replaygain 1.0.1

// As mentioned elsewhere, this is a literal line-by-line translation of the af_replaygain.c from ffmpeg.
// In the few places where is isn't (because I dropped or reimplemented code), their code remains as comments.
// Also, that copyright notice is theirs (obviously).
// Original code is here: https://github.com/FFmpeg/FFmpeg/blob/6aaac24d72a7da631173209841a3944fcb4a3309/libavfilter/af_replaygain.c

/*
 * Copyright (c) 1998 - 2009 Conifer Software
 *
 * 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
 */

const HISTOGRAM_SLOTS: usize = 12000;
const BUTTER_ORDER: usize = 2;
const YULE_ORDER: usize = 10;

pub struct ReplayGainFreqInfo {
    //sample_rate: i32,
    byule: [f64; YULE_ORDER + 1],
    ayule: [f64; YULE_ORDER + 1],
    bbutter: [f64; BUTTER_ORDER + 1],
    abutter: [f64; BUTTER_ORDER + 1],
}


pub fn freq_to_info(freq: usize) -> Option<ReplayGainFreqInfo> {
    Some(match freq {
        192000 => ReplayGainFreqInfo {
            byule: [  0.01184742123123, -0.04631092400086,  0.06584226961238,
                      -0.02165588522478, -0.05656260778952,  0.08607493592760,
                      -0.03375544339786, -0.04216579932754,  0.06416711490648,
                      -0.03444708260844,  0.00697275872241 ],
            ayule: [  1.00000000000000, -5.24727318348167, 10.60821585192244,
                      -8.74127665810413, -1.33906071371683,  8.07972882096606,
                      -5.46179918950847,  0.54318070652536,  0.87450969224280,
                      -0.34656083539754,  0.03034796843589 ],
            bbutter: [  0.99653501465135, -1.99307002930271,  0.99653501465135 ],
            abutter: [  1.00000000000000, -1.99305802314321,  0.99308203546221 ],
        },
        176400 => ReplayGainFreqInfo {
            byule: [  0.00268568524529, -0.00852379426080,  0.00852704191347,
                      0.00146116310295, -0.00950855828762,  0.00625449515499,
                      0.00116183868722, -0.00362461417136,  0.00203961000134,
                      -0.00050664587933,  0.00004327455427 ],
            ayule: [  1.00000000000000, -5.57512782763045, 12.44291056065794,
                      -12.87462799681221,  3.08554846961576,  6.62493459880692,
                      -7.07662766313248,  2.51175542736441,  0.06731510802735,
                      -0.24567753819213,  0.03961404162376 ],
            bbutter: [  0.99622916581118, -1.99245833162236,  0.99622916581118 ],
            abutter: [  1.00000000000000, -1.99244411238133,  0.99247255086339 ],
        },
        144000 => ReplayGainFreqInfo {
            byule: [  0.00639682359450, -0.02556437970955,  0.04230854400938,
                      -0.03722462201267,  0.01718514827295,  0.00610592243009,
                      -0.03065965747365,  0.04345745003539, -0.03298592681309,
                      0.01320937236809, -0.00220304127757 ],
            ayule: [  1.00000000000000, -6.14814623523425, 15.80002457141566,
                      -20.78487587686937, 11.98848552310315,  3.36462015062606,
                      -10.22419868359470,  6.65599702146473, -1.67141861110485,
                      -0.05417956536718,  0.07374767867406 ],
            bbutter: [  0.99538268958706, -1.99076537917413,  0.99538268958706 ],
            abutter: [  1.00000000000000, -1.99074405950505,  0.99078669884321 ],
        },
        128000 => ReplayGainFreqInfo {
            byule: [  0.00553120584305, -0.02112620545016,  0.03549076243117,
                      -0.03362498312306,  0.01425867248183,  0.01344686928787,
                      -0.03392770787836,  0.03464136459530, -0.02039116051549,
                      0.00667420794705, -0.00093763762995 ],
            ayule: [  1.00000000000000, -6.14581710839925, 16.04785903675838,
                      -22.19089131407749, 15.24756471580286, -0.52001440400238,
                      -8.00488641699940,  6.60916094768855, -2.37856022810923,
                      0.33106947986101,  0.00459820832036 ],
            bbutter: [  0.99480702681278, -1.98961405362557,  0.99480702681278 ],
            abutter: [  1.00000000000000, -1.98958708647324,  0.98964102077790 ],
        },
        112000 => ReplayGainFreqInfo {
            byule: [  0.00528778718259, -0.01893240907245,  0.03185982561867,
                      -0.02926260297838,  0.00715743034072,  0.01985743355827,
                      -0.03222614850941,  0.02565681978192, -0.01210662313473,
                      0.00325436284541, -0.00044173593001 ],
            ayule: [  1.00000000000000, -6.24932108456288, 17.42344320538476,
                      -27.86819709054896, 26.79087344681326,-13.43711081485123,
                      -0.66023612948173,  6.03658091814935, -4.24926577030310,
                      1.40829268709186, -0.19480852628112 ],
            bbutter: [  0.99406737810867, -1.98813475621734,  0.99406737810867 ],
            abutter: [  1.00000000000000, -1.98809955990514,  0.98816995252954 ],
        },
        96000 => ReplayGainFreqInfo {
            byule: [  0.00588138296683, -0.01613559730421,  0.02184798954216,
                      -0.01742490405317,  0.00464635643780,  0.01117772513205,
                      -0.02123865824368,  0.01959354413350, -0.01079720643523,
                      0.00352183686289, -0.00063124341421 ],
            ayule: [  1.00000000000000, -5.97808823642008, 16.21362507964068,
                      -25.72923730652599, 25.40470663139513,-14.66166287771134,
                      2.81597484359752,  2.51447125969733, -2.23575306985286,
                      0.75788151036791, -0.10078025199029 ],
            bbutter: [  0.99308203517541, -1.98616407035082,  0.99308203517541 ],
            abutter: [  1.00000000000000, -1.98611621154089,  0.98621192916075 ],
        },
        88200 => ReplayGainFreqInfo {
            byule: [  0.02667482047416, -0.11377479336097,  0.23063167910965,
                      -0.30726477945593,  0.33188520686529, -0.33862680249063,
                      0.31807161531340, -0.23730796929880,  0.12273894790371,
                      -0.03840017967282,  0.00549673387936 ],
            ayule: [  1.00000000000000, -6.31836451657302, 18.31351310801799,
                      -31.88210014815921, 36.53792146976740,-28.23393036467559,
                      14.24725258227189, -4.04670980012854,  0.18865757280515,
                      0.25420333563908, -0.06012333531065 ],
            bbutter: [  0.99247255046129, -1.98494510092259,  0.99247255046129 ],
            abutter: [  1.00000000000000, -1.98488843762335,  0.98500176422183 ],
        },
        64000 => ReplayGainFreqInfo {
            byule: [  0.02613056568174, -0.08128786488109,  0.14937282347325,
                      -0.21695711675126,  0.25010286673402, -0.23162283619278,
                      0.17424041833052, -0.10299599216680,  0.04258696481981,
                      -0.00977952936493,  0.00105325558889 ],
            ayule: [  1.00000000000000, -5.73625477092119, 16.15249794355035,
                      -29.68654912464508, 39.55706155674083,-39.82524556246253,
                      30.50605345013009,-17.43051772821245,  7.05154573908017,
                      -1.80783839720514,  0.22127840210813 ],
            bbutter: [  0.98964101933472, -1.97928203866944,  0.98964101933472 ],
            abutter: [  1.00000000000000, -1.97917472731009,  0.97938935002880 ],
        },
        56000 => ReplayGainFreqInfo {
            byule: [  0.03144914734085, -0.06151729206963,  0.08066788708145,
                      -0.09737939921516,  0.08943210803999, -0.06989984672010,
                      0.04926972841044, -0.03161257848451,  0.01456837493506,
                      -0.00316015108496,  0.00132807215875 ],
            ayule: [  1.00000000000000, -4.87377313090032, 12.03922160140209,
                      -20.10151118381395, 25.10388534415171,-24.29065560815903,
                      18.27158469090663,-10.45249552560593,  4.30319491872003,
                      -1.13716992070185,  0.14510733527035 ],
            bbutter: [  0.98816995007392, -1.97633990014784,  0.98816995007392 ],
            abutter: [  1.00000000000000, -1.97619994516973,  0.97647985512594 ],
        },
        48000 => ReplayGainFreqInfo {
            byule: [  0.03857599435200, -0.02160367184185, -0.00123395316851,
                      -0.00009291677959, -0.01655260341619,  0.02161526843274,
                      -0.02074045215285,  0.00594298065125,  0.00306428023191,
                      0.00012025322027,  0.00288463683916 ],
            ayule: [  1.00000000000000, -3.84664617118067,  7.81501653005538,
                      -11.34170355132042, 13.05504219327545,-12.28759895145294,
                      9.48293806319790, -5.87257861775999,  2.75465861874613,
                      -0.86984376593551,  0.13919314567432 ],
            bbutter: [  0.98621192462708, -1.97242384925416,  0.98621192462708 ],
            abutter: [  1.00000000000000, -1.97223372919527,  0.97261396931306 ],
        },
        44100 => ReplayGainFreqInfo {
            byule: [  0.05418656406430, -0.02911007808948, -0.00848709379851,
                      -0.00851165645469, -0.00834990904936,  0.02245293253339,
                      -0.02596338512915,  0.01624864962975, -0.00240879051584,
                      0.00674613682247, -0.00187763777362 ],
            ayule: [  1.00000000000000, -3.47845948550071,  6.36317777566148,
                      -8.54751527471874,  9.47693607801280, -8.81498681370155,
                      6.85401540936998, -4.39470996079559,  2.19611684890774,
                      -0.75104302451432,  0.13149317958808 ],
            bbutter: [  0.98500175787242, -1.97000351574484,  0.98500175787242 ],
            abutter: [  1.00000000000000, -1.96977855582618,  0.97022847566350 ],
        },
        37800 => ReplayGainFreqInfo {
            byule: [  0.08717879977844, -0.01000374016172, -0.06265852122368,
                      -0.01119328800950, -0.00114279372960,  0.02081333954769,
                      -0.01603261863207,  0.01936763028546,  0.00760044736442,
                      -0.00303979112271, -0.00075088605788 ],
            ayule: [  1.00000000000000, -2.62816311472146,  3.53734535817992,
                      -3.81003448678921,  3.91291636730132, -3.53518605896288,
                      2.71356866157873, -1.86723311846592,  1.12075382367659,
                      -0.48574086886890,  0.11330544663849 ],
            bbutter: [  0.98252400815195, -1.96504801630391,  0.98252400815195 ],
            abutter: [  1.00000000000000, -1.96474258269041,  0.96535344991740 ],
        },
        32000 => ReplayGainFreqInfo {
            byule: [  0.15457299681924, -0.09331049056315, -0.06247880153653,
                      0.02163541888798, -0.05588393329856,  0.04781476674921,
                      0.00222312597743,  0.03174092540049, -0.01390589421898,
                      0.00651420667831, -0.00881362733839 ],
            ayule: [  1.00000000000000, -2.37898834973084,  2.84868151156327,
                      -2.64577170229825,  2.23697657451713, -1.67148153367602,
                      1.00595954808547, -0.45953458054983,  0.16378164858596,
                      -0.05032077717131,  0.02347897407020 ],
            bbutter: [  0.97938932735214, -1.95877865470428,  0.97938932735214 ],
            abutter: [  1.00000000000000, -1.95835380975398,  0.95920349965459 ],
        },
        24000 => ReplayGainFreqInfo {
            byule: [  0.30296907319327, -0.22613988682123, -0.08587323730772,
                      0.03282930172664, -0.00915702933434, -0.02364141202522,
                      -0.00584456039913,  0.06276101321749, -0.00000828086748,
                      0.00205861885564, -0.02950134983287 ],
            ayule: [  1.00000000000000, -1.61273165137247,  1.07977492259970,
                      -0.25656257754070, -0.16276719120440, -0.22638893773906,
                      0.39120800788284, -0.22138138954925,  0.04500235387352,
                      0.02005851806501,  0.00302439095741 ],
            bbutter: [  0.97531843204928, -1.95063686409857,  0.97531843204928 ],
            abutter: [  1.00000000000000, -1.95002759149878,  0.95124613669835 ],
        },
        22050 => ReplayGainFreqInfo {
            byule: [  0.33642304856132, -0.25572241425570, -0.11828570177555,
                      0.11921148675203, -0.07834489609479, -0.00469977914380,
                      -0.00589500224440,  0.05724228140351,  0.00832043980773,
                      -0.01635381384540, -0.01760176568150 ],
            ayule: [  1.00000000000000, -1.49858979367799,  0.87350271418188,
                      0.12205022308084, -0.80774944671438,  0.47854794562326,
                      -0.12453458140019, -0.04067510197014,  0.08333755284107,
                      -0.04237348025746,  0.02977207319925 ],
            bbutter: [  0.97316523498161, -1.94633046996323,  0.97316523498161 ],
            abutter: [  1.00000000000000, -1.94561023566527,  0.94705070426118 ],
        },
        18900 => ReplayGainFreqInfo {
            byule: [  0.38524531015142, -0.27682212062067, -0.09980181488805,
                      0.09951486755646, -0.08934020156622, -0.00322369330199,
                      -0.00110329090689,  0.03784509844682,  0.01683906213303,
                      -0.01147039862572, -0.01941767987192 ],
            ayule: [  1.00000000000000, -1.29708918404534,  0.90399339674203,
                      -0.29613799017877, -0.42326645916207,  0.37934887402200,
                      -0.37919795944938,  0.23410283284785, -0.03892971758879,
                      0.00403009552351,  0.03640166626278 ],
            bbutter: [  0.96535326815829, -1.93070653631658,  0.96535326815829 ],
            abutter: [  1.00000000000000, -1.92950577983524,  0.93190729279793 ],
        },
        16000 => ReplayGainFreqInfo {
            byule: [  0.44915256608450, -0.14351757464547, -0.22784394429749,
                      -0.01419140100551,  0.04078262797139, -0.12398163381748,
                      0.04097565135648,  0.10478503600251, -0.01863887810927,
                      -0.03193428438915,  0.00541907748707 ],
            ayule: [  1.00000000000000, -0.62820619233671,  0.29661783706366,
                      -0.37256372942400,  0.00213767857124, -0.42029820170918,
                      0.22199650564824,  0.00613424350682,  0.06747620744683,
                      0.05784820375801,  0.03222754072173 ],
            bbutter: [  0.96454515552826, -1.92909031105652,  0.96454515552826 ],
            abutter: [  1.00000000000000, -1.92783286977036,  0.93034775234268 ],
        },
        12000 => ReplayGainFreqInfo {
            byule: [  0.56619470757641, -0.75464456939302,  0.16242137742230,
                      0.16744243493672, -0.18901604199609,  0.30931782841830,
                      -0.27562961986224,  0.00647310677246,  0.08647503780351,
                      -0.03788984554840, -0.00588215443421 ],
            ayule: [  1.00000000000000, -1.04800335126349,  0.29156311971249,
                      -0.26806001042947,  0.00819999645858,  0.45054734505008,
                      -0.33032403314006,  0.06739368333110, -0.04784254229033,
                      0.01639907836189,  0.01807364323573 ],
            bbutter: [  0.96009142950541, -1.92018285901082,  0.96009142950541 ],
            abutter: [  1.00000000000000, -1.91858953033784,  0.92177618768381 ],
        },
        11025 => ReplayGainFreqInfo {
            byule: [  0.58100494960553, -0.53174909058578, -0.14289799034253,
                      0.17520704835522,  0.02377945217615,  0.15558449135573,
                      -0.25344790059353,  0.01628462406333,  0.06920467763959,
                      -0.03721611395801, -0.00749618797172 ],
            ayule: [  1.00000000000000, -0.51035327095184, -0.31863563325245,
                      -0.20256413484477,  0.14728154134330,  0.38952639978999,
                      -0.23313271880868, -0.05246019024463, -0.02505961724053,
                      0.02442357316099,  0.01818801111503 ],
            bbutter: [  0.95856916599601, -1.91713833199203,  0.95856916599601 ],
            abutter: [  1.00000000000000, -1.91542108074780,  0.91885558323625 ],
        },
        8000 => ReplayGainFreqInfo {
            byule: [  0.53648789255105, -0.42163034350696, -0.00275953611929,
                      0.04267842219415, -0.10214864179676,  0.14590772289388,
                      -0.02459864859345, -0.11202315195388, -0.04060034127000,
                      0.04788665548180, -0.02217936801134 ],
            ayule: [  1.00000000000000, -0.25049871956020, -0.43193942311114,
                      -0.03424681017675, -0.04678328784242,  0.26408300200955,
                      0.15113130533216, -0.17556493366449, -0.18823009262115,
                      0.05477720428674,  0.04704409688120 ],
            bbutter: [  0.94597685600279, -1.89195371200558,  0.94597685600279 ],
            abutter: [  1.00000000000000, -1.88903307939452,  0.89487434461664 ],
        },
        _ => return None,
    })
}

pub struct ReplayGainContext {
    histogram: [u32; HISTOGRAM_SLOTS],
    peak: f32,
    yule_hist_i: usize,
    butter_hist_i: usize,

    // these should be ptrs but we do things differently
    yule_coeff_a: [f64; YULE_ORDER + 1],
    yule_coeff_b: [f64; YULE_ORDER + 1],
    butter_coeff_a: [f64; BUTTER_ORDER + 1],
    butter_coeff_b: [f64; BUTTER_ORDER + 1],

    yule_hist_a: [f32; 256],
    yule_hist_b: [f32; 256],
    butter_hist_a: [f32; 256],
    butter_hist_b: [f32; 256],
}

/*
static int query_formats(AVFilterContext *ctx)
{
    AVFilterFormats *formats = NULL;
    AVFilterChannelLayouts *layout = NULL;
    int i, ret;

    if ((ret = ff_add_format                 (&formats, AV_SAMPLE_FMT_FLT  )) < 0 ||
        (ret = ff_set_common_formats         (ctx     , formats            )) < 0 ||
        (ret = ff_add_channel_layout         (&layout , AV_CH_LAYOUT_STEREO)) < 0 ||
        (ret = ff_set_common_channel_layouts (ctx     , layout             )) < 0)
        return ret;

    formats = NULL;
    for (i = 0; i < FF_ARRAY_ELEMS(freqinfos); i++) {
        if ((ret = ff_add_format(&formats, freqinfos[i].sample_rate)) < 0)
            return ret;
    }

    return ff_set_common_samplerates(ctx, formats);
}
*/

//static int config_input(AVFilterLink *inlink)
//{
//    AVFilterContext *ctx = inlink->dst;
//    ReplayGainContext *s = ctx->priv;
//    int i;
pub fn init_context(fi: &ReplayGainFreqInfo) -> ReplayGainContext {
//    for (i = 0; i < FF_ARRAY_ELEMS(freqinfos); i++) {
//        if (freqinfos[i].sample_rate == inlink->sample_rate)
//            break;
//    }
//    av_assert0(i < FF_ARRAY_ELEMS(freqinfos));


//    s->yule_coeff_a   = freqinfos[i].ayule;
//    s->yule_coeff_b   = freqinfos[i].byule;
//    s->butter_coeff_a = freqinfos[i].abutter;
//    s->butter_coeff_b = freqinfos[i].bbutter;

//    s->yule_hist_i   = 20;
//    s->butter_hist_i = 4;
    ReplayGainContext {
        yule_coeff_a: fi.ayule,
        yule_coeff_b: fi.byule,
        butter_coeff_a: fi.abutter,
        butter_coeff_b: fi.bbutter,

        yule_hist_i: 20,
        butter_hist_i: 4,

        // defaults
        histogram: [0; HISTOGRAM_SLOTS],
        peak: 0f32,
        yule_hist_a: [0f32; 256],
        yule_hist_b: [0f32; 256],
        butter_hist_a: [0f32; 256],
        butter_hist_b: [0f32; 256],
    }
//    inlink->partial_buf_size =
//    inlink->min_samples =
//    inlink->max_samples = inlink->sample_rate / 20;

//    return 0;
}

/*
 * Update largest absolute sample value.
 */
//static void calc_stereo_peak(const float *samples, int nb_samples,
//                             float *peak_p)
fn calc_stereo_peak(samples: &[f32], peak_p: &mut f32) {
    assert!(samples.len() % 2 == 0);

//    let peak = 0f32;
//
//    while (nb_samples--) {
//        if (samples[0] > peak)
//            peak = samples[0];
//        else if (-samples[0] > peak)
//            peak = -samples[0];
//
//        if (samples[1] > peak)
//            peak = samples[1];
//        else if (-samples[1] > peak)
//            peak = -samples[1];
//
//        samples += 2;
//    }
//
//    *peak_p = FFMAX(peak, *peak_p);

    *peak_p = samples.iter().cloned().map(f32::abs).fold(*peak_p, f32::max)
}

/*
 * Calculate stereo RMS level. Minimum value is about -100 dB for
 * digital silence. The 90 dB offset is to compensate for the
 * normalized float range and 3 dB is for stereo samples.
 */
//static double calc_stereo_rms(const float *samples, int nb_samples)
//{
fn calc_stereo_rms(samples: &[f32]) -> f64 {
    assert!(samples.len() % 2 == 0);
    //int count = nb_samples;
    //double sum = 1e-16;

    //while (count--) {
    //    sum += samples[0] * samples[0] + samples[1] * samples[1];
    //    samples += 2;
    //}
    let mut sum: f64 = samples.iter().map(|&x| (x * x) as f64).sum();
    sum += 1e-16f64; // for some reason???

    //return 10 * log10 (sum / nb_samples) + 90.0 - 3.0;
    10f64 * (sum / (samples.len() / 2) as f64).log10() + 90f64 - 3f64
}

/*
 * Optimized implementation of 2nd-order IIR stereo filter.
 */
//static void butter_filter_stereo_samples(ReplayGainContext *s,
//                                         float *samples, int nb_samples)
//{
fn butter_filter_stereo_samples(ctx: &mut ReplayGainContext, samples: &mut [f32]) {
    assert!(samples.len() % 2 == 0);

//    const double *coeff_a = s->butter_coeff_a;
    let coeff_a = &ctx.butter_coeff_a;
//    const double *coeff_b = s->butter_coeff_b;
    let coeff_b = &ctx.butter_coeff_b;
//    float *hist_a   = s->butter_hist_a;
//    float *hist_b   = s->butter_hist_b;
//    double left, right;
//    int i, j;

//    i = s->butter_hist_i;
    let mut i = ctx.butter_hist_i;

//    // If filter history is very small magnitude, clear it completely
//    // to prevent denormals from rattling around in there forever
//    // (slowing us down).

//    for (j = -4; j < 0; ++j)
//        if (fabs(hist_a[i + j]) > 1e-10 || fabs(hist_b[i + j]) > 1e-10)
//            break;

//    if (!j) {
//        memset(s->butter_hist_a, 0, sizeof(s->butter_hist_a));
//        memset(s->butter_hist_b, 0, sizeof(s->butter_hist_b));
//    }

    if !ctx.butter_hist_a[i - 4..][..4].iter()
        .chain(ctx.butter_hist_b[i - 4..][..4].iter())
        .any(|&x| x.abs() > 1e-10f32)
    {
        ctx.butter_hist_a = [0f32; 256];
        ctx.butter_hist_b = [0f32; 256];
    }

    let hist_a = &mut ctx.butter_hist_a;
    let hist_b = &mut ctx.butter_hist_b;

//    while (nb_samples--) {
    for samples in samples.chunks_mut(2) {
        let tmp0 = samples[0];
        let tmp1 = samples[1];
        hist_b[i    ] = tmp0;
        hist_b[i + 1] = tmp1;
        let mut left: f64   = tmp0 as f64 * coeff_b[0];
        let mut right: f64  = tmp1 as f64 * coeff_b[0];
        left  += hist_b[i - 2] as f64 * coeff_b[1] - hist_a[i - 2] as f64 * coeff_a[1];
        right += hist_b[i - 1] as f64 * coeff_b[1] - hist_a[i - 1] as f64 * coeff_a[1];
        left  += hist_b[i - 4] as f64 * coeff_b[2] - hist_a[i - 4] as f64 * coeff_a[2];
        right += hist_b[i - 3] as f64 * coeff_b[2] - hist_a[i - 3] as f64 * coeff_a[2];
        hist_a[i    ] = left as f32;
        hist_a[i + 1] = right as f32;
        samples[0] = left as f32;
        samples[1] = right as f32;
//        samples += 2;

        i += 2;
        if i == 256 {
//            memcpy(hist_a, hist_a + 252, sizeof(*hist_a) * 4);
//            memcpy(hist_b, hist_b + 252, sizeof(*hist_b) * 4);
            fn copy_4_to_front(x: &mut [f32]) {
                let (a, b) = x.split_at_mut(4);
                a.copy_from_slice(&b[b.len() - 4 ..]);
            }
            copy_4_to_front(hist_a);
            copy_4_to_front(hist_b);
            i = 4;
        }
    }

    ctx.butter_hist_i = i;
}

/*
 * Optimized implementation of 10th-order IIR stereo filter.
 */
//static void yule_filter_stereo_samples(ReplayGainContext *s, const float *src,
//                                       float *dst, int nb_samples)
//{
fn yule_filter_stereo_samples(ctx: &mut ReplayGainContext, src: &[f32], dst: &mut [f32]) {
    assert!(src.len() % 2 == 0);
    assert!(src.len() == dst.len());

//    const double *coeff_a = s->yule_coeff_a;
    let coeff_a = &ctx.yule_coeff_a;
//    const double *coeff_b = s->yule_coeff_b;
    let coeff_b = &ctx.yule_coeff_b;
//    float *hist_a   = s->yule_hist_a;
//    float *hist_b   = s->yule_hist_b;
//    double left, right;
//    int i, j;

//    //i = s->yule_hist_i;
    let mut i = ctx.yule_hist_i;

//    // If filter history is very small magnitude, clear it completely to
//    // prevent denormals from rattling around in there forever
//    // (slowing us down).

//    for (j = -20; j < 0; ++j)
//        if (fabs(hist_a[i + j]) > 1e-10 || fabs(hist_b[i + j]) > 1e-10)
//            break;
//    if (!j) {
//        memset(s->yule_hist_a, 0, sizeof(s->yule_hist_a));
//        memset(s->yule_hist_b, 0, sizeof(s->yule_hist_b));
//    }
    if !ctx.yule_hist_a[i - 4..][..4].iter()
        .chain(ctx.yule_hist_b[i - 4..][..4].iter())
        .any(|&x| x.abs() > 1e-10f32)
    {
        ctx.yule_hist_a = [0f32; 256];
        ctx.yule_hist_b = [0f32; 256];
    }

    let hist_a = &mut ctx.yule_hist_a;
    let hist_b = &mut ctx.yule_hist_b;
//    while (nb_samples--) {
    for (src, dst) in src.chunks(2).zip(dst.chunks_mut(2)) {
        hist_b[i] = src[0];
        hist_b[i + 1] = src[1];
        let mut left: f64   = src[0] as f64 * coeff_b[0];
        let mut right: f64  = src[1]as f64 * coeff_b[0];
        left  += hist_b[i -  2] as f64 * coeff_b[ 1] - hist_a[i -  2] as f64 * coeff_a[1 ];
        right += hist_b[i -  1] as f64 * coeff_b[ 1] - hist_a[i -  1] as f64 * coeff_a[1 ];
        left  += hist_b[i -  4] as f64 * coeff_b[ 2] - hist_a[i -  4] as f64 * coeff_a[2 ];
        right += hist_b[i -  3] as f64 * coeff_b[ 2] - hist_a[i -  3] as f64 * coeff_a[2 ];
        left  += hist_b[i -  6] as f64 * coeff_b[ 3] - hist_a[i -  6] as f64 * coeff_a[3 ];
        right += hist_b[i -  5] as f64 * coeff_b[ 3] - hist_a[i -  5] as f64 * coeff_a[3 ];
        left  += hist_b[i -  8] as f64 * coeff_b[ 4] - hist_a[i -  8] as f64 * coeff_a[4 ];
        right += hist_b[i -  7] as f64 * coeff_b[ 4] - hist_a[i -  7] as f64 * coeff_a[4 ];
        left  += hist_b[i - 10] as f64 * coeff_b[ 5] - hist_a[i - 10] as f64 * coeff_a[5 ];
        right += hist_b[i -  9] as f64 * coeff_b[ 5] - hist_a[i -  9] as f64 * coeff_a[5 ];
        left  += hist_b[i - 12] as f64 * coeff_b[ 6] - hist_a[i - 12] as f64 * coeff_a[6 ];
        right += hist_b[i - 11] as f64 * coeff_b[ 6] - hist_a[i - 11] as f64 * coeff_a[6 ];
        left  += hist_b[i - 14] as f64 * coeff_b[ 7] - hist_a[i - 14] as f64 * coeff_a[7 ];
        right += hist_b[i - 13] as f64 * coeff_b[ 7] - hist_a[i - 13] as f64 * coeff_a[7 ];
        left  += hist_b[i - 16] as f64 * coeff_b[ 8] - hist_a[i - 16] as f64 * coeff_a[8 ];
        right += hist_b[i - 15] as f64 * coeff_b[ 8] - hist_a[i - 15] as f64 * coeff_a[8 ];
        left  += hist_b[i - 18] as f64 * coeff_b[ 9] - hist_a[i - 18] as f64 * coeff_a[9 ];
        right += hist_b[i - 17] as f64 * coeff_b[ 9] - hist_a[i - 17] as f64 * coeff_a[9 ];
        left  += hist_b[i - 20] as f64 * coeff_b[10] - hist_a[i - 20] as f64 * coeff_a[10];
        right += hist_b[i - 19] as f64 * coeff_b[10] - hist_a[i - 19] as f64 * coeff_a[10];
        hist_a[i    ] = left as f32;
        hist_a[i + 1] = right as f32;
        dst[0] = left as f32;
        dst[1] = right as f32;
//        src += 2;
//        dst += 2;

        i += 2;
        if i == 256 {
//            memcpy(hist_a, hist_a + 236, sizeof(*hist_a) * 20);
//            memcpy(hist_b, hist_b + 236, sizeof(*hist_b) * 20);
            fn copy_20_to_front(x: &mut [f32]) {
                let (a, b) = x.split_at_mut(20);
                a.copy_from_slice(&b[b.len() - 20 ..]);
            }
            copy_20_to_front(hist_a);
            copy_20_to_front(hist_b);
            i = 20;
        }
    }

    ctx.yule_hist_i = i;
}

/*
 * Calculate the ReplayGain value from the specified loudness histogram;
 * clip to -24 / +64 dB.
 */
//static float calc_replaygain(uint32_t *histogram)
//{
fn calc_replaygain(histogram: &[u32; HISTOGRAM_SLOTS]) -> f32 {
//    uint32_t loud_count = 0, total_windows = 0;
//    float gain;
//    int i;

//    for (i = 0; i < HISTOGRAM_SLOTS; i++)
//        total_windows += histogram [i];
    let total_windows = histogram.iter().sum();

//    while (i--)
//        if ((loud_count += histogram [i]) * 20 >= total_windows)
//            break;
    let mut i = HISTOGRAM_SLOTS;
    let mut loud_count = 0;
    while { i -= 1; i > 0 } {
        loud_count += histogram[i];
        if loud_count * 20 >= total_windows {
            break;
        }
    }

    let gain = (64.54f64 - i as f64 / 100.0f64) as f32;
    clip(gain, -24.0, 64.0)
}

//static int filter_frame(AVFilterLink *inlink, AVFrame *in)
//{
pub fn filter_frame(ctx: &mut ReplayGainContext, frame: &[f32]) {
//    AVFilterContext *ctx = inlink->dst;
//    AVFilterLink *outlink = ctx->outputs[0];
//    ReplayGainContext *s = ctx->priv;
//    uint32_t level;
//    AVFrame *out;
//
//    out = ff_get_audio_buffer(inlink, in->nb_samples);
//    if (!out) {
//        av_frame_free(&in);
//        return AVERROR(ENOMEM);
//    }

    let mut buf = frame.to_vec();
    calc_stereo_peak(frame, &mut ctx.peak);
    yule_filter_stereo_samples(ctx, frame, &mut buf[..]);
    butter_filter_stereo_samples(ctx, &mut buf[..]);
    let level = (100f64 * calc_stereo_rms(&buf[..])).floor() as usize;

    ctx.histogram[clip(level, 0, HISTOGRAM_SLOTS - 1)] += 1;
//    return ff_filter_frame(outlink, in);
}

//}
//static av_cold void uninit(AVFilterContext *ctx)
//{
pub fn finish(ctx: &mut ReplayGainContext) -> (f32, f32) {
    let gain = calc_replaygain(&ctx.histogram);
    // and THIS is literally the one reason why this entire crate exists:
    // here we RETURN those values instead of fucking __P R I N T I N G__ them
    // wtf were they thinking!?
//    av_log(ctx, AV_LOG_INFO, "track_gain = %+.2f dB\n", gain);
//    av_log(ctx, AV_LOG_INFO, "track_peak = %.6f\n", s->peak);
    (gain, ctx.peak)
}

/*
static const AVFilterPad replaygain_inputs[] = {
    {
        .name         = "default",
        .type         = AVMEDIA_TYPE_AUDIO,
        .filter_frame = filter_frame,
        .config_props = config_input,
    },
    { NULL }
};

static const AVFilterPad replaygain_outputs[] = {
    {
        .name = "default",
        .type = AVMEDIA_TYPE_AUDIO,
    },
    { NULL }
};

AVFilter ff_af_replaygain = {
    .name          = "replaygain",
    .description   = NULL_IF_CONFIG_SMALL("ReplayGain scanner."),
    .query_formats = query_formats,
    .uninit        = uninit,
    .priv_size     = sizeof(ReplayGainContext),
    .inputs        = replaygain_inputs,
    .outputs       = replaygain_outputs,
};


 */



// little helper
fn clip<T : PartialOrd<T>>(val: T, min: T, max: T) -> T {
    if val < min {
        min
    } else if val > max {
        max
    } else {
        val
    }
}