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 "libavcore/audioconvert.h"

#include "libavutil/crc.h"

#include "avcodec.h"

#include "put_bits.h"

#include "dsputil.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

/** 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

    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

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

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

} AC3Block;

/**

 * AC-3 encoder private context.

 */

typedef struct AC3EncodeContext {

    PutBitContext pb;                       ///< bitstream writer context

    DSPContext dsp;

    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];

    /* 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;

    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;

    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(SampleType *output, const SampleType *input,

                         const SampleType *window, int n);

static int normalize_samples(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->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 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];

            CoefType *coef = block->mdct_coef[ch];

            int exp_shift  = block->exp_shift[ch];

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

                int e;

                int v = abs(SCALE_COEF(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 1000

/**

 * 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;

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

        exp_diff = s->dsp.sad[0](NULL, exp[blk], exp[blk-1], 16, 16);

        if (exp_diff > EXP_DIFF_THRESHOLD)

            exp_strategy[blk] = EXP_NEW;

        else

            exp_strategy[blk] = EXP_REUSE;

    }

    emms_c();

    /* 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)

{

    uint8_t *exp1[AC3_MAX_CHANNELS][AC3_MAX_BLOCKS];

    uint8_t exp_str1[AC3_MAX_CHANNELS][AC3_MAX_BLOCKS];

    int ch, blk;

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

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

            exp1[ch][blk]     = s->blocks[blk].exp[ch];

            exp_str1[ch][blk] = s->blocks[blk].exp_strategy[ch];

        }

        compute_exp_strategy_ch(s, exp_str1[ch], exp1[ch]);

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

            s->blocks[blk].exp_strategy[ch] = exp_str1[ch][blk];

    }

    if (s->lfe_on) {

        ch = s->lfe_channel;

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

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

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

    }

}

/**

 * Set each encoded exponent in a block to the minimum of itself and the

 * exponent in the same frequency bin of a following block.

 * exp[i] = min(exp[i], exp1[i]

 */

static void exponent_min(uint8_t *exp, uint8_t *exp1, int n)

{

    int i;

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

        if (exp1[i] < exp[i])

            exp[i] = exp1[i];

    }

}

/**

 * 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, blk2, ch;

    AC3Block *block, *block1, *block2;

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

        blk = 0;

        block = &s->blocks[0];

        while (blk < AC3_MAX_BLOCKS) {

            blk1 = blk + 1;

            block1 = block + 1;

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

            while (blk1 < AC3_MAX_BLOCKS && block1->exp_strategy[ch] == EXP_REUSE) {

                exponent_min(block->exp[ch], block1->exp[ch], s->nb_coefs[ch]);

                blk1++;

                block1++;

            }

            encode_exponents_blk_ch(block->exp[ch], s->nb_coefs[ch],

                                    block->exp_strategy[ch]);

            /* copy encoded exponents for reuse case */

            block2 = block + 1;

            for (blk2 = blk+1; blk2 < blk1; blk2++, block2++) {

                memcpy(block2->exp[ch], block->exp[ch],

                       s->nb_coefs[ch] * sizeof(uint8_t));

            }

            blk = blk1;

            block = block1;

        }

    }

}

/**

 * 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++) {

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

                continue;

            }

            group_size = block->exp_strategy[ch] + (block->exp_strategy[ch] == EXP_D45);

            nb_groups = exponent_group_tab[block->exp_strategy[ch]-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);

}

/**

 * 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

     *   no rematrixing

     *   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 */

            if (!blk)

                frame_bits += 4;

        }

        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      = 4;

    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++) {

        uint8_t *exp_strategy = s->blocks[blk].exp_strategy;

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

            if (exp_strategy[ch] != 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 (block->exp_strategy[ch] != 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 (block->exp_strategy[ch] == 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);

    FFSWAP(uint8_t *, s->bap_buffer, s->bap1_buffer);

    for (snr_incr = 64; snr_incr > 0; snr_incr >>= 2) {

        while (snr_offset + snr_incr <= 1023 &&

               bit_alloc(s, snr_offset + snr_incr) <= bits_left) {

            snr_offset += snr_incr;

            FFSWAP(uint8_t *, s->bap_buffer, s->bap1_buffer);

        }

    }

    FFSWAP(uint8_t *, s->bap_buffer, s->bap1_buffer);

    reset_block_bap(s);

    s->coarse_snr_offset = snr_offset >> 4;

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

        s->fine_snr_offset[ch] = snr_offset & 0xF;

    return 0;

}

/**

 * 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;

}

/**

 * Perform bit allocation search.

 * Finds the SNR offset value that maximizes quality and fits in the specified

 * frame size.  Output is the SNR offset and a set of bit allocation pointers

 * used to quantize the mantissas.

 */

static int compute_bit_allocation(AC3EncodeContext *s)

{

    int ret;

    count_frame_bits(s);

    bit_alloc_masking(s);

    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;

}

/**

 * Symmetric quantization on 'levels' levels.

 */

static inline int sym_quant(int c, int e, int levels)

{

    int v;

    if (c >= 0) {

        v = (levels * (c << e)) >> 24;

        v = (v + 1) >> 1;

        v = (levels >> 1) + v;

    } else {

        v = (levels * ((-c) << e)) >> 24;

        v = (v + 1) >> 1;

        v = (levels >> 1) - v;

    }

    assert(v >= 0 && v < levels);

    return v;

}

/**

 * Asymmetric quantization on 2^qbits levels.

 */

static inline int asym_quant(int c, int e, int qbits)

{

    int lshift, m, v;

    lshift = e + qbits - 24;

    if (lshift >= 0)

        v = c << lshift;

    else

        v = c >> (-lshift);

    /* rounding */

    v = (v + 1) >> 1;

    m = (1 << (qbits-1));

    if (v >= m)

        v = m - 1;

    assert(v >= -m);

    return v & ((1 << qbits)-1);

}

/**

 * Quantize a set of mantissas for a single channel in a single block.

 */

static void quantize_mantissas_blk_ch(AC3EncodeContext *s, CoefType *mdct_coef,

                                      int8_t exp_shift, uint8_t *exp,

                                      uint8_t *bap, uint16_t *qmant, int n)

{

    int i;

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

        int v;

        int c = SCALE_COEF(mdct_coef[i]);

        int e = exp[i] - exp_shift;

        int b = bap[i];

        switch (b) {

        case 0:

            v = 0;

            break;

        case 1:

            v = sym_quant(c, e, 3);

            switch (s->mant1_cnt) {

            case 0:

                s->qmant1_ptr = &qmant[i];

                v = 9 * v;

                s->mant1_cnt = 1;

                break;

            case 1:

                *s->qmant1_ptr += 3 * v;

                s->mant1_cnt = 2;

                v = 128;

                break;

            default:

                *s->qmant1_ptr += v;

                s->mant1_cnt = 0;

                v = 128;

                break;

            }

            break;

        case 2:

            v = sym_quant(c, e, 5);

            switch (s->mant2_cnt) {

            case 0:

                s->qmant2_ptr = &qmant[i];

                v = 25 * v;

                s->mant2_cnt = 1;

                break;

            case 1:

                *s->qmant2_ptr += 5 * v;

                s->mant2_cnt = 2;

                v = 128;

                break;

            default:

                *s->qmant2_ptr += v;

                s->mant2_cnt = 0;

                v = 128;

                break;

            }

            break;

        case 3:

            v = sym_quant(c, e, 7);

            break;

        case 4:

            v = sym_quant(c, e, 11);

            switch (s->mant4_cnt) {

            case 0:

                s->qmant4_ptr = &qmant[i];

                v = 11 * v;

                s->mant4_cnt = 1;

                break;

            default:

                *s->qmant4_ptr += v;

                s->mant4_cnt = 0;

                v = 128;

                break;

            }

            break;

        case 5:

            v = sym_quant(c, e, 15);

            break;

        case 14:

            v = asym_quant(c, e, 14);

            break;

        case 15:

            v = asym_quant(c, e, 16);

            break;

        default:

            v = asym_quant(c, e, b - 1);

            break;

        }

        qmant[i] = v;

    }

}

/**

 * Quantize mantissas using coefficients, exponents, and bit allocation pointers.

 */

static void quantize_mantissas(AC3EncodeContext *s)

{

    int blk, ch;

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

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

        s->mant1_cnt  = s->mant2_cnt  = s->mant4_cnt  = 0;

        s->qmant1_ptr = s->qmant2_ptr = s->qmant4_ptr = NULL;

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

            quantize_mantissas_blk_ch(s, block->mdct_coef[ch], block->exp_shift[ch],

                                      block->exp[ch], block->bap[ch],

                                      block->qmant[ch], s->nb_coefs[ch]);

        }

    }

}

/**

 * Write the AC-3 frame header to the output bitstream.

 */

static void output_frame_header(AC3EncodeContext *s)

{

    put_bits(&s->pb, 16, 0x0b77);   /* frame header */

    put_bits(&s->pb, 16, 0);        /* crc1: will be filled later */

    put_bits(&s->pb, 2,  s->bit_alloc.sr_code);

    put_bits(&s->pb, 6,  s->frame_size_code + (s->frame_size - s->frame_size_min) / 2);

    put_bits(&s->pb, 5,  s->bitstream_id);

    put_bits(&s->pb, 3,  s->bitstream_mode);

    put_bits(&s->pb, 3,  s->channel_mode);

    if ((s->channel_mode & 0x01) && s->channel_mode != AC3_CHMODE_MONO)

        put_bits(&s->pb, 2, 1);     /* XXX -4.5 dB */

    if (s->channel_mode & 0x04)

        put_bits(&s->pb, 2, 1);     /* XXX -6 dB */

    if (s->channel_mode == AC3_CHMODE_STEREO)

        put_bits(&s->pb, 2, 0);     /* surround not indicated */

    put_bits(&s->pb, 1, s->lfe_on); /* LFE */

    put_bits(&s->pb, 5, 31);        /* dialog norm: -31 db */

    put_bits(&s->pb, 1, 0);         /* no compression control word */

    put_bits(&s->pb, 1, 0);         /* no lang code */

    put_bits(&s->pb, 1, 0);         /* no audio production info */

    put_bits(&s->pb, 1, 0);         /* no copyright */

    put_bits(&s->pb, 1, 1);         /* original bitstream */

    put_bits(&s->pb, 1, 0);         /* no time code 1 */

    put_bits(&s->pb, 1, 0);         /* no time code 2 */

    put_bits(&s->pb, 1, 0);         /* no additional bit stream info */

}

/**

 * Write one audio block to the output bitstream.

 */

static void output_audio_block(AC3EncodeContext *s, int block_num)

{

    int ch, i, baie, rbnd;

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

    /* block switching */

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

        put_bits(&s->pb, 1, 0);

    /* dither flags */

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

        put_bits(&s->pb, 1, 1);

    /* dynamic range codes */

    put_bits(&s->pb, 1, 0);

    /* channel coupling */

    if (!block_num) {

        put_bits(&s->pb, 1, 1); /* coupling strategy present */

        put_bits(&s->pb, 1, 0); /* no coupling strategy */

    } else {

        put_bits(&s->pb, 1, 0); /* no new coupling strategy */

    }

    /* stereo rematrixing */

    if (s->channel_mode == AC3_CHMODE_STEREO) {

        if (!block_num) {

            /* first block must define rematrixing (rematstr) */

            put_bits(&s->pb, 1, 1);

            /* dummy rematrixing rematflg(1:4)=0 */

            for (rbnd = 0; rbnd < 4; rbnd++)

                put_bits(&s->pb, 1, 0);

        } else {

            /* no matrixing (but should be used in the future) */

            put_bits(&s->pb, 1, 0);

        }

    }

    /* exponent strategy */

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

        put_bits(&s->pb, 2, block->exp_strategy[ch]);

    if (s->lfe_on)

        put_bits(&s->pb, 1, block->exp_strategy[s->lfe_channel]);

    /* bandwidth */

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

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

            put_bits(&s->pb, 6, s->bandwidth_code[ch]);

    }

    /* exponents */

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

        int nb_groups;

        if (block->exp_strategy[ch] == EXP_REUSE)

            continue;

        /* DC exponent */

        put_bits(&s->pb, 4, block->grouped_exp[ch][0]);

        /* exponent groups */

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

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

            put_bits(&s->pb, 7, block->grouped_exp[ch][i]);

        /* gain range info */

        if (ch != s->lfe_channel)

            put_bits(&s->pb, 2, 0);

    }

    /* bit allocation info */

    baie = (block_num == 0);

    put_bits(&s->pb, 1, baie);

    if (baie) {

        put_bits(&s->pb, 2, s->slow_decay_code);

        put_bits(&s->pb, 2, s->fast_decay_code);

        put_bits(&s->pb, 2, s->slow_gain_code);

        put_bits(&s->pb, 2, s->db_per_bit_code);

        put_bits(&s->pb, 3, s->floor_code);

    }

    /* snr offset */

    put_bits(&s->pb, 1, baie);

    if (baie) {

        put_bits(&s->pb, 6, s->coarse_snr_offset);

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

            put_bits(&s->pb, 4, s->fine_snr_offset[ch]);

            put_bits(&s->pb, 3, s->fast_gain_code[ch]);

        }

    }

    put_bits(&s->pb, 1, 0); /* no delta bit allocation */

    put_bits(&s->pb, 1, 0); /* no data to skip */

    /* mantissas */

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

        int b, q;

        for (i = 0; i < s->nb_coefs[ch]; i++) {

            q = block->qmant[ch][i];

            b = block->bap[ch][i];

            switch (b) {

            case 0:                                         break;

            case 1: if (q != 128) put_bits(&s->pb,   5, q); break;

            case 2: if (q != 128) put_bits(&s->pb,   7, q); break;

            case 3:               put_bits(&s->pb,   3, q); break;

            case 4: if (q != 128) put_bits(&s->pb,   7, q); break;

            case 14:              put_bits(&s->pb,  14, q); break;

            case 15:              put_bits(&s->pb,  16, q); break;

            default:              put_bits(&s->pb, b-1, q); break;

            }

        }

    }

}

/** CRC-16 Polynomial */

#define CRC16_POLY ((1 << 0) | (1 << 2) | (1 << 15) | (1 << 16))

static unsigned int mul_poly(unsigned int a, unsigned int b, unsigned int poly)

{

    unsigned int c;

    c = 0;

    while (a) {

        if (a & 1)

            c ^= b;

        a = a >> 1;

        b = b << 1;

        if (b & (1 << 16))

            b ^= poly;

    }

    return c;

}

static unsigned int pow_poly(unsigned int a, unsigned int n, unsigned int poly)

{

    unsigned int r;

    r = 1;

    while (n) {

        if (n & 1)

            r = mul_poly(r, a, poly);

        a = mul_poly(a, a, poly);

        n >>= 1;

    }

    return r;

}

/**

 * Fill the end of the frame with 0's and compute the two CRCs.

 */

static void output_frame_end(AC3EncodeContext *s)

{

    const AVCRC *crc_ctx = av_crc_get_table(AV_CRC_16_ANSI);

    int frame_size_58, pad_bytes, crc1, crc2_partial, crc2, crc_inv;

    uint8_t *frame;

    frame_size_58 = ((s->frame_size >> 2) + (s->frame_size >> 4)) << 1;

    /* pad the remainder of the frame with zeros */

    flush_put_bits(&s->pb);

    frame = s->pb.buf;

    pad_bytes = s->frame_size - (put_bits_ptr(&s->pb) - frame) - 2;

    assert(pad_bytes >= 0);

    if (pad_bytes > 0)

        memset(put_bits_ptr(&s->pb), 0, pad_bytes);

    /* compute crc1 */

    /* this is not so easy because it is at the beginning of the data... */