// SPDX-FileCopyrightText: fabian "ryg" giesen
// SPDX-License-Identifier: MIT

// stb_dxt.h - v1.12 - DXT1/DXT5 compressor

#include <stb_dxt.h>

#include <stdlib.h>
#include <string.h>

#if !defined(STBD_FABS)
#include <math.h>
#endif

#ifndef STBD_FABS
#define STBD_FABS(x) fabs(x)
#endif

static const unsigned char stb__OMatch5[256][2] = {
    {0, 0},   {0, 0},   {0, 1},   {0, 1},   {1, 0},   {1, 0},   {1, 0},   {1, 1},   {1, 1},
    {1, 1},   {1, 2},   {0, 4},   {2, 1},   {2, 1},   {2, 1},   {2, 2},   {2, 2},   {2, 2},
    {2, 3},   {1, 5},   {3, 2},   {3, 2},   {4, 0},   {3, 3},   {3, 3},   {3, 3},   {3, 4},
    {3, 4},   {3, 4},   {3, 5},   {4, 3},   {4, 3},   {5, 2},   {4, 4},   {4, 4},   {4, 5},
    {4, 5},   {5, 4},   {5, 4},   {5, 4},   {6, 3},   {5, 5},   {5, 5},   {5, 6},   {4, 8},
    {6, 5},   {6, 5},   {6, 5},   {6, 6},   {6, 6},   {6, 6},   {6, 7},   {5, 9},   {7, 6},
    {7, 6},   {8, 4},   {7, 7},   {7, 7},   {7, 7},   {7, 8},   {7, 8},   {7, 8},   {7, 9},
    {8, 7},   {8, 7},   {9, 6},   {8, 8},   {8, 8},   {8, 9},   {8, 9},   {9, 8},   {9, 8},
    {9, 8},   {10, 7},  {9, 9},   {9, 9},   {9, 10},  {8, 12},  {10, 9},  {10, 9},  {10, 9},
    {10, 10}, {10, 10}, {10, 10}, {10, 11}, {9, 13},  {11, 10}, {11, 10}, {12, 8},  {11, 11},
    {11, 11}, {11, 11}, {11, 12}, {11, 12}, {11, 12}, {11, 13}, {12, 11}, {12, 11}, {13, 10},
    {12, 12}, {12, 12}, {12, 13}, {12, 13}, {13, 12}, {13, 12}, {13, 12}, {14, 11}, {13, 13},
    {13, 13}, {13, 14}, {12, 16}, {14, 13}, {14, 13}, {14, 13}, {14, 14}, {14, 14}, {14, 14},
    {14, 15}, {13, 17}, {15, 14}, {15, 14}, {16, 12}, {15, 15}, {15, 15}, {15, 15}, {15, 16},
    {15, 16}, {15, 16}, {15, 17}, {16, 15}, {16, 15}, {17, 14}, {16, 16}, {16, 16}, {16, 17},
    {16, 17}, {17, 16}, {17, 16}, {17, 16}, {18, 15}, {17, 17}, {17, 17}, {17, 18}, {16, 20},
    {18, 17}, {18, 17}, {18, 17}, {18, 18}, {18, 18}, {18, 18}, {18, 19}, {17, 21}, {19, 18},
    {19, 18}, {20, 16}, {19, 19}, {19, 19}, {19, 19}, {19, 20}, {19, 20}, {19, 20}, {19, 21},
    {20, 19}, {20, 19}, {21, 18}, {20, 20}, {20, 20}, {20, 21}, {20, 21}, {21, 20}, {21, 20},
    {21, 20}, {22, 19}, {21, 21}, {21, 21}, {21, 22}, {20, 24}, {22, 21}, {22, 21}, {22, 21},
    {22, 22}, {22, 22}, {22, 22}, {22, 23}, {21, 25}, {23, 22}, {23, 22}, {24, 20}, {23, 23},
    {23, 23}, {23, 23}, {23, 24}, {23, 24}, {23, 24}, {23, 25}, {24, 23}, {24, 23}, {25, 22},
    {24, 24}, {24, 24}, {24, 25}, {24, 25}, {25, 24}, {25, 24}, {25, 24}, {26, 23}, {25, 25},
    {25, 25}, {25, 26}, {24, 28}, {26, 25}, {26, 25}, {26, 25}, {26, 26}, {26, 26}, {26, 26},
    {26, 27}, {25, 29}, {27, 26}, {27, 26}, {28, 24}, {27, 27}, {27, 27}, {27, 27}, {27, 28},
    {27, 28}, {27, 28}, {27, 29}, {28, 27}, {28, 27}, {29, 26}, {28, 28}, {28, 28}, {28, 29},
    {28, 29}, {29, 28}, {29, 28}, {29, 28}, {30, 27}, {29, 29}, {29, 29}, {29, 30}, {29, 30},
    {30, 29}, {30, 29}, {30, 29}, {30, 30}, {30, 30}, {30, 30}, {30, 31}, {30, 31}, {31, 30},
    {31, 30}, {31, 30}, {31, 31}, {31, 31},
};
static const unsigned char stb__OMatch6[256][2] = {
    {0, 0},   {0, 1},   {1, 0},   {1, 1},   {1, 1},   {1, 2},   {2, 1},   {2, 2},   {2, 2},
    {2, 3},   {3, 2},   {3, 3},   {3, 3},   {3, 4},   {4, 3},   {4, 4},   {4, 4},   {4, 5},
    {5, 4},   {5, 5},   {5, 5},   {5, 6},   {6, 5},   {6, 6},   {6, 6},   {6, 7},   {7, 6},
    {7, 7},   {7, 7},   {7, 8},   {8, 7},   {8, 8},   {8, 8},   {8, 9},   {9, 8},   {9, 9},
    {9, 9},   {9, 10},  {10, 9},  {10, 10}, {10, 10}, {10, 11}, {11, 10}, {8, 16},  {11, 11},
    {11, 12}, {12, 11}, {9, 17},  {12, 12}, {12, 13}, {13, 12}, {11, 16}, {13, 13}, {13, 14},
    {14, 13}, {12, 17}, {14, 14}, {14, 15}, {15, 14}, {14, 16}, {15, 15}, {15, 16}, {16, 14},
    {16, 15}, {17, 14}, {16, 16}, {16, 17}, {17, 16}, {18, 15}, {17, 17}, {17, 18}, {18, 17},
    {20, 14}, {18, 18}, {18, 19}, {19, 18}, {21, 15}, {19, 19}, {19, 20}, {20, 19}, {20, 20},
    {20, 20}, {20, 21}, {21, 20}, {21, 21}, {21, 21}, {21, 22}, {22, 21}, {22, 22}, {22, 22},
    {22, 23}, {23, 22}, {23, 23}, {23, 23}, {23, 24}, {24, 23}, {24, 24}, {24, 24}, {24, 25},
    {25, 24}, {25, 25}, {25, 25}, {25, 26}, {26, 25}, {26, 26}, {26, 26}, {26, 27}, {27, 26},
    {24, 32}, {27, 27}, {27, 28}, {28, 27}, {25, 33}, {28, 28}, {28, 29}, {29, 28}, {27, 32},
    {29, 29}, {29, 30}, {30, 29}, {28, 33}, {30, 30}, {30, 31}, {31, 30}, {30, 32}, {31, 31},
    {31, 32}, {32, 30}, {32, 31}, {33, 30}, {32, 32}, {32, 33}, {33, 32}, {34, 31}, {33, 33},
    {33, 34}, {34, 33}, {36, 30}, {34, 34}, {34, 35}, {35, 34}, {37, 31}, {35, 35}, {35, 36},
    {36, 35}, {36, 36}, {36, 36}, {36, 37}, {37, 36}, {37, 37}, {37, 37}, {37, 38}, {38, 37},
    {38, 38}, {38, 38}, {38, 39}, {39, 38}, {39, 39}, {39, 39}, {39, 40}, {40, 39}, {40, 40},
    {40, 40}, {40, 41}, {41, 40}, {41, 41}, {41, 41}, {41, 42}, {42, 41}, {42, 42}, {42, 42},
    {42, 43}, {43, 42}, {40, 48}, {43, 43}, {43, 44}, {44, 43}, {41, 49}, {44, 44}, {44, 45},
    {45, 44}, {43, 48}, {45, 45}, {45, 46}, {46, 45}, {44, 49}, {46, 46}, {46, 47}, {47, 46},
    {46, 48}, {47, 47}, {47, 48}, {48, 46}, {48, 47}, {49, 46}, {48, 48}, {48, 49}, {49, 48},
    {50, 47}, {49, 49}, {49, 50}, {50, 49}, {52, 46}, {50, 50}, {50, 51}, {51, 50}, {53, 47},
    {51, 51}, {51, 52}, {52, 51}, {52, 52}, {52, 52}, {52, 53}, {53, 52}, {53, 53}, {53, 53},
    {53, 54}, {54, 53}, {54, 54}, {54, 54}, {54, 55}, {55, 54}, {55, 55}, {55, 55}, {55, 56},
    {56, 55}, {56, 56}, {56, 56}, {56, 57}, {57, 56}, {57, 57}, {57, 57}, {57, 58}, {58, 57},
    {58, 58}, {58, 58}, {58, 59}, {59, 58}, {59, 59}, {59, 59}, {59, 60}, {60, 59}, {60, 60},
    {60, 60}, {60, 61}, {61, 60}, {61, 61}, {61, 61}, {61, 62}, {62, 61}, {62, 62}, {62, 62},
    {62, 63}, {63, 62}, {63, 63}, {63, 63},
};

static int stb__Mul8Bit(int a, int b) {
    int t = a * b + 128;
    return (t + (t >> 8)) >> 8;
}

static void stb__From16Bit(unsigned char* out, unsigned short v) {
    int rv = (v & 0xf800) >> 11;
    int gv = (v & 0x07e0) >> 5;
    int bv = (v & 0x001f) >> 0;

    // expand to 8 bits via bit replication
    out[0] = static_cast<unsigned char>((rv * 33) >> 2);
    out[1] = static_cast<unsigned char>((gv * 65) >> 4);
    out[2] = static_cast<unsigned char>((bv * 33) >> 2);
    out[3] = 0;
}

static unsigned short stb__As16Bit(int r, int g, int b) {
    return (unsigned short)((stb__Mul8Bit(r, 31) << 11) + (stb__Mul8Bit(g, 63) << 5) +
                            stb__Mul8Bit(b, 31));
}

// linear interpolation at 1/3 point between a and b, using desired rounding
// type
static int stb__Lerp13(int a, int b) {
#ifdef STB_DXT_USE_ROUNDING_BIAS
    // with rounding bias
    return a + stb__Mul8Bit(b - a, 0x55);
#else
    // without rounding bias
    // replace "/ 3" by "* 0xaaab) >> 17" if your compiler sucks or you really
    // need every ounce of speed.
    return (2 * a + b) / 3;
#endif
}

// linear interpolation at 1/2 point between a and b
static int stb__Lerp12(int a, int b) {
    return (a + b) / 2;
}

// lerp RGB color
static void stb__Lerp13RGB(unsigned char* out, unsigned char* p1, unsigned char* p2) {
    out[0] = (unsigned char)stb__Lerp13(p1[0], p2[0]);
    out[1] = (unsigned char)stb__Lerp13(p1[1], p2[1]);
    out[2] = (unsigned char)stb__Lerp13(p1[2], p2[2]);
}

static void stb__Lerp12RGB(unsigned char* out, unsigned char* p1, unsigned char* p2) {
    out[0] = (unsigned char)stb__Lerp12(p1[0], p2[0]);
    out[1] = (unsigned char)stb__Lerp12(p1[1], p2[1]);
    out[2] = (unsigned char)stb__Lerp12(p1[2], p2[2]);
}

/****************************************************************************/

static void stb__Eval4Colors(unsigned char* color, unsigned short c0, unsigned short c1) {
    stb__From16Bit(color + 0, c0);
    stb__From16Bit(color + 4, c1);
    stb__Lerp13RGB(color + 8, color + 0, color + 4);
    stb__Lerp13RGB(color + 12, color + 4, color + 0);
}

static void stb__Eval3Colors(unsigned char* color, unsigned short c0, unsigned short c1) {
    stb__From16Bit(color + 0, c0);
    stb__From16Bit(color + 4, c1);
    stb__Lerp12RGB(color + 8, color + 0, color + 4);
}

// The color matching function
static unsigned int stb__MatchColorsBlock(unsigned char* block, unsigned char* color) {
    unsigned int mask = 0;
    int dirr = color[0 * 4 + 0] - color[1 * 4 + 0];
    int dirg = color[0 * 4 + 1] - color[1 * 4 + 1];
    int dirb = color[0 * 4 + 2] - color[1 * 4 + 2];
    int dots[16];
    int stops[4];
    int i;
    int c0Point, halfPoint, c3Point;

    for (i = 0; i < 16; i++)
        dots[i] = block[i * 4 + 0] * dirr + block[i * 4 + 1] * dirg + block[i * 4 + 2] * dirb;

    for (i = 0; i < 4; i++)
        stops[i] = color[i * 4 + 0] * dirr + color[i * 4 + 1] * dirg + color[i * 4 + 2] * dirb;

    // think of the colors as arranged on a line; project point onto that line,
    // then choose next color out of available ones. we compute the crossover
    // points for "best color in top half"/"best in bottom half" and then the same
    // inside that subinterval.
    //
    // relying on this 1d approximation isn't always optimal in terms of euclidean
    // distance, but it's very close and a lot faster.
    // http://cbloomrants.blogspot.com/2008/12/12-08-08-dxtc-summary.html

    c0Point = (stops[1] + stops[3]);
    halfPoint = (stops[3] + stops[2]);
    c3Point = (stops[2] + stops[0]);

    for (i = 15; i >= 0; i--) {
        int dot = dots[i] * 2;
        mask <<= 2;

        if (dot < halfPoint)
            mask |= (dot < c0Point) ? 1 : 3;
        else
            mask |= (dot < c3Point) ? 2 : 0;
    }

    return mask;
}

static unsigned int stb__MatchColorsAlphaBlock(unsigned char* block, unsigned char* color) {
    unsigned int mask = 0;
    int dirr = color[0 * 4 + 0] - color[1 * 4 + 0];
    int dirg = color[0 * 4 + 1] - color[1 * 4 + 1];
    int dirb = color[0 * 4 + 2] - color[1 * 4 + 2];
    int dots[16];
    int stops[3];
    int i;
    int c0Point, c2Point;

    for (i = 0; i < 16; i++)
        dots[i] = block[i * 4 + 0] * dirr + block[i * 4 + 1] * dirg + block[i * 4 + 2] * dirb;

    for (i = 0; i < 3; i++)
        stops[i] = color[i * 4 + 0] * dirr + color[i * 4 + 1] * dirg + color[i * 4 + 2] * dirb;

    c0Point = (stops[1] + stops[2]);
    c2Point = (stops[2] + stops[0]);

    for (i = 15; i >= 0; i--) {
        int dot = dots[i] * 2;
        mask <<= 2;

        if (block[i * 4 + 3] == 0)
            mask |= 3;
        else if (dot < c2Point)
            mask |= (dot < c0Point) ? 0 : 2;
        else
            mask |= (dot < c0Point) ? 1 : 0;
    }

    return mask;
}

static void stb__ReorderColors(unsigned short* pmax16, unsigned short* pmin16) {
    if (*pmin16 < *pmax16) {
        unsigned short t = *pmin16;
        *pmin16 = *pmax16;
        *pmax16 = t;
    }
}

static void stb__FinalizeColors(unsigned short* pmax16, unsigned short* pmin16,
                                unsigned int* pmask) {
    if (*pmax16 < *pmin16) {
        unsigned short t = *pmin16;
        *pmin16 = *pmax16;
        *pmax16 = t;
        *pmask ^= 0x55555555;
    }
}

// The color optimization function. (Clever code, part 1)
static void stb__OptimizeColorsBlock(unsigned char* block, unsigned short* pmax16,
                                     unsigned short* pmin16) {
    int mind, maxd;
    unsigned char *minp, *maxp;
    double magn;
    int v_r, v_g, v_b;
    static const int nIterPower = 4;
    float covf[6], vfr, vfg, vfb;

    // determine color distribution
    int cov[6];
    int mu[3], min[3], max[3];
    int ch, i, iter;

    for (ch = 0; ch < 3; ch++) {
        const unsigned char* bp = ((const unsigned char*)block) + ch;
        int muv, minv, maxv;

        muv = minv = maxv = bp[0];
        for (i = 4; i < 64; i += 4) {
            muv += bp[i];
            if (bp[i] < minv)
                minv = bp[i];
            else if (bp[i] > maxv)
                maxv = bp[i];
        }

        mu[ch] = (muv + 8) >> 4;
        min[ch] = minv;
        max[ch] = maxv;
    }

    // determine covariance matrix
    for (i = 0; i < 6; i++)
        cov[i] = 0;

    for (i = 0; i < 16; i++) {
        int r = block[i * 4 + 0] - mu[0];
        int g = block[i * 4 + 1] - mu[1];
        int b = block[i * 4 + 2] - mu[2];

        cov[0] += r * r;
        cov[1] += r * g;
        cov[2] += r * b;
        cov[3] += g * g;
        cov[4] += g * b;
        cov[5] += b * b;
    }

    // convert covariance matrix to float, find principal axis via power iter
    for (i = 0; i < 6; i++)
        covf[i] = static_cast<float>(cov[i]) / 255.0f;

    vfr = (float)(max[0] - min[0]);
    vfg = (float)(max[1] - min[1]);
    vfb = (float)(max[2] - min[2]);

    for (iter = 0; iter < nIterPower; iter++) {
        float r = vfr * covf[0] + vfg * covf[1] + vfb * covf[2];
        float g = vfr * covf[1] + vfg * covf[3] + vfb * covf[4];
        float b = vfr * covf[2] + vfg * covf[4] + vfb * covf[5];

        vfr = r;
        vfg = g;
        vfb = b;
    }

    magn = STBD_FABS(vfr);
    if (STBD_FABS(vfg) > magn)
        magn = STBD_FABS(vfg);
    if (STBD_FABS(vfb) > magn)
        magn = STBD_FABS(vfb);

    if (magn < 4.0f) { // too small, default to luminance
        v_r = 299;     // JPEG YCbCr luma coefs, scaled by 1000.
        v_g = 587;
        v_b = 114;
    } else {
        magn = 512.0 / magn;
        v_r = (int)(vfr * magn);
        v_g = (int)(vfg * magn);
        v_b = (int)(vfb * magn);
    }

    minp = maxp = block;
    mind = maxd = block[0] * v_r + block[1] * v_g + block[2] * v_b;
    // Pick colors at extreme points
    for (i = 1; i < 16; i++) {
        int dot = block[i * 4 + 0] * v_r + block[i * 4 + 1] * v_g + block[i * 4 + 2] * v_b;

        if (dot < mind) {
            mind = dot;
            minp = block + i * 4;
        }

        if (dot > maxd) {
            maxd = dot;
            maxp = block + i * 4;
        }
    }

    *pmax16 = stb__As16Bit(maxp[0], maxp[1], maxp[2]);
    *pmin16 = stb__As16Bit(minp[0], minp[1], minp[2]);
    stb__ReorderColors(pmax16, pmin16);
}

static void stb__OptimizeColorsAlphaBlock(unsigned char* block, unsigned short* pmax16,
                                          unsigned short* pmin16) {
    int mind, maxd;
    unsigned char *minp, *maxp;
    double magn;
    int v_r, v_g, v_b;
    static const int nIterPower = 4;
    float covf[6], vfr, vfg, vfb;

    // determine color distribution
    int cov[6];
    int mu[3], min[3], max[3];
    int ch, i, iter;

    for (ch = 0; ch < 3; ch++) {
        const unsigned char* bp = ((const unsigned char*)block) + ch;
        int muv = 0, minv = 256, maxv = -1;
        int num = 0;

        for (i = 0; i < 64; i += 4) {
            if (bp[3 - ch] == 0) {
                continue;
            }

            muv += bp[i];
            if (bp[i] < minv)
                minv = bp[i];
            else if (bp[i] > maxv)
                maxv = bp[i];

            num++;
        }

        mu[ch] = num > 0 ? (muv + 8) / num : 0;
        min[ch] = minv;
        max[ch] = maxv;
    }

    // determine covariance matrix
    for (i = 0; i < 6; i++)
        cov[i] = 0;

    for (i = 0; i < 16; i++) {
        if (block[i * 4 + 3] == 0) {
            continue;
        }

        int r = block[i * 4 + 0] - mu[0];
        int g = block[i * 4 + 1] - mu[1];
        int b = block[i * 4 + 2] - mu[2];

        cov[0] += r * r;
        cov[1] += r * g;
        cov[2] += r * b;
        cov[3] += g * g;
        cov[4] += g * b;
        cov[5] += b * b;
    }

    // convert covariance matrix to float, find principal axis via power iter
    for (i = 0; i < 6; i++)
        covf[i] = static_cast<float>(cov[i]) / 255.0f;

    vfr = (float)(max[0] - min[0]);
    vfg = (float)(max[1] - min[1]);
    vfb = (float)(max[2] - min[2]);

    for (iter = 0; iter < nIterPower; iter++) {
        float r = vfr * covf[0] + vfg * covf[1] + vfb * covf[2];
        float g = vfr * covf[1] + vfg * covf[3] + vfb * covf[4];
        float b = vfr * covf[2] + vfg * covf[4] + vfb * covf[5];

        vfr = r;
        vfg = g;
        vfb = b;
    }

    magn = STBD_FABS(vfr);
    if (STBD_FABS(vfg) > magn)
        magn = STBD_FABS(vfg);
    if (STBD_FABS(vfb) > magn)
        magn = STBD_FABS(vfb);

    if (magn < 4.0f) { // too small, default to luminance
        v_r = 299;     // JPEG YCbCr luma coefs, scaled by 1000.
        v_g = 587;
        v_b = 114;
    } else {
        magn = 512.0 / magn;
        v_r = (int)(vfr * magn);
        v_g = (int)(vfg * magn);
        v_b = (int)(vfb * magn);
    }

    minp = maxp = NULL;
    mind = 0x7fffffff;
    maxd = -0x80000000;

    // Pick colors at extreme points
    for (i = 0; i < 16; i++) {
        if (block[i * 4 + 3] == 0) {
            continue;
        }

        int dot = block[i * 4 + 0] * v_r + block[i * 4 + 1] * v_g + block[i * 4 + 2] * v_b;

        if (dot < mind) {
            mind = dot;
            minp = block + i * 4;
        }

        if (dot > maxd) {
            maxd = dot;
            maxp = block + i * 4;
        }
    }

    if (!maxp) {
        // all alpha, no color
        *pmin16 = 0xffff;
        *pmax16 = 0;
    } else {
        // endpoint colors found
        *pmax16 = stb__As16Bit(maxp[0], maxp[1], maxp[2]);
        *pmin16 = stb__As16Bit(minp[0], minp[1], minp[2]);

        if (*pmax16 == *pmin16) {
            // modify the endpoints to indicate presence of an alpha block
            if (*pmax16 > 0) {
                (*pmax16)--;
            } else {
                (*pmin16)++;
            }
        }

        stb__ReorderColors(pmax16, pmin16);
    }
}

static const float stb__midpoints5[32] = {
    0.015686f, 0.047059f, 0.078431f, 0.111765f, 0.145098f, 0.176471f, 0.207843f, 0.241176f,
    0.274510f, 0.305882f, 0.337255f, 0.370588f, 0.403922f, 0.435294f, 0.466667f, 0.5f,
    0.533333f, 0.564706f, 0.596078f, 0.629412f, 0.662745f, 0.694118f, 0.725490f, 0.758824f,
    0.792157f, 0.823529f, 0.854902f, 0.888235f, 0.921569f, 0.952941f, 0.984314f, 1.0f};

static const float stb__midpoints6[64] = {
    0.007843f, 0.023529f, 0.039216f, 0.054902f, 0.070588f, 0.086275f, 0.101961f, 0.117647f,
    0.133333f, 0.149020f, 0.164706f, 0.180392f, 0.196078f, 0.211765f, 0.227451f, 0.245098f,
    0.262745f, 0.278431f, 0.294118f, 0.309804f, 0.325490f, 0.341176f, 0.356863f, 0.372549f,
    0.388235f, 0.403922f, 0.419608f, 0.435294f, 0.450980f, 0.466667f, 0.482353f, 0.500000f,
    0.517647f, 0.533333f, 0.549020f, 0.564706f, 0.580392f, 0.596078f, 0.611765f, 0.627451f,
    0.643137f, 0.658824f, 0.674510f, 0.690196f, 0.705882f, 0.721569f, 0.737255f, 0.754902f,
    0.772549f, 0.788235f, 0.803922f, 0.819608f, 0.835294f, 0.850980f, 0.866667f, 0.882353f,
    0.898039f, 0.913725f, 0.929412f, 0.945098f, 0.960784f, 0.976471f, 0.992157f, 1.0f};

static unsigned short stb__Quantize5(float x) {
    unsigned short q;
    x = x < 0 ? 0 : x > 1 ? 1 : x; // saturate
    q = (unsigned short)(x * 31);
    q += (x > stb__midpoints5[q]);
    return q;
}

static unsigned short stb__Quantize6(float x) {
    unsigned short q;
    x = x < 0 ? 0 : x > 1 ? 1 : x; // saturate
    q = (unsigned short)(x * 63);
    q += (x > stb__midpoints6[q]);
    return q;
}

// The refinement function. (Clever code, part 2)
// Tries to optimize colors to suit block contents better.
// (By solving a least squares system via normal equations+Cramer's rule)
static int stb__RefineBlock(unsigned char* block, unsigned short* pmax16, unsigned short* pmin16,
                            unsigned int mask) {
    static const int w1Tab[4] = {3, 0, 2, 1};
    static const int prods[4] = {0x090000, 0x000900, 0x040102, 0x010402};
    // ^some magic to save a lot of multiplies in the accumulating loop...
    // (precomputed products of weights for least squares system, accumulated
    // inside one 32-bit register)

    float f;
    unsigned short oldMin, oldMax, min16, max16;
    int i, akku = 0, xx, xy, yy;
    int At1_r, At1_g, At1_b;
    int At2_r, At2_g, At2_b;
    unsigned int cm = mask;

    oldMin = *pmin16;
    oldMax = *pmax16;

    if ((mask ^ (mask << 2)) < 4) // all pixels have the same index?
    {
        // yes, linear system would be singular; solve using optimal
        // single-color match on average color
        int r = 8, g = 8, b = 8;
        for (i = 0; i < 16; ++i) {
            r += block[i * 4 + 0];
            g += block[i * 4 + 1];
            b += block[i * 4 + 2];
        }

        r >>= 4;
        g >>= 4;
        b >>= 4;

        max16 = static_cast<unsigned short>((stb__OMatch5[r][0] << 11) | (stb__OMatch6[g][0] << 5) |
                                            stb__OMatch5[b][0]);
        min16 = static_cast<unsigned short>((stb__OMatch5[r][1] << 11) | (stb__OMatch6[g][1] << 5) |
                                            stb__OMatch5[b][1]);
    } else {
        At1_r = At1_g = At1_b = 0;
        At2_r = At2_g = At2_b = 0;
        for (i = 0; i < 16; ++i, cm >>= 2) {
            int step = cm & 3;
            int w1 = w1Tab[step];
            int r = block[i * 4 + 0];
            int g = block[i * 4 + 1];
            int b = block[i * 4 + 2];

            akku += prods[step];
            At1_r += w1 * r;
            At1_g += w1 * g;
            At1_b += w1 * b;
            At2_r += r;
            At2_g += g;
            At2_b += b;
        }

        At2_r = 3 * At2_r - At1_r;
        At2_g = 3 * At2_g - At1_g;
        At2_b = 3 * At2_b - At1_b;

        // extract solutions and decide solvability
        xx = akku >> 16;
        yy = (akku >> 8) & 0xff;
        xy = (akku >> 0) & 0xff;

        f = 3.0f / 255.0f / static_cast<float>(xx * yy - xy * xy);

        max16 = static_cast<unsigned short>(
            stb__Quantize5(static_cast<float>(At1_r * yy - At2_r * xy) * f) << 11);
        max16 |= static_cast<unsigned short>(
            stb__Quantize6(static_cast<float>(At1_g * yy - At2_g * xy) * f) << 5);
        max16 |= static_cast<unsigned short>(
            stb__Quantize5(static_cast<float>(At1_b * yy - At2_b * xy) * f) << 0);

        min16 = static_cast<unsigned short>(
            stb__Quantize5(static_cast<float>(At2_r * xx - At1_r * xy) * f) << 11);
        min16 |= static_cast<unsigned short>(
            stb__Quantize6(static_cast<float>(At2_g * xx - At1_g * xy) * f) << 5);
        min16 |= static_cast<unsigned short>(
            stb__Quantize5(static_cast<float>(At2_b * xx - At1_b * xy) * f) << 0);
    }

    *pmin16 = min16;
    *pmax16 = max16;
    stb__ReorderColors(pmax16, pmin16);

    return oldMin != min16 || oldMax != max16;
}

// Color block compression
static void stb__CompressColorBlock(unsigned char* dest, unsigned char* block, int alpha,
                                    int mode) {
    unsigned int mask;
    int i;
    int refinecount;
    unsigned short max16, min16;
    unsigned char color[4 * 4];

    refinecount = (mode & STB_DXT_HIGHQUAL) ? 2 : 1;

    // check if block is constant
    for (i = 1; i < 16; i++)
        if (((unsigned int*)block)[i] != ((unsigned int*)block)[0])
            break;

    if (i == 16 && block[3] == 0 && alpha) { // constant alpha
        mask = 0xffffffff;
        max16 = 0;
        min16 = 0xffff;
    } else if (i == 16) { // constant color
        int r = block[0], g = block[1], b = block[2];
        mask = 0xaaaaaaaa;
        max16 = static_cast<unsigned short>((stb__OMatch5[r][0] << 11) | (stb__OMatch6[g][0] << 5) |
                                            stb__OMatch5[b][0]);
        min16 = static_cast<unsigned short>((stb__OMatch5[r][1] << 11) | (stb__OMatch6[g][1] << 5) |
                                            stb__OMatch5[b][1]);
    } else if (alpha) {
        stb__OptimizeColorsAlphaBlock(block, &max16, &min16);
        stb__Eval3Colors(color, max16, min16);
        mask = stb__MatchColorsAlphaBlock(block, color);
    } else {
        // first step: PCA+map along principal axis
        stb__OptimizeColorsBlock(block, &max16, &min16);
        if (max16 != min16) {
            stb__Eval4Colors(color, max16, min16);
            mask = stb__MatchColorsBlock(block, color);
        } else
            mask = 0;

        // third step: refine (multiple times if requested)
        for (i = 0; i < refinecount; i++) {
            unsigned int lastmask = mask;

            if (stb__RefineBlock(block, &max16, &min16, mask)) {
                if (max16 != min16) {
                    stb__Eval4Colors(color, max16, min16);
                    mask = stb__MatchColorsBlock(block, color);
                } else {
                    mask = 0;
                    break;
                }
            }

            if (mask == lastmask)
                break;
        }
    }

    // write the color block
    if (!alpha)
        stb__FinalizeColors(&max16, &min16, &mask);

    dest[0] = (unsigned char)(max16);
    dest[1] = (unsigned char)(max16 >> 8);
    dest[2] = (unsigned char)(min16);
    dest[3] = (unsigned char)(min16 >> 8);
    dest[4] = (unsigned char)(mask);
    dest[5] = (unsigned char)(mask >> 8);
    dest[6] = (unsigned char)(mask >> 16);
    dest[7] = (unsigned char)(mask >> 24);
}

// Alpha block compression (this is easy for a change)
static void stb__CompressAlphaBlock(unsigned char* dest, unsigned char* src, int stride) {
    int i, dist, bias, dist4, dist2, bits, mask;

    // find min/max color
    int mn, mx;
    mn = mx = src[0];

    for (i = 1; i < 16; i++) {
        if (src[i * stride] < mn)
            mn = src[i * stride];
        else if (src[i * stride] > mx)
            mx = src[i * stride];
    }

    // encode them
    dest[0] = (unsigned char)mx;
    dest[1] = (unsigned char)mn;
    dest += 2;

    // determine bias and emit color indices
    // given the choice of mx/mn, these indices are optimal:
    // http://fgiesen.wordpress.com/2009/12/15/dxt5-alpha-block-index-determination/
    dist = mx - mn;
    dist4 = dist * 4;
    dist2 = dist * 2;
    bias = (dist < 8) ? (dist - 1) : (dist / 2 + 2);
    bias -= mn * 7;
    bits = 0, mask = 0;

    for (i = 0; i < 16; i++) {
        int a = src[i * stride] * 7 + bias;
        int ind, t;

        // select index. this is a "linear scale" lerp factor between 0 (val=min)
        // and 7 (val=max).
        t = (a >= dist4) ? -1 : 0;
        ind = t & 4;
        a -= dist4 & t;
        t = (a >= dist2) ? -1 : 0;
        ind += t & 2;
        a -= dist2 & t;
        ind += (a >= dist);

        // turn linear scale into DXT index (0/1 are extremal pts)
        ind = -ind & 7;
        ind ^= (2 > ind);

        // write index
        mask |= ind << bits;
        if ((bits += 3) >= 8) {
            *dest++ = (unsigned char)mask;
            mask >>= 8;
            bits -= 8;
        }
    }
}

void stb_compress_bc1_block(unsigned char* dest, const unsigned char* src, int alpha, int mode) {
    stb__CompressColorBlock(dest, (unsigned char*)src, alpha, mode);
}

void stb_compress_bc3_block(unsigned char* dest, const unsigned char* src, int mode) {
    unsigned char data[16][4];
    int i;

    stb__CompressAlphaBlock(dest, (unsigned char*)src + 3, 4);
    dest += 8;
    // make a new copy of the data in which alpha is opaque,
    // because code uses a fast test for color constancy
    memcpy(data, src, 4 * 16);
    for (i = 0; i < 16; ++i)
        data[i][3] = 255;
    src = &data[0][0];

    stb__CompressColorBlock(dest, (unsigned char*)src, 0, mode);
}
