static int vorbis_parse_audio_packet(vorbis_context *vc)
{
    GetBitContext *gb = &vc->gb;
    FFTContext *mdct;
    unsigned previous_window = vc->previous_window;
    unsigned mode_number, blockflag, blocksize;
    int i, j;
    uint8_t no_residue[255];
    uint8_t do_not_decode[255];
    vorbis_mapping *mapping;
    float *ch_res_ptr   = vc->channel_residues;
    float *ch_floor_ptr = vc->channel_floors;
    uint8_t res_chan[255];
    unsigned res_num = 0;
    int retlen  = 0;
    unsigned ch_left = vc->audio_channels;
    unsigned vlen;

    if (get_bits1(gb)) {
        av_log(vc->avccontext, AV_LOG_ERROR, "Not a Vorbis I audio packet.\n");
        return AVERROR_INVALIDDATA; // packet type not audio
    }

    if (vc->mode_count == 1) {
        mode_number = 0;
    } else {
        GET_VALIDATED_INDEX(mode_number, ilog(vc->mode_count-1), vc->mode_count)
    }
    vc->mode_number = mode_number;
    mapping = &vc->mappings[vc->modes[mode_number].mapping];

    av_dlog(NULL, " Mode number: %u , mapping: %d , blocktype %d\n", mode_number,
            vc->modes[mode_number].mapping, vc->modes[mode_number].blockflag);

    blockflag = vc->modes[mode_number].blockflag;
    blocksize = vc->blocksize[blockflag];
    vlen = blocksize / 2;
    if (blockflag)
        skip_bits(gb, 2); // previous_window, next_window

    memset(ch_res_ptr,   0, sizeof(float) * vc->audio_channels * vlen); //FIXME can this be removed ?
    memset(ch_floor_ptr, 0, sizeof(float) * vc->audio_channels * vlen); //FIXME can this be removed ?

// Decode floor

    for (i = 0; i < vc->audio_channels; ++i) {
        vorbis_floor *floor;
        int ret;
        if (mapping->submaps > 1) {
            floor = &vc->floors[mapping->submap_floor[mapping->mux[i]]];
        } else {
            floor = &vc->floors[mapping->submap_floor[0]];
        }

        ret = floor->decode(vc, &floor->data, ch_floor_ptr);

        if (ret < 0) {
            av_log(vc->avccontext, AV_LOG_ERROR, "Invalid codebook in vorbis_floor_decode.\n");
            return AVERROR_INVALIDDATA;
        }
        no_residue[i] = ret;
        ch_floor_ptr += vlen;
    }

// Nonzero vector propagate

    for (i = mapping->coupling_steps - 1; i >= 0; --i) {
        if (!(no_residue[mapping->magnitude[i]] & no_residue[mapping->angle[i]])) {
            no_residue[mapping->magnitude[i]] = 0;
            no_residue[mapping->angle[i]]     = 0;
        }
    }

// Decode residue

    for (i = 0; i < mapping->submaps; ++i) {
        vorbis_residue *residue;
        unsigned ch = 0;
        int ret;

        for (j = 0; j < vc->audio_channels; ++j) {
            if ((mapping->submaps == 1) || (i == mapping->mux[j])) {
                res_chan[j] = res_num;
                if (no_residue[j]) {
                    do_not_decode[ch] = 1;
                } else {
                    do_not_decode[ch] = 0;
                }
                ++ch;
                ++res_num;
            }
        }
        residue = &vc->residues[mapping->submap_residue[i]];
        if (ch_left < ch) {
            av_log(vc->avccontext, AV_LOG_ERROR, "Too many channels in vorbis_floor_decode.\n");
            return -1;
        }
        if (ch) {
            ret = vorbis_residue_decode(vc, residue, ch, do_not_decode, ch_res_ptr, vlen, ch_left);
            if (ret < 0)
                return ret;
        }

        ch_res_ptr += ch * vlen;
        ch_left -= ch;
    }

// Inverse coupling

    for (i = mapping->coupling_steps - 1; i >= 0; --i) { //warning: i has to be signed
        float *mag, *ang;

        mag = vc->channel_residues+res_chan[mapping->magnitude[i]] * blocksize / 2;
        ang = vc->channel_residues+res_chan[mapping->angle[i]]     * blocksize / 2;
        vc->dsp.vorbis_inverse_coupling(mag, ang, blocksize / 2);
    }

// Dotproduct, MDCT

    mdct = &vc->mdct[blockflag];

    for (j = vc->audio_channels-1;j >= 0; j--) {
        ch_floor_ptr = vc->channel_floors   + j           * blocksize / 2;
        ch_res_ptr   = vc->channel_residues + res_chan[j] * blocksize / 2;
        vc->dsp.vector_fmul(ch_floor_ptr, ch_floor_ptr, ch_res_ptr, blocksize / 2);
        mdct->imdct_half(mdct, ch_res_ptr, ch_floor_ptr);
    }

// Overlap/add, save data for next overlapping  FPMATH

    retlen = (blocksize + vc->blocksize[previous_window]) / 4;
    for (j = 0; j < vc->audio_channels; j++) {
        unsigned bs0 = vc->blocksize[0];
        unsigned bs1 = vc->blocksize[1];
        float *residue    = vc->channel_residues + res_chan[j] * blocksize / 2;
        float *saved      = vc->saved + j * bs1 / 4;
        float *ret        = vc->channel_floors + j * retlen;
        float *buf        = residue;
        const float *win  = vc->win[blockflag & previous_window];

        if (blockflag == previous_window) {
            vc->dsp.vector_fmul_window(ret, saved, buf, win, blocksize / 4);
        } else if (blockflag > previous_window) {
            vc->dsp.vector_fmul_window(ret, saved, buf, win, bs0 / 4);
            memcpy(ret+bs0/2, buf+bs0/4, ((bs1-bs0)/4) * sizeof(float));
        } else {
            memcpy(ret, saved, ((bs1 - bs0) / 4) * sizeof(float));
            vc->dsp.vector_fmul_window(ret + (bs1 - bs0) / 4, saved + (bs1 - bs0) / 4, buf, win, bs0 / 4);
        }
        memcpy(saved, buf + blocksize / 4, blocksize / 4 * sizeof(float));
    }

    vc->previous_window = blockflag;
    return retlen;
}