BcnDecode.c

/*
 * The Python Imaging Library
 *
 * decoder for DXTn-compressed data
 *
 * Format documentation:
 *   https://web.archive.org/web/20170802060935/http://oss.sgi.com/projects/ogl-sample/registry/EXT/texture_compression_s3tc.txt
 *
 * The contents of this file are in the public domain (CC0)
 * Full text of the CC0 license:
 *   https://creativecommons.org/publicdomain/zero/1.0/
 */

#include "Imaging.h"

#include "Bcn.h"

typedef struct {
    UINT8 r, g, b, a;
} rgba;

typedef struct {
    UINT8 l;
} lum;

typedef struct {
    UINT16 c0, c1;
    UINT32 lut;
} bc1_color;

typedef struct {
    UINT8 a0, a1;
    UINT8 lut[6];
} bc3_alpha;

typedef struct {
    INT8 a0, a1;
    UINT8 lut[6];
} bc5s_alpha;

#define LOAD16(p) (p)[0] | ((p)[1] << 8)

#define LOAD32(p) (p)[0] | ((p)[1] << 8) | ((p)[2] << 16) | ((p)[3] << 24)

static void
bc1_color_load(bc1_color *dst, const UINT8 *src) {
    dst->c0 = LOAD16(src);
    dst->c1 = LOAD16(src + 2);
    dst->lut = LOAD32(src + 4);
}

static rgba
decode_565(UINT16 x) {
    rgba c;
    int r, g, b;
    r = (x & 0xf800) >> 8;
    r |= r >> 5;
    c.r = r;
    g = (x & 0x7e0) >> 3;
    g |= g >> 6;
    c.g = g;
    b = (x & 0x1f) << 3;
    b |= b >> 5;
    c.b = b;
    c.a = 0xff;
    return c;
}

static void
decode_bc1_color(rgba *dst, const UINT8 *src, int separate_alpha) {
    bc1_color col;
    rgba p[4];
    int n, cw;
    UINT16 r0, g0, b0, r1, g1, b1;
    bc1_color_load(&col, src);

    p[0] = decode_565(col.c0);
    r0 = p[0].r;
    g0 = p[0].g;
    b0 = p[0].b;
    p[1] = decode_565(col.c1);
    r1 = p[1].r;
    g1 = p[1].g;
    b1 = p[1].b;


    /* NOTE: BC2 and BC3 reuse BC1 color blocks but always act like c0 > c1 */
    if (col.c0 > col.c1 || separate_alpha) {
        p[2].r = (2 * r0 + 1 * r1) / 3;
        p[2].g = (2 * g0 + 1 * g1) / 3;
        p[2].b = (2 * b0 + 1 * b1) / 3;
        p[2].a = 0xff;
        p[3].r = (1 * r0 + 2 * r1) / 3;
        p[3].g = (1 * g0 + 2 * g1) / 3;
        p[3].b = (1 * b0 + 2 * b1) / 3;
        p[3].a = 0xff;
    } else {
        p[2].r = (r0 + r1) / 2;
        p[2].g = (g0 + g1) / 2;
        p[2].b = (b0 + b1) / 2;
        p[2].a = 0xff;
        p[3].r = 0;
        p[3].g = 0;
        p[3].b = 0;
        p[3].a = 0;
    }
    for (n = 0; n < 16; n++) {
        cw = 3 & (col.lut >> (2 * n));
        dst[n] = p[cw];
    }
}

static void
decode_bc3_alpha(char *dst, const UINT8 *src, int stride, int o, int sign) {
    UINT16 a0, a1;
    UINT8 a[8];
    int n, lut1, lut2, aw;
    if (sign == 1) {
        bc5s_alpha b;
        memcpy(&b, src, sizeof(bc5s_alpha));
        a0 = b.a0 + 128;
        a1 = b.a1 + 128;
        lut1 = b.lut[0] | (b.lut[1] << 8) | (b.lut[2] << 16);
        lut2 = b.lut[3] | (b.lut[4] << 8) | (b.lut[5] << 16);
    } else {
        bc3_alpha b;
        memcpy(&b, src, sizeof(bc3_alpha));
        a0 = b.a0;
        a1 = b.a1;
        lut1 = b.lut[0] | (b.lut[1] << 8) | (b.lut[2] << 16);
        lut2 = b.lut[3] | (b.lut[4] << 8) | (b.lut[5] << 16);
    }

    a[0] = (UINT8)a0;
    a[1] = (UINT8)a1;
    if (a0 > a1) {
        a[2] = (6 * a0 + 1 * a1) / 7;
        a[3] = (5 * a0 + 2 * a1) / 7;
        a[4] = (4 * a0 + 3 * a1) / 7;
        a[5] = (3 * a0 + 4 * a1) / 7;
        a[6] = (2 * a0 + 5 * a1) / 7;
        a[7] = (1 * a0 + 6 * a1) / 7;
    } else {
        a[2] = (4 * a0 + 1 * a1) / 5;
        a[3] = (3 * a0 + 2 * a1) / 5;
        a[4] = (2 * a0 + 3 * a1) / 5;
        a[5] = (1 * a0 + 4 * a1) / 5;
        a[6] = 0;
        a[7] = 0xff;
    }
    for (n = 0; n < 8; n++) {
        aw = 7 & (lut1 >> (3 * n));
        dst[stride * n + o] = a[aw];
    }
    for (n = 0; n < 8; n++) {
        aw = 7 & (lut2 >> (3 * n));
        dst[stride * (8 + n) + o] = a[aw];
    }
}

static void
decode_bc1_block(rgba *col, const UINT8 *src) {
    decode_bc1_color(col, src, 0);
}

static void
decode_bc2_block(rgba *col, const UINT8 *src) {
    int n, bitI, byI, av;
    decode_bc1_color(col, src + 8, 1);
    for (n = 0; n < 16; n++) {
        bitI = n * 4;
        byI = bitI >> 3;
        av = 0xf & (src[byI] >> (bitI & 7));
        av = (av << 4) | av;
        col[n].a = av;
    }
}

static void
decode_bc3_block(rgba *col, const UINT8 *src) {
    decode_bc1_color(col, src + 8, 1);
    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 3, 0);
}

static void
decode_bc4_block(lum *col, const UINT8 *src) {
    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 0, 0);
}

static void
decode_bc5_block(rgba *col, const UINT8 *src, int sign) {
    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 0, sign);
    decode_bc3_alpha((char *)col, src + 8, sizeof(col[0]), 1, sign);
}

/* BC6 and BC7 are described here:
   https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_texture_compression_bptc.txt
 */

static UINT8
get_bit(const UINT8 *src, int bit) {
    int by = bit >> 3;
    bit &= 7;
    return (src[by] >> bit) & 1;
}

static UINT8
get_bits(const UINT8 *src, int bit, int count) {
    UINT8 v;
    int x;
    int by = bit >> 3;
    bit &= 7;
    if (!count) {
        return 0;
    }
    if (bit + count <= 8) {
        v = (src[by] >> bit) & ((1 << count) - 1);
    } else {
        x = src[by] | (src[by + 1] << 8);
        v = (x >> bit) & ((1 << count) - 1);
    }
    return v;
}

/* BC7 */
typedef struct {
    char ns;
    char pb;
    char rb;
    char isb;
    char cb;
    char ab;
    char epb;
    char spb;
    char ib;
    char ib2;
} bc7_mode_info;

static const bc7_mode_info bc7_modes[] = {
    {3, 4, 0, 0, 4, 0, 1, 0, 3, 0},
    {2, 6, 0, 0, 6, 0, 0, 1, 3, 0},
    {3, 6, 0, 0, 5, 0, 0, 0, 2, 0},
    {2, 6, 0, 0, 7, 0, 1, 0, 2, 0},
    {1, 0, 2, 1, 5, 6, 0, 0, 2, 3},
    {1, 0, 2, 0, 7, 8, 0, 0, 2, 2},
    {1, 0, 0, 0, 7, 7, 1, 0, 4, 0},
    {2, 6, 0, 0, 5, 5, 1, 0, 2, 0}};

/* Subset indices:
   Table.P2, 1 bit per index */
static const UINT16 bc7_si2[] = {
    0xcccc, 0x8888, 0xeeee, 0xecc8, 0xc880, 0xfeec, 0xfec8, 0xec80, 0xc800, 0xffec,
    0xfe80, 0xe800, 0xffe8, 0xff00, 0xfff0, 0xf000, 0xf710, 0x008e, 0x7100, 0x08ce,
    0x008c, 0x7310, 0x3100, 0x8cce, 0x088c, 0x3110, 0x6666, 0x366c, 0x17e8, 0x0ff0,
    0x718e, 0x399c, 0xaaaa, 0xf0f0, 0x5a5a, 0x33cc, 0x3c3c, 0x55aa, 0x9696, 0xa55a,
    0x73ce, 0x13c8, 0x324c, 0x3bdc, 0x6996, 0xc33c, 0x9966, 0x0660, 0x0272, 0x04e4,
    0x4e40, 0x2720, 0xc936, 0x936c, 0x39c6, 0x639c, 0x9336, 0x9cc6, 0x817e, 0xe718,
    0xccf0, 0x0fcc, 0x7744, 0xee22};

/* Table.P3, 2 bits per index */
static const UINT32 bc7_si3[] = {
    0xaa685050, 0x6a5a5040, 0x5a5a4200, 0x5450a0a8, 0xa5a50000, 0xa0a05050, 0x5555a0a0,
    0x5a5a5050, 0xaa550000, 0xaa555500, 0xaaaa5500, 0x90909090, 0x94949494, 0xa4a4a4a4,
    0xa9a59450, 0x2a0a4250, 0xa5945040, 0x0a425054, 0xa5a5a500, 0x55a0a0a0, 0xa8a85454,
    0x6a6a4040, 0xa4a45000, 0x1a1a0500, 0x0050a4a4, 0xaaa59090, 0x14696914, 0x69691400,
    0xa08585a0, 0xaa821414, 0x50a4a450, 0x6a5a0200, 0xa9a58000, 0x5090a0a8, 0xa8a09050,
    0x24242424, 0x00aa5500, 0x24924924, 0x24499224, 0x50a50a50, 0x500aa550, 0xaaaa4444,
    0x66660000, 0xa5a0a5a0, 0x50a050a0, 0x69286928, 0x44aaaa44, 0x66666600, 0xaa444444,
    0x54a854a8, 0x95809580, 0x96969600, 0xa85454a8, 0x80959580, 0xaa141414, 0x96960000,
    0xaaaa1414, 0xa05050a0, 0xa0a5a5a0, 0x96000000, 0x40804080, 0xa9a8a9a8, 0xaaaaaa44,
    0x2a4a5254};

/* Anchor indices:
   Table.A2 */
static const char bc7_ai0[] = {
    15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 2,  8, 2, 2, 8,
    8,  15, 2,  8,  2,  2,  8,  8,  2,  2,  15, 15, 6,  8,  2,  8,  15, 15, 2, 8, 2, 2,
    2,  15, 15, 6,  6,  2,  6,  8,  15, 15, 2,  2,  15, 15, 15, 15, 15, 2,  2, 15};

/* Table.A3a */
static const char bc7_ai1[] = {
    3,  3,  15, 15, 8, 3,  15, 15, 8,  8,  6, 6,  6, 5,  3, 3,  3,  3,  8,  15, 3,  3,
    6,  10, 5,  8,  8, 6,  8,  5,  15, 15, 8, 15, 3, 5,  6, 10, 8,  15, 15, 3,  15, 5,
    15, 15, 15, 15, 3, 15, 5,  5,  5,  8,  5, 10, 5, 10, 8, 13, 15, 12, 3,  3};

/* Table.A3b */
static const char bc7_ai2[] = {15, 8,  8,  3,  15, 15, 3,  8,  15, 15, 15, 15, 15,
                               15, 15, 8,  15, 8,  15, 3,  15, 8,  15, 8,  3,  15,
                               6,  10, 15, 15, 10, 8,  15, 3,  15, 10, 10, 8,  9,
                               10, 6,  15, 8,  15, 3,  6,  6,  8,  15, 3,  15, 15,
                               15, 15, 15, 15, 15, 15, 15, 15, 3,  15, 15, 8};

/* Interpolation weights */
static const char bc7_weights2[] = {0, 21, 43, 64};
static const char bc7_weights3[] = {0, 9, 18, 27, 37, 46, 55, 64};
static const char bc7_weights4[] = {
    0, 4, 9, 13, 17, 21, 26, 30, 34, 38, 43, 47, 51, 55, 60, 64};

static const char *
bc7_get_weights(int n) {
    if (n == 2) {
        return bc7_weights2;
    }
    if (n == 3) {
        return bc7_weights3;
    }
    return bc7_weights4;
}

static int
bc7_get_subset(int ns, int partition, int n) {
    if (ns == 2) {
        return 1 & (bc7_si2[partition] >> n);
    }
    if (ns == 3) {
        return 3 & (bc7_si3[partition] >> (2 * n));
    }
    return 0;
}

static UINT8
expand_quantized(UINT8 v, int bits) {
    v = v << (8 - bits);
    return v | (v >> bits);
}

static void
bc7_lerp(rgba *dst, const rgba *e, int s0, int s1) {
    int t0 = 64 - s0;
    int t1 = 64 - s1;
    dst->r = (UINT8)((t0 * e[0].r + s0 * e[1].r + 32) >> 6);
    dst->g = (UINT8)((t0 * e[0].g + s0 * e[1].g + 32) >> 6);
    dst->b = (UINT8)((t0 * e[0].b + s0 * e[1].b + 32) >> 6);
    dst->a = (UINT8)((t1 * e[0].a + s1 * e[1].a + 32) >> 6);
}

static void
decode_bc7_block(rgba *col, const UINT8 *src) {
    rgba endpoints[6];
    int bit = 0, cibit, aibit;
    int mode = src[0];
    int i, j;
    int numep, cb, ab, ib, ib2, i0, i1, s;
    UINT8 index_sel, partition, rotation, val;
    const char *cw, *aw;
    const bc7_mode_info *info;

    /* mode is the number of unset bits before the first set bit: */
    if (!mode) {
        /* degenerate case when no bits set */
        for (i = 0; i < 16; i++) {
            col[i].r = col[i].g = col[i].b = 0;
            col[i].a = 255;
        }
        return;
    }
    while (!(mode & (1 << bit++)))
        ;
    mode = bit - 1;
    info = &bc7_modes[mode];
    /* color selection bits: {subset}{endpoint} */
    cb = info->cb;
    ab = info->ab;
    cw = bc7_get_weights(info->ib);
    aw = bc7_get_weights((ab && info->ib2) ? info->ib2 : info->ib);

#define LOAD(DST, N)             \
    DST = get_bits(src, bit, N); \
    bit += N;
    LOAD(partition, info->pb);
    LOAD(rotation, info->rb);
    LOAD(index_sel, info->isb);
    numep = info->ns << 1;

    /* red */
    for (i = 0; i < numep; i++) {
        LOAD(val, cb);
        endpoints[i].r = val;
    }

    /* green */
    for (i = 0; i < numep; i++) {
        LOAD(val, cb);
        endpoints[i].g = val;
    }

    /* blue */
    for (i = 0; i < numep; i++) {
        LOAD(val, cb);
        endpoints[i].b = val;
    }

    /* alpha */
    for (i = 0; i < numep; i++) {
        if (ab) {
            LOAD(val, ab);
        } else {
            val = 255;
        }
        endpoints[i].a = val;
    }

    /* p-bits */
#define ASSIGN_P(x) x = (x << 1) | val
    if (info->epb) {
        /* per endpoint */
        cb++;
        if (ab) {
            ab++;
        }
        for (i = 0; i < numep; i++) {
            LOAD(val, 1);
            ASSIGN_P(endpoints[i].r);
            ASSIGN_P(endpoints[i].g);
            ASSIGN_P(endpoints[i].b);
            if (ab) {
                ASSIGN_P(endpoints[i].a);
            }
        }
    }
    if (info->spb) {
        /* per subset */
        cb++;
        if (ab) {
            ab++;
        }
        for (i = 0; i < numep; i += 2) {
            LOAD(val, 1);
            for (j = 0; j < 2; j++) {
                ASSIGN_P(endpoints[i + j].r);
                ASSIGN_P(endpoints[i + j].g);
                ASSIGN_P(endpoints[i + j].b);
                if (ab) {
                    ASSIGN_P(endpoints[i + j].a);
                }
            }
        }
    }
#undef ASSIGN_P
#define EXPAND(x, b) x = expand_quantized(x, b)
    for (i = 0; i < numep; i++) {
        EXPAND(endpoints[i].r, cb);
        EXPAND(endpoints[i].g, cb);
        EXPAND(endpoints[i].b, cb);
        if (ab) {
            EXPAND(endpoints[i].a, ab);
        }
    }
#undef EXPAND
#undef LOAD
    cibit = bit;
    aibit = cibit + 16 * info->ib - info->ns;
    for (i = 0; i < 16; i++) {
        s = bc7_get_subset(info->ns, partition, i) << 1;
        ib = info->ib;
        if (i == 0) {
            ib--;
        } else if (info->ns == 2) {
            if (i == bc7_ai0[partition]) {
                ib--;
            }
        } else if (info->ns == 3) {
            if (i == bc7_ai1[partition]) {
                ib--;
            } else if (i == bc7_ai2[partition]) {
                ib--;
            }
        }
        i0 = get_bits(src, cibit, ib);
        cibit += ib;

        if (ab && info->ib2) {
            ib2 = info->ib2;
            if (ib2 && i == 0) {
                ib2--;
            }
            i1 = get_bits(src, aibit, ib2);
            aibit += ib2;
            if (index_sel) {
                bc7_lerp(&col[i], &endpoints[s], aw[i1], cw[i0]);
            } else {
                bc7_lerp(&col[i], &endpoints[s], cw[i0], aw[i1]);
            }
        } else {
            bc7_lerp(&col[i], &endpoints[s], cw[i0], cw[i0]);
        }
#define ROTATE(x, y) \
    val = x;         \
    x = y;           \
    y = val
        if (rotation == 1) {
            ROTATE(col[i].r, col[i].a);
        } else if (rotation == 2) {
            ROTATE(col[i].g, col[i].a);
        } else if (rotation == 3) {
            ROTATE(col[i].b, col[i].a);
        }
#undef ROTATE
    }
}

/* BC6 */
typedef struct {
    char ns;  /* number of subsets (also called regions) */
    char tr;  /* whether endpoints are delta-compressed */
    char pb;  /* partition bits */
    char epb; /* endpoint bits */
    char rb;  /* red bits (delta) */
    char gb;  /* green bits (delta) */
    char bb;  /* blue bits (delta) */
} bc6_mode_info;

static const bc6_mode_info bc6_modes[] = {
    // 00
    {2, 1, 5, 10, 5, 5, 5},
    // 01
    {2, 1, 5, 7, 6, 6, 6},
    // 10
    {2, 1, 5, 11, 5, 4, 4},
    {2, 1, 5, 11, 4, 5, 4},
    {2, 1, 5, 11, 4, 4, 5},
    {2, 1, 5, 9, 5, 5, 5},
    {2, 1, 5, 8, 6, 5, 5},
    {2, 1, 5, 8, 5, 6, 5},
    {2, 1, 5, 8, 5, 5, 6},
    {2, 0, 5, 6, 6, 6, 6},
    // 11
    {1, 0, 0, 10, 10, 10, 10},
    {1, 1, 0, 11, 9, 9, 9},
    {1, 1, 0, 12, 8, 8, 8},
    {1, 1, 0, 16, 4, 4, 4}};

/* Table.F, encoded as a sequence of bit indices */
static const UINT8 bc6_bit_packings[][75] = {
    {116, 132, 180, 0,   1,   2,   3,   4,   5,   6,   7,   8,   9,   16,  17,
     18,  19,  20,  21,  22,  23,  24,  25,  32,  33,  34,  35,  36,  37,  38,
     39,  40,  41,  48,  49,  50,  51,  52,  164, 112, 113, 114, 115, 64,  65,
     66,  67,  68,  176, 160, 161, 162, 163, 80,  81,  82,  83,  84,  177, 128,
     129, 130, 131, 96,  97,  98,  99,  100, 178, 144, 145, 146, 147, 148, 179},
    {117, 164, 165, 0,  1,   2,   3,   4,   5,   6,   176, 177, 132, 16,  17,
     18,  19,  20,  21, 22,  133, 178, 116, 32,  33,  34,  35,  36,  37,  38,
     179, 181, 180, 48, 49,  50,  51,  52,  53,  112, 113, 114, 115, 64,  65,
     66,  67,  68,  69, 160, 161, 162, 163, 80,  81,  82,  83,  84,  85,  128,
     129, 130, 131, 96, 97,  98,  99,  100, 101, 144, 145, 146, 147, 148, 149},
    {0,   1,   2,   3,   4,   5,   6,   7,   8,   9,   16,  17,  18,  19,  20,
     21,  22,  23,  24,  25,  32,  33,  34,  35,  36,  37,  38,  39,  40,  41,
     48,  49,  50,  51,  52,  10,  112, 113, 114, 115, 64,  65,  66,  67,  26,
     176, 160, 161, 162, 163, 80,  81,  82,  83,  42,  177, 128, 129, 130, 131,
     96,  97, 98,  99,  100, 178, 144, 145, 146, 147, 148, 179},
    {0,  1,   2,   3,   4,   5,   6,   7,   8,   9,   16,  17,  18,  19,  20,
     21, 22,  23,  24,  25,  32,  33,  34,  35,  36,  37,  38,  39,  40,  41,
     48, 49,  50,  51,  10,  164, 112, 113, 114, 115, 64,  65,  66,  67,  68,
     26, 160, 161, 162, 163, 80,  81,  82,  83,  42,  177, 128, 129, 130, 131,
     96, 97,  98,  99,  176, 178, 144, 145, 146, 147, 116, 179},
    {0,   1,   2,   3,   4,   5,   6,   7,   8,   9,   16,  17,  18,  19,  20,
     21,  22,  23,  24,  25,  32,  33,  34,  35,  36,  37,  38,  39,  40,  41,
     48,  49,  50,  51,  10,  132, 112, 113, 114, 115, 64,  65,  66,  67,  26,
     176, 160, 161, 162, 163, 80,  81,  82,  83,  84,  42,  128, 129, 130, 131,
     96,  97,  98,  99,  177, 178, 144, 145, 146, 147, 180, 179},
    {0,   1,   2,   3,   4,   5,   6,   7,   8,   132, 16,  17,  18,  19,  20,
     21,  22,  23,  24,  116, 32,  33,  34,  35,  36,  37,  38,  39,  40,  180,
     48,  49,  50,  51,  52,  164, 112, 113, 114, 115, 64,  65,  66,  67,  68,
     176, 160, 161, 162, 163, 80,  81,  82,  83,  84,  177, 128, 129, 130, 131,
     96,  97,  98,  99,  100, 178, 144, 145, 146, 147, 148, 179},
    {0,   1,   2,   3,   4,   5,   6,   7,   164, 132, 16,  17,  18,  19,  20,
     21,  22,  23,  178, 116, 32,  33,  34,  35,  36,  37,  38,  39,  179, 180,
     48,  49,  50,  51,  52,  53,  112, 113, 114, 115, 64,  65,  66,  67,  68,
     176, 160, 161, 162, 163, 80,  81,  82,  83,  84,  177, 128, 129, 130, 131,
     96,  97,  98,  99,  100, 101, 144, 145, 146, 147, 148, 149},
    {0,  1,   2,   3,   4,   5,   6,   7,   176, 132, 16,  17,  18,  19,  20,
     21, 22,  23,  117, 116, 32,  33,  34,  35,  36,  37,  38,  39,  165, 180,
     48, 49,  50,  51,  52,  164, 112, 113, 114, 115, 64,  65,  66,  67,  68,
     69, 160, 161, 162, 163, 80,  81,  82,  83,  84,  177, 128, 129, 130, 131,
     96, 97,  98,  99,  100, 178, 144, 145, 146, 147, 148, 179},
    {0,   1,   2,   3,   4,   5,   6,   7,   177, 132, 16,  17,  18,  19,  20,
     21,  22,  23,  133, 116, 32,  33,  34,  35,  36,  37,  38,  39,  181, 180,
     48,  49,  50,  51,  52,  164, 112, 113, 114, 115, 64,  65,  66,  67,  68,
     176, 160, 161, 162, 163, 80,  81,  82,  83,  84,  85,  128, 129, 130, 131,
     96,  97,  98,  99,  100, 178, 144, 145, 146, 147, 148, 179},
    {0,  1,   2,   3,   4,   5,   164, 176, 177, 132, 16,  17,  18,  19,  20,
     21, 117, 133, 178, 116, 32,  33,  34,  35,  36,  37,  165, 179, 181, 180,
     48, 49,  50,  51,  52,  53,  112, 113, 114, 115, 64,  65,  66,  67,  68,
     69, 160, 161, 162, 163, 80,  81,  82,  83,  84,  85,  128, 129, 130, 131,
     96, 97,  98,  99,  100, 101, 144, 145, 146, 147, 148, 149},
    {0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
     32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57,
     64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89},
    {0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
     32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 48, 49, 50, 51, 52, 53, 54, 55, 56, 10,
     64, 65, 66, 67, 68, 69, 70, 71, 72, 26, 80, 81, 82, 83, 84, 85, 86, 87, 88, 42},
    {0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
     32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 48, 49, 50, 51, 52, 53, 54, 55, 11, 10,
     64, 65, 66, 67, 68, 69, 70, 71, 27, 26, 80, 81, 82, 83, 84, 85, 86, 87, 43, 42},
    {0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
     32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 48, 49, 50, 51, 15, 14, 13, 12, 11, 10,
     64, 65, 66, 67, 31, 30, 29, 28, 27, 26, 80, 81, 82, 83, 47, 46, 45, 44, 43, 42}};

static void
bc6_sign_extend(UINT16 *v, int prec) {
    int x = *v;
    if (x & (1 << (prec - 1))) {
        x |= -1 << prec;
    }
    *v = (UINT16)x;
}

static int
bc6_unquantize(UINT16 v, int prec, int sign) {
    int s = 0;
    int x;
    if (!sign) {
        x = v;
        if (prec >= 15) {
            return x;
        }
        if (x == 0) {
            return 0;
        }
        if (x == ((1 << prec) - 1)) {
            return 0xffff;
        }
        return ((x << 15) + 0x4000) >> (prec - 1);
    } else {
        x = (INT16)v;
        if (prec >= 16) {
            return x;
        }
        if (x < 0) {
            s = 1;
            x = -x;
        }

        if (x != 0) {
            if (x >= ((1 << (prec - 1)) - 1)) {
                x = 0x7fff;
            } else {
                x = ((x << 15) + 0x4000) >> (prec - 1);
            }
        }

        if (s) {
            return -x;
        }
        return x;
    }
}

static float
half_to_float(UINT16 h) {
    /* https://gist.github.com/rygorous/2144712 */
    union {
        UINT32 u;
        float f;
    } o, m;
    m.u = 0x77800000;
    o.u = (h & 0x7fff) << 13;
    o.f *= m.f;
    m.u = 0x47800000;
    if (o.f >= m.f) {
        o.u |= 255 << 23;
    }
    o.u |= (h & 0x8000) << 16;
    return o.f;
}

static float
bc6_finalize(int v, int sign) {
    if (sign) {
        if (v < 0) {
            v = ((-v) * 31) / 32;
            return half_to_float((UINT16)(0x8000 | v));
        } else {
            return half_to_float((UINT16)((v * 31) / 32));
        }
    } else {
        return half_to_float((UINT16)((v * 31) / 64));
    }
}

static UINT8
bc6_clamp(float value) {
    if (value < 0.0f) {
        return 0;
    } else if (value > 1.0f) {
        return 255;
    } else {
        return (UINT8) (value * 255.0f);
    }
}

static void
bc6_lerp(rgba *col, int *e0, int *e1, int s, int sign) {
    int r, g, b;
    int t = 64 - s;
    r = (e0[0] * t + e1[0] * s) >> 6;
    g = (e0[1] * t + e1[1] * s) >> 6;
    b = (e0[2] * t + e1[2] * s) >> 6;
    col->r = bc6_clamp(bc6_finalize(r, sign));
    col->g = bc6_clamp(bc6_finalize(g, sign));
    col->b = bc6_clamp(bc6_finalize(b, sign));
}

static void
decode_bc6_block(rgba *col, const UINT8 *src, int sign) {
    UINT16 endpoints[12]; /* storage for r0, g0, b0, r1, ... */
    int ueps[12];
    int i, i0, ib2, di, dw, mask, numep, s;
    UINT8 partition;
    const bc6_mode_info *info;
    const char *cw;
    int bit = 5;
    int epbits = 75;
    int ib = 3;
    int mode = src[0] & 0x1f;
    if ((mode & 3) == 0 || (mode & 3) == 1) {
        mode &= 3;
        bit = 2;
    } else if ((mode & 3) == 2) {
        mode = 2 + (mode >> 2);
        epbits = 72;
    } else {
        mode = 10 + (mode >> 2);
        epbits = 60;
        ib = 4;
    }
    if (mode >= 14) {
        /* invalid block */
        memset(col, 0, 16 * sizeof(col[0]));
        return;
    }
    info = &bc6_modes[mode];
    cw = bc7_get_weights(ib);
    numep = info->ns == 2 ? 12 : 6;
    for (i = 0; i < 12; i++) {
        endpoints[i] = 0;
    }
    for (i = 0; i < epbits; i++) {
        di = bc6_bit_packings[mode][i];
        dw = di >> 4;
        di &= 15;
        endpoints[dw] |= (UINT16)get_bit(src, bit + i) << di;
    }
    bit += epbits;
    partition = get_bits(src, bit, info->pb);
    bit += info->pb;
    mask = (1 << info->epb) - 1;
    if (sign) { /* sign-extend e0 if signed */
        bc6_sign_extend(&endpoints[0], info->epb);
        bc6_sign_extend(&endpoints[1], info->epb);
        bc6_sign_extend(&endpoints[2], info->epb);
    }
    if (sign || info->tr) { /* sign-extend e1,2,3 if signed or deltas */
        for (i = 3; i < numep; i += 3) {
            bc6_sign_extend(&endpoints[i], info->rb);
            bc6_sign_extend(&endpoints[i + 1], info->gb);
            bc6_sign_extend(&endpoints[i + 2], info->bb);
        }
    }
    if (info->tr) { /* apply deltas */
        for (i = 3; i < numep; i += 3) {
            endpoints[i] = (endpoints[i] + endpoints[0]) & mask;
            endpoints[i + 1] = (endpoints[i + 1] + endpoints[1]) & mask;
            endpoints[i + 2] = (endpoints[i + 2] + endpoints[2]) & mask;
        }
    }
    for (i = 0; i < numep; i++) {
        ueps[i] = bc6_unquantize(endpoints[i], info->epb, sign);
    }
    for (i = 0; i < 16; i++) {
        s = bc7_get_subset(info->ns, partition, i) * 6;
        ib2 = ib;
        if (i == 0) {
            ib2--;
        } else if (info->ns == 2) {
            if (i == bc7_ai0[partition]) {
                ib2--;
            }
        }
        i0 = get_bits(src, bit, ib2);
        bit += ib2;

        bc6_lerp(&col[i], &ueps[s], &ueps[s + 3], cw[i0], sign);
    }
}

static void
put_block(Imaging im, ImagingCodecState state, const char *col, int sz, int C) {
    int width = state->xsize;
    int height = state->ysize;
    int xmax = width + state->xoff;
    int ymax = height + state->yoff;
    int j, i, y, x;
    char *dst;
    for (j = 0; j < 4; j++) {
        y = state->y + j;
        if (C) {
            if (y >= height) {
                continue;
            }
            if (state->ystep < 0) {
                y = state->yoff + ymax - y - 1;
            }
            dst = im->image[y];
            for (i = 0; i < 4; i++) {
                x = state->x + i;
                if (x >= width) {
                    continue;
                }
                memcpy(dst + sz * x, col + sz * (j * 4 + i), sz);
            }
        } else {
            if (state->ystep < 0) {
                y = state->yoff + ymax - y - 1;
            }
            x = state->x;
            dst = im->image[y] + sz * x;
            memcpy(dst, col + sz * (j * 4), 4 * sz);
        }
    }
    state->x += 4;
    if (state->x >= xmax) {
        state->y += 4;
        state->x = state->xoff;
    }
}

static int
decode_bcn(
    Imaging im, ImagingCodecState state, const UINT8 *src, int bytes, int N, int C, char *pixel_format) {
    int ymax = state->ysize + state->yoff;
    const UINT8 *ptr = src;
    switch (N) {
#define DECODE_LOOP(NN, SZ, TY, ...)                                    \
    case NN:                                                            \
        while (bytes >= SZ) {                                           \
            TY col[16];                                                 \
            memset(col, 0, 16 * sizeof(col[0]));                        \
            decode_bc##NN##_block(col, ptr);                            \
            put_block(im, state, (const char *)col, sizeof(col[0]), C); \
            ptr += SZ;                                                  \
            bytes -= SZ;                                                \
            if (state->y >= ymax) {                                     \
                return -1;                                              \
            }                                                           \
        }                                                               \
        break

        DECODE_LOOP(1, 8, rgba);
        DECODE_LOOP(2, 16, rgba);
        DECODE_LOOP(3, 16, rgba);
        DECODE_LOOP(4, 8, lum);
        case 5:
        {
            int sign = strcmp(pixel_format, "BC5S") == 0 ? 1 : 0;
            while (bytes >= 16) {
                rgba col[16];
                memset(col, sign ? 128 : 0, 16 * sizeof(col[0]));
                decode_bc5_block(col, ptr, sign);
                put_block(im, state, (const char *)col, sizeof(col[0]), C);
                ptr += 16;
                bytes -= 16;
                if (state->y >= ymax) {
                    return -1;
                }
            }
            break;
        }
        case 6:
        {
            int sign = strcmp(pixel_format, "BC6HS") == 0 ? 1 : 0;
            while (bytes >= 16) {
                rgba col[16];
                decode_bc6_block(col, ptr, sign);
                put_block(im, state, (const char *)col, sizeof(col[0]), C);
                ptr += 16;
                bytes -= 16;
                if (state->y >= ymax) {
                    return -1;
                }
            }
            break;
        }
        DECODE_LOOP(7, 16, rgba);
#undef DECODE_LOOP
    }
    return (int)(ptr - src);
}

int
ImagingBcnDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t bytes) {
    int N = state->state & 0xf;
    int width = state->xsize;
    int height = state->ysize;
    int C = (width & 3) | (height & 3) ? 1 : 0;
    char *pixel_format = ((BCNSTATE *)state->context)->pixel_format;
    return decode_bcn(im, state, buf, bytes, N, C, pixel_format);
}