/* SPDX-License-Identifier: BSD-3-Clause
 * Copyright 2009 Google Inc. All rights reserved. */

/** \file
 * \ingroup imbdds
 * This file comes from the chromium project, adapted to Blender to add DDS
 * flipping to OpenGL convention for Blender.
 */

#include "IMB_imbuf_types.h"

#include <cstring>

#include <BlockDXT.h>
#include <ColorBlock.h>
#include <Common.h>
#include <FlipDXT.h>
#include <Stream.h>

/* A function that flips a DXTC block. */
using FlipBlockFunction = void (*)(uint8_t *block);

/* Flips a full DXT1 block in the y direction. */
static void FlipDXT1BlockFull(uint8_t *block)
{
  /* A DXT1 block layout is:
   * [0-1] color0.
   * [2-3] color1.
   * [4-7] color bitmap, 2 bits per pixel.
   * So each of the 4-7 bytes represents one line, flipping a block is just
   * flipping those bytes. */
  uint8_t tmp = block[4];
  block[4] = block[7];
  block[7] = tmp;
  tmp = block[5];
  block[5] = block[6];
  block[6] = tmp;
}

/* Flips the first 2 lines of a DXT1 block in the y direction. */
static void FlipDXT1BlockHalf(uint8_t *block)
{
  /* See layout above. */
  uint8_t tmp = block[4];
  block[4] = block[5];
  block[5] = tmp;
}

/* Flips a full DXT3 block in the y direction. */
static void FlipDXT3BlockFull(uint8_t *block)
{
  /* A DXT3 block layout is:
   * [0-7]    alpha bitmap, 4 bits per pixel.
   * [8-15] a DXT1 block. */

  /* We can flip the alpha bits at the byte level (2 bytes per line). */
  uint8_t tmp = block[0];

  block[0] = block[6];
  block[6] = tmp;
  tmp = block[1];
  block[1] = block[7];
  block[7] = tmp;
  tmp = block[2];
  block[2] = block[4];
  block[4] = tmp;
  tmp = block[3];
  block[3] = block[5];
  block[5] = tmp;

  /* And flip the DXT1 block using the above function. */
  FlipDXT1BlockFull(block + 8);
}

/* Flips the first 2 lines of a DXT3 block in the y direction. */
static void FlipDXT3BlockHalf(uint8_t *block)
{
  /* See layout above. */
  uint8_t tmp = block[0];

  block[0] = block[2];
  block[2] = tmp;
  tmp = block[1];
  block[1] = block[3];
  block[3] = tmp;
  FlipDXT1BlockHalf(block + 8);
}

/* Flips a full DXT5 block in the y direction. */
static void FlipDXT5BlockFull(uint8_t *block)
{
  /* A DXT5 block layout is:
   * [0]      alpha0.
   * [1]      alpha1.
   * [2-7]    alpha bitmap, 3 bits per pixel.
   * [8-15] a DXT1 block. */

  /* The alpha bitmap doesn't easily map lines to bytes, so we have to
   * interpret it correctly.  Extracted from
   * http://www.opengl.org/registry/specs/EXT/texture_compression_s3tc.txt :
   *
   *   The 6 "bits" bytes of the block are decoded into one 48-bit integer:
   *
   *       bits = bits_0 + 256 * (bits_1 + 256 * (bits_2 + 256 * (bits_3 +
   *                                   256 * (bits_4 + 256 * bits_5))))
   *
   *   bits is a 48-bit unsigned integer, from which a three-bit control code
   *   is extracted for a texel at location (x,y) in the block using:
   *
   *           code(x,y) = bits[3*(4*y+x)+1..3*(4*y+x)+0]
   *
   *   where bit 47 is the most significant and bit 0 is the least
   *   significant bit. */
  unsigned int line_0_1 = block[2] + 256 * (block[3] + 256 * block[4]);
  unsigned int line_2_3 = block[5] + 256 * (block[6] + 256 * block[7]);
  /* swap lines 0 and 1 in line_0_1. */
  unsigned int line_1_0 = ((line_0_1 & 0x000fff) << 12) | ((line_0_1 & 0xfff000) >> 12);
  /* swap lines 2 and 3 in line_2_3. */
  unsigned int line_3_2 = ((line_2_3 & 0x000fff) << 12) | ((line_2_3 & 0xfff000) >> 12);

  block[2] = line_3_2 & 0xff;
  block[3] = (line_3_2 & 0xff00) >> 8;
  block[4] = (line_3_2 & 0xff0000) >> 16;
  block[5] = line_1_0 & 0xff;
  block[6] = (line_1_0 & 0xff00) >> 8;
  block[7] = (line_1_0 & 0xff0000) >> 16;

  /* And flip the DXT1 block using the above function. */
  FlipDXT1BlockFull(block + 8);
}

/* Flips the first 2 lines of a DXT5 block in the y direction. */
static void FlipDXT5BlockHalf(uint8_t *block)
{
  /* See layout above. */
  unsigned int line_0_1 = block[2] + 256 * (block[3] + 256 * block[4]);
  unsigned int line_1_0 = ((line_0_1 & 0x000fff) << 12) | ((line_0_1 & 0xfff000) >> 12);
  block[2] = line_1_0 & 0xff;
  block[3] = (line_1_0 & 0xff00) >> 8;
  block[4] = (line_1_0 & 0xff0000) >> 16;
  FlipDXT1BlockHalf(block + 8);
}

int FlipDXTCImage(unsigned int width,
                  unsigned int height,
                  unsigned int levels,
                  int fourcc,
                  uint8_t *data,
                  int data_size,
                  unsigned int *r_num_valid_levels)
{
  *r_num_valid_levels = 0;

  /* Must have valid dimensions. */
  if (width == 0 || height == 0) {
    return 0;
  }
  /* Height must be a power-of-two. */
  if ((height & (height - 1)) != 0) {
    return 0;
  }

  FlipBlockFunction full_block_function;
  FlipBlockFunction half_block_function;
  unsigned int block_bytes = 0;

  switch (fourcc) {
    case FOURCC_DXT1:
      full_block_function = FlipDXT1BlockFull;
      half_block_function = FlipDXT1BlockHalf;
      block_bytes = 8;
      break;
    case FOURCC_DXT3:
      full_block_function = FlipDXT3BlockFull;
      half_block_function = FlipDXT3BlockHalf;
      block_bytes = 16;
      break;
    case FOURCC_DXT5:
      full_block_function = FlipDXT5BlockFull;
      half_block_function = FlipDXT5BlockHalf;
      block_bytes = 16;
      break;
    default:
      return 0;
  }

  *r_num_valid_levels = levels;

  unsigned int mip_width = width;
  unsigned int mip_height = height;

  const uint8_t *data_end = data + data_size;

  for (unsigned int i = 0; i < levels; i++) {
    unsigned int blocks_per_row = (mip_width + 3) / 4;
    unsigned int blocks_per_col = (mip_height + 3) / 4;
    unsigned int blocks = blocks_per_row * blocks_per_col;

    if (data + block_bytes * blocks > data_end) {
      /* Stop flipping when running out of data to be modified, avoiding possible buffer overrun
       * on a malformed files. */
      *r_num_valid_levels = i;
      break;
    }

    if (mip_height == 1) {
      /* no flip to do, and we're done. */
      break;
    }
    if (mip_height == 2) {
      /* flip the first 2 lines in each block. */
      for (unsigned int i = 0; i < blocks_per_row; i++) {
        half_block_function(data + i * block_bytes);
      }
    }
    else {
      /* flip each block. */
      for (unsigned int i = 0; i < blocks; i++) {
        full_block_function(data + i * block_bytes);
      }

      /* Swap each block line in the first half of the image with the
       * corresponding one in the second half.
       * note that this is a no-op if mip_height is 4. */
      unsigned int row_bytes = block_bytes * blocks_per_row;
      uint8_t *temp_line = new uint8_t[row_bytes];

      for (unsigned int y = 0; y < blocks_per_col / 2; y++) {
        uint8_t *line1 = data + y * row_bytes;
        uint8_t *line2 = data + (blocks_per_col - y - 1) * row_bytes;

        memcpy(temp_line, line1, row_bytes);
        memcpy(line1, line2, row_bytes);
        memcpy(line2, temp_line, row_bytes);
      }

      delete[] temp_line;
    }

    /* mip levels are contiguous. */
    data += block_bytes * blocks;
    mip_width = MAX(1U, mip_width >> 1);
    mip_height = MAX(1U, mip_height >> 1);
  }

  return 1;
}