import sharp from "sharp";

// Empty sky faces are valid when looking up. A surface map or the view straight
// down from a surface capture must contain terrain, even if other faces are sky.
export async function assertTerrainImage(bytes, label) {
  const { data, info } = await sharp(bytes)
    .resize(64, 64, { fit: "fill" })
    .toColourspace("srgb")
    .removeAlpha()
    .raw()
    .toBuffer({ resolveWithObject: true });
  const sky = [0x95, 0xc4, 0xeb];
  let terrainPixels = 0;
  for (let i = 0; i < data.length; i += info.channels) {
    if (
      Math.abs(data[i] - sky[0]) > 12 ||
      Math.abs(data[i + 1] - sky[1]) > 12 ||
      Math.abs(data[i + 2] - sky[2]) > 12
    )
      terrainPixels++;
  }
  if (terrainPixels < 4) {
    throw new Error(
      `${label} contains only sky. Check Minecraft asset loading, capture coordinates and graphics errors; this render will not be submitted as a playable map.`,
    );
  }
  return terrainPixels;
}