PeerStreamer

/*

 * The simplest AC-3 encoder

 * Copyright (c) 2000 Fabrice Bellard

 * Copyright (c) 2006-2010 Justin Ruggles <justin.ruggles@gmail.com>

 * Copyright (c) 2006-2010 Prakash Punnoor <prakash@punnoor.de>

 *

 * This file is part of FFmpeg.

 *

 * FFmpeg is free software; you can redistribute it and/or

 * modify it under the terms of the GNU Lesser General Public

 * License as published by the Free Software Foundation; either

 * version 2.1 of the License, or (at your option) any later version.

 *

 * FFmpeg is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 * Lesser General Public License for more details.

 *

 * You should have received a copy of the GNU Lesser General Public

 * License along with FFmpeg; if not, write to the Free Software

 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

 */

/**

 * @file

 * The simplest AC-3 encoder.

 */

//#define DEBUG

#include "libavutil/audioconvert.h"

#include "libavutil/crc.h"

#include "avcodec.h"

#include "put_bits.h"

#include "dsputil.h"

#include "ac3dsp.h"

#include "ac3.h"

#include "audioconvert.h"

#ifndef CONFIG_AC3ENC_FLOAT

#define CONFIG_AC3ENC_FLOAT 0

#endif

/** Maximum number of exponent groups. +1 for separate DC exponent. */

#define AC3_MAX_EXP_GROUPS 85

/* stereo rematrixing algorithms */

#define AC3_REMATRIXING_IS_STATIC 0x1

#define AC3_REMATRIXING_SUMS    0

#define AC3_REMATRIXING_NONE    1

#define AC3_REMATRIXING_ALWAYS  3

/** Scale a float value by 2^bits and convert to an integer. */

#define SCALE_FLOAT(a, bits) lrintf((a) * (float)(1 << (bits)))

#if CONFIG_AC3ENC_FLOAT

#include "ac3enc_float.h"

#else

#include "ac3enc_fixed.h"

#endif

/**

 * Data for a single audio block.

 */

typedef struct AC3Block {

    uint8_t  **bap;                             ///< bit allocation pointers (bap)

    CoefType **mdct_coef;                       ///< MDCT coefficients

    int32_t  **fixed_coef;                      ///< fixed-point MDCT coefficients

    uint8_t  **exp;                             ///< original exponents

    uint8_t  **grouped_exp;                     ///< grouped exponents

    int16_t  **psd;                             ///< psd per frequency bin

    int16_t  **band_psd;                        ///< psd per critical band

    int16_t  **mask;                            ///< masking curve

    uint16_t **qmant;                           ///< quantized mantissas

    int8_t   exp_shift[AC3_MAX_CHANNELS];       ///< exponent shift values

    uint8_t  new_rematrixing_strategy;          ///< send new rematrixing flags in this block

    uint8_t  rematrixing_flags[4];              ///< rematrixing flags

} AC3Block;

/**

 * AC-3 encoder private context.

 */

typedef struct AC3EncodeContext {

    PutBitContext pb;                       ///< bitstream writer context

    DSPContext dsp;

    AC3DSPContext ac3dsp;                   ///< AC-3 optimized functions

    AC3MDCTContext mdct;                    ///< MDCT context

    AC3Block blocks[AC3_MAX_BLOCKS];        ///< per-block info

    int bitstream_id;                       ///< bitstream id                           (bsid)

    int bitstream_mode;                     ///< bitstream mode                         (bsmod)

    int bit_rate;                           ///< target bit rate, in bits-per-second

    int sample_rate;                        ///< sampling frequency, in Hz

    int frame_size_min;                     ///< minimum frame size in case rounding is necessary

    int frame_size;                         ///< current frame size in bytes

    int frame_size_code;                    ///< frame size code                        (frmsizecod)

    uint16_t crc_inv[2];

    int bits_written;                       ///< bit count    (used to avg. bitrate)

    int samples_written;                    ///< sample count (used to avg. bitrate)

    int fbw_channels;                       ///< number of full-bandwidth channels      (nfchans)

    int channels;                           ///< total number of channels               (nchans)

    int lfe_on;                             ///< indicates if there is an LFE channel   (lfeon)

    int lfe_channel;                        ///< channel index of the LFE channel

    int channel_mode;                       ///< channel mode                           (acmod)

    const uint8_t *channel_map;             ///< channel map used to reorder channels

    int cutoff;                             ///< user-specified cutoff frequency, in Hz

    int bandwidth_code[AC3_MAX_CHANNELS];   ///< bandwidth code (0 to 60)               (chbwcod)

    int nb_coefs[AC3_MAX_CHANNELS];

    int rematrixing;                        ///< determines how rematrixing strategy is calculated

    /* bitrate allocation control */

    int slow_gain_code;                     ///< slow gain code                         (sgaincod)

    int slow_decay_code;                    ///< slow decay code                        (sdcycod)

    int fast_decay_code;                    ///< fast decay code                        (fdcycod)

    int db_per_bit_code;                    ///< dB/bit code                            (dbpbcod)

    int floor_code;                         ///< floor code                             (floorcod)

    AC3BitAllocParameters bit_alloc;        ///< bit allocation parameters

    int coarse_snr_offset;                  ///< coarse SNR offsets                     (csnroffst)

    int fast_gain_code[AC3_MAX_CHANNELS];   ///< fast gain codes (signal-to-mask ratio) (fgaincod)

    int fine_snr_offset[AC3_MAX_CHANNELS];  ///< fine SNR offsets                       (fsnroffst)

    int frame_bits_fixed;                   ///< number of non-coefficient bits for fixed parameters

    int frame_bits;                         ///< all frame bits except exponents and mantissas

    int exponent_bits;                      ///< number of bits used for exponents

    /* mantissa encoding */

    int mant1_cnt, mant2_cnt, mant4_cnt;    ///< mantissa counts for bap=1,2,4

    uint16_t *qmant1_ptr, *qmant2_ptr, *qmant4_ptr; ///< mantissa pointers for bap=1,2,4

    SampleType **planar_samples;

    uint8_t *bap_buffer;

    uint8_t *bap1_buffer;

    CoefType *mdct_coef_buffer;

    int32_t *fixed_coef_buffer;

    uint8_t *exp_buffer;

    uint8_t *grouped_exp_buffer;

    int16_t *psd_buffer;

    int16_t *band_psd_buffer;

    int16_t *mask_buffer;

    uint16_t *qmant_buffer;

    uint8_t exp_strategy[AC3_MAX_CHANNELS][AC3_MAX_BLOCKS]; ///< exponent strategies

    DECLARE_ALIGNED(16, SampleType, windowed_samples)[AC3_WINDOW_SIZE];

} AC3EncodeContext;

/* prototypes for functions in ac3enc_fixed.c and ac3enc_float.c */

static av_cold void mdct_end(AC3MDCTContext *mdct);

static av_cold int mdct_init(AVCodecContext *avctx, AC3MDCTContext *mdct,

                             int nbits);

static void mdct512(AC3MDCTContext *mdct, CoefType *out, SampleType *in);

static void apply_window(DSPContext *dsp, SampleType *output, const SampleType *input,

                         const SampleType *window, int n);

static int normalize_samples(AC3EncodeContext *s);

static void scale_coefficients(AC3EncodeContext *s);

/**

 * LUT for number of exponent groups.

 * exponent_group_tab[exponent strategy-1][number of coefficients]

 */

static uint8_t exponent_group_tab[3][256];

/**

 * List of supported channel layouts.

 */

static const int64_t ac3_channel_layouts[] = {

     AV_CH_LAYOUT_MONO,

     AV_CH_LAYOUT_STEREO,

     AV_CH_LAYOUT_2_1,

     AV_CH_LAYOUT_SURROUND,

     AV_CH_LAYOUT_2_2,

     AV_CH_LAYOUT_QUAD,

     AV_CH_LAYOUT_4POINT0,

     AV_CH_LAYOUT_5POINT0,

     AV_CH_LAYOUT_5POINT0_BACK,

    (AV_CH_LAYOUT_MONO     | AV_CH_LOW_FREQUENCY),

    (AV_CH_LAYOUT_STEREO   | AV_CH_LOW_FREQUENCY),

    (AV_CH_LAYOUT_2_1      | AV_CH_LOW_FREQUENCY),

    (AV_CH_LAYOUT_SURROUND | AV_CH_LOW_FREQUENCY),

    (AV_CH_LAYOUT_2_2      | AV_CH_LOW_FREQUENCY),

    (AV_CH_LAYOUT_QUAD     | AV_CH_LOW_FREQUENCY),

    (AV_CH_LAYOUT_4POINT0  | AV_CH_LOW_FREQUENCY),

     AV_CH_LAYOUT_5POINT1,

     AV_CH_LAYOUT_5POINT1_BACK,

     0

};

/**

 * Adjust the frame size to make the average bit rate match the target bit rate.

 * This is only needed for 11025, 22050, and 44100 sample rates.

 */

static void adjust_frame_size(AC3EncodeContext *s)

{

    while (s->bits_written >= s->bit_rate && s->samples_written >= s->sample_rate) {

        s->bits_written    -= s->bit_rate;

        s->samples_written -= s->sample_rate;

    }

    s->frame_size = s->frame_size_min +

                    2 * (s->bits_written * s->sample_rate < s->samples_written * s->bit_rate);

    s->bits_written    += s->frame_size * 8;

    s->samples_written += AC3_FRAME_SIZE;

}

/**

 * Deinterleave input samples.

 * Channels are reordered from FFmpeg's default order to AC-3 order.

 */

static void deinterleave_input_samples(AC3EncodeContext *s,

                                       const SampleType *samples)

{

    int ch, i;

    /* deinterleave and remap input samples */

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

        const SampleType *sptr;

        int sinc;

        /* copy last 256 samples of previous frame to the start of the current frame */

        memcpy(&s->planar_samples[ch][0], &s->planar_samples[ch][AC3_FRAME_SIZE],

               AC3_BLOCK_SIZE * sizeof(s->planar_samples[0][0]));

        /* deinterleave */

        sinc = s->channels;

        sptr = samples + s->channel_map[ch];

        for (i = AC3_BLOCK_SIZE; i < AC3_FRAME_SIZE+AC3_BLOCK_SIZE; i++) {

            s->planar_samples[ch][i] = *sptr;

            sptr += sinc;

        }

    }

}

/**

 * Apply the MDCT to input samples to generate frequency coefficients.

 * This applies the KBD window and normalizes the input to reduce precision

 * loss due to fixed-point calculations.

 */

static void apply_mdct(AC3EncodeContext *s)

{

    int blk, ch;

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

        for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

            AC3Block *block = &s->blocks[blk];

            const SampleType *input_samples = &s->planar_samples[ch][blk * AC3_BLOCK_SIZE];

            apply_window(&s->dsp, s->windowed_samples, input_samples, s->mdct.window, AC3_WINDOW_SIZE);

            block->exp_shift[ch] = normalize_samples(s);

            mdct512(&s->mdct, block->mdct_coef[ch], s->windowed_samples);

        }

    }

}

/**

 * Initialize stereo rematrixing.

 * If the strategy does not change for each frame, set the rematrixing flags.

 */

static void rematrixing_init(AC3EncodeContext *s)

{

    if (s->channel_mode == AC3_CHMODE_STEREO)

        s->rematrixing = AC3_REMATRIXING_SUMS;

    else

        s->rematrixing = AC3_REMATRIXING_NONE;

    /* NOTE: AC3_REMATRIXING_ALWAYS might be used in

             the future in conjunction with channel coupling. */

    if (s->rematrixing & AC3_REMATRIXING_IS_STATIC) {

        int flag = (s->rematrixing == AC3_REMATRIXING_ALWAYS);

        s->blocks[0].new_rematrixing_strategy = 1;

        memset(s->blocks[0].rematrixing_flags, flag,

               sizeof(s->blocks[0].rematrixing_flags));

    }

}

/**

 * Determine rematrixing flags for each block and band.

 */

static void compute_rematrixing_strategy(AC3EncodeContext *s)

{

    int nb_coefs;

    int blk, bnd, i;

    AC3Block *block, *block0;

    if (s->rematrixing & AC3_REMATRIXING_IS_STATIC)

        return;

    nb_coefs = FFMIN(s->nb_coefs[0], s->nb_coefs[1]);

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        block = &s->blocks[blk];

        block->new_rematrixing_strategy = !blk;

        for (bnd = 0; bnd < 4; bnd++) {

            /* calculate calculate sum of squared coeffs for one band in one block */

            int start = ff_ac3_rematrix_band_tab[bnd];

            int end   = FFMIN(nb_coefs, ff_ac3_rematrix_band_tab[bnd+1]);

            CoefSumType sum[4] = {0,};

            for (i = start; i < end; i++) {

                CoefType lt = block->mdct_coef[0][i];

                CoefType rt = block->mdct_coef[1][i];

                CoefType md = lt + rt;

                CoefType sd = lt - rt;

                sum[0] += lt * lt;

                sum[1] += rt * rt;

                sum[2] += md * md;

                sum[3] += sd * sd;

            }

            /* compare sums to determine if rematrixing will be used for this band */

            if (FFMIN(sum[2], sum[3]) < FFMIN(sum[0], sum[1]))

                block->rematrixing_flags[bnd] = 1;

            else

                block->rematrixing_flags[bnd] = 0;

            /* determine if new rematrixing flags will be sent */

            if (blk &&

                block->rematrixing_flags[bnd] != block0->rematrixing_flags[bnd]) {

                block->new_rematrixing_strategy = 1;

            }

        }

        block0 = block;

    }

}

/**

 * Apply stereo rematrixing to coefficients based on rematrixing flags.

 */

static void apply_rematrixing(AC3EncodeContext *s)

{

    int nb_coefs;

    int blk, bnd, i;

    int start, end;

    uint8_t *flags;

    if (s->rematrixing == AC3_REMATRIXING_NONE)

        return;

    nb_coefs = FFMIN(s->nb_coefs[0], s->nb_coefs[1]);

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        AC3Block *block = &s->blocks[blk];

        if (block->new_rematrixing_strategy)

            flags = block->rematrixing_flags;

        for (bnd = 0; bnd < 4; bnd++) {

            if (flags[bnd]) {

                start = ff_ac3_rematrix_band_tab[bnd];

                end   = FFMIN(nb_coefs, ff_ac3_rematrix_band_tab[bnd+1]);

                for (i = start; i < end; i++) {

                    int32_t lt = block->fixed_coef[0][i];

                    int32_t rt = block->fixed_coef[1][i];

                    block->fixed_coef[0][i] = (lt + rt) >> 1;

                    block->fixed_coef[1][i] = (lt - rt) >> 1;

                }

            }

        }

    }

}

/**

 * Initialize exponent tables.

 */

static av_cold void exponent_init(AC3EncodeContext *s)

{

    int i;

    for (i = 73; i < 256; i++) {

        exponent_group_tab[0][i] = (i - 1) /  3;

        exponent_group_tab[1][i] = (i + 2) /  6;

        exponent_group_tab[2][i] = (i + 8) / 12;

    }

    /* LFE */

    exponent_group_tab[0][7] = 2;

}

/**

 * Extract exponents from the MDCT coefficients.

 * This takes into account the normalization that was done to the input samples

 * by adjusting the exponents by the exponent shift values.

 */

static void extract_exponents(AC3EncodeContext *s)

{

    int blk, ch, i;

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

        for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

            AC3Block *block = &s->blocks[blk];

            uint8_t *exp   = block->exp[ch];

            int32_t *coef = block->fixed_coef[ch];

            int exp_shift  = block->exp_shift[ch];

            for (i = 0; i < AC3_MAX_COEFS; i++) {

                int e;

                int v = abs(coef[i]);

                if (v == 0)

                    e = 24;

                else {

                    e = 23 - av_log2(v) + exp_shift;

                    if (e >= 24) {

                        e = 24;

                        coef[i] = 0;

                    }

                }

                exp[i] = e;

            }

        }

    }

}

/**

 * Exponent Difference Threshold.

 * New exponents are sent if their SAD exceed this number.

 */

#define EXP_DIFF_THRESHOLD 500

/**

 * Calculate exponent strategies for all blocks in a single channel.

 */

static void compute_exp_strategy_ch(AC3EncodeContext *s, uint8_t *exp_strategy,

                                    uint8_t *exp)

{

    int blk, blk1;

    int exp_diff;

    /* estimate if the exponent variation & decide if they should be

       reused in the next frame */

    exp_strategy[0] = EXP_NEW;

    exp += AC3_MAX_COEFS;

    for (blk = 1; blk < AC3_MAX_BLOCKS; blk++) {

        exp_diff = s->dsp.sad[0](NULL, exp, exp - AC3_MAX_COEFS, 16, 16);

        if (exp_diff > EXP_DIFF_THRESHOLD)

            exp_strategy[blk] = EXP_NEW;

        else

            exp_strategy[blk] = EXP_REUSE;

        exp += AC3_MAX_COEFS;

    }

    /* now select the encoding strategy type : if exponents are often

       recoded, we use a coarse encoding */

    blk = 0;

    while (blk < AC3_MAX_BLOCKS) {

        blk1 = blk + 1;

        while (blk1 < AC3_MAX_BLOCKS && exp_strategy[blk1] == EXP_REUSE)

            blk1++;

        switch (blk1 - blk) {

        case 1:  exp_strategy[blk] = EXP_D45; break;

        case 2:

        case 3:  exp_strategy[blk] = EXP_D25; break;

        default: exp_strategy[blk] = EXP_D15; break;

        }

        blk = blk1;

    }

}

/**

 * Calculate exponent strategies for all channels.

 * Array arrangement is reversed to simplify the per-channel calculation.

 */

static void compute_exp_strategy(AC3EncodeContext *s)

{

    int ch, blk;

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

        compute_exp_strategy_ch(s, s->exp_strategy[ch], s->blocks[0].exp[ch]);

    }

    if (s->lfe_on) {

        ch = s->lfe_channel;

        s->exp_strategy[ch][0] = EXP_D15;

        for (blk = 1; blk < AC3_MAX_BLOCKS; blk++)

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

    }

}

/**

 * Update the exponents so that they are the ones the decoder will decode.

 */

static void encode_exponents_blk_ch(uint8_t *exp, int nb_exps, int exp_strategy)

{

    int nb_groups, i, k;

    nb_groups = exponent_group_tab[exp_strategy-1][nb_exps] * 3;

    /* for each group, compute the minimum exponent */

    switch(exp_strategy) {

    case EXP_D25:

        for (i = 1, k = 1; i <= nb_groups; i++) {

            uint8_t exp_min = exp[k];

            if (exp[k+1] < exp_min)

                exp_min = exp[k+1];

            exp[i] = exp_min;

            k += 2;

        }

        break;

    case EXP_D45:

        for (i = 1, k = 1; i <= nb_groups; i++) {

            uint8_t exp_min = exp[k];

            if (exp[k+1] < exp_min)

                exp_min = exp[k+1];

            if (exp[k+2] < exp_min)

                exp_min = exp[k+2];

            if (exp[k+3] < exp_min)

                exp_min = exp[k+3];

            exp[i] = exp_min;

            k += 4;

        }

        break;

    }

    /* constraint for DC exponent */

    if (exp[0] > 15)

        exp[0] = 15;

    /* decrease the delta between each groups to within 2 so that they can be

       differentially encoded */

    for (i = 1; i <= nb_groups; i++)

        exp[i] = FFMIN(exp[i], exp[i-1] + 2);

    i--;

    while (--i >= 0)

        exp[i] = FFMIN(exp[i], exp[i+1] + 2);

    /* now we have the exponent values the decoder will see */

    switch (exp_strategy) {

    case EXP_D25:

        for (i = nb_groups, k = nb_groups * 2; i > 0; i--) {

            uint8_t exp1 = exp[i];

            exp[k--] = exp1;

            exp[k--] = exp1;

        }

        break;

    case EXP_D45:

        for (i = nb_groups, k = nb_groups * 4; i > 0; i--) {

            exp[k] = exp[k-1] = exp[k-2] = exp[k-3] = exp[i];

            k -= 4;

        }

        break;

    }

}

/**

 * Encode exponents from original extracted form to what the decoder will see.

 * This copies and groups exponents based on exponent strategy and reduces

 * deltas between adjacent exponent groups so that they can be differentially

 * encoded.

 */

static void encode_exponents(AC3EncodeContext *s)

{

    int blk, blk1, ch;

    uint8_t *exp, *exp1, *exp_strategy;

    int nb_coefs, num_reuse_blocks;

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

        exp          = s->blocks[0].exp[ch];

        exp_strategy = s->exp_strategy[ch];

        nb_coefs     = s->nb_coefs[ch];

        blk = 0;

        while (blk < AC3_MAX_BLOCKS) {

            blk1 = blk + 1;

            /* count the number of EXP_REUSE blocks after the current block */

            while (blk1 < AC3_MAX_BLOCKS && exp_strategy[blk1] == EXP_REUSE)

                blk1++;

            num_reuse_blocks = blk1 - blk - 1;

            /* for the EXP_REUSE case we select the min of the exponents */

            s->ac3dsp.ac3_exponent_min(exp, num_reuse_blocks, nb_coefs);

            encode_exponents_blk_ch(exp, nb_coefs, exp_strategy[blk]);

            /* copy encoded exponents for reuse case */

            exp1 = exp + AC3_MAX_COEFS;

            while (blk < blk1-1) {

                memcpy(exp1, exp, nb_coefs * sizeof(*exp));

                exp1 += AC3_MAX_COEFS;

                blk++;

            }

            blk = blk1;

            exp = exp1;

        }

    }

}

/**

 * Group exponents.

 * 3 delta-encoded exponents are in each 7-bit group. The number of groups

 * varies depending on exponent strategy and bandwidth.

 */

static void group_exponents(AC3EncodeContext *s)

{

    int blk, ch, i;

    int group_size, nb_groups, bit_count;

    uint8_t *p;

    int delta0, delta1, delta2;

    int exp0, exp1;

    bit_count = 0;

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        AC3Block *block = &s->blocks[blk];

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

            int exp_strategy = s->exp_strategy[ch][blk];

            if (exp_strategy == EXP_REUSE)

                continue;

            group_size = exp_strategy + (exp_strategy == EXP_D45);

            nb_groups = exponent_group_tab[exp_strategy-1][s->nb_coefs[ch]];

            bit_count += 4 + (nb_groups * 7);

            p = block->exp[ch];

            /* DC exponent */

            exp1 = *p++;

            block->grouped_exp[ch][0] = exp1;

            /* remaining exponents are delta encoded */

            for (i = 1; i <= nb_groups; i++) {

                /* merge three delta in one code */

                exp0   = exp1;

                exp1   = p[0];

                p     += group_size;

                delta0 = exp1 - exp0 + 2;

                exp0   = exp1;

                exp1   = p[0];

                p     += group_size;

                delta1 = exp1 - exp0 + 2;

                exp0   = exp1;

                exp1   = p[0];

                p     += group_size;

                delta2 = exp1 - exp0 + 2;

                block->grouped_exp[ch][i] = ((delta0 * 5 + delta1) * 5) + delta2;

            }

        }

    }

    s->exponent_bits = bit_count;

}

/**

 * Calculate final exponents from the supplied MDCT coefficients and exponent shift.

 * Extract exponents from MDCT coefficients, calculate exponent strategies,

 * and encode final exponents.

 */

static void process_exponents(AC3EncodeContext *s)

{

    extract_exponents(s);

    compute_exp_strategy(s);

    encode_exponents(s);

    group_exponents(s);

    emms_c();

}

/**

 * Count frame bits that are based solely on fixed parameters.

 * This only has to be run once when the encoder is initialized.

 */

static void count_frame_bits_fixed(AC3EncodeContext *s)

{

    static const int frame_bits_inc[8] = { 0, 0, 2, 2, 2, 4, 2, 4 };

    int blk;

    int frame_bits;

    /* assumptions:

     *   no dynamic range codes

     *   no channel coupling

     *   bit allocation parameters do not change between blocks

     *   SNR offsets do not change between blocks

     *   no delta bit allocation

     *   no skipped data

     *   no auxilliary data

     */

    /* header size */

    frame_bits = 65;

    frame_bits += frame_bits_inc[s->channel_mode];

    /* audio blocks */

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        frame_bits += s->fbw_channels * 2 + 2; /* blksw * c, dithflag * c, dynrnge, cplstre */

        if (s->channel_mode == AC3_CHMODE_STEREO) {

            frame_bits++; /* rematstr */

        }

        frame_bits += 2 * s->fbw_channels; /* chexpstr[2] * c */

        if (s->lfe_on)

            frame_bits++; /* lfeexpstr */

        frame_bits++; /* baie */

        frame_bits++; /* snr */

        frame_bits += 2; /* delta / skip */

    }

    frame_bits++; /* cplinu for block 0 */

    /* bit alloc info */

    /* sdcycod[2], fdcycod[2], sgaincod[2], dbpbcod[2], floorcod[3] */

    /* csnroffset[6] */

    /* (fsnoffset[4] + fgaincod[4]) * c */

    frame_bits += 2*4 + 3 + 6 + s->channels * (4 + 3);

    /* auxdatae, crcrsv */

    frame_bits += 2;

    /* CRC */

    frame_bits += 16;

    s->frame_bits_fixed = frame_bits;

}

/**

 * Initialize bit allocation.

 * Set default parameter codes and calculate parameter values.

 */

static void bit_alloc_init(AC3EncodeContext *s)

{

    int ch;

    /* init default parameters */

    s->slow_decay_code = 2;

    s->fast_decay_code = 1;

    s->slow_gain_code  = 1;

    s->db_per_bit_code = 3;

    s->floor_code      = 7;

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

        s->fast_gain_code[ch] = 4;

    /* initial snr offset */

    s->coarse_snr_offset = 40;

    /* compute real values */

    /* currently none of these values change during encoding, so we can just

       set them once at initialization */

    s->bit_alloc.slow_decay = ff_ac3_slow_decay_tab[s->slow_decay_code] >> s->bit_alloc.sr_shift;

    s->bit_alloc.fast_decay = ff_ac3_fast_decay_tab[s->fast_decay_code] >> s->bit_alloc.sr_shift;

    s->bit_alloc.slow_gain  = ff_ac3_slow_gain_tab[s->slow_gain_code];

    s->bit_alloc.db_per_bit = ff_ac3_db_per_bit_tab[s->db_per_bit_code];

    s->bit_alloc.floor      = ff_ac3_floor_tab[s->floor_code];

    count_frame_bits_fixed(s);

}

/**

 * Count the bits used to encode the frame, minus exponents and mantissas.

 * Bits based on fixed parameters have already been counted, so now we just

 * have to add the bits based on parameters that change during encoding.

 */

static void count_frame_bits(AC3EncodeContext *s)

{

    int blk, ch;

    int frame_bits = 0;

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        /* stereo rematrixing */

        if (s->channel_mode == AC3_CHMODE_STEREO &&

            s->blocks[blk].new_rematrixing_strategy) {

            frame_bits += 4;

        }

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

            if (s->exp_strategy[ch][blk] != EXP_REUSE)

                frame_bits += 6 + 2; /* chbwcod[6], gainrng[2] */

        }

    }

    s->frame_bits = s->frame_bits_fixed + frame_bits;

}

/**

 * Calculate the number of bits needed to encode a set of mantissas.

 */

static int compute_mantissa_size(int mant_cnt[5], uint8_t *bap, int nb_coefs)

{

    int bits, b, i;

    bits = 0;

    for (i = 0; i < nb_coefs; i++) {

        b = bap[i];

        if (b <= 4) {

            // bap=1 to bap=4 will be counted in compute_mantissa_size_final

            mant_cnt[b]++;

        } else if (b <= 13) {

            // bap=5 to bap=13 use (bap-1) bits

            bits += b - 1;

        } else {

            // bap=14 uses 14 bits and bap=15 uses 16 bits

            bits += (b == 14) ? 14 : 16;

        }

    }

    return bits;

}

/**

 * Finalize the mantissa bit count by adding in the grouped mantissas.

 */

static int compute_mantissa_size_final(int mant_cnt[5])

{

    // bap=1 : 3 mantissas in 5 bits

    int bits = (mant_cnt[1] / 3) * 5;

    // bap=2 : 3 mantissas in 7 bits

    // bap=4 : 2 mantissas in 7 bits

    bits += ((mant_cnt[2] / 3) + (mant_cnt[4] >> 1)) * 7;

    // bap=3 : each mantissa is 3 bits

    bits += mant_cnt[3] * 3;

    return bits;

}

/**

 * Calculate masking curve based on the final exponents.

 * Also calculate the power spectral densities to use in future calculations.

 */

static void bit_alloc_masking(AC3EncodeContext *s)

{

    int blk, ch;

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        AC3Block *block = &s->blocks[blk];

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

            /* We only need psd and mask for calculating bap.

               Since we currently do not calculate bap when exponent

               strategy is EXP_REUSE we do not need to calculate psd or mask. */

            if (s->exp_strategy[ch][blk] != EXP_REUSE) {

                ff_ac3_bit_alloc_calc_psd(block->exp[ch], 0,

                                          s->nb_coefs[ch],

                                          block->psd[ch], block->band_psd[ch]);

                ff_ac3_bit_alloc_calc_mask(&s->bit_alloc, block->band_psd[ch],

                                           0, s->nb_coefs[ch],

                                           ff_ac3_fast_gain_tab[s->fast_gain_code[ch]],

                                           ch == s->lfe_channel,

                                           DBA_NONE, 0, NULL, NULL, NULL,

                                           block->mask[ch]);

            }

        }

    }

}

/**

 * Ensure that bap for each block and channel point to the current bap_buffer.

 * They may have been switched during the bit allocation search.

 */

static void reset_block_bap(AC3EncodeContext *s)

{

    int blk, ch;

    if (s->blocks[0].bap[0] == s->bap_buffer)

        return;

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

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

            s->blocks[blk].bap[ch] = &s->bap_buffer[AC3_MAX_COEFS * (blk * s->channels + ch)];

        }

    }

}

/**

 * Run the bit allocation with a given SNR offset.

 * This calculates the bit allocation pointers that will be used to determine

 * the quantization of each mantissa.

 * @return the number of bits needed for mantissas if the given SNR offset is

 *         is used.

 */

static int bit_alloc(AC3EncodeContext *s, int snr_offset)

{

    int blk, ch;

    int mantissa_bits;

    int mant_cnt[5];

    snr_offset = (snr_offset - 240) << 2;

    reset_block_bap(s);

    mantissa_bits = 0;

    for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {

        AC3Block *block = &s->blocks[blk];

        // initialize grouped mantissa counts. these are set so that they are

        // padded to the next whole group size when bits are counted in

        // compute_mantissa_size_final

        mant_cnt[0] = mant_cnt[3] = 0;

        mant_cnt[1] = mant_cnt[2] = 2;

        mant_cnt[4] = 1;

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

            /* Currently the only bit allocation parameters which vary across

               blocks within a frame are the exponent values.  We can take

               advantage of that by reusing the bit allocation pointers

               whenever we reuse exponents. */

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

                memcpy(block->bap[ch], s->blocks[blk-1].bap[ch], AC3_MAX_COEFS);

            } else {

                ff_ac3_bit_alloc_calc_bap(block->mask[ch], block->psd[ch], 0,

                                          s->nb_coefs[ch], snr_offset,

                                          s->bit_alloc.floor, ff_ac3_bap_tab,

                                          block->bap[ch]);

            }

            mantissa_bits += compute_mantissa_size(mant_cnt, block->bap[ch], s->nb_coefs[ch]);

        }

        mantissa_bits += compute_mantissa_size_final(mant_cnt);

    }

    return mantissa_bits;

}

/**

 * Constant bitrate bit allocation search.

 * Find the largest SNR offset that will allow data to fit in the frame.

 */

static int cbr_bit_allocation(AC3EncodeContext *s)

{

    int ch;

    int bits_left;

    int snr_offset, snr_incr;

    bits_left = 8 * s->frame_size - (s->frame_bits + s->exponent_bits);

    snr_offset = s->coarse_snr_offset << 4;

    /* if previous frame SNR offset was 1023, check if current frame can also

       use SNR offset of 1023. if so, skip the search. */

    if ((snr_offset | s->fine_snr_offset[0]) == 1023) {

        if (bit_alloc(s, 1023) <= bits_left)

            return 0;

    }

    while (snr_offset >= 0 &&

           bit_alloc(s, snr_offset) > bits_left) {

        snr_offset -= 64;

    }

    if (snr_offset < 0)

        return AVERROR(EINVAL);