ac3dec.c

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        if(!s->dither_flag[ch])
            s->dither_all = 0;
    }

    /* dynamic range */
    i = !(s->channel_mode);
    do {
        if(get_bits1(gbc)) {
            s->dynamic_range[i] = ((dynamic_range_tab[get_bits(gbc, 8)]-1.0) *
                                  s->avctx->drc_scale)+1.0;
        } else if(blk == 0) {
            s->dynamic_range[i] = 1.0f;
        }
    } while(i--);

    /* coupling strategy */
    if (get_bits1(gbc)) {
        memset(bit_alloc_stages, 3, AC3_MAX_CHANNELS);
        s->cpl_in_use = get_bits1(gbc);
        if (s->cpl_in_use) {
            /* coupling in use */
            int cpl_begin_freq, cpl_end_freq;

            /* determine which channels are coupled */
            for (ch = 1; ch <= fbw_channels; ch++)
                s->channel_in_cpl[ch] = get_bits1(gbc);

            /* phase flags in use */
            if (channel_mode == AC3_CHMODE_STEREO)
                s->phase_flags_in_use = get_bits1(gbc);

            /* coupling frequency range and band structure */
            cpl_begin_freq = get_bits(gbc, 4);
            cpl_end_freq = get_bits(gbc, 4);
            if (3 + cpl_end_freq - cpl_begin_freq < 0) {
                av_log(s->avctx, AV_LOG_ERROR, "3+cplendf = %d < cplbegf = %d\n", 3+cpl_end_freq, cpl_begin_freq);
                return -1;
            }
            s->num_cpl_bands = s->num_cpl_subbands = 3 + cpl_end_freq - cpl_begin_freq;
            s->start_freq[CPL_CH] = cpl_begin_freq * 12 + 37;
            s->end_freq[CPL_CH] = cpl_end_freq * 12 + 73;
            for (bnd = 0; bnd < s->num_cpl_subbands - 1; bnd++) {
                if (get_bits1(gbc)) {
                    s->cpl_band_struct[bnd] = 1;
                    s->num_cpl_bands--;
                }
            }
            s->cpl_band_struct[s->num_cpl_subbands-1] = 0;
        } else {
            /* coupling not in use */
            for (ch = 1; ch <= fbw_channels; ch++)
                s->channel_in_cpl[ch] = 0;
        }
    }

    /* coupling coordinates */
    if (s->cpl_in_use) {
        int cpl_coords_exist = 0;

        for (ch = 1; ch <= fbw_channels; ch++) {
            if (s->channel_in_cpl[ch]) {
                if (get_bits1(gbc)) {
                    int master_cpl_coord, cpl_coord_exp, cpl_coord_mant;
                    cpl_coords_exist = 1;
                    master_cpl_coord = 3 * get_bits(gbc, 2);
                    for (bnd = 0; bnd < s->num_cpl_bands; bnd++) {
                        cpl_coord_exp = get_bits(gbc, 4);
                        cpl_coord_mant = get_bits(gbc, 4);
                        if (cpl_coord_exp == 15)
                            s->cpl_coords[ch][bnd] = cpl_coord_mant / 16.0f;
                        else
                            s->cpl_coords[ch][bnd] = (cpl_coord_mant + 16.0f) / 32.0f;
                        s->cpl_coords[ch][bnd] *= scale_factors[cpl_coord_exp + master_cpl_coord];
                    }
                }
            }
        }
        /* phase flags */
        if (channel_mode == AC3_CHMODE_STEREO && cpl_coords_exist) {
            for (bnd = 0; bnd < s->num_cpl_bands; bnd++) {
                s->phase_flags[bnd] = s->phase_flags_in_use? get_bits1(gbc) : 0;
            }
        }
    }

    /* stereo rematrixing strategy and band structure */
    if (channel_mode == AC3_CHMODE_STEREO) {
        if (get_bits1(gbc)) {
            s->num_rematrixing_bands = 4;
            if(s->cpl_in_use && s->start_freq[CPL_CH] <= 61)
                s->num_rematrixing_bands -= 1 + (s->start_freq[CPL_CH] == 37);
            for(bnd=0; bnd<s->num_rematrixing_bands; bnd++)
                s->rematrixing_flags[bnd] = get_bits1(gbc);
        }
    }

    /* exponent strategies for each channel */
    s->exp_strategy[CPL_CH] = EXP_REUSE;
    s->exp_strategy[s->lfe_ch] = EXP_REUSE;
    for (ch = !s->cpl_in_use; ch <= s->channels; ch++) {
        if(ch == s->lfe_ch)
            s->exp_strategy[ch] = get_bits(gbc, 1);
        else
            s->exp_strategy[ch] = get_bits(gbc, 2);
        if(s->exp_strategy[ch] != EXP_REUSE)
            bit_alloc_stages[ch] = 3;
    }

    /* channel bandwidth */
    for (ch = 1; ch <= fbw_channels; ch++) {
        s->start_freq[ch] = 0;
        if (s->exp_strategy[ch] != EXP_REUSE) {
            int prev = s->end_freq[ch];
            if (s->channel_in_cpl[ch])
                s->end_freq[ch] = s->start_freq[CPL_CH];
            else {
                int bandwidth_code = get_bits(gbc, 6);
                if (bandwidth_code > 60) {
                    av_log(s->avctx, AV_LOG_ERROR, "bandwidth code = %d > 60", bandwidth_code);
                    return -1;
                }
                s->end_freq[ch] = bandwidth_code * 3 + 73;
            }
            if(blk > 0 && s->end_freq[ch] != prev)
                memset(bit_alloc_stages, 3, AC3_MAX_CHANNELS);
        }
    }
    s->start_freq[s->lfe_ch] = 0;
    s->end_freq[s->lfe_ch] = 7;

    /* decode exponents for each channel */
    for (ch = !s->cpl_in_use; ch <= s->channels; ch++) {
        if (s->exp_strategy[ch] != EXP_REUSE) {
            int group_size, num_groups;
            group_size = 3 << (s->exp_strategy[ch] - 1);
            if(ch == CPL_CH)
                num_groups = (s->end_freq[ch] - s->start_freq[ch]) / group_size;
            else if(ch == s->lfe_ch)
                num_groups = 2;
            else
                num_groups = (s->end_freq[ch] + group_size - 4) / group_size;
            s->dexps[ch][0] = get_bits(gbc, 4) << !ch;
            decode_exponents(gbc, s->exp_strategy[ch], num_groups, s->dexps[ch][0],
                             &s->dexps[ch][s->start_freq[ch]+!!ch]);
            if(ch != CPL_CH && ch != s->lfe_ch)
                skip_bits(gbc, 2); /* skip gainrng */
        }
    }

    /* bit allocation information */
    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=!s->cpl_in_use; ch<=s->channels; ch++) {
            bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
        }
    }

    /* signal-to-noise ratio offsets and fast gains (signal-to-mask ratios) */
    if (get_bits1(gbc)) {
        int csnr;
        csnr = (get_bits(gbc, 6) - 15) << 4;
        for (ch = !s->cpl_in_use; ch <= s->channels; ch++) { /* snr offset and fast gain */
            s->snr_offset[ch] = (csnr + get_bits(gbc, 4)) << 2;
            s->fast_gain[ch] = ff_ac3_fast_gain_tab[get_bits(gbc, 3)];
        }
        memset(bit_alloc_stages, 3, AC3_MAX_CHANNELS);
    }

    /* coupling leak information */
    if (s->cpl_in_use && get_bits1(gbc)) {
        s->bit_alloc_params.cpl_fast_leak = get_bits(gbc, 3);
        s->bit_alloc_params.cpl_slow_leak = get_bits(gbc, 3);
        bit_alloc_stages[CPL_CH] = FFMAX(bit_alloc_stages[CPL_CH], 2);
    }

    /* delta bit allocation information */
    if (get_bits1(gbc)) {
        /* delta bit allocation exists (strategy) */
        for (ch = !s->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 = !s->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);
                }
            }
        }
    } else if(blk == 0) {
        for(ch=0; ch<=s->channels; ch++) {
            s->dba_mode[ch] = DBA_NONE;
        }
    }

    /* Bit allocation */
    for(ch=!s->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 */
            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]);
        }
        if(bit_alloc_stages[ch] > 0) {
            /* Compute bit allocation */
            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,
                                      s->bap[ch]);
        }
    }

    /* unused dummy data */
    if (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. */
    if (get_transform_coeffs(s)) {
        av_log(s->avctx, AV_LOG_ERROR, "Error in routine get_transform_coeffs\n");
        return -1;
    }

    /* 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 = 2.0f * s->mul_bias;
        if(s->channel_mode == AC3_CHMODE_DUALMONO) {
            gain *= s->dynamic_range[ch-1];
        } else {
            gain *= s->dynamic_range[0];
        }
        for(i=0; i<s->end_freq[ch]; i++) {
            s->transform_coeffs[ch][i] *= gain;
        }
    }

    do_imdct(s);

    /* downmix output if needed */
    if(s->channels != s->out_channels && !((s->output_mode & AC3_OUTPUT_LFEON) &&
            s->fbw_channels == s->out_channels)) {
        ac3_downmix(s);
    }

    /* convert float to 16-bit integer */
    for(ch=0; ch<s->out_channels; ch++) {
        for(i=0; i<256; i++) {
            s->output[ch][i] += s->add_bias;
        }
        s->dsp.float_to_int16(s->int_output[ch], s->output[ch], 256);
    }

    return 0;
}

/**
 * Decode a single AC-3 frame.
 */
static int ac3_decode_frame(AVCodecContext * avctx, void *data, int *data_size, uint8_t *buf, int buf_size)
{
    AC3DecodeContext *s = avctx->priv_data;
    int16_t *out_samples = (int16_t *)data;
    int i, blk, ch, err;

    /* initialize the GetBitContext with the start of valid AC-3 Frame */
    init_get_bits(&s->gbc, buf, buf_size * 8);

    /* parse the syncinfo */
    err = ac3_parse_header(s);
    if(err) {
        switch(err) {
            case AC3_PARSE_ERROR_SYNC:
                av_log(avctx, AV_LOG_ERROR, "frame sync error\n");
                break;
            case AC3_PARSE_ERROR_BSID:
                av_log(avctx, AV_LOG_ERROR, "invalid bitstream id\n");
                break;
            case AC3_PARSE_ERROR_SAMPLE_RATE:
                av_log(avctx, AV_LOG_ERROR, "invalid sample rate\n");
                break;
            case AC3_PARSE_ERROR_FRAME_SIZE:
                av_log(avctx, AV_LOG_ERROR, "invalid frame size\n");
                break;
            default:
                av_log(avctx, AV_LOG_ERROR, "invalid header\n");
                break;
        }
        return -1;
    }

    /* check that reported frame size fits in input buffer */
    if(s->frame_size > buf_size) {
        av_log(avctx, AV_LOG_ERROR, "incomplete frame\n");
        return -1;
    }

    /* check for crc mismatch */
    if(avctx->error_resilience >= 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");
            return -1;
        }
        /* TODO: error concealment */
    }

    avctx->sample_rate = s->sample_rate;
    avctx->bit_rate = s->bit_rate;

    /* channel config */
    s->out_channels = s->channels;
    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;
    }
    avctx->channels = s->out_channels;

    /* 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);
    }

    /* parse the audio blocks */
    for (blk = 0; blk < NB_BLOCKS; blk++) {
        if (ac3_parse_audio_block(s, blk)) {
            av_log(avctx, AV_LOG_ERROR, "error parsing the audio block\n");
            *data_size = 0;
            return s->frame_size;
        }
        for (i = 0; i < 256; i++)
            for (ch = 0; ch < s->out_channels; ch++)
                *(out_samples++) = s->int_output[ch][i];
    }
    *data_size = NB_BLOCKS * 256 * avctx->channels * sizeof (int16_t);
    return s->frame_size;
}

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

    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,
};