07Acid dissolve — a reaction rule that erases the material it meets

<main class="stage">
<div class="cd is-demo" id="cdRoot">
  <p class="cd__hint"><span>Acid dissolves soil and rock, carving a passage</span></p>
  <div class="cd__field" id="cdField" role="application" aria-label="Acid Dissolve cellular automaton demo" tabindex="0">
    <canvas class="cd__cv" id="cdCv"></canvas>
  </div>
  <div class="cd__bar">
    <span class="cd__meta"><b class="cd__val" id="cdN">30</b>% corrosion chance · acid-soil-rock reaction</span>
    <span class="cd__meta cd__meta--dim">canvas · ImageData putImageData · requestAnimationFrame</span>
  </div>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// 산 부식(07). @keyframes 을 아예 두지 않는다 —
// 움직임은 전부 캔버스 rAF 루프(산 스포너 → 부식 → 낙하 → 잔광)가 만든다.
// 무대는 어스름 잉크. 파스텔 고정 팔레트를 --sand-* 로 편 공통 규격(01~09 동일).

$duration: $dur-loop;
$easing: $ease-spring;
$color: $subject-lilac;

@include stage($stage-ink);

// ── 편 공통 파스텔 팔레트(BD26 vivid 0 실측 통과값 — 다른 색 금지) ──
$sand-night: #201c17;   // 밤 지하 배경
$sand-mist:  #d8d2c6;   // 먼지 — 힌트 글자
$sand-glow:  #f2e2c4;   // 산 하이라이트
$sand-grain: #d9b98a;   // 모래 셰이드 A
$sand-gold:  #ecd9b0;   // 모래 셰이드 B
$sand-water: #a9c8dd;   // (공용) 물
$sand-ice:   #cfe3ee;   // (공용) 얼음
$sand-fire:  #ffc9ad;   // (공용) 불
$sand-flame: #ffe0c4;   // (공용) 불꽃 코어
$sand-ash:   #b9b2a6;   // (공용) 재
$sand-smoke: #8e8879;   // (공용) 연기
$sand-plant: #9fd6ad;   // (공용) 풀
$sand-leaf:  #c4e6c9;   // (공용) 잎
$sand-acid:  #c7dd8f;   // 산 — 라임 파스텔
$sand-stone: #8a8578;   // 돌
$sand-wood:  #b98a63;   // 흙(목톤)

.cd {
  // 렌더러가 읽는 색 — JS 는 getComputedStyle 로 --sand-* 를 받아 쓴다
  --sand-night: #{$sand-night};
  --sand-mist:  #{$sand-mist};
  --sand-glow:  #{$sand-glow};
  --sand-grain: #{$sand-grain};
  --sand-gold:  #{$sand-gold};
  --sand-water: #{$sand-water};
  --sand-ice:   #{$sand-ice};
  --sand-fire:  #{$sand-fire};
  --sand-flame: #{$sand-flame};
  --sand-ash:   #{$sand-ash};
  --sand-smoke: #{$sand-smoke};
  --sand-plant: #{$sand-plant};
  --sand-leaf:  #{$sand-leaf};
  --sand-acid:  #{$sand-acid};
  --sand-stone: #{$sand-stone};
  --sand-wood:  #{$sand-wood};

  display: flex; flex-direction: column; gap: $sp-2;
  width: 100%; max-width: 424px;
}

.cd__hint {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: $sand-mist;
}

.cd__field {
  position: relative; height: 200px;
  border-radius: $r-card; overflow: hidden;
  background: $sand-night;
  box-shadow: inset 0 0 0 1px rgba($sand-mist, .18);
}
.cd__field:focus-visible { outline: 2px solid $subject-lilac; outline-offset: 2px; }

.cd__cv { display: block; width: 100%; height: 100%; }

.cd__bar { display: flex; align-items: baseline; justify-content: space-between; gap: $sp-2; }
.cd__meta { font-size: 10px; font-weight: 500; color: rgba($sand-mist, .72); }
.cd__val { color: $subject-lilac; font-weight: 700; }
.cd__meta--dim { font-size: 10px; }

@media (max-width: 340px) {
  .cd { gap: $sp-1; }
  .cd__field { height: 110px; }
  .cd__meta--dim { display: none; }
}

// reduce — rAF 를 돌리지 않고 부식이 한참 진행된 한 장만 남는다.
@media (prefers-reduced-motion: reduce) {
  .cd.is-still .cd__field { cursor: default !important; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.cd {
  --sand-night: #201c17;
  --sand-mist: #d8d2c6;
  --sand-glow: #f2e2c4;
  --sand-grain: #d9b98a;
  --sand-gold: #ecd9b0;
  --sand-water: #a9c8dd;
  --sand-ice: #cfe3ee;
  --sand-fire: #ffc9ad;
  --sand-flame: #ffe0c4;
  --sand-ash: #b9b2a6;
  --sand-smoke: #8e8879;
  --sand-plant: #9fd6ad;
  --sand-leaf: #c4e6c9;
  --sand-acid: #c7dd8f;
  --sand-stone: #8a8578;
  --sand-wood: #b98a63;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.cd__hint {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: #d8d2c6;
}

.cd__field {
  position: relative;
  height: 200px;
  border-radius: 16px;
  overflow: hidden;
  background: #201c17;
  box-shadow: inset 0 0 0 1px rgba(216, 210, 198, 0.18);
}

.cd__field:focus-visible {
  outline: 2px solid #b9a0ff;
  outline-offset: 2px;
}

.cd__cv {
  display: block;
  width: 100%;
  height: 100%;
}

.cd__bar {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 8px;
}

.cd__meta {
  font-size: 10px;
  font-weight: 500;
  color: rgba(216, 210, 198, 0.72);
}

.cd__val {
  color: #b9a0ff;
  font-weight: 700;
}

.cd__meta--dim {
  font-size: 10px;
}

@media (max-width: 340px) {
  .cd {
    gap: 4px;
  }
  .cd__field {
    height: 110px;
  }
  .cd__meta--dim {
    display: none;
  }
}
@media (prefers-reduced-motion: reduce) {
  .cd.is-still .cd__field {
    cursor: default !important;
  }
}

// 산 부식(07). 재료 코드: 0 빈칸 1 모래 10 흙 11 돌 12 산.
// 산 칸은 물처럼 아래→대각→좌우로 이동하되, 이동 전 아래·좌·우 이웃이 흙/돌/모래면 그 이웃을 지우고(부식)
// 확률 EAT_P 로 산 자신도 소모돼 사라진다. 3~4 단 계단형 지형(흙+돌) 위 얇은 모래층을 상단 좌·우
// 스포너에서 떨어지는 산이 뚫고 내려가며 지하 통로를 판다.
(function () {
  // ── 변형 상수 블록 ──
  var EAT_P = 0.3;      // 부식 성공 시 산이 소모될 확률 — 낮추면 산이 오래 남아 통로가 굵어진다
  var SPAWN_EVERY = 7; // 산 스포너 간격(틱) — 줄이면 산이 더 자주 떨어진다
  // ────────────────────────────

  var EMPTY = 0, SAND = 1, DIRT = 10, STONE = 11, ACID = 12;
  var root = document.getElementById('cdRoot');
  var field = document.getElementById('cdField');
  var cv = document.getElementById('cdCv');
  var ctx = cv.getContext('2d');

  function makeRand(seed) {
    var st = seed >>> 0;
    return function () {
      st = (st + 0x6D2B79F5) >>> 0;
      var t = st;
      t = Math.imul(t ^ (t >>> 15), t | 1);
      t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
      return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
    };
  }
  var rand = makeRand(0x318A07);

  function hexToRgb(hex) {
    hex = hex.trim();
    if (hex.length === 4) hex = '#' + hex[1] + hex[1] + hex[2] + hex[2] + hex[3] + hex[3];
    var n = parseInt(hex.slice(1), 16);
    return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var CELL = 4;
  var cols = 0, rows = 0, grid = null;
  var COL = {};
  // 지형 연출 — 프레임0 은 빈 무대에서 시작해 TERRAIN_TICKS 틱에 걸쳐 계단 지형이 위에서부터 굳는다.
  // 채워지는 지형 자체가 누적면적을 만들고(장면의 절반 이상), 굳는 동안 모래가 흘러내리는 낙하도 겹친다.
  var TERRAIN_TICKS = 115;

  function readColors() {
    var css = getComputedStyle(root);
    COL.night = hexToRgb(css.getPropertyValue('--sand-night'));
    COL.dirt = hexToRgb(css.getPropertyValue('--sand-wood'));
    COL.stone = hexToRgb(css.getPropertyValue('--sand-stone'));
    COL.sandA = hexToRgb(css.getPropertyValue('--sand-grain'));
    COL.sandB = hexToRgb(css.getPropertyValue('--sand-gold'));
    COL.acidA = hexToRgb(css.getPropertyValue('--sand-acid'));
    COL.acidB = hexToRgb(css.getPropertyValue('--sand-glow'));
    COL.mist = hexToRgb(css.getPropertyValue('--sand-mist'));
  }

  function idx(x, y) { return y * cols + x; }
  function get(x, y) {
    if (x < 0 || x >= cols || y < 0 || y >= rows) return STONE;
    return grid[idx(x, y)];
  }
  function set(x, y, v) {
    if (x < 0 || x >= cols || y < 0 || y >= rows) return;
    grid[idx(x, y)] = v;
  }

  function buildTerrainFull() {
    // 3~4 단 계단형 흙+돌 선반 — x 를 4 구간으로 나눠 단마다 깊이를 다르게
    var seg = Math.max(1, Math.floor(cols / 4));
    for (var x = 0; x < cols; x++) {
      var tier = Math.min(3, Math.floor(x / seg));
      var top = Math.floor(rows * (0.32 + tier * 0.15));
      for (var y = top; y < rows; y++) {
        var depth = y - top;
        set(x, y, depth < 2 ? DIRT : STONE);
      }
      // 선반 위 얇은 모래층 1~2 칸
      var sandDepth = 1 + (rand() < 0.4 ? 1 : 0);
      for (var sy = 0; sy < sandDepth; sy++) {
        var ty = top - 1 - sy;
        if (ty >= 0) set(x, ty, SAND);
      }
    }
  }

  // 지형 낙하 연출 — 각 칸에 '굳는 시각' 을 배정하고 update 마다 그 시각이 지난 칸만 채운다.
  // 위쪽 행부터 먼저 굳고 x 는 좌우에서 안쪽으로(협곡이 벌어지듯) 차오른다.
  var solidAt = null;
  function buildTerrainStagger() {
    solidAt = new Float32Array(cols * rows);
    var seg = Math.max(1, Math.floor(cols / 4));
    for (var x = 0; x < cols; x++) {
      var tier = Math.min(3, Math.floor(x / seg));
      var top = Math.floor(rows * (0.32 + tier * 0.15));
      var edgeDist = Math.min(x, cols - 1 - x) / (cols / 2); // 0(가장자리) → 1(중앙)
      for (var y = top; y < rows; y++) {
        var depthFrac = (y - top) / Math.max(1, rows - top);
        // 가장자리·얕은 쪽 먼저, 중앙 심부가 마지막에 굳는다
        solidAt[idx(x, y)] = TERRAIN_TICKS * (0.15 + 0.85 * edgeDist * 0.55 + 0.45 * depthFrac) * (0.85 + 0.3 * rand());
      }
      var sandDepth = 1 + (rand() < 0.4 ? 1 : 0);
      for (var sy = 0; sy < sandDepth; sy++) {
        var ty = top - 1 - sy;
        if (ty >= 0) solidAt[idx(x, ty)] = TERRAIN_TICKS * (0.7 + 0.5 * rand()) * (0.4 + 0.6 * edgeDist);
      }
    }
  }

  function resolveSolid(tick) {
    if (!solidAt) return;
    for (var y = 0; y < rows; y++) {
      for (var x = 0; x < cols; x++) {
        var i = idx(x, y);
        if (solidAt[i] > 0 && solidAt[i] <= tick && grid[i] === EMPTY) {
          // 굳는 시각에 도달한 칸 — 흙 표층(위 2칸) / 돌 심부 / 모래 얇은 층
          var seg = Math.max(1, Math.floor(cols / 4));
          var tier = Math.min(3, Math.floor(x / seg));
          var top = Math.floor(rows * (0.32 + tier * 0.15));
          if (y < top) grid[i] = SAND;
          else if (y - top < 2) grid[i] = DIRT;
          else grid[i] = STONE;
        }
      }
    }
  }

  function resetGrid() {
    grid = new Uint8Array(cols * rows);
    buildTerrainStagger();
  }

  function spawnAcid(tick) {
    if (tick % SPAWN_EVERY !== 0) return;
    var leftX = 1 + Math.floor(rand() * 3);
    var rightX = cols - 2 - Math.floor(rand() * 3);
    if (get(leftX, 0) === EMPTY) set(leftX, 0, ACID);
    if (get(rightX, 0) === EMPTY) set(rightX, 0, ACID);
  }

  function isErodible(m) { return m === DIRT || m === STONE || m === SAND; }

  function moveInto(x, y, nx, ny, m) {
    if (get(nx, ny) !== EMPTY) return false;
    set(x, y, EMPTY);
    set(nx, ny, m);
    return true;
  }

  function updateAcid(x, y, dir) {
    var m = get(x, y);
    if (m !== ACID) return;
    var spots = [[x, y + 1], [x - 1, y], [x + 1, y]];
    var melted = false;
    for (var i = 0; i < spots.length; i++) {
      var nx = spots[i][0], ny = spots[i][1];
      if (isErodible(get(nx, ny))) { set(nx, ny, EMPTY); melted = true; }
    }
    if (melted && rand() < EAT_P) { set(x, y, EMPTY); return; }
    if (moveInto(x, y, x, y + 1, ACID)) return;
    var first = dir > 0 ? -1 : 1;
    if (moveInto(x, y, x + first, y + 1, ACID)) return;
    if (moveInto(x, y, x - first, y + 1, ACID)) return;
    if (moveInto(x, y, x + first, y, ACID)) return;
    moveInto(x, y, x - first, y, ACID);
  }

  function updateSand(x, y, dir) {
    var m = get(x, y);
    if (m !== SAND) return;
    if (moveInto(x, y, x, y + 1, SAND)) return;
    var first = dir > 0 ? -1 : 1;
    if (moveInto(x, y, x + first, y + 1, SAND)) return;
    moveInto(x, y, x - first, y + 1, SAND);
  }

  var tick = 0;
  function update() {
    tick++;
    resolveSolid(tick);
    spawnAcid(tick);
    var dir = (tick % 2 === 0) ? 1 : -1;
    for (var y = rows - 1; y >= 0; y--) {
      if (dir > 0) {
        for (var x = 0; x < cols; x++) {
          var m = get(x, y);
          if (m === ACID) updateAcid(x, y, dir);
          else if (m === SAND) updateSand(x, y, dir);
        }
      } else {
        for (var x2 = cols - 1; x2 >= 0; x2--) {
          var m2 = get(x2, y);
          if (m2 === ACID) updateAcid(x2, y, dir);
          else if (m2 === SAND) updateSand(x2, y, dir);
        }
      }
    }
  }

  var buf = null, imgData = null;
  function ensureBuf() {
    imgData = ctx.createImageData(cv.width, cv.height);
    buf = imgData.data;
  }

  function shade(c, amt) {
    return [
      Math.max(0, Math.min(255, c[0] + amt)),
      Math.max(0, Math.min(255, c[1] + amt)),
      Math.max(0, Math.min(255, c[2] + amt))
    ];
  }

  function draw(t) {
    if (!buf) ensureBuf();
    var W = cv.width, H = cv.height;
    var wave = Math.sin(t * 0.00055) * 0.5 + 0.5;
    var cx = W * (0.5 + 0.34 * Math.sin(t * 0.00042));
    var cy = H * (0.5 + 0.34 * Math.cos(t * 0.00036));
    var maxD = Math.hypot(W, H) * 0.6;
    for (var y = 0; y < rows; y++) {
      for (var x = 0; x < cols; x++) {
        var m = grid[idx(x, y)];
        var px0 = x * CELL, py0 = y * CELL;
        var d = Math.hypot(px0 - cx, py0 - cy) / maxD;
        var glow = Math.max(0, 1 - d) * (0.08 + 0.05 * wave);
        var col;
        if (m === EMPTY) {
          col = shade(COL.night, Math.round(glow * 40));
        } else if (m === DIRT) {
          col = shade(COL.dirt, ((x * 7 + y * 13) % 5) - 2 + Math.round(glow * 14));
        } else if (m === STONE) {
          col = shade(COL.stone, ((x * 11 + y * 5) % 5) - 2 + Math.round(glow * 10));
        } else if (m === SAND) {
          col = ((x + y) % 3 === 0) ? shade(COL.sandA, Math.round(glow * 18)) : shade(COL.sandB, Math.round(glow * 18));
        } else if (m === ACID) {
          var flick = Math.sin(t * 0.02 + x * 0.7 + y * 0.5) * 0.5 + 0.5;
          col = shade(flick > 0.5 ? COL.acidB : COL.acidA, Math.round(glow * 20));
        } else {
          col = COL.night;
        }
        for (var cyi = 0; cyi < CELL; cyi++) {
          var rowOff = ((py0 + cyi) * W + px0) * 4;
          for (var cxi = 0; cxi < CELL; cxi++) {
            var o = rowOff + cxi * 4;
            buf[o] = col[0]; buf[o + 1] = col[1]; buf[o + 2] = col[2]; buf[o + 3] = 255;
          }
        }
      }
    }
    ctx.putImageData(imgData, 0, 0);
  }

  function fit() {
    readColors();
    var r = field.getBoundingClientRect();
    cv.width = Math.max(1, Math.round(r.width));
    cv.height = Math.max(1, Math.round(r.height));
    cols = Math.max(1, Math.floor(cv.width / CELL));
    rows = Math.max(1, Math.floor(cv.height / CELL));
    buf = null;
    resetGrid();
  }

  function stillFrame() {
    buildTerrainFull();
    solidAt = null;
    for (var i = 0; i < 90; i++) update();
    draw(1500);
    root.classList.add('is-still');
  }

  var last = 0, acc = 0;
  var STEP = 1000 / 60;
  function frame(now) {
    if (!last) last = now;
    acc += now - last;
    last = now;
    var guard = 0;
    while (acc >= STEP && guard < 6) { update(); acc -= STEP; guard++; }
    draw(now);
    requestAnimationFrame(frame);
  }

  window.addEventListener('resize', fit);
  fit();
  if (reduce) { stillFrame(); } else { requestAnimationFrame(frame); }
})();

// AcidDissolve — vanilla 판(run/318/demo/07-sand-acid-dissolve-react/index.html) 의 React 포트.
// 산 칸이 아래·양옆 이웃을 지우고(부식) 확률로 자신도 소모되며, 물처럼 아래→대각→좌우로 이동한다.
// 계단형 흙·돌 선반 위 얇은 모래층을 상단 좌·우 스포너의 산이 뚫고 지하 통로를 파는 장면.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./AcidDissolve.module.scss";

export type AcidDissolveProps = {
  duration?: string;
  easing?: string;
  color?: string;
  eatP?: number;        // 부식 성공 시 산이 소모될 확률(0~1)
  spawnEvery?: number;  // 산 스포너 간격(틱)
  cellPx?: number;      // 셀 크기(px)
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type RGB = [number, number, number];
type Colors = {
  night: RGB; dirt: RGB; stone: RGB; sandA: RGB; sandB: RGB;
  acidA: RGB; acidB: RGB; mist: RGB;
};

const FIELD_LABEL = "산 부식 셀 오토마타 데모";
const EMPTY = 0, SAND = 1, DIRT = 10, STONE = 11, ACID = 12;

function makePRNG(seed: number): () => number {
  let s = seed >>> 0;
  return function () {
    s = (s + 0x6d2b79f5) >>> 0;
    let t = s;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

export default function AcidDissolve({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#b9a0ff",
  eatP = 0.3,
  spawnEvery = 7,
  cellPx = 4,
}: AcidDissolveProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const cvRef = useRef<HTMLCanvasElement | null>(null);
  const ctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const colRef = useRef<Colors | null>(null);
  const gridRef = useRef(new Uint8Array(0)) as Box<Uint8Array>;
  const dimRef = useRef({ cols: 0, rows: 0 }) as Box<{ cols: number; rows: number }>;
  const cellRef = useRef(cellPx) as Box<number>;
  const eatRef = useRef(eatP) as Box<number>;
  const spawnRef = useRef(spawnEvery) as Box<number>;
  const randRef = useRef(makePRNG(0x318a07)) as Box<() => number>;
  const tickRef = useRef(0) as Box<number>;
  const imgRef = useRef<ImageData | null>(null) as Box<ImageData | null>;
  const rafRef = useRef(0) as Box<number>;
  const [still, setStill] = useState(false);
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

  function reduced(): boolean {
    return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
  }

  function tokRGB(root: HTMLDivElement, name: string): RGB {
    const s = getComputedStyle(root).getPropertyValue(name).trim();
    const hx = s.length === 4
      ? s.charAt(1) + s.charAt(1) + s.charAt(2) + s.charAt(2) + s.charAt(3) + s.charAt(3)
      : s.slice(1);
    return [parseInt(hx.slice(0, 2), 16), parseInt(hx.slice(2, 4), 16), parseInt(hx.slice(4, 6), 16)];
  }

  function get(x: number, y: number): number {
    const { cols, rows } = dimRef.current;
    if (x < 0 || x >= cols || y < 0 || y >= rows) return STONE;
    return gridRef.current[y * cols + x];
  }
  function set(x: number, y: number, v: number): void {
    const { cols, rows } = dimRef.current;
    if (x < 0 || x >= cols || y < 0 || y >= rows) return;
    gridRef.current[y * cols + x] = v;
  }

  function buildTerrain(): void {
    const rand = randRef.current;
    const { cols, rows } = dimRef.current;
    const seg = Math.max(1, Math.floor(cols / 4));
    for (let x = 0; x < cols; x++) {
      const tier = Math.min(3, Math.floor(x / seg));
      const top = Math.floor(rows * (0.32 + tier * 0.15));
      for (let y = top; y < rows; y++) {
        const depth = y - top;
        set(x, y, depth < 2 ? DIRT : STONE);
      }
      const sandDepth = 1 + (rand() < 0.4 ? 1 : 0);
      for (let sy = 0; sy < sandDepth; sy++) {
        const ty = top - 1 - sy;
        if (ty >= 0) set(x, ty, SAND);
      }
    }
  }

  function isErodible(m: number): boolean { return m === DIRT || m === STONE || m === SAND; }

  function moveInto(x: number, y: number, nx: number, ny: number, m: number): boolean {
    if (get(nx, ny) !== EMPTY) return false;
    set(x, y, EMPTY);
    set(nx, ny, m);
    return true;
  }

  function updateAcid(x: number, y: number, dir: number): void {
    const rand = randRef.current;
    const spots: [number, number][] = [[x, y + 1], [x - 1, y], [x + 1, y]];
    let melted = false;
    for (let i = 0; i < spots.length; i++) {
      if (isErodible(get(spots[i][0], spots[i][1]))) { set(spots[i][0], spots[i][1], EMPTY); melted = true; }
    }
    if (melted && rand() < eatRef.current) { set(x, y, EMPTY); return; }
    if (moveInto(x, y, x, y + 1, ACID)) return;
    const first = dir > 0 ? -1 : 1;
    if (moveInto(x, y, x + first, y + 1, ACID)) return;
    if (moveInto(x, y, x - first, y + 1, ACID)) return;
    if (moveInto(x, y, x + first, y, ACID)) return;
    moveInto(x, y, x - first, y, ACID);
  }

  function updateSand(x: number, y: number, dir: number): void {
    if (moveInto(x, y, x, y + 1, SAND)) return;
    const first = dir > 0 ? -1 : 1;
    if (moveInto(x, y, x + first, y + 1, SAND)) return;
    moveInto(x, y, x - first, y + 1, SAND);
  }

  function update(): void {
    const rand = randRef.current;
    const { cols, rows } = dimRef.current;
    tickRef.current++;
    const tick = tickRef.current;
    if (tick % spawnRef.current === 0) {
      const leftX = 1 + Math.floor(rand() * 3);
      const rightX = cols - 2 - Math.floor(rand() * 3);
      if (get(leftX, 0) === EMPTY) set(leftX, 0, ACID);
      if (get(rightX, 0) === EMPTY) set(rightX, 0, ACID);
    }
    const dir = tick % 2 === 0 ? 1 : -1;
    for (let y = rows - 1; y >= 0; y--) {
      if (dir > 0) {
        for (let x = 0; x < cols; x++) {
          const m = get(x, y);
          if (m === ACID) updateAcid(x, y, dir);
          else if (m === SAND) updateSand(x, y, dir);
        }
      } else {
        for (let x = cols - 1; x >= 0; x--) {
          const m = get(x, y);
          if (m === ACID) updateAcid(x, y, dir);
          else if (m === SAND) updateSand(x, y, dir);
        }
      }
    }
  }

  function shade(c: RGB, amt: number): RGB {
    return [
      Math.max(0, Math.min(255, c[0] + amt)),
      Math.max(0, Math.min(255, c[1] + amt)),
      Math.max(0, Math.min(255, c[2] + amt)),
    ];
  }

  function draw(t: number): void {
    const ctx = ctxRef.current;
    const col = colRef.current;
    if (!ctx || !col) return;
    const W = sizeRef.current.w, H = sizeRef.current.h;
    const CELL = cellRef.current;
    const { cols, rows } = dimRef.current;
    let img = imgRef.current;
    if (!img || img.width !== W || img.height !== H) {
      img = ctx.createImageData(W, H);
      imgRef.current = img;
    }
    const buf = img.data;
    const grid = gridRef.current;
    const wave = Math.sin(t * 0.00055) * 0.5 + 0.5;
    const cx = W * (0.5 + 0.34 * Math.sin(t * 0.00042));
    const cy = H * (0.5 + 0.34 * Math.cos(t * 0.00036));
    const maxD = Math.hypot(W, H) * 0.6;
    for (let y = 0; y < rows; y++) {
      for (let x = 0; x < cols; x++) {
        const m = grid[y * cols + x];
        const px0 = x * CELL, py0 = y * CELL;
        const d = Math.hypot(px0 - cx, py0 - cy) / maxD;
        const glow = Math.max(0, 1 - d) * (0.08 + 0.05 * wave);
        let c: RGB;
        if (m === EMPTY) {
          c = shade(col.night, Math.round(glow * 40));
        } else if (m === DIRT) {
          c = shade(col.dirt, ((x * 7 + y * 13) % 5) - 2 + Math.round(glow * 14));
        } else if (m === STONE) {
          c = shade(col.stone, ((x * 11 + y * 5) % 5) - 2 + Math.round(glow * 10));
        } else if (m === SAND) {
          c = ((x + y) % 3 === 0)
            ? shade(col.sandA, Math.round(glow * 18))
            : shade(col.sandB, Math.round(glow * 18));
        } else {
          const flick = Math.sin(t * 0.02 + x * 0.7 + y * 0.5) * 0.5 + 0.5;
          c = shade(flick > 0.5 ? col.acidB : col.acidA, Math.round(glow * 20));
        }
        for (let cyi = 0; cyi < CELL; cyi++) {
          const rowOff = ((py0 + cyi) * W + px0) * 4;
          for (let cxi = 0; cxi < CELL; cxi++) {
            const o = rowOff + cxi * 4;
            buf[o] = c[0]; buf[o + 1] = c[1]; buf[o + 2] = c[2]; buf[o + 3] = 255;
          }
        }
      }
    }
    ctx.putImageData(img, 0, 0);
  }

  function fit(): void {
    const field = fieldRef.current;
    const cv = cvRef.current;
    if (!field || !cv) return;
    const ctx = cv.getContext("2d");
    if (!ctx) return;
    const r = field.getBoundingClientRect();
    sizeRef.current.w = Math.max(1, Math.round(r.width));
    sizeRef.current.h = Math.max(1, Math.round(r.height));
    cv.width = sizeRef.current.w;
    cv.height = sizeRef.current.h;
    ctxRef.current = ctx;
    const CELL = cellRef.current;
    dimRef.current.cols = Math.max(1, Math.floor(sizeRef.current.w / CELL));
    dimRef.current.rows = Math.max(1, Math.floor(sizeRef.current.h / CELL));
    gridRef.current = new Uint8Array(dimRef.current.cols * dimRef.current.rows);
    imgRef.current = null;
    buildTerrain();
  }

  let last = 0, acc = 0;
  const STEP = 1000 / 60;
  function frame(now: number): void {
    if (!last) last = now;
    acc += now - last;
    last = now;
    let guard = 0;
    while (acc >= STEP && guard < 6) { update(); acc -= STEP; guard++; }
    draw(now);
    rafRef.current = requestAnimationFrame(frame);
  }

  useEffect(() => {
    const root = rootRef.current;
    if (!root) return;
    cellRef.current = cellPx;
    eatRef.current = eatP;
    spawnRef.current = spawnEvery;
    randRef.current = makePRNG(0x318a07);
    tickRef.current = 0;
    colRef.current = {
      night: tokRGB(root, "--sand-night"),
      dirt: tokRGB(root, "--sand-wood"),
      stone: tokRGB(root, "--sand-stone"),
      sandA: tokRGB(root, "--sand-grain"),
      sandB: tokRGB(root, "--sand-gold"),
      acidA: tokRGB(root, "--sand-acid"),
      acidB: tokRGB(root, "--sand-glow"),
      mist: tokRGB(root, "--sand-mist"),
    };
    fit();
    function onResize(): void { last = 0; acc = 0; fit(); }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      for (let i = 0; i < 90; i++) update();
      draw(1500);
      setStill(true);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [eatP, spawnEvery, cellPx]);

  const rootCls = still ? `${styles.root} ${styles.isStill}` : styles.root;

  return (
    <main className={styles.stage}>
      <div className={rootCls} ref={rootRef} style={vars}>
        <p className={styles.hint}>
          <span>산이 흙과 돌을 지우며 통로를 팝니다</span>
        </p>
        <div className={styles.field} ref={fieldRef} role="application" aria-label={FIELD_LABEL} tabIndex={0}>
          <canvas className={styles.cv} ref={cvRef}></canvas>
        </div>
        <div className={styles.bar}>
          <span className={styles.meta}><b className={styles.val}>{Math.round(eatP * 100)}</b>% 부식 확률 · 산-흙-돌 반응</span>
          <span className={styles.metaDim}>canvas · ImageData putImageData · requestAnimationFrame</span>
        </div>
      </div>
    </main>
  );
}