALL("WebM"),
    .p.mime_type       = "video/webm",
    .p.extensions      = "webm",
    .priv_data_size    = sizeof(MatroskaMuxContext),
    .p.audio_codec     = CONFIG_LIBOPUS_ENCODER ? AV_CODEC_ID_OPUS : AV_CODEC_ID_VORBIS,
    .p.video_codec     = CONFIG_LIBVPX_VP9_ENCODER? AV_CODEC_ID_VP9 : AV_CODEC_ID_VP8,
    .p.subtitle_codec  = AV_CODEC_ID_WEBVTT,
    .init              = mkv_init,
    .deinit            = mkv_deinit,
    .write_header      = mkv_write_header,
    .write_packet      = mkv_write_flush_packet,
    .write_trailer     = mkv_write_trailer,
    .query_codec       = webm_query_codec,
    .check_bitstream   = mkv_check_bitstream,
    .p.flags           = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |
#if FF_API_ALLOW_FLUSH
                         AVFMT_TS_NONSTRICT | AVFMT_ALLOW_FLUSH,
#else
                         AVFMT_TS_NONSTRICT,
#endif
    .p.priv_class      = &matroska_webm_class,
    .flags_internal    = FF_OFMT_FLAG_ALLOW_FLUSH,
};
#endif

#if CONFIG_MATROSKA_AUDIO_MUXER
const FFOutputFormat ff_matroska_audio_muxer = {
    .p.name            = "matroska",
    .p.long_name       = NULL_IF_CONFIG_SMALL("Matroska Audio"),
    .p.mime_type       = "audio/x-matroska",
    .p.extensions      = "mka",
    .priv_data_size    = sizeof(MatroskaMuxContext),
    .p.audio_codec     = CONFIG_LIBVORBIS_ENCODER ?
                         AV_CODEC_ID_VORBIS : AV_CODEC_ID_AC3,
    .p.video_codec     = AV_CODEC_ID_NONE,
    .init              = mkv_init,
    .deinit            = mkv_deinit,
    .write_header      = mkv_write_header,
    .write_packet      = mkv_write_flush_packet,
    .write_trailer     = mkv_write_trailer,
    .check_bitstream   = mkv_check_bitstream,
#if FF_API_ALLOW_FLUSH
    .p.flags           = AVFMT_GLOBALHEADER | AVFMT_TS_NONSTRICT |
                         AVFMT_ALLOW_FLUSH,
#else
    .p.flags           = AVFMT_GLOBALHEADER | AVFMT_TS_NONSTRICT,
#endif
    .p.codec_tag       = (const AVCodecTag* const []){
        ff_codec_wav_tags, additional_audio_tags, 0
    },
    .p.priv_class      = &matroska_webm_class,
    .flags_internal    = FF_OFMT_FLAG_ALLOW_FLUSH,
};
#endif