// Exploration only: never changes grains or the random stream.
const FOG = 2;
const FOG_SENSE = 24; // Reveal nearby ground through rock, in grains.
const FOG_SENSE_FADE = 12;
let fog, FW, FH;
function fogAlloc() { FW = Math.ceil(W / FOG); FH = Math.ceil(H / FOG); fog = new Uint8Array(FW * FH); }
function fogMark(x, y, v) {
  if (x < 0 || y < 0 || x >= W || y >= H) return;
  const i = Math.floor(y / FOG) * FW + Math.floor(x / FOG);
  if (v > fog[i]) fog[i] = v;
}
function fogSeen(x, y) {
  if (x < 0 || y < 0 || x >= W || y >= H) return 255;
  return fog[Math.floor(y / FOG) * FW + Math.floor(x / FOG)];
}
function fogInit() {
  for (let x = 0; x < W; x++) for (let y = 0; y < Math.min(H, surfY[x] + 16); y++) fogMark(x, y, y <= surfY[x] ? 255 : (surfY[x] + 16 - y) * 255 / 16);
}
function fogLook(cx, cy) {
  for (let a = 0; a < 256; a++) {
    const dx = Math.cos(a * Math.PI / 128), dy = Math.sin(a * Math.PI / 128);
    let hit = -1;
    for (let d = 0; d < 48; d++) {
      const x = Math.floor(cx + dx * d), y = Math.floor(cy + dy * d);
      if (x < 0 || y < 0 || x >= W || y >= H) break;
      const ph = PH[mat[y * W + x]];
      if (hit < 0 && (ph === PH_SOLID || ph === PH_POWDER)) hit = d;
      // into rock, sight fades out over 8 grains from however bright it arrived
      const v = Math.min(255, (48 - d) * 255 / 16) * (hit < 0 ? 1 : (8 - d + hit) / 8);
      if (v <= 0) break;
      fogMark(x, y, v);
    }
  }
  // one fog cell at a time, measured from its centre; full inside 12 grains, fading over the outer 12 grains
  const R2 = FOG_SENSE * FOG_SENSE, in2 = (FOG_SENSE - FOG_SENSE_FADE) ** 2;
  const fy0 = Math.max(0, Math.floor((cy - FOG_SENSE) / FOG)), fy1 = Math.min(FH - 1, Math.floor((cy + FOG_SENSE) / FOG));
  const fx0 = Math.max(0, Math.floor((cx - FOG_SENSE) / FOG)), fx1 = Math.min(FW - 1, Math.floor((cx + FOG_SENSE) / FOG));
  for (let fy = fy0; fy <= fy1; fy++) for (let fx = fx0; fx <= fx1; fx++) {
    const dx = (fx + 0.5) * FOG - cx, dy = (fy + 0.5) * FOG - cy, d2 = dx * dx + dy * dy, i = fy * FW + fx;
    if (d2 >= R2) continue;
    const v = d2 <= in2 ? 255 : (FOG_SENSE - Math.sqrt(d2)) * 255 / FOG_SENSE_FADE;
    if (v > fog[i]) fog[i] = v;
  }
}
function fogRevealAround(x, y, r) {
  for (let yy = Math.max(0, Math.floor(y - r)); yy <= Math.min(H - 1, y + r); yy++) for (let xx = Math.max(0, Math.floor(x - r)); xx <= Math.min(W - 1, x + r); xx++) {
    const d2 = (xx - x) ** 2 + (yy - y) ** 2;
    if (d2 >= r * r) continue;
    fogMark(xx, yy, r >= 6 && d2 <= (r - 6) ** 2 ? 255 : Math.min(255, (r - Math.sqrt(d2)) * 255 / 6));
  }
}
function fogRevealAll() { fog.fill(255); }