"""
A Pillow loader for .dds files (S3TC-compressed aka DXTC)
Jerome Leclanche <jerome@leclan.ch>

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 hereby released in the public domain (CC0)
Full text of the CC0 license:
  https://creativecommons.org/publicdomain/zero/1.0/
"""

import struct
from io import BytesIO

from PIL import Image, ImageFile

# Magic ("DDS ")
DDS_MAGIC = 0x20534444

# DDS flags
DDSD_CAPS = 0x1
DDSD_HEIGHT = 0x2
DDSD_WIDTH = 0x4
DDSD_PITCH = 0x8
DDSD_PIXELFORMAT = 0x1000
DDSD_MIPMAPCOUNT = 0x20000
DDSD_LINEARSIZE = 0x80000
DDSD_DEPTH = 0x800000

# DDS caps
DDSCAPS_COMPLEX = 0x8
DDSCAPS_TEXTURE = 0x1000
DDSCAPS_MIPMAP = 0x400000

DDSCAPS2_CUBEMAP = 0x200
DDSCAPS2_CUBEMAP_POSITIVEX = 0x400
DDSCAPS2_CUBEMAP_NEGATIVEX = 0x800
DDSCAPS2_CUBEMAP_POSITIVEY = 0x1000
DDSCAPS2_CUBEMAP_NEGATIVEY = 0x2000
DDSCAPS2_CUBEMAP_POSITIVEZ = 0x4000
DDSCAPS2_CUBEMAP_NEGATIVEZ = 0x8000
DDSCAPS2_VOLUME = 0x200000

# Pixel Format
DDPF_ALPHAPIXELS = 0x1
DDPF_ALPHA = 0x2
DDPF_FOURCC = 0x4
DDPF_PALETTEINDEXED8 = 0x20
DDPF_RGB = 0x40
DDPF_LUMINANCE = 0x20000


# dds.h

DDS_FOURCC = DDPF_FOURCC
DDS_RGB = DDPF_RGB
DDS_RGBA = DDPF_RGB | DDPF_ALPHAPIXELS
DDS_LUMINANCE = DDPF_LUMINANCE
DDS_LUMINANCEA = DDPF_LUMINANCE | DDPF_ALPHAPIXELS
DDS_ALPHA = DDPF_ALPHA
DDS_PAL8 = DDPF_PALETTEINDEXED8

DDS_HEADER_FLAGS_TEXTURE = DDSD_CAPS | DDSD_HEIGHT | DDSD_WIDTH | DDSD_PIXELFORMAT
DDS_HEADER_FLAGS_MIPMAP = DDSD_MIPMAPCOUNT
DDS_HEADER_FLAGS_VOLUME = DDSD_DEPTH
DDS_HEADER_FLAGS_PITCH = DDSD_PITCH
DDS_HEADER_FLAGS_LINEARSIZE = DDSD_LINEARSIZE

DDS_HEIGHT = DDSD_HEIGHT
DDS_WIDTH = DDSD_WIDTH

DDS_SURFACE_FLAGS_TEXTURE = DDSCAPS_TEXTURE
DDS_SURFACE_FLAGS_MIPMAP = DDSCAPS_COMPLEX | DDSCAPS_MIPMAP
DDS_SURFACE_FLAGS_CUBEMAP = DDSCAPS_COMPLEX

DDS_CUBEMAP_POSITIVEX = DDSCAPS2_CUBEMAP | DDSCAPS2_CUBEMAP_POSITIVEX
DDS_CUBEMAP_NEGATIVEX = DDSCAPS2_CUBEMAP | DDSCAPS2_CUBEMAP_NEGATIVEX
DDS_CUBEMAP_POSITIVEY = DDSCAPS2_CUBEMAP | DDSCAPS2_CUBEMAP_POSITIVEY
DDS_CUBEMAP_NEGATIVEY = DDSCAPS2_CUBEMAP | DDSCAPS2_CUBEMAP_NEGATIVEY
DDS_CUBEMAP_POSITIVEZ = DDSCAPS2_CUBEMAP | DDSCAPS2_CUBEMAP_POSITIVEZ
DDS_CUBEMAP_NEGATIVEZ = DDSCAPS2_CUBEMAP | DDSCAPS2_CUBEMAP_NEGATIVEZ


# DXT1
DXT1_FOURCC = 0x31545844

# DXT3
DXT3_FOURCC = 0x33545844

# DXT5
DXT5_FOURCC = 0x35545844


def _decode565(bits):
    a = ((bits >> 11) & 0x1F) << 3
    b = ((bits >> 5) & 0x3F) << 2
    c = (bits & 0x1F) << 3
    return a, b, c


def _c2a(a, b):
    return (2 * a + b) // 3


def _c2b(a, b):
    return (a + b) // 2


def _c3(a, b):
    return (2 * b + a) // 3


def _dxt1(data, width, height):
    # TODO implement this function as pixel format in decode.c
    ret = bytearray(4 * width * height)

    for y in range(0, height, 4):
        for x in range(0, width, 4):
            color0, color1, bits = struct.unpack("<HHI", data.read(8))

            r0, g0, b0 = _decode565(color0)
            r1, g1, b1 = _decode565(color1)

            # Decode this block into 4x4 pixels
            for j in range(4):
                for i in range(4):
                    # get next control op and generate a pixel
                    control = bits & 3
                    bits = bits >> 2
                    if control == 0:
                        r, g, b = r0, g0, b0
                    elif control == 1:
                        r, g, b = r1, g1, b1
                    elif control == 2:
                        if color0 > color1:
                            r, g, b = _c2a(r0, r1), _c2a(g0, g1), _c2a(b0, b1)
                        else:
                            r, g, b = _c2b(r0, r1), _c2b(g0, g1), _c2b(b0, b1)
                    elif control == 3:
                        if color0 > color1:
                            r, g, b = _c3(r0, r1), _c3(g0, g1), _c3(b0, b1)
                        else:
                            r, g, b = 0, 0, 0

                    idx = 4 * ((y + j) * width + x + i)
                    ret[idx : idx + 4] = struct.pack("4B", r, g, b, 255)

    return bytes(ret)


def _dxtc_alpha(a0, a1, ac0, ac1, ai):
    if ai <= 12:
        ac = (ac0 >> ai) & 7
    elif ai == 15:
        ac = (ac0 >> 15) | ((ac1 << 1) & 6)
    else:
        ac = (ac1 >> (ai - 16)) & 7

    if ac == 0:
        alpha = a0
    elif ac == 1:
        alpha = a1
    elif a0 > a1:
        alpha = ((8 - ac) * a0 + (ac - 1) * a1) // 7
    elif ac == 6:
        alpha = 0
    elif ac == 7:
        alpha = 0xFF
    else:
        alpha = ((6 - ac) * a0 + (ac - 1) * a1) // 5

    return alpha


def _dxt5(data, width, height):
    # TODO implement this function as pixel format in decode.c
    ret = bytearray(4 * width * height)

    for y in range(0, height, 4):
        for x in range(0, width, 4):
            a0, a1, ac0, ac1, c0, c1, code = struct.unpack("<2BHI2HI", data.read(16))

            r0, g0, b0 = _decode565(c0)
            r1, g1, b1 = _decode565(c1)

            for j in range(4):
                for i in range(4):
                    ai = 3 * (4 * j + i)
                    alpha = _dxtc_alpha(a0, a1, ac0, ac1, ai)

                    cc = (code >> 2 * (4 * j + i)) & 3
                    if cc == 0:
                        r, g, b = r0, g0, b0
                    elif cc == 1:
                        r, g, b = r1, g1, b1
                    elif cc == 2:
                        r, g, b = _c2a(r0, r1), _c2a(g0, g1), _c2a(b0, b1)
                    elif cc == 3:
                        r, g, b = _c3(r0, r1), _c3(g0, g1), _c3(b0, b1)

                    idx = 4 * ((y + j) * width + x + i)
                    ret[idx : idx + 4] = struct.pack("4B", r, g, b, alpha)

    return bytes(ret)


class DdsImageFile(ImageFile.ImageFile):
    format = "DDS"
    format_description = "DirectDraw Surface"

    def _open(self):
        magic, header_size = struct.unpack("<II", self.fp.read(8))
        if header_size != 124:
            raise OSError(f"Unsupported header size {repr(header_size)}")
        header_bytes = self.fp.read(header_size - 4)
        if len(header_bytes) != 120:
            raise OSError(f"Incomplete header: {len(header_bytes)} bytes")
        header = BytesIO(header_bytes)

        flags, height, width = struct.unpack("<3I", header.read(12))
        self._size = (width, height)
        self.mode = "RGBA"

        pitch, depth, mipmaps = struct.unpack("<3I", header.read(12))
        struct.unpack("<11I", header.read(44))  # reserved

        # pixel format
        pfsize, pfflags = struct.unpack("<2I", header.read(8))
        fourcc = header.read(4)
        bitcount, rmask, gmask, bmask, amask = struct.unpack("<5I", header.read(20))

        if fourcc == b"DXT1":
            self.decoder = "DXT1"
        elif fourcc == b"DXT5":
            self.decoder = "DXT5"
        else:
            raise NotImplementedError(f"Unimplemented pixel format {fourcc}")

        self.tile = [(self.decoder, (0, 0) + self.size, 0, (self.mode, 0, 1))]

    def load_seek(self, pos):
        pass


class DXT1Decoder(ImageFile.PyDecoder):
    _pulls_fd = True

    def decode(self, buffer):
        try:
            self.set_as_raw(_dxt1(self.fd, self.state.xsize, self.state.ysize))
        except struct.error as e:
            raise OSError("Truncated DDS file") from e
        return 0, 0


class DXT5Decoder(ImageFile.PyDecoder):
    _pulls_fd = True

    def decode(self, buffer):
        try:
            self.set_as_raw(_dxt5(self.fd, self.state.xsize, self.state.ysize))
        except struct.error as e:
            raise OSError("Truncated DDS file") from e
        return 0, 0


Image.register_decoder("DXT1", DXT1Decoder)
Image.register_decoder("DXT5", DXT5Decoder)


def _validate(prefix):
    return prefix[:4] == b"DDS "


Image.register_open(DdsImageFile.format, DdsImageFile, _validate)
Image.register_extension(DdsImageFile.format, ".dds")
