PeerStreamer

 * Downgrade exponent strategies to reduce the bits used by the exponents.

 * This is a fallback for when bit allocation fails with the normal exponent

 * strategies.  Each time this function is run it only downgrades the

 * strategy in 1 channel of 1 block.

 * @return non-zero if downgrade was unsuccessful

 */

static int downgrade_exponents(AC3EncodeContext *s)

{

    int ch, blk;

    for (ch = 0; ch < s->fbw_channels; ch++) {

        for (blk = AC3_MAX_BLOCKS-1; blk >= 0; blk--) {

            if (s->blocks[blk].exp_strategy[ch] == EXP_D15) {

                s->blocks[blk].exp_strategy[ch] = EXP_D25;

                return 0;

            }

        }

    }

    for (ch = 0; ch < s->fbw_channels; ch++) {

        for (blk = AC3_MAX_BLOCKS-1; blk >= 0; blk--) {

            if (s->blocks[blk].exp_strategy[ch] == EXP_D25) {

                s->blocks[blk].exp_strategy[ch] = EXP_D45;

                return 0;

            }

        }

    }

    for (ch = 0; ch < s->fbw_channels; ch++) {

        /* block 0 cannot reuse exponents, so only downgrade D45 to REUSE if

           the block number > 0 */

        for (blk = AC3_MAX_BLOCKS-1; blk > 0; blk--) {

            if (s->blocks[blk].exp_strategy[ch] > EXP_REUSE) {

                s->blocks[blk].exp_strategy[ch] = EXP_REUSE;

                return 0;

            }

        }

    }

    return -1;

}

/**

 * Reduce the bandwidth to reduce the number of bits used for a given SNR offset.

 * This is a second fallback for when bit allocation still fails after exponents

 * have been downgraded.

 * @return non-zero if bandwidth reduction was unsuccessful

 */

static int reduce_bandwidth(AC3EncodeContext *s, int min_bw_code)

{

    int ch;

    if (s->bandwidth_code[0] > min_bw_code) {

        for (ch = 0; ch < s->fbw_channels; ch++) {

            s->bandwidth_code[ch]--;

            s->nb_coefs[ch] = s->bandwidth_code[ch] * 3 + 73;

        }

        return 0;

    }

    return -1;

}

/**

    int ret;

    ret = cbr_bit_allocation(s);

    while (ret) {

        /* fallback 1: downgrade exponents */

        if (!downgrade_exponents(s)) {

            extract_exponents(s);

            encode_exponents(s);

            group_exponents(s);

            ret = compute_bit_allocation(s);

            continue;

        }

        /* fallback 2: reduce bandwidth */

        /* only do this if the user has not specified a specific cutoff

           frequency */

        if (!s->cutoff && !reduce_bandwidth(s, 0)) {

            process_exponents(s);

            ret = compute_bit_allocation(s);

            continue;

        }

        /* fallbacks were not enough... */

        break;

    }

    return ret;