ac3dec.c

            }
            if(ch != CPL_CH && ch != s->lfe_ch)
                skip_bits(gbc, 2); /* skip gainrng */
        }
    }

    /* bit allocation information */
    if (s->bit_allocation_syntax) {
        if (get_bits1(gbc)) {
            s->bit_alloc_params.slow_decay = ff_ac3_slow_decay_tab[get_bits(gbc, 2)] >> s->bit_alloc_params.sr_shift;
            s->bit_alloc_params.fast_decay = ff_ac3_fast_decay_tab[get_bits(gbc, 2)] >> s->bit_alloc_params.sr_shift;
            s->bit_alloc_params.slow_gain  = ff_ac3_slow_gain_tab[get_bits(gbc, 2)];
            s->bit_alloc_params.db_per_bit = ff_ac3_db_per_bit_tab[get_bits(gbc, 2)];
            s->bit_alloc_params.floor  = ff_ac3_floor_tab[get_bits(gbc, 3)];
            for(ch=!cpl_in_use; ch<=s->channels; ch++)
                bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
        } else if (!blk) {
            av_log(s->avctx, AV_LOG_ERROR, "new bit allocation info must be present in block 0\n");
            return -1;
        }
    }

    /* signal-to-noise ratio offsets and fast gains (signal-to-mask ratios) */
    if(!s->eac3 || !blk){
        if(s->snr_offset_strategy && get_bits1(gbc)) {
            int snr = 0;
            int csnr;
            csnr = (get_bits(gbc, 6) - 15) << 4;
            for (i = ch = !cpl_in_use; ch <= s->channels; ch++) {
                /* snr offset */
                if (ch == i || s->snr_offset_strategy == 2)
                    snr = (csnr + get_bits(gbc, 4)) << 2;
                /* run at least last bit allocation stage if snr offset changes */
                if(blk && s->snr_offset[ch] != snr) {
                    bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 1);
                }
                s->snr_offset[ch] = snr;

                /* fast gain (normal AC-3 only) */
                if (!s->eac3) {
                    int prev = s->fast_gain[ch];
                    s->fast_gain[ch] = ff_ac3_fast_gain_tab[get_bits(gbc, 3)];
                    /* run last 2 bit allocation stages if fast gain changes */
                    if(blk && prev != s->fast_gain[ch])
                        bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
                }
            }
        } else if (!s->eac3 && !blk) {
            av_log(s->avctx, AV_LOG_ERROR, "new snr offsets must be present in block 0\n");
            return -1;
        }
    }

    /* fast gain (E-AC-3 only) */
    if (s->fast_gain_syntax && get_bits1(gbc)) {
        for (ch = !cpl_in_use; ch <= s->channels; ch++) {
            int prev = s->fast_gain[ch];
            s->fast_gain[ch] = ff_ac3_fast_gain_tab[get_bits(gbc, 3)];
            /* run last 2 bit allocation stages if fast gain changes */
            if(blk && prev != s->fast_gain[ch])
                bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
        }
    } else if (s->eac3 && !blk) {
        for (ch = !cpl_in_use; ch <= s->channels; ch++)
            s->fast_gain[ch] = ff_ac3_fast_gain_tab[4];
    }

    /* E-AC-3 to AC-3 converter SNR offset */
    if (s->frame_type == EAC3_FRAME_TYPE_INDEPENDENT && get_bits1(gbc)) {
        skip_bits(gbc, 10); // skip converter snr offset
    }

    /* coupling leak information */
    if (cpl_in_use) {
        if (s->first_cpl_leak || get_bits1(gbc)) {
            int fl = get_bits(gbc, 3);
            int sl = get_bits(gbc, 3);
            /* run last 2 bit allocation stages for coupling channel if
               coupling leak changes */
            if(blk && (fl != s->bit_alloc_params.cpl_fast_leak ||
                       sl != s->bit_alloc_params.cpl_slow_leak)) {
                bit_alloc_stages[CPL_CH] = FFMAX(bit_alloc_stages[CPL_CH], 2);
            }
            s->bit_alloc_params.cpl_fast_leak = fl;
            s->bit_alloc_params.cpl_slow_leak = sl;
        } else if (!s->eac3 && !blk) {
            av_log(s->avctx, AV_LOG_ERROR, "new coupling leak info must be present in block 0\n");
            return -1;
        }
        s->first_cpl_leak = 0;
    }

    /* delta bit allocation information */
    if (s->dba_syntax && get_bits1(gbc)) {
        /* delta bit allocation exists (strategy) */
        for (ch = !cpl_in_use; ch <= fbw_channels; ch++) {
            s->dba_mode[ch] = get_bits(gbc, 2);
            if (s->dba_mode[ch] == DBA_RESERVED) {
                av_log(s->avctx, AV_LOG_ERROR, "delta bit allocation strategy reserved\n");
                return -1;
            }
            bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
        }
        /* channel delta offset, len and bit allocation */
        for (ch = !cpl_in_use; ch <= fbw_channels; ch++) {
            if (s->dba_mode[ch] == DBA_NEW) {
                s->dba_nsegs[ch] = get_bits(gbc, 3);
                for (seg = 0; seg <= s->dba_nsegs[ch]; seg++) {
                    s->dba_offsets[ch][seg] = get_bits(gbc, 5);
                    s->dba_lengths[ch][seg] = get_bits(gbc, 4);
                    s->dba_values[ch][seg] = get_bits(gbc, 3);
                }
                /* run last 2 bit allocation stages if new dba values */
                bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
            }
        }
    } else if(blk == 0) {
        for(ch=0; ch<=s->channels; ch++) {
            s->dba_mode[ch] = DBA_NONE;
        }
    }

    /* Bit allocation */
    for(ch=!cpl_in_use; ch<=s->channels; ch++) {
        if(bit_alloc_stages[ch] > 2) {
            /* Exponent mapping into PSD and PSD integration */
            ff_ac3_bit_alloc_calc_psd(s->dexps[ch],
                                      s->start_freq[ch], s->end_freq[ch],
                                      s->psd[ch], s->band_psd[ch]);
        }
        if(bit_alloc_stages[ch] > 1) {
            /* Compute excitation function, Compute masking curve, and
               Apply delta bit allocation */
            if (ff_ac3_bit_alloc_calc_mask(&s->bit_alloc_params, s->band_psd[ch],
                                           s->start_freq[ch], s->end_freq[ch],
                                           s->fast_gain[ch], (ch == s->lfe_ch),
                                           s->dba_mode[ch], s->dba_nsegs[ch],
                                           s->dba_offsets[ch], s->dba_lengths[ch],
                                           s->dba_values[ch], s->mask[ch])) {
                av_log(s->avctx, AV_LOG_ERROR, "error in bit allocation\n");
                return -1;
            }
        }
        if(bit_alloc_stages[ch] > 0) {
            /* Compute bit allocation */
            const uint8_t *bap_tab = s->channel_uses_aht[ch] ?
                                     ff_eac3_hebap_tab : ff_ac3_bap_tab;
            ff_ac3_bit_alloc_calc_bap(s->mask[ch], s->psd[ch],
                                      s->start_freq[ch], s->end_freq[ch],
                                      s->snr_offset[ch],
                                      s->bit_alloc_params.floor,
                                      bap_tab, s->bap[ch]);
        }
    }

    /* unused dummy data */
    if (s->skip_syntax && get_bits1(gbc)) {
        int skipl = get_bits(gbc, 9);
        while(skipl--)
            skip_bits(gbc, 8);
    }

    /* unpack the transform coefficients
       this also uncouples channels if coupling is in use. */
    decode_transform_coeffs(s, blk);

    /* TODO: generate enhanced coupling coordinates and uncouple */

    /* TODO: apply spectral extension */

    /* recover coefficients if rematrixing is in use */
    if(s->channel_mode == AC3_CHMODE_STEREO)
        do_rematrixing(s);

    /* apply scaling to coefficients (headroom, dynrng) */
    for(ch=1; ch<=s->channels; ch++) {
        float gain = s->mul_bias / 4194304.0f;
        if(s->channel_mode == AC3_CHMODE_DUALMONO) {
            gain *= s->dynamic_range[ch-1];
        } else {
            gain *= s->dynamic_range[0];
        }
        s->dsp.int32_to_float_fmul_scalar(s->transform_coeffs[ch], s->fixed_coeffs[ch], gain, 256);
    }

    /* downmix and MDCT. order depends on whether block switching is used for
       any channel in this block. this is because coefficients for the long
       and short transforms cannot be mixed. */
    downmix_output = s->channels != s->out_channels &&
                     !((s->output_mode & AC3_OUTPUT_LFEON) &&
                     s->fbw_channels == s->out_channels);
    if(different_transforms) {
        /* the delay samples have already been downmixed, so we upmix the delay
           samples in order to reconstruct all channels before downmixing. */
        if(s->downmixed) {
            s->downmixed = 0;
            ac3_upmix_delay(s);
        }

        do_imdct(s, s->channels);

        if(downmix_output) {
            s->dsp.ac3_downmix(s->output, s->downmix_coeffs, s->out_channels, s->fbw_channels, 256);
        }
    } else {
        if(downmix_output) {
            s->dsp.ac3_downmix(s->transform_coeffs+1, s->downmix_coeffs, s->out_channels, s->fbw_channels, 256);
        }

        if(downmix_output && !s->downmixed) {
            s->downmixed = 1;
            s->dsp.ac3_downmix(s->delay, s->downmix_coeffs, s->out_channels, s->fbw_channels, 128);
        }

        do_imdct(s, s->out_channels);
    }

    return 0;
}

/**
 * Decode a single AC-3 frame.
 */
static int ac3_decode_frame(AVCodecContext * avctx, void *data, int *data_size,
                            AVPacket *avpkt)
{
    const uint8_t *buf = avpkt->data;
    int buf_size = avpkt->size;
    AC3DecodeContext *s = avctx->priv_data;
    int16_t *out_samples = (int16_t *)data;
    int blk, ch, err;
    const uint8_t *channel_map;
    const float *output[AC3_MAX_CHANNELS];

    /* initialize the GetBitContext with the start of valid AC-3 Frame */
    if (s->input_buffer) {
        /* copy input buffer to decoder context to avoid reading past the end
           of the buffer, which can be caused by a damaged input stream. */
        memcpy(s->input_buffer, buf, FFMIN(buf_size, AC3_FRAME_BUFFER_SIZE));
        init_get_bits(&s->gbc, s->input_buffer, buf_size * 8);
    } else {
        init_get_bits(&s->gbc, buf, buf_size * 8);
    }

    /* parse the syncinfo */
    *data_size = 0;
    err = parse_frame_header(s);

    /* check that reported frame size fits in input buffer */
    if(s->frame_size > buf_size) {
        av_log(avctx, AV_LOG_ERROR, "incomplete frame\n");
        err = AAC_AC3_PARSE_ERROR_FRAME_SIZE;
    }

    /* check for crc mismatch */
    if(err != AAC_AC3_PARSE_ERROR_FRAME_SIZE && avctx->error_recognition >= FF_ER_CAREFUL) {
        if(av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, &buf[2], s->frame_size-2)) {
            av_log(avctx, AV_LOG_ERROR, "frame CRC mismatch\n");
            err = AAC_AC3_PARSE_ERROR_CRC;
        }
    }

    if(err && err != AAC_AC3_PARSE_ERROR_CRC) {
        switch(err) {
            case AAC_AC3_PARSE_ERROR_SYNC:
                av_log(avctx, AV_LOG_ERROR, "frame sync error\n");
                return -1;
            case AAC_AC3_PARSE_ERROR_BSID:
                av_log(avctx, AV_LOG_ERROR, "invalid bitstream id\n");
                break;
            case AAC_AC3_PARSE_ERROR_SAMPLE_RATE:
                av_log(avctx, AV_LOG_ERROR, "invalid sample rate\n");
                break;
            case AAC_AC3_PARSE_ERROR_FRAME_SIZE:
                av_log(avctx, AV_LOG_ERROR, "invalid frame size\n");
                break;
            case AAC_AC3_PARSE_ERROR_FRAME_TYPE:
                /* skip frame if CRC is ok. otherwise use error concealment. */
                /* TODO: add support for substreams and dependent frames */
                if(s->frame_type == EAC3_FRAME_TYPE_DEPENDENT || s->substreamid) {
                    av_log(avctx, AV_LOG_ERROR, "unsupported frame type : skipping frame\n");
                    return s->frame_size;
                } else {
                    av_log(avctx, AV_LOG_ERROR, "invalid frame type\n");
                }
                break;
            default:
                av_log(avctx, AV_LOG_ERROR, "invalid header\n");
                break;
        }
    }

    /* if frame is ok, set audio parameters */
    if (!err) {
        avctx->sample_rate = s->sample_rate;
        avctx->bit_rate = s->bit_rate;

        /* channel config */
        s->out_channels = s->channels;
        s->output_mode = s->channel_mode;
        if(s->lfe_on)
            s->output_mode |= AC3_OUTPUT_LFEON;
        if (avctx->request_channels > 0 && avctx->request_channels <= 2 &&
                avctx->request_channels < s->channels) {
            s->out_channels = avctx->request_channels;
            s->output_mode  = avctx->request_channels == 1 ? AC3_CHMODE_MONO : AC3_CHMODE_STEREO;
            s->channel_layout = ff_ac3_channel_layout_tab[s->output_mode];
        }
        avctx->channels = s->out_channels;
        avctx->channel_layout = s->channel_layout;

        /* set downmixing coefficients if needed */
        if(s->channels != s->out_channels && !((s->output_mode & AC3_OUTPUT_LFEON) &&
                s->fbw_channels == s->out_channels)) {
            set_downmix_coeffs(s);
        }
    } else if (!s->out_channels) {
        s->out_channels = avctx->channels;
        if(s->out_channels < s->channels)
            s->output_mode  = s->out_channels == 1 ? AC3_CHMODE_MONO : AC3_CHMODE_STEREO;
    }

    /* decode the audio blocks */
    channel_map = ff_ac3_dec_channel_map[s->output_mode & ~AC3_OUTPUT_LFEON][s->lfe_on];
    for (ch = 0; ch < s->out_channels; ch++)
        output[ch] = s->output[channel_map[ch]];
    for (blk = 0; blk < s->num_blocks; blk++) {
        if (!err && decode_audio_block(s, blk)) {
            av_log(avctx, AV_LOG_ERROR, "error decoding the audio block\n");
            err = 1;
        }
        s->dsp.float_to_int16_interleave(out_samples, output, 256, s->out_channels);
        out_samples += 256 * s->out_channels;
    }
    *data_size = s->num_blocks * 256 * avctx->channels * sizeof (int16_t);
    return s->frame_size;
}

/**
 * Uninitialize the AC-3 decoder.
 */
static av_cold int ac3_decode_end(AVCodecContext *avctx)
{
    AC3DecodeContext *s = avctx->priv_data;
    ff_mdct_end(&s->imdct_512);
    ff_mdct_end(&s->imdct_256);

    av_freep(&s->input_buffer);

    return 0;
}

AVCodec ac3_decoder = {
    .name = "ac3",
    .type = CODEC_TYPE_AUDIO,
    .id = CODEC_ID_AC3,
    .priv_data_size = sizeof (AC3DecodeContext),
    .init = ac3_decode_init,
    .close = ac3_decode_end,
    .decode = ac3_decode_frame,
    .long_name = NULL_IF_CONFIG_SMALL("ATSC A/52A (AC-3)"),
};

AVCodec eac3_decoder = {
    .name = "eac3",
    .type = CODEC_TYPE_AUDIO,
    .id = CODEC_ID_EAC3,
    .priv_data_size = sizeof (AC3DecodeContext),
    .init = ac3_decode_init,
    .close = ac3_decode_end,
    .decode = ac3_decode_frame,
    .long_name = NULL_IF_CONFIG_SMALL("ATSC A/52B (AC-3, E-AC-3)"),
};