/src/ffmpeg/libavcodec/get_bits.h

Source
/*
 * Copyright (c) 2004 Michael Niedermayer <michaelni@gmx.at>
 *
 * 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
 */

/**
 * @file
 * bitstream reader API header.
 */

#ifndef AVCODEC_GET_BITS_H
#define AVCODEC_GET_BITS_H

#include <stdint.h>

#include "libavutil/common.h"
#include "libavutil/intreadwrite.h"
#include "libavutil/avassert.h"

#include "defs.h"
#include "mathops.h"
#include "vlc.h"

/*
 * Safe bitstream reading:
 * optionally, the get_bits API can check to ensure that we
 * don't read past input buffer boundaries. This is protected
 * with CONFIG_SAFE_BITSTREAM_READER at the global level, and
 * then below that with UNCHECKED_BITSTREAM_READER at the per-
 * decoder level. This means that decoders that check internally
 * can "#define UNCHECKED_BITSTREAM_READER 1" to disable
 * overread checks.
 * Boundary checking causes a minor performance penalty so for
 * applications that won't want/need this, it can be disabled
 * globally using "#define CONFIG_SAFE_BITSTREAM_READER 0".
 */
#ifndef UNCHECKED_BITSTREAM_READER
#define UNCHECKED_BITSTREAM_READER !CONFIG_SAFE_BITSTREAM_READER
#endif

#ifndef CACHED_BITSTREAM_READER
#define CACHED_BITSTREAM_READER 0
#endif

#if CACHED_BITSTREAM_READER

// we always want the LE implementation, to provide get_bits_le()
#define BITSTREAM_LE

#ifndef BITSTREAM_READER_LE
# define BITSTREAM_BE
# define BITSTREAM_DEFAULT_BE
#endif

#include "bitstream.h"

#undef BITSTREAM_LE
#undef BITSTREAM_BE
#undef BITSTREAM_DEFAULT_BE

typedef BitstreamContext GetBitContext;

#define get_bits_count      bits_tell
#define get_bits_bytesize   bits_bytesize
#define get_bits_left       bits_left
#define skip_bits_long      bits_skip
#define skip_bits           bits_skip
#define get_bits            bits_read_nz
#define get_bitsz           bits_read
#define get_bits_long       bits_read
#define get_bits1           bits_read_bit
#define get_bits64          bits_read_64
#define get_xbits           bits_read_xbits
#define get_sbits           bits_read_signed_nz
#define get_sbits_long      bits_read_signed
#define show_bits           bits_peek
#define show_bits_long      bits_peek
#define init_get_bits       bits_init
#define init_get_bits8      bits_init8
#define align_get_bits      bits_align
#define get_vlc2            bits_read_vlc
#define get_vlc_multi       bits_read_vlc_multi

#define init_get_bits8_le(s, buffer, byte_size) bits_init8_le((BitstreamContextLE*)s, buffer, byte_size)
#define get_bits_le(s, n)                       bits_read_le((BitstreamContextLE*)s, n)

#define show_bits1(s)       bits_peek(s, 1)
#define skip_bits1(s)       bits_skip(s, 1)

#define skip_1stop_8data_bits bits_skip_1stop_8data

#else   // CACHED_BITSTREAM_READER

typedef struct GetBitContext {
    const uint8_t *buffer;
    int index;
    int size_in_bits;
    int size_in_bits_plus8;
} GetBitContext;

static inline unsigned int get_bits(GetBitContext *s, int n);
static inline void skip_bits(GetBitContext *s, int n);
static inline unsigned int show_bits(GetBitContext *s, int n);

/* Bitstream reader API docs:
 * name
 *   arbitrary name which is used as prefix for the internal variables
 *
 * gb
 *   getbitcontext
 *
 * OPEN_READER(name, gb)
 *   load gb into local variables
 *
 * CLOSE_READER(name, gb)
 *   store local vars in gb
 *
 * UPDATE_CACHE(name, gb)
 *   Refill the internal cache from the bitstream.
 *   After this call at least MIN_CACHE_BITS will be available.
 *
 * GET_CACHE(name, gb)
 *   Will output the contents of the internal cache,
 *   next bit is MSB of 32 or 64 bits (FIXME 64 bits).
 *
 * SHOW_UBITS(name, gb, num)
 *   Will return the next num bits.
 *
 * SHOW_SBITS(name, gb, num)
 *   Will return the next num bits and do sign extension.
 *
 * SKIP_BITS(name, gb, num)
 *   Will skip over the next num bits.
 *   Note, this is equivalent to SKIP_CACHE; SKIP_COUNTER.
 *
 * SKIP_CACHE(name, gb, num)
 *   Will remove the next num bits from the cache (note SKIP_COUNTER
 *   MUST be called before UPDATE_CACHE / CLOSE_READER).
 *
 * SKIP_COUNTER(name, gb, num)
 *   Will increment the internal bit counter (see SKIP_CACHE & SKIP_BITS).
 *
 * LAST_SKIP_BITS(name, gb, num)
 *   Like SKIP_BITS, to be used if next call is UPDATE_CACHE or CLOSE_READER.
 *
 * BITS_LEFT(name, gb)
 *   Return the number of bits left
 *
 * For examples see get_bits, show_bits, skip_bits, get_vlc.
 */

#define MIN_CACHE_BITS 25

#define OPEN_READER_NOSIZE_NOCACHE(name, gb)    \
    unsigned int name ## _index = (gb)->index

#define OPEN_READER_NOSIZE(name, gb)            \
    OPEN_READER_NOSIZE_NOCACHE(name, gb);       \
    unsigned int name ## _cache

#if UNCHECKED_BITSTREAM_READER
#define OPEN_READER(name, gb) OPEN_READER_NOSIZE(name, gb)
#define OPEN_READER_SIZE(name, gb) ((void)0)
#define BITS_AVAILABLE(name, gb) 1
#else
#define OPEN_READER_SIZE(name, gb) unsigned int name ## _size_plus8 = (gb)->size_in_bits_plus8
#define OPEN_READER(name, gb)                   \
    OPEN_READER_NOSIZE(name, gb);               \
    OPEN_READER_SIZE(name, gb)

#define BITS_AVAILABLE(name, gb) name ## _index < name ## _size_plus8
#endif

#define CLOSE_READER(name, gb) (gb)->index = name ## _index

#define UPDATE_CACHE_BE_EXT(name, gb, bits, dst_bits) name ## _cache = \
    AV_RB ## bits((gb)->buffer + (name ## _index >> 3)) << (name ## _index & 7) >> (bits - dst_bits)

#define UPDATE_CACHE_LE_EXT(name, gb, bits, dst_bits) name ## _cache = \
    (uint ## dst_bits ## _t)(AV_RL ## bits((gb)->buffer + (name ## _index >> 3)) >> (name ## _index & 7))

/* Using these two macros ensures that 32 bits are available. */
# define UPDATE_CACHE_LE_32(name, gb) UPDATE_CACHE_LE_EXT(name, (gb), 64, 32)
# define UPDATE_CACHE_BE_32(name, gb) UPDATE_CACHE_BE_EXT(name, (gb), 64, 32)

# define UPDATE_CACHE_LE(name, gb) UPDATE_CACHE_LE_EXT(name, (gb), 32, 32)
# define UPDATE_CACHE_BE(name, gb) UPDATE_CACHE_BE_EXT(name, (gb), 32, 32)

#ifdef BITSTREAM_READER_LE

# define UPDATE_CACHE(name, gb) UPDATE_CACHE_LE(name, gb)
# define UPDATE_CACHE_32(name, gb) UPDATE_CACHE_LE_32(name, (gb))

# define SKIP_CACHE(name, gb, num) name ## _cache >>= (num)

#else

# define UPDATE_CACHE(name, gb) UPDATE_CACHE_BE(name, gb)
# define UPDATE_CACHE_32(name, gb) UPDATE_CACHE_BE_32(name, (gb))

# define SKIP_CACHE(name, gb, num) name ## _cache <<= (num)

#endif

#if UNCHECKED_BITSTREAM_READER
#   define SKIP_COUNTER(name, gb, num) name ## _index += (num)
#else
#   define SKIP_COUNTER(name, gb, num) \
    name ## _index = FFMIN(name ## _size_plus8, name ## _index + (num))
#endif

#define BITS_LEFT(name, gb) ((int)((gb)->size_in_bits - name ## _index))

#define SKIP_BITS(name, gb, num)                \
    do {                                        \
        SKIP_CACHE(name, gb, num);              \
        SKIP_COUNTER(name, gb, num);            \
    } while (0)

#define LAST_SKIP_BITS(name, gb, num) SKIP_COUNTER(name, gb, num)

#define SHOW_UBITS_LE(name, gb, num) zero_extend(name ## _cache, num)
#define SHOW_SBITS_LE(name, gb, num) sign_extend(name ## _cache, num)

#define SHOW_UBITS_BE(name, gb, num) NEG_USR32(name ## _cache, num)
#define SHOW_SBITS_BE(name, gb, num) NEG_SSR32(name ## _cache, num)

#ifdef BITSTREAM_READER_LE
#   define SHOW_UBITS(name, gb, num) SHOW_UBITS_LE(name, gb, num)
#   define SHOW_SBITS(name, gb, num) SHOW_SBITS_LE(name, gb, num)
#else
#   define SHOW_UBITS(name, gb, num) SHOW_UBITS_BE(name, gb, num)
#   define SHOW_SBITS(name, gb, num) SHOW_SBITS_BE(name, gb, num)
#endif

#define GET_CACHE(name, gb) ((uint32_t) name ## _cache)


static inline int get_bits_count(const GetBitContext *s)
{
    return s->index;
}

/**
 * Get the size of the GetBitContext's buffer in bytes.
 *
 * @param s        the GetBitContext
 * @param round_up If set, the number of bits will be rounded up to full bytes;
 *                 this does not matter if the number of bits is known to be
 *                 a multiple of eight, e.g. if the GetBitContext has been
 *                 initialized with init_get_bits8.
 */
static inline int get_bits_bytesize(const GetBitContext *s, int round_up)
{
    return (s->size_in_bits + (round_up ? 7 : 0)) >> 3;
}

/**
 * Skips the specified number of bits.
 * @param n the number of bits to skip,
 *          For the UNCHECKED_BITSTREAM_READER this must not cause the distance
 *          from the start to overflow int32_t. Staying within the bitstream + padding
 *          is sufficient, too.
 */
static inline void skip_bits_long(GetBitContext *s, int n)
{
#if UNCHECKED_BITSTREAM_READER
    s->index += n;
#else
    s->index += av_clip(n, -s->index, s->size_in_bits_plus8 - s->index);
#endif
}

/**
 * Read MPEG-1 dc-style VLC (sign bit + mantissa with no MSB).
 * if MSB not set it is negative
 * @param n length in bits
 */
static inline int get_xbits(GetBitContext *s, int n)
{
    register int sign;
    register int32_t cache;
    OPEN_READER(re, s);
    av_assert2(n>0 && n<=25);
    UPDATE_CACHE(re, s);
    cache = GET_CACHE(re, s);
    sign  = ~cache >> 31;
    LAST_SKIP_BITS(re, s, n);
    CLOSE_READER(re, s);
    return (NEG_USR32(sign ^ cache, n) ^ sign) - sign;
}

static inline int get_xbits_le(GetBitContext *s, int n)
{
    register int sign;
    register int32_t cache;
    OPEN_READER(re, s);
    av_assert2(n>0 && n<=25);
    UPDATE_CACHE_LE(re, s);
    cache = GET_CACHE(re, s);
    sign  = sign_extend(~cache, n) >> 31;
    LAST_SKIP_BITS(re, s, n);
    CLOSE_READER(re, s);
    return (zero_extend(sign ^ cache, n) ^ sign) - sign;
}

static inline int get_sbits(GetBitContext *s, int n)
{
    register int tmp;
    OPEN_READER(re, s);
    av_assert2(n>0 && n<=25);
    UPDATE_CACHE(re, s);
    tmp = SHOW_SBITS(re, s, n);
    LAST_SKIP_BITS(re, s, n);
    CLOSE_READER(re, s);
    return tmp;
}

/**
 * Read 1-25 bits.
 */
static inline unsigned int get_bits(GetBitContext *s, int n)
{
    register unsigned int tmp;
    OPEN_READER(re, s);
    av_assert2(n>0 && n<=25);
    UPDATE_CACHE(re, s);
    tmp = SHOW_UBITS(re, s, n);
    LAST_SKIP_BITS(re, s, n);
    CLOSE_READER(re, s);
    av_assert2(tmp < UINT64_C(1) << n);
    return tmp;
}

/**
 * Read 0-25 bits.
 */
static av_always_inline int get_bitsz(GetBitContext *s, int n)
{
    return n ? get_bits(s, n) : 0;
}

static inline unsigned int get_bits_le(GetBitContext *s, int n)
{
    register int tmp;
    OPEN_READER(re, s);
    av_assert2(n>0 && n<=25);
    UPDATE_CACHE_LE(re, s);
    tmp = SHOW_UBITS_LE(re, s, n);
    LAST_SKIP_BITS(re, s, n);
    CLOSE_READER(re, s);
    return tmp;
}

/**
 * Show 1-25 bits.
 */
static inline unsigned int show_bits(GetBitContext *s, int n)
{
    register unsigned int tmp;
    OPEN_READER_NOSIZE(re, s);
    av_assert2(n>0 && n<=25);
    UPDATE_CACHE(re, s);
    tmp = SHOW_UBITS(re, s, n);
    return tmp;
}

static inline void skip_bits(GetBitContext *s, int n)
{
    OPEN_READER_NOSIZE_NOCACHE(re, s);
    OPEN_READER_SIZE(re, s);
    LAST_SKIP_BITS(re, s, n);
    CLOSE_READER(re, s);
}

static inline unsigned int get_bits1(GetBitContext *s)
{
    unsigned int index = s->index;
    uint8_t result     = s->buffer[index >> 3];
#ifdef BITSTREAM_READER_LE
    result >>= index & 7;
    result  &= 1;
#else
    result <<= index & 7;
    result >>= 8 - 1;
#endif
#if !UNCHECKED_BITSTREAM_READER
    if (s->index < s->size_in_bits_plus8)
#endif
        index++;
    s->index = index;

    return result;
}

static inline unsigned int show_bits1(GetBitContext *s)
{
    return show_bits(s, 1);
}

static inline void skip_bits1(GetBitContext *s)
{
    skip_bits(s, 1);
}

/**
 * Read 0-32 bits.
 */
static inline unsigned int get_bits_long(GetBitContext *s, int n)
{
    av_assert2(n>=0 && n<=32);
    if (!n) {
        return 0;
    } else if ((!HAVE_FAST_64BIT || av_builtin_constant_p(n <= MIN_CACHE_BITS))
               && n <= MIN_CACHE_BITS) {
        return get_bits(s, n);
    } else {
#if HAVE_FAST_64BIT
        unsigned tmp;
        OPEN_READER(re, s);
        UPDATE_CACHE_32(re, s);
        tmp = SHOW_UBITS(re, s, n);
        LAST_SKIP_BITS(re, s, n);
        CLOSE_READER(re, s);
        return tmp;
#else
#ifdef BITSTREAM_READER_LE
        unsigned ret = get_bits(s, 16);
        return ret | (get_bits(s, n - 16) << 16);
#else
        unsigned ret = get_bits(s, 16) << (n - 16);
        return ret | get_bits(s, n - 16);
#endif
#endif
    }
}

/**
 * Read 0-64 bits.
 */
static inline uint64_t get_bits64(GetBitContext *s, int n)
{
    if (n <= 32) {
        return get_bits_long(s, n);
    } else {
#ifdef BITSTREAM_READER_LE
        uint64_t ret = get_bits_long(s, 32);
        return ret | (uint64_t) get_bits_long(s, n - 32) << 32;
#else
        uint64_t ret = (uint64_t) get_bits_long(s, n - 32) << 32;
        return ret | get_bits_long(s, 32);
#endif
    }
}

/**
 * Read 0-32 bits as a signed integer.
 */
static inline int get_sbits_long(GetBitContext *s, int n)
{
    // sign_extend(x, 0) is undefined
    if (!n)
        return 0;

    return sign_extend(get_bits_long(s, n), n);
}

/**
 * Read 0-64 bits as a signed integer.
 */
static inline int64_t get_sbits64(GetBitContext *s, int n)
{
    // sign_extend(x, 0) is undefined
    if (!n)
        return 0;

    return sign_extend64(get_bits64(s, n), n);
}

/**
 * Show 0-32 bits.
 */
static inline unsigned int show_bits_long(GetBitContext *s, int n)
{
    if (n <= MIN_CACHE_BITS) {
        return show_bits(s, n);
    } else {
        GetBitContext gb = *s;
        return get_bits_long(&gb, n);
    }
}


/**
 * Initialize GetBitContext.
 * @param buffer bitstream buffer, must be AV_INPUT_BUFFER_PADDING_SIZE bytes
 *        larger than the actual read bits because some optimized bitstream
 *        readers read 32 or 64 bit at once and could read over the end
 * @param bit_size the size of the buffer in bits
 * @return 0 on success, AVERROR_INVALIDDATA if the buffer_size would overflow.
 */
static inline int init_get_bits(GetBitContext *s, const uint8_t *buffer,
                                int bit_size)
{
    int ret = 0;

    if (bit_size >= INT_MAX - FFMAX(7, AV_INPUT_BUFFER_PADDING_SIZE*8) || bit_size < 0 || !buffer) {
        bit_size    = 0;
        buffer      = NULL;
        ret         = AVERROR_INVALIDDATA;
    }

    s->buffer             = buffer;
    s->size_in_bits       = bit_size;
    s->size_in_bits_plus8 = bit_size + 8;
    s->index              = 0;

    return ret;
}

/**
 * Initialize GetBitContext.
 * @param buffer bitstream buffer, must be AV_INPUT_BUFFER_PADDING_SIZE bytes
 *        larger than the actual read bits because some optimized bitstream
 *        readers read 32 or 64 bit at once and could read over the end
 * @param byte_size the size of the buffer in bytes
 * @return 0 on success, AVERROR_INVALIDDATA if the buffer_size would overflow.
 */
static inline int init_get_bits8(GetBitContext *s, const uint8_t *buffer,
                                 int byte_size)
{
    if (byte_size > INT_MAX / 8 || byte_size < 0)
        byte_size = -1;
    return init_get_bits(s, buffer, byte_size * 8);
}

static inline int init_get_bits8_le(GetBitContext *s, const uint8_t *buffer,
                                    int byte_size)
{
    if (byte_size > INT_MAX / 8 || byte_size < 0)
        byte_size = -1;
    return init_get_bits(s, buffer, byte_size * 8);
}

static inline const uint8_t *align_get_bits(GetBitContext *s)
{
    int n = -get_bits_count(s) & 7;
    if (n)
        skip_bits(s, n);
    return s->buffer + (s->index >> 3);
}

/**
 * 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.
 */
#define GET_VLC(code, name, gb, table, bits, max_depth)         \
    do {                                                        \
        unsigned idx_ = SHOW_UBITS(name, gb, bits);             \
        code          = table[idx_].sym;                        \
        int        n_ = table[idx_].len;                        \
                                                                \
        if (max_depth > 1 && n_ < 0) {                          \
            LAST_SKIP_BITS(name, gb, bits);                     \
            UPDATE_CACHE(name, gb);                             \
                                                                \
            int nb__bits = -n_;                                 \
                                                                \
            idx_ = SHOW_UBITS(name, gb, nb__bits) + code;       \
            code = table[idx_].sym;                             \
            n_   = table[idx_].len;                             \
            if (max_depth > 2 && n_ < 0) {                      \
                LAST_SKIP_BITS(name, gb, nb__bits);             \
                UPDATE_CACHE(name, gb);                         \
                                                                \
                nb__bits = -n_;                                 \
                                                                \
                idx_ = SHOW_UBITS(name, gb, nb__bits) + code;   \
                code = table[idx_].sym;                         \
                n_   = table[idx_].len;                         \
            }                                                   \
        }                                                       \
        SKIP_BITS(name, gb, n_);                                \
    } while (0)

#define GET_RL_VLC(level, run, name, gb, table, bits,  \
                   max_depth, need_update)                      \
    do {                                                        \
        unsigned idx_ = SHOW_UBITS(name, gb, bits);             \
        level         = table[idx_].level;                      \
        int        n_ = table[idx_].len8;                       \
                                                                \
        if (max_depth > 1 && n_ < 0) {                          \
            SKIP_BITS(name, gb, bits);                          \
            if (need_update) {                                  \
                UPDATE_CACHE(name, gb);                         \
            }                                                   \
                                                                \
            int nb__bits = -n_;                                 \
                                                                \
            idx_  = SHOW_UBITS(name, gb, nb__bits) + level;     \
            level = table[idx_].level;                          \
            n_    = table[idx_].len8;                           \
            if (max_depth > 2 && n_ < 0) {                      \
                LAST_SKIP_BITS(name, gb, nb__bits);             \
                if (need_update) {                              \
                    UPDATE_CACHE(name, gb);                     \
                }                                               \
                nb__bits = -n_;                                 \
                                                                \
                idx_  = SHOW_UBITS(name, gb, nb__bits) + level; \
                level = table[idx_].level;                      \
                n_    = table[idx_].len8;                       \
            }                                                   \
        }                                                       \
        run = table[idx_].run;                                  \
        SKIP_BITS(name, gb, n_);                                \
    } while (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 vlc_init()
 * @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
 * @returns the code parsed or -1 if no vlc matches
 */
static av_always_inline int get_vlc2(GetBitContext *s, const VLCElem *table,
                                     int bits, int max_depth)
{
    int code;

    OPEN_READER(re, s);
    UPDATE_CACHE(re, s);

    GET_VLC(code, re, s, table, bits, max_depth);

    CLOSE_READER(re, s);

    return code;
}

static inline int get_vlc_multi(GetBitContext *s, uint8_t *dst,
                                av_unused const VLC_MULTI_ELEM *const Jtable,
                                const VLCElem *const table,
                                const int bits, const int max_depth,
                                av_unused const int symbols_size)
{
    dst[0] = get_vlc2(s, table, bits, max_depth);
    return 1;
}

static inline int decode012(GetBitContext *gb)
{
    int n;
    n = get_bits1(gb);
    if (n == 0)
        return 0;
    else
        return get_bits1(gb) + 1;
}

static inline int decode210(GetBitContext *gb)
{
    if (get_bits1(gb))
        return 0;
    else
        return 2 - get_bits1(gb);
}

static inline int get_bits_left(GetBitContext *gb)
{
    return gb->size_in_bits - get_bits_count(gb);
}

static inline int skip_1stop_8data_bits(GetBitContext *gb)
{
    if (get_bits_left(gb) <= 0)
        return AVERROR_INVALIDDATA;

    while (get_bits1(gb)) {
        skip_bits(gb, 8);
        if (get_bits_left(gb) <= 0)
            return AVERROR_INVALIDDATA;
    }

    return 0;
}

#endif // CACHED_BITSTREAM_READER

#endif /* AVCODEC_GET_BITS_H */

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Created: 2026-07-25 07:52