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lavc: add a new bitstream reader to replace get_bits

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The new bit reader features a simpler API and an implementation without
stacks of nested macros.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
This commit is contained in:
Alexandra Hájková 2016-04-22 09:43:52 +02:00 committed by Anton Khirnov
parent e18ba2dfd2
commit dd3d7ddf2a
1 changed files with 387 additions and 0 deletions
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libavcodec/bitstream.h Normal file
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/*
* Copyright (c) 2016 Alexandra Hájková
*
* This file is part of Libav.
*
* Libav 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.
*
* Libav 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 Libav; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
/**
* @file
* functions for reading bits from a buffer
*/
#ifndef AVCODEC_BITSTREAM_H
#define AVCODEC_BITSTREAM_H
#include <stdint.h>
#include "libavutil/common.h"
#include "libavutil/intreadwrite.h"
#include "libavutil/log.h"
#include "mathops.h"
#include "vlc.h"
typedef struct BitstreamContext {
uint64_t bits; // stores bits read from the buffer
const uint8_t *buffer, *buffer_end;
const uint8_t *ptr; // position inside a buffer
unsigned bits_left; // number of bits left in bits field
unsigned size_in_bits;
} BitstreamContext;
static inline void refill_64(BitstreamContext *bc)
{
if (bc->ptr >= bc->buffer_end)
return;
#ifdef BITSTREAM_READER_LE
bc->bits = AV_RL64(bc->ptr);
#else
bc->bits = AV_RB64(bc->ptr);
#endif
bc->ptr += 8;
bc->bits_left = 64;
}
static inline void refill_32(BitstreamContext *bc)
{
if (bc->ptr >= bc->buffer_end)
return;
#ifdef BITSTREAM_READER_LE
bc->bits = (uint64_t)AV_RL32(bc->ptr) << bc->bits_left | bc->bits;
#else
bc->bits = bc->bits | (uint64_t)AV_RB32(bc->ptr) << (32 - bc->bits_left);
#endif
bc->ptr += 4;
bc->bits_left += 32;
}
/* Initialize BitstreamContext. Input buffer must have an additional zero
* padding of AV_INPUT_BUFFER_PADDING_SIZE bytes at the end. */
static inline int bitstream_init(BitstreamContext *bc, const uint8_t *buffer,
unsigned bit_size)
{
unsigned buffer_size;
if (bit_size > INT_MAX - 7 || !buffer) {
buffer =
bc->buffer =
bc->ptr = NULL;
bc->bits_left = 0;
return AVERROR_INVALIDDATA;
}
buffer_size = (bit_size + 7) >> 3;
bc->buffer = buffer;
bc->buffer_end = buffer + buffer_size;
bc->ptr = bc->buffer;
bc->size_in_bits = bit_size;
bc->bits_left = 0;
bc->bits = 0;
refill_64(bc);
return 0;
}
/* Initialize BitstreamContext with buffer size in bytes instead of bits. */
static inline int bitstream_init8(BitstreamContext *bc, const uint8_t *buffer,
unsigned byte_size)
{
if (byte_size > INT_MAX / 8)
return AVERROR_INVALIDDATA;
return bitstream_init(bc, buffer, byte_size * 8);
}
/* Return number of bits already read. */
static inline int bitstream_tell(const BitstreamContext *bc)
{
return (bc->ptr - bc->buffer) * 8 - bc->bits_left;
}
/* Return buffer size in bits. */
static inline int bitstream_tell_size(const BitstreamContext *bc)
{
return bc->size_in_bits;
}
/* Return the number of the bits left in a buffer. */
static inline int bitstream_bits_left(const BitstreamContext *bc)
{
return (bc->buffer - bc->ptr) * 8 + bc->size_in_bits + bc->bits_left;
}
static inline uint64_t get_val(BitstreamContext *bc, unsigned n)
{
#ifdef BITSTREAM_READER_LE
uint64_t ret = bc->bits & ((UINT64_C(1) << n) - 1);
bc->bits >>= n;
#else
uint64_t ret = bc->bits >> (64 - n);
bc->bits <<= n;
#endif
bc->bits_left -= n;
return ret;
}
/* Return one bit from the buffer. */
static inline unsigned bitstream_read_bit(BitstreamContext *bc)
{
if (!bc->bits_left)
refill_64(bc);
return get_val(bc, 1);
}
/* Return n bits from the buffer. n has to be in the 0-63 range. */
static inline uint64_t bitstream_read_63(BitstreamContext *bc, unsigned n)
{
uint64_t ret = 0;
#ifdef BITSTREAM_READER_LE
uint64_t left = 0;
#endif
if (!n)
return 0;
if (n > bc->bits_left) {
n -= bc->bits_left;
#ifdef BITSTREAM_READER_LE
left = bc->bits_left;
#endif
ret = get_val(bc, bc->bits_left);
refill_64(bc);
}
#ifdef BITSTREAM_READER_LE
ret = get_val(bc, n) << left | ret;
#else
ret = get_val(bc, n) | ret << n;
#endif
return ret;
}
/* Return n bits from the buffer. n has to be in the 0-32 range. */
static inline uint32_t bitstream_read(BitstreamContext *bc, unsigned n)
{
if (!n)
return 0;
if (n > bc->bits_left) {
refill_32(bc);
if (bc->bits_left < 32)
bc->bits_left = n;
}
return get_val(bc, n);
}
/* Return n bits from the buffer as a signed integer.
* n has to be in the 0-32 range. */
static inline int32_t bitstream_read_signed(BitstreamContext *bc, unsigned n)
{
return sign_extend(bitstream_read(bc, n), n);
}
static inline unsigned show_val(const BitstreamContext *bc, unsigned n)
{
#ifdef BITSTREAM_READER_LE
return bc->bits & ((UINT64_C(1) << n) - 1);
#else
return bc->bits >> (64 - n);
#endif
}
/* Return n bits from the buffer, but do not change the buffer state.
* n has to be in the 0-32 range. */
static inline unsigned bitstream_peek(BitstreamContext *bc, unsigned n)
{
if (n > bc->bits_left)
refill_32(bc);
return show_val(bc, n);
}
/* Return n bits from the buffer as a signed integer, but do not change the
* buffer state. n has to be in the 0-32 range. */
static inline int bitstream_peek_signed(BitstreamContext *bc, unsigned n)
{
return sign_extend(bitstream_peek(bc, n), n);
}
static inline void skip_remaining(BitstreamContext *bc, unsigned n)
{
#ifdef BITSTREAM_READER_LE
bc->bits >>= n;
#else
bc->bits <<= n;
#endif
bc->bits_left -= n;
}
/* Skip n bits in the buffer. */
static inline void bitstream_skip(BitstreamContext *bc, unsigned n)
{
if (n <= bc->bits_left)
skip_remaining(bc, n);
else {
n -= bc->bits_left;
skip_remaining(bc, bc->bits_left);
if (n >= 64) {
unsigned skip = n / 8;
n -= skip * 8;
bc->ptr += skip;
}
refill_64(bc);
if (n)
skip_remaining(bc, n);
}
}
/* Seek to the given bit position. */
static inline void bitstream_seek(BitstreamContext *bc, unsigned pos)
{
bc->ptr = bc->buffer;
bc->bits = 0;
bc->bits_left = 0;
bitstream_skip(bc, pos);
}
/* Skip bits to a byte boundary. */
static inline const uint8_t *bitstream_align(BitstreamContext *bc)
{
unsigned n = -bitstream_tell(bc) & 7;
if (n)
bitstream_skip(bc, n);
return bc->buffer + (bitstream_tell(bc) >> 3);
}
/* Read MPEG-1 dc-style VLC (sign bit + mantissa with no MSB).
* If MSB not set it is negative. */
static inline int bitstream_read_xbits(BitstreamContext *bc, unsigned length)
{
int32_t cache = bitstream_peek(bc, 32);
int sign = ~cache >> 31;
skip_remaining(bc, length);
return ((((uint32_t)(sign ^ cache)) >> (32 - length)) ^ sign) - sign;
}
/* Return the LUT element for the given bitstream configuration. */
static inline int set_idx(BitstreamContext *bc, int code, int *n, int *nb_bits,
VLC_TYPE (*table)[2])
{
unsigned idx;
*nb_bits = -*n;
idx = bitstream_peek(bc, *nb_bits) + code;
*n = table[idx][1];
return table[idx][0];
}
/**
* Parse a VLC code.
* @param bits is the number of bits which will be read at once, must be
* identical to nb_bits in init_vlc()
* @param max_depth is the number of times bits bits must be read to completely
* read the longest VLC code
* = (max_vlc_length + bits - 1) / bits
* If the VLC code is invalid and max_depth = 1, then no bits will be removed.
* If the VLC code is invalid and max_depth > 1, then the number of bits removed
* is undefined. */
static inline int bitstream_read_vlc(BitstreamContext *bc, VLC_TYPE (*table)[2],
int bits, int max_depth)
{
int nb_bits;
unsigned idx = bitstream_peek(bc, bits);
int code = table[idx][0];
int n = table[idx][1];
if (max_depth > 1 && n < 0) {
skip_remaining(bc, bits);
code = set_idx(bc, code, &n, &nb_bits, table);
if (max_depth > 2 && n < 0) {
skip_remaining(bc, nb_bits);
code = set_idx(bc, code, &n, &nb_bits, table);
}
}
skip_remaining(bc, n);
return code;
}
#define BITSTREAM_RL_VLC(level, run, bc, table, bits, max_depth) \
do { \
int n, nb_bits; \
unsigned index = bitstream_peek(bc, bits); \
level = table[index].level; \
n = table[index].len; \
\
if (max_depth > 1 && n < 0) { \
bitstream_skip(bc, bits); \
\
nb_bits = -n; \
\
index = bitstream_peek(bc, nb_bits) + level; \
level = table[index].level; \
n = table[index].len; \
if (max_depth > 2 && n < 0) { \
bitstream_skip(bc, nb_bits); \
nb_bits = -n; \
\
index = bitstream_peek(bc, nb_bits) + level; \
level = table[index].level; \
n = table[index].len; \
} \
} \
run = table[index].run; \
bitstream_skip(bc, n); \
} while (0)
/* Return decoded truncated unary code for the values 0, 1, 2. */
static inline int bitstream_decode012(BitstreamContext *bc)
{
if (!bitstream_read_bit(bc))
return 0;
else
return bitstream_read_bit(bc) + 1;
}
/* Return decoded truncated unary code for the values 2, 1, 0. */
static inline int bitstream_decode210(BitstreamContext *bc)
{
if (bitstream_read_bit(bc))
return 0;
else
return 2 - bitstream_read_bit(bc);
}
/* Read sign bit and flip the sign of the provided value accordingly. */
static inline int bitstream_apply_sign(BitstreamContext *bc, int val)
{
int sign = bitstream_read_signed(bc, 1);
return (val ^ sign) - sign;
}
#endif /* AVCODEC_BITSTREAM_H */