import { createHash } from "node:crypto";

const sha256 = (bytes) => createHash("sha256").update(bytes).digest("hex");

/** Validate a legacy DXT1 DDS and describe every source mip without decoding or resampling it. */
export function inspectNativeWaterDxt1Dds(bytes) {
  if (!Buffer.isBuffer(bytes)) bytes = Buffer.from(bytes);
  if (bytes.length < 128 || bytes.toString("ascii", 0, 4) !== "DDS ") {
    throw new Error("Native water frame is not a complete DDS file.");
  }
  const headerSize = bytes.readUInt32LE(4);
  const height = bytes.readUInt32LE(12);
  const width = bytes.readUInt32LE(16);
  const pixelFormatSize = bytes.readUInt32LE(76);
  const formatFourCC = bytes.toString("ascii", 84, 88);
  const mipCount = Math.max(1, bytes.readUInt32LE(28));
  if (headerSize !== 124 || pixelFormatSize !== 32) {
    throw new Error(`Unsupported DDS header sizes ${headerSize}/${pixelFormatSize}.`);
  }
  if (formatFourCC !== "DXT1") throw new Error(`Unsupported native water frame format ${formatFourCC}.`);
  if (width < 1 || height < 1 || width % 4 !== 0 || height % 4 !== 0) {
    throw new Error(`Native water DXT1 dimensions must be positive multiples of four; got ${width}x${height}.`);
  }
  const maxMipCount = 1 + Math.floor(Math.log2(Math.max(width, height)));
  if (mipCount > maxMipCount) throw new Error(`DDS declares ${mipCount} mips for ${width}x${height}.`);

  let offset = 128;
  let mipWidth = width;
  let mipHeight = height;
  const mips = [];
  for (let level = 0; level < mipCount; level += 1) {
    const byteLength = Math.ceil(mipWidth / 4) * Math.ceil(mipHeight / 4) * 8;
    const end = offset + byteLength;
    if (end > bytes.length) throw new Error(`DDS mip ${level} is truncated.`);
    const data = bytes.subarray(offset, end);
    mips.push({
      level,
      width: mipWidth,
      height: mipHeight,
      offset,
      byteLength,
      sha256: sha256(data),
    });
    offset = end;
    mipWidth = Math.max(1, Math.floor(mipWidth / 2));
    mipHeight = Math.max(1, Math.floor(mipHeight / 2));
  }
  if (offset !== bytes.length) {
    throw new Error(`DDS has ${bytes.length - offset} trailing bytes after its declared mip chain.`);
  }
  return {
    width,
    height,
    mipCount,
    formatFourCC,
    headerBytes: 128,
    fileBytes: bytes.length,
    sha256: sha256(bytes),
    mips,
    exactMipChainConsumption: offset === bytes.length,
  };
}