ffmpeg中aac的adts源码走读

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static int adts_write_frame_header(ADTSContext *ctx,
                                   uint8_t *buf, int size, int pce_size)
{
    PutBitContext pb;

    unsigned full_frame_size = (unsigned)ADTS_HEADER_SIZE + size + pce_size;
    if (full_frame_size > ADTS_MAX_FRAME_BYTES) {
        av_log(NULL, AV_LOG_ERROR, "ADTS frame size too large: %u (max %d)n",
               full_frame_size, ADTS_MAX_FRAME_BYTES);
        return AVERROR_INVALIDDATA;
    }

    init_put_bits(&pb, buf, ADTS_HEADER_SIZE);

    /* adts_fixed_header */
    put_bits(&pb, 12, 0xfff);   /* syncword */
    put_bits(&pb, 1, ctx->mpeg_id); /* ID */
    put_bits(&pb, 2, 0);        /* layer */
    put_bits(&pb, 1, 1);        /* protection_absent */
    put_bits(&pb, 2, ctx->objecttype); /* profile_objecttype */
    put_bits(&pb, 4, ctx->sample_rate_index);
    put_bits(&pb, 1, 0);        /* private_bit */
    put_bits(&pb, 3, ctx->channel_conf); /* channel_configuration */
    put_bits(&pb, 1, 0);        /* original_copy */
    put_bits(&pb, 1, 0);        /* home */

    /* adts_variable_header */
    put_bits(&pb, 1, 0);        /* copyright_identification_bit */
    put_bits(&pb, 1, 0);        /* copyright_identification_start */
    put_bits(&pb, 13, full_frame_size); /* aac_frame_length */
    put_bits(&pb, 11, 0x7ff);   /* adts_buffer_fullness */
    put_bits(&pb, 2, 0);        /* number_of_raw_data_blocks_in_frame */

    flush_put_bits(&pb);

    return 0;
}

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typedef uint64_t BitBuf;
typedef struct PutBitContext {
    BitBuf bit_buf;
    int bit_left;
    uint8_t *buf, *buf_ptr, *buf_end;
} PutBitContext;

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static inline void init_put_bits(PutBitContext *s, uint8_t *buffer,
                                 int buffer_size)
{
    if (buffer_size < 0) {
        buffer_size = 0;
        buffer      = NULL;
    }

    s->buf          = buffer;
    s->buf_end      = s->buf + buffer_size;
    s->buf_ptr      = s->buf;
    s->bit_left     = BUF_BITS;
    s->bit_buf      = 0;
}

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static inline void put_bits(PutBitContext *s, int n, BitBuf value)
{
    av_assert2(n <= 31 && value < (1UL << n));
    put_bits_no_assert(s, n, value);
}
static inline void put_bits_no_assert(PutBitContext *s, int n, BitBuf value)
{
    BitBuf bit_buf;
    int bit_left;

    bit_buf  = s->bit_buf;
    bit_left = s->bit_left;
    if (n < bit_left) {
        bit_buf     = (bit_buf << n) | value;
        bit_left   -= n;
    } else {
        bit_buf   <<= bit_left;
        bit_buf    |= value >> (n - bit_left);
        if (s->buf_end - s->buf_ptr >= sizeof(BitBuf)) {
            // AV_WBBUF(s->buf_ptr, bit_buf);
            *((__unaligned uint64_t*)(s->buf_ptr)) = _byteswap_uint64(bit_buf);
            s->buf_ptr += sizeof(BitBuf);
        } else {
            av_log(NULL, AV_LOG_ERROR, "Internal error, put_bits buffer too smalln");
            av_assert2(0);
        }
        bit_left   += BUF_BITS - n;
        bit_buf     = value;
    }
    s->bit_buf  = bit_buf;
    s->bit_left = bit_left;
}

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int ff_adts_header_parse(GetBitContext *gbc, AACADTSHeaderInfo *hdr)
{
    int size, rdb, ch, sr;
    int aot, crc_abs;

    if (get_bits(gbc, 12) != 0xfff)
        return AAC_AC3_PARSE_ERROR_SYNC;

    skip_bits1(gbc);             /* id */
    skip_bits(gbc, 2);           /* layer */
    crc_abs = get_bits1(gbc);    /* protection_absent */
    aot     = get_bits(gbc, 2);  /* profile_objecttype */
    sr      = get_bits(gbc, 4);  /* sample_frequency_index */
    if (!avpriv_mpeg4audio_sample_rates[sr])
        return AAC_AC3_PARSE_ERROR_SAMPLE_RATE;
    skip_bits1(gbc);             /* private_bit */
    ch = get_bits(gbc, 3);       /* channel_configuration */

    skip_bits1(gbc);             /* original/copy */
    skip_bits1(gbc);             /* home */

    /* adts_variable_header */
    skip_bits1(gbc);             /* copyright_identification_bit */
    skip_bits1(gbc);             /* copyright_identification_start */
    size = get_bits(gbc, 13);    /* aac_frame_length */
    if (size < AV_AAC_ADTS_HEADER_SIZE)
        return AAC_AC3_PARSE_ERROR_FRAME_SIZE;

    skip_bits(gbc, 11);          /* adts_buffer_fullness */
    rdb = get_bits(gbc, 2);      /* number_of_raw_data_blocks_in_frame */

    hdr->object_type    = aot + 1;
    hdr->chan_config    = ch;
    hdr->crc_absent     = crc_abs;
    hdr->num_aac_frames = rdb + 1;
    hdr->sampling_index = sr;
    hdr->sample_rate    = avpriv_mpeg4audio_sample_rates[sr];
    hdr->samples        = (rdb + 1) * 1024;
    hdr->bit_rate       = size * 8 * hdr->sample_rate / hdr->samples;
    hdr->frame_length   = size;

    return size;
}

// 将宏定义展开
static inline unsigned int get_bits(GetBitContext *s, int n)
{
    register unsigned int tmp;
    unsigned int re_index = s->index;
	unsigned int re_cache;
	unsigned int re_size_plus8 = s->size_in_bits_plus8;
	re_cache = av_bswap32(*((const __unaligned uint32_t*)(s->buffer + (re_index >> 3)))) << (re_index & 7);
	tmp = ((uint32_t)(re_cache))>>(32-(n));
	re_index = re_size_plus8 > re_index + n ? re_index + n : re_size_plus8;
	s->index = re_index;
    return tmp;
}