FFmpeg/libavformat/matroskaenc.c

/*
 * Matroska file 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 "avformat.h"
#include "md5.h"
#include "riff.h"
#include "xiph.h"
#include "matroska.h"

typedef struct mkv_seekhead_entry {
    unsigned int    elementid;
    uint64_t        segmentpos;
} mkv_seekhead_entry;

typedef struct mkv_seekhead {
    offset_t                filepos;
    offset_t                segment_offset;     ///< the file offset to the beginning of the segment
    int                     reserved_size;      ///< -1 if appending to file
    int                     max_entries;
    mkv_seekhead_entry      *entries;
    int                     num_entries;
} mkv_seekhead;

typedef struct {
    uint64_t        pts;
    int             tracknum;
    offset_t        cluster_pos;        ///< file offset of the cluster containing the block
} mkv_cuepoint;

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

typedef struct MatroskaMuxContext {
    offset_t        segment;
    offset_t        segment_offset;
    offset_t        segment_uid;
    offset_t        cluster;
    offset_t        cluster_pos;        ///< file offset of the current cluster
    uint64_t        cluster_pts;
    offset_t        duration_offset;
    uint64_t        duration;
    mkv_seekhead    *main_seekhead;
    mkv_seekhead    *cluster_seekhead;
    mkv_cues        *cues;

    struct AVMD5    *md5_ctx;
} MatroskaMuxContext;

static int ebml_id_size(unsigned int id)
{
    return (av_log2(id+1)-1)/7+1;
}

static void put_ebml_id(ByteIOContext *pb, unsigned int id)
{
    int i = ebml_id_size(id);
    while (i--)
        put_byte(pb, id >> (i*8));
}

/**
 * Write an EBML size meaning "unknown size"
 *
 * @param bytes The number of bytes the size should occupy. Maximum of 8.
 */
static void put_ebml_size_unknown(ByteIOContext *pb, int bytes)
{
    uint64_t value = 0;
    int i;

    bytes = FFMIN(bytes, 8);
    for (i = 0; i < bytes*7 + 1; i++)
        value |= 1ULL << i;
    for (i = bytes-1; i >= 0; i--)
        put_byte(pb, value >> i*8);
}

// XXX: test this thoroughly and get rid of minbytes hack (currently needed to
// use up all of the space reserved in start_ebml_master)
static void put_ebml_size(ByteIOContext *pb, uint64_t size, int minbytes)
{
    int i, bytes = minbytes;

    // sizes larger than this are currently undefined in EBML
    // so write "unknown" size
    if (size >= (1ULL<<56)-1) {
        put_ebml_size_unknown(pb, 1);
        return;
    }

    while ((size+1) >> bytes*7) bytes++;

    put_byte(pb, (0x80 >> (bytes-1)) | (size >> (bytes-1)*8));
    for (i = bytes - 2; i >= 0; i--)
        put_byte(pb, size >> i*8);
}

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

    put_ebml_id(pb, elementid);
    put_ebml_size(pb, bytes, 0);
    for (i = bytes - 1; i >= 0; i--)
        put_byte(pb, val >> i*8);
}

static void put_ebml_float(ByteIOContext *pb, unsigned int elementid, double val)
{
    put_ebml_id(pb, elementid);
    put_ebml_size(pb, 8, 0);
    put_be64(pb, av_dbl2int(val));
}

static void put_ebml_binary(ByteIOContext *pb, unsigned int elementid,
                            const uint8_t *buf, int size)
{
    put_ebml_id(pb, elementid);
    put_ebml_size(pb, size, 0);
    put_buffer(pb, buf, size);
}

static void put_ebml_string(ByteIOContext *pb, unsigned int elementid, const char *str)
{
    put_ebml_binary(pb, elementid, str, strlen(str));
}

/**
 * Writes a void element of a given size. Useful for reserving space in the file to be
 * written to later.
 *
 * @param size The amount of space to reserve, which must be at least 2.
 */
static void put_ebml_void(ByteIOContext *pb, uint64_t size)
{
    offset_t currentpos = url_ftell(pb);

    if (size < 2)
        return;

    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)
        put_ebml_size(pb, size-1, 0);
    else
        put_ebml_size(pb, size-9, 8);
    url_fseek(pb, currentpos + size, SEEK_SET);
}

static offset_t start_ebml_master(ByteIOContext *pb, unsigned int elementid)
{
    put_ebml_id(pb, elementid);
    // XXX: this always reserves the maximum needed space to store any size value
    // we should be smarter (additional parameter for expected size?)
    put_ebml_size_unknown(pb, 8);
    return url_ftell(pb);
}

static void end_ebml_master(ByteIOContext *pb, offset_t start)
{
    offset_t pos = url_ftell(pb);

    url_fseek(pb, start - 8, SEEK_SET);
    put_ebml_size(pb, pos - start, 8);
    url_fseek(pb, pos, SEEK_SET);
}

static void put_xiph_size(ByteIOContext *pb, int size)
{
    int i;
    for (i = 0; i < size / 255; i++)
        put_byte(pb, 255);
    put_byte(pb, size % 255);
}

/**
 * Initialize a mkv_seekhead element to be ready to index level 1 Matroska elements.
 * If a maximum number of elements is specified, enough space will be reserved at
 * the current file location to write a seek head of that size.
 *
 * @param segment_offset the absolute offset into the file that the segment begins
 * @param numelements the maximum number of elements that will be indexed by this
 *                    seek head, 0 if unlimited.
 */
static mkv_seekhead * mkv_start_seekhead(ByteIOContext *pb, offset_t segment_offset, int numelements)
{
    mkv_seekhead *new_seekhead = av_mallocz(sizeof(mkv_seekhead));
    if (new_seekhead == NULL)
        return NULL;

    new_seekhead->segment_offset = segment_offset;

    if (numelements > 0) {
        new_seekhead->filepos = url_ftell(pb);
        // 21 bytes max for a seek entry, 10 bytes max for the SeekHead ID and size,
        // and 3 bytes to guarantee that an EBML void element will fit afterwards
        // XXX: 28 bytes right now because begin_ebml_master() reserves more than necessary
        new_seekhead->reserved_size = numelements * 28 + 13;
        new_seekhead->max_entries = numelements;
        put_ebml_void(pb, new_seekhead->reserved_size);
    }
    return new_seekhead;
}

static int mkv_add_seekhead_entry(mkv_seekhead *seekhead, unsigned int elementid, uint64_t filepos)
{
    mkv_seekhead_entry *entries = seekhead->entries;
    int new_entry = seekhead->num_entries;

    // don't store more elements than we reserved space for
    if (seekhead->max_entries > 0 && seekhead->max_entries <= seekhead->num_entries)
        return -1;

    entries = av_realloc(entries, (seekhead->num_entries + 1) * sizeof(mkv_seekhead_entry));
    if (entries == NULL)
        return -1;

    entries[new_entry].elementid = elementid;
    entries[new_entry].segmentpos = filepos - seekhead->segment_offset;

    seekhead->entries = entries;
    seekhead->num_entries++;

    return 0;
}

/**
 * Write the seek head to the file and free it. If a maximum number of elements was
 * specified to mkv_start_seekhead(), the seek head will be written at the location
 * reserved for it. Otherwise, it is written at the current location in the file.
 *
 * @return the file offset where the seekhead was written
 */
static offset_t mkv_write_seekhead(ByteIOContext *pb, mkv_seekhead *seekhead)
{
    offset_t metaseek, seekentry, currentpos;
    int i;

    currentpos = url_ftell(pb);

    if (seekhead->reserved_size > 0)
        url_fseek(pb, seekhead->filepos, SEEK_SET);

    metaseek = start_ebml_master(pb, MATROSKA_ID_SEEKHEAD);
    for (i = 0; i < seekhead->num_entries; i++) {
        mkv_seekhead_entry *entry = &seekhead->entries[i];

        seekentry = start_ebml_master(pb, MATROSKA_ID_SEEKENTRY);

        put_ebml_id(pb, MATROSKA_ID_SEEKID);
        put_ebml_size(pb, ebml_id_size(entry->elementid), 0);
        put_ebml_id(pb, entry->elementid);

        put_ebml_uint(pb, MATROSKA_ID_SEEKPOSITION, entry->segmentpos);
        end_ebml_master(pb, seekentry);
    }
    end_ebml_master(pb, metaseek);

    if (seekhead->reserved_size > 0) {
        uint64_t remaining = seekhead->filepos + seekhead->reserved_size - url_ftell(pb);
        put_ebml_void(pb, remaining);
        url_fseek(pb, currentpos, SEEK_SET);

        currentpos = seekhead->filepos;
    }
    av_free(seekhead->entries);
    av_free(seekhead);

    return currentpos;
}

static mkv_cues * mkv_start_cues(offset_t segment_offset)
{
    mkv_cues *cues = av_mallocz(sizeof(mkv_cues));
    if (cues == NULL)
        return NULL;

    cues->segment_offset = segment_offset;
    return cues;
}

static int mkv_add_cuepoint(mkv_cues *cues, AVPacket *pkt, offset_t cluster_pos)
{
    mkv_cuepoint *entries = cues->entries;
    int new_entry = cues->num_entries;

    entries = av_realloc(entries, (cues->num_entries + 1) * sizeof(mkv_cuepoint));
    if (entries == NULL)
        return -1;

    entries[new_entry].pts = pkt->pts;
    entries[new_entry].tracknum = pkt->stream_index + 1;
    entries[new_entry].cluster_pos = cluster_pos - cues->segment_offset;

    cues->entries = entries;
    cues->num_entries++;
    return 0;
}

static offset_t mkv_write_cues(ByteIOContext *pb, mkv_cues *cues)
{
    offset_t currentpos, cues_element;
    int i, j;

    currentpos = url_ftell(pb);
    cues_element = start_ebml_master(pb, MATROSKA_ID_CUES);

    for (i = 0; i < cues->num_entries; i++) {
        offset_t cuepoint, track_positions;
        mkv_cuepoint *entry = &cues->entries[i];
        uint64_t pts = entry->pts;

        cuepoint = start_ebml_master(pb, MATROSKA_ID_POINTENTRY);
        put_ebml_uint(pb, MATROSKA_ID_CUETIME, pts);

        // put all the entries from different tracks that have the exact same
        // timestamp into the same CuePoint
        for (j = 0; j < cues->num_entries - i && entry[j].pts == pts; j++) {
            track_positions = start_ebml_master(pb, MATROSKA_ID_CUETRACKPOSITION);
            put_ebml_uint(pb, MATROSKA_ID_CUETRACK          , entry[j].tracknum   );
            put_ebml_uint(pb, MATROSKA_ID_CUECLUSTERPOSITION, entry[j].cluster_pos);
            end_ebml_master(pb, track_positions);
        }
        i += j - 1;
        end_ebml_master(pb, cuepoint);
    }
    end_ebml_master(pb, cues_element);

    av_free(cues->entries);
    av_free(cues);
    return currentpos;
}

static int put_xiph_codecpriv(ByteIOContext *pb, AVCodecContext *codec)
{
    offset_t codecprivate;
    uint8_t *header_start[3];
    int header_len[3];
    int first_header_size;
    int j;

    if (codec->codec_id == CODEC_ID_VORBIS)
        first_header_size = 30;
    else
        first_header_size = 42;

    if (ff_split_xiph_headers(codec->extradata, codec->extradata_size,
                              first_header_size, header_start, header_len) < 0) {
        av_log(codec, AV_LOG_ERROR, "Extradata corrupt.\n");
        return -1;
    }

    codecprivate = start_ebml_master(pb, MATROSKA_ID_CODECPRIVATE);
    put_byte(pb, 2);                    // number packets - 1
    for (j = 0; j < 2; j++) {
        put_xiph_size(pb, header_len[j]);
    }
    for (j = 0; j < 3; j++)
        put_buffer(pb, header_start[j], header_len[j]);
    end_ebml_master(pb, codecprivate);

    return 0;
}

#define FLAC_STREAMINFO_SIZE 34

static int put_flac_codecpriv(ByteIOContext *pb, AVCodecContext *codec)
{
    offset_t codecpriv = start_ebml_master(pb, MATROSKA_ID_CODECPRIVATE);

    // if the extradata_size is greater than FLAC_STREAMINFO_SIZE,
    // assume that it's in Matroska's format already
    if (codec->extradata_size < FLAC_STREAMINFO_SIZE) {
        av_log(codec, AV_LOG_ERROR, "Invalid FLAC extradata\n");
        return -1;
    } else if (codec->extradata_size == FLAC_STREAMINFO_SIZE) {
        // only the streaminfo packet
        put_byte(pb, 0);
        put_xiph_size(pb, codec->extradata_size);
        av_log(codec, AV_LOG_ERROR, "Only one packet\n");
    }
    put_buffer(pb, codec->extradata, codec->extradata_size);
    end_ebml_master(pb, codecpriv);
    return 0;
}

static void get_aac_sample_rates(AVCodecContext *codec, int *sample_rate, int *output_sample_rate)
{
    static const int aac_sample_rates[] = {
        96000, 88200, 64000, 48000, 44100, 32000,
        24000, 22050, 16000, 12000, 11025,  8000,
    };
    int sri;

    if (codec->extradata_size < 2) {
        av_log(codec, AV_LOG_WARNING, "no aac extradata, unable to determine sample rate\n");
        return;
    }

    sri = ((codec->extradata[0] << 1) & 0xE) | (codec->extradata[1] >> 7);
    if (sri > 12) {
        av_log(codec, AV_LOG_WARNING, "aac samplerate index out of bounds\n");
        return;
    }
    *sample_rate = aac_sample_rates[sri];

    // if sbr, get output sample rate as well
    if (codec->extradata_size == 5) {
        sri = (codec->extradata[4] >> 3) & 0xF;
        if (sri > 12) {
            av_log(codec, AV_LOG_WARNING, "aac output samplerate index out of bounds\n");
            return;
        }
        *output_sample_rate = aac_sample_rates[sri];
    }
}

static int mkv_write_tracks(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    ByteIOContext *pb = &s->pb;
    offset_t tracks;
    int i, j;

    if (mkv_add_seekhead_entry(mkv->main_seekhead, MATROSKA_ID_TRACKS, url_ftell(pb)) < 0)
        return -1;

    tracks = start_ebml_master(pb, MATROSKA_ID_TRACKS);
    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];
        AVCodecContext *codec = st->codec;
        offset_t subinfo, track;
        int native_id = 0;
        int bit_depth = av_get_bits_per_sample(codec->codec_id);
        int sample_rate = codec->sample_rate;
        int output_sample_rate = 0;

        if (codec->codec_id == CODEC_ID_AAC)
            get_aac_sample_rates(codec, &sample_rate, &output_sample_rate);

        track = start_ebml_master(pb, MATROSKA_ID_TRACKENTRY);
        put_ebml_uint (pb, MATROSKA_ID_TRACKNUMBER     , i + 1);
        put_ebml_uint (pb, MATROSKA_ID_TRACKUID        , i + 1);
        put_ebml_uint (pb, MATROSKA_ID_TRACKFLAGLACING , 0);    // no lacing (yet)

        if (st->language[0])
            put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE, st->language);
        else
            put_ebml_string(pb, MATROSKA_ID_TRACKLANGUAGE, "und");

        // look for a codec id string specific to mkv to use, if none are found, use AVI codes
        for (j = 0; ff_mkv_codec_tags[j].id != CODEC_ID_NONE; j++) {
            if (ff_mkv_codec_tags[j].id == codec->codec_id) {
                put_ebml_string(pb, MATROSKA_ID_CODECID, ff_mkv_codec_tags[j].str);
                native_id = 1;
                break;
            }
        }

        if (native_id) {
            if (codec->codec_id == CODEC_ID_VORBIS || codec->codec_id == CODEC_ID_THEORA) {
                if (put_xiph_codecpriv(pb, codec) < 0)
                    return -1;
            } else if (codec->codec_id == CODEC_ID_FLAC) {
                if (put_flac_codecpriv(pb, codec) < 0)
                    return -1;
            } else if (codec->extradata_size) {
                put_ebml_binary(pb, MATROSKA_ID_CODECPRIVATE, codec->extradata, codec->extradata_size);
            }
        }

        switch (codec->codec_type) {
            case CODEC_TYPE_VIDEO:
                put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, MATROSKA_TRACK_TYPE_VIDEO);

                if (!native_id) {
                    offset_t bmp_header;
                    // if there is no mkv-specific codec id, use VFW mode
                    if (!codec->codec_tag)
                        codec->codec_tag = codec_get_tag(codec_bmp_tags, codec->codec_id);

                    put_ebml_string(pb, MATROSKA_ID_CODECID, MATROSKA_CODEC_ID_VIDEO_VFW_FOURCC);
                    bmp_header = start_ebml_master(pb, MATROSKA_ID_CODECPRIVATE);
                    put_bmp_header(pb, codec, codec_bmp_tags, 0);
                    end_ebml_master(pb, bmp_header);
                }
                subinfo = start_ebml_master(pb, MATROSKA_ID_TRACKVIDEO);
                // XXX: interlace flag?
                put_ebml_uint (pb, MATROSKA_ID_VIDEOPIXELWIDTH , codec->width);
                put_ebml_uint (pb, MATROSKA_ID_VIDEOPIXELHEIGHT, codec->height);
                if (codec->sample_aspect_ratio.num) {
                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , codec->sample_aspect_ratio.num);
                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, codec->sample_aspect_ratio.den);
                }
                end_ebml_master(pb, subinfo);
                break;

            case CODEC_TYPE_AUDIO:
                put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, MATROSKA_TRACK_TYPE_AUDIO);

                if (!native_id) {
                    offset_t wav_header;
                    // no mkv-specific ID, use ACM mode
                    codec->codec_tag = codec_get_tag(codec_wav_tags, codec->codec_id);
                    if (!codec->codec_tag) {
                        av_log(s, AV_LOG_ERROR, "no codec id found for stream %d", i);
                        return -1;
                    }

                    put_ebml_string(pb, MATROSKA_ID_CODECID, MATROSKA_CODEC_ID_AUDIO_ACM);
                    wav_header = start_ebml_master(pb, MATROSKA_ID_CODECPRIVATE);
                    put_wav_header(pb, codec);
                    end_ebml_master(pb, wav_header);
                }
                subinfo = start_ebml_master(pb, MATROSKA_ID_TRACKAUDIO);
                put_ebml_uint  (pb, MATROSKA_ID_AUDIOCHANNELS    , codec->channels);
                put_ebml_float (pb, MATROSKA_ID_AUDIOSAMPLINGFREQ, sample_rate);
                if (output_sample_rate)
                    put_ebml_float(pb, MATROSKA_ID_AUDIOOUTSAMPLINGFREQ, output_sample_rate);
                if (bit_depth)
                    put_ebml_uint(pb, MATROSKA_ID_AUDIOBITDEPTH, bit_depth);
                end_ebml_master(pb, subinfo);
                break;

                case CODEC_TYPE_SUBTITLE:
                    put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, MATROSKA_TRACK_TYPE_SUBTITLE);
                    break;
            default:
                av_log(s, AV_LOG_ERROR, "Only audio and video are supported for Matroska.");
                break;
        }
        end_ebml_master(pb, track);

        // ms precision is the de-facto standard timescale for mkv files
        av_set_pts_info(st, 64, 1, 1000);
    }
    end_ebml_master(pb, tracks);
    return 0;
}

static int mkv_write_header(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    ByteIOContext *pb = &s->pb;
    offset_t ebml_header, segment_info;

    mkv->md5_ctx = av_mallocz(av_md5_size);
    av_md5_init(mkv->md5_ctx);

    ebml_header = start_ebml_master(pb, EBML_ID_HEADER);
    put_ebml_uint   (pb, EBML_ID_EBMLVERSION        ,           1);
    put_ebml_uint   (pb, EBML_ID_EBMLREADVERSION    ,           1);
    put_ebml_uint   (pb, EBML_ID_EBMLMAXIDLENGTH    ,           4);
    put_ebml_uint   (pb, EBML_ID_EBMLMAXSIZELENGTH  ,           8);
    put_ebml_string (pb, EBML_ID_DOCTYPE            ,  "matroska");
    put_ebml_uint   (pb, EBML_ID_DOCTYPEVERSION     ,           2);
    put_ebml_uint   (pb, EBML_ID_DOCTYPEREADVERSION ,           2);
    end_ebml_master(pb, ebml_header);

    mkv->segment = start_ebml_master(pb, MATROSKA_ID_SEGMENT);
    mkv->segment_offset = url_ftell(pb);

    // we write 2 seek heads - one at the end of the file to point to each cluster, and
    // one at the beginning to point to all other level one elements (including the seek
    // head at the end of the file), which isn't more than 10 elements if we only write one
    // of each other currently defined level 1 element
    mkv->main_seekhead    = mkv_start_seekhead(pb, mkv->segment_offset, 10);
    mkv->cluster_seekhead = mkv_start_seekhead(pb, mkv->segment_offset, 0);

    if (mkv_add_seekhead_entry(mkv->main_seekhead, MATROSKA_ID_INFO, url_ftell(pb)) < 0)
        return -1;

    segment_info = start_ebml_master(pb, MATROSKA_ID_INFO);
    put_ebml_uint(pb, MATROSKA_ID_TIMECODESCALE, 1000000);
    if (strlen(s->title))
        put_ebml_string(pb, MATROSKA_ID_TITLE, s->title);
    if (!(s->streams[0]->codec->flags & CODEC_FLAG_BITEXACT)) {
        put_ebml_string(pb, MATROSKA_ID_MUXINGAPP , LIBAVFORMAT_IDENT);
        put_ebml_string(pb, MATROSKA_ID_WRITINGAPP, LIBAVFORMAT_IDENT);

        // reserve space to write the segment UID later
        mkv->segment_uid = url_ftell(pb);
        put_ebml_void(pb, 19);
    }

    // reserve space for the duration
    mkv->duration = 0;
    mkv->duration_offset = url_ftell(pb);
    put_ebml_void(pb, 11);                  // assumes double-precision float to be written
    end_ebml_master(pb, segment_info);

    if (mkv_write_tracks(s) < 0)
        return -1;

    if (mkv_add_seekhead_entry(mkv->cluster_seekhead, MATROSKA_ID_CLUSTER, url_ftell(pb)) < 0)
        return -1;

    mkv->cluster_pos = url_ftell(pb);
    mkv->cluster = start_ebml_master(pb, MATROSKA_ID_CLUSTER);
    put_ebml_uint(pb, MATROSKA_ID_CLUSTERTIMECODE, 0);
    mkv->cluster_pts = 0;

    mkv->cues = mkv_start_cues(mkv->segment_offset);
    if (mkv->cues == NULL)
        return -1;

    return 0;
}

static void mkv_write_block(AVFormatContext *s, unsigned int blockid, AVPacket *pkt, int flags)
{
    MatroskaMuxContext *mkv = s->priv_data;
    ByteIOContext *pb = &s->pb;

    av_log(s, AV_LOG_DEBUG, "Writing block at offset %llu, size %d, pts %lld, dts %lld, duration %d, flags %d\n",
           url_ftell(pb), pkt->size, pkt->pts, pkt->dts, pkt->duration, flags);
    put_ebml_id(pb, blockid);
    put_ebml_size(pb, pkt->size + 4, 0);
    put_byte(pb, 0x80 | (pkt->stream_index + 1));     // this assumes stream_index is less than 126
    put_be16(pb, pkt->pts - mkv->cluster_pts);
    put_byte(pb, flags);
    put_buffer(pb, pkt->data, pkt->size);
}

static int mkv_write_packet(AVFormatContext *s, AVPacket *pkt)
{
    MatroskaMuxContext *mkv = s->priv_data;
    ByteIOContext *pb = &s->pb;
    AVCodecContext *codec = s->streams[pkt->stream_index]->codec;
    int keyframe = !!(pkt->flags & PKT_FLAG_KEY);

    // start a new cluster every 5 MB or 5 sec
    if (url_ftell(pb) > mkv->cluster + 5*1024*1024 || pkt->pts > mkv->cluster_pts + 5000) {
        av_log(s, AV_LOG_DEBUG, "Starting new cluster at offset %llu bytes, pts %llu\n", url_ftell(pb), pkt->pts);
        end_ebml_master(pb, mkv->cluster);

        if (mkv_add_seekhead_entry(mkv->cluster_seekhead, MATROSKA_ID_CLUSTER, url_ftell(pb)) < 0)
            return -1;

        mkv->cluster_pos = url_ftell(pb);
        mkv->cluster = start_ebml_master(pb, MATROSKA_ID_CLUSTER);
        put_ebml_uint(pb, MATROSKA_ID_CLUSTERTIMECODE, pkt->pts);
        mkv->cluster_pts = pkt->pts;
        av_md5_update(mkv->md5_ctx, pkt->data, FFMIN(200, pkt->size));
    }

    if (codec->codec_type != CODEC_TYPE_SUBTITLE) {
        mkv_write_block(s, MATROSKA_ID_SIMPLEBLOCK, pkt, keyframe << 7);
    } else {
        offset_t blockgroup = start_ebml_master(pb, MATROSKA_ID_BLOCKGROUP);
        mkv_write_block(s, MATROSKA_ID_BLOCK, pkt, 0);
        put_ebml_uint(pb, MATROSKA_ID_DURATION, pkt->duration);
        end_ebml_master(pb, blockgroup);
    }

    if (codec->codec_type == CODEC_TYPE_VIDEO && keyframe) {
        if (mkv_add_cuepoint(mkv->cues, pkt, mkv->cluster_pos) < 0)
            return -1;
    }

    mkv->duration = pkt->pts + pkt->duration;
    return 0;
}

static int mkv_write_trailer(AVFormatContext *s)
{
    MatroskaMuxContext *mkv = s->priv_data;
    ByteIOContext *pb = &s->pb;
    offset_t currentpos, second_seekhead, cuespos;

    end_ebml_master(pb, mkv->cluster);

    cuespos = mkv_write_cues(pb, mkv->cues);
    second_seekhead = mkv_write_seekhead(pb, mkv->cluster_seekhead);

    mkv_add_seekhead_entry(mkv->main_seekhead, MATROSKA_ID_CUES    , cuespos);
    mkv_add_seekhead_entry(mkv->main_seekhead, MATROSKA_ID_SEEKHEAD, second_seekhead);
    mkv_write_seekhead(pb, mkv->main_seekhead);

    // update the duration
    av_log(s, AV_LOG_DEBUG, "end duration = %llu\n", mkv->duration);
    currentpos = url_ftell(pb);
    url_fseek(pb, mkv->duration_offset, SEEK_SET);
    put_ebml_float(pb, MATROSKA_ID_DURATION, mkv->duration);

    // write the md5sum of some frames as the segment UID
    if (!(s->streams[0]->codec->flags & CODEC_FLAG_BITEXACT)) {
        uint8_t segment_uid[16];
        av_md5_final(mkv->md5_ctx, segment_uid);
        url_fseek(pb, mkv->segment_uid, SEEK_SET);
        put_ebml_binary(pb, MATROSKA_ID_SEGMENTUID, segment_uid, 16);
    }
    url_fseek(pb, currentpos, SEEK_SET);

    end_ebml_master(pb, mkv->segment);
    av_free(mkv->md5_ctx);
    return 0;
}

AVOutputFormat matroska_muxer = {
    "matroska",
    "Matroska File Format",
    "video/x-matroska",
    "mkv",
    sizeof(MatroskaMuxContext),
    CODEC_ID_MP2,
    CODEC_ID_MPEG4,
    mkv_write_header,
    mkv_write_packet,
    mkv_write_trailer,
    .codec_tag = (const AVCodecTag*[]){codec_bmp_tags, codec_wav_tags, 0},
};

AVOutputFormat matroska_audio_muxer = {
    "matroska",
    "Matroska File Format",
    "audio/x-matroska",
    "mka",
    sizeof(MatroskaMuxContext),
    CODEC_ID_MP2,
    CODEC_ID_NONE,
    mkv_write_header,
    mkv_write_packet,
    mkv_write_trailer,
    .codec_tag = (const AVCodecTag*[]){codec_wav_tags, 0},
};