08Hourglass timer — walls channeling the fall into a readable time display

<main class="stage">
<div class="cd is-demo" id="cdRoot">
  <p class="cd__hint"><span>Passing the narrow neck, the top empties and the bottom fills</span></p>
  <div class="cd__field" id="cdField" role="application" aria-label="Hourglass 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">3</b>cells neck width · hourglass fall</span>
    <span class="cd__meta cd__meta--dim">canvas · fillRect · requestAnimationFrame</span>
  </div>
</div>
</main>

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

// 모래시계 타이머(08). @keyframes 을 아예 두지 않는다 —
// 움직임은 전부 캔버스 rAF 루프(낙하 + 대각 미끄짐)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;
$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;

.cd {
  --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 절반쯤 낙하가 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;
  }
}

// 모래시계 타이머(08). 재료 코드: 0 빈칸 1 모래 3 벽(고정).
// 위 원뿔 + 아래 역원뿔 모양으로 벽을 1 칸 두께 계단선으로 그려 목 폭 NECK 칸을 만들고,
// 위칸을 모래로 가득 채운다. 규칙은 기본 낙하(아래 비었으면 스왑) + 대각 미끄짐(SLIDE_P) 뿐 —
// 벽이 유도하는 것만으로 위가 비고 아래가 차는 모래시계 리듬이 나온다.
(function () {
  // ── 변형 상수 블록 ──
  var NECK = 3;       // 목 폭(칸) — 늘리면 낙하가 빨라져 시계가 급하게 다 찬다
  var SLIDE_P = 1.0;  // 사면 대각 미끄짐 확률 — 낮추면 모래가 봉긋하게 쌓인다
  // ────────────────────────────

  var EMPTY = 0, SAND = 1, WALL = 3;
  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(0x318A08);

  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];
  }
  function rgba(c, a) { return 'rgba(' + c[0] + ',' + c[1] + ',' + c[2] + ',' + a + ')'; }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var CELL = 4;
  var cols = 0, rows = 0, grid = null;
  var COL = {};
  var leftWall = null, rightWall = null; // 행별 벽 칼럼(좌/우)

  function readColors() {
    var css = getComputedStyle(root);
    COL.night = hexToRgb(css.getPropertyValue('--sand-night'));
    COL.mist = hexToRgb(css.getPropertyValue('--sand-mist'));
    COL.ice = hexToRgb(css.getPropertyValue('--sand-ice'));
    COL.sandA = hexToRgb(css.getPropertyValue('--sand-grain'));
    COL.sandB = hexToRgb(css.getPropertyValue('--sand-gold'));
    COL.glow = hexToRgb(css.getPropertyValue('--sand-glow'));
  }

  function idx(x, y) { return y * cols + x; }
  function get(x, y) {
    if (x < 0 || x >= cols || y < 0 || y >= rows) return WALL;
    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 buildHourglass() {
    var cx = cols / 2;
    var neckHalf = NECK / 2;
    var edgeMargin = Math.max(1, Math.round(cols * 0.04)); // 화면 폭 대부분을 쓰도록 가장자리 여백 최소화
    var wideHalf = cols / 2 - edgeMargin;
    var neckBand = Math.max(2, Math.round(rows * 0.05));
    var mid = rows / 2;
    leftWall = new Int16Array(rows);
    rightWall = new Int16Array(rows);
    for (var y = 0; y < rows; y++) {
      var half;
      if (Math.abs(y - mid) <= neckBand) {
        half = neckHalf;
      } else if (y < mid) {
        var f = 1 - (mid - neckBand - y) / Math.max(1, mid - neckBand);
        half = neckHalf + (wideHalf - neckHalf) * Math.max(0, Math.min(1, 1 - f));
      } else {
        var f2 = (y - mid - neckBand) / Math.max(1, rows - 1 - mid - neckBand);
        half = neckHalf + (wideHalf - neckHalf) * Math.max(0, Math.min(1, f2));
      }
      leftWall[y] = Math.round(cx - half);
      rightWall[y] = Math.round(cx + half);
    }
    for (var yy = 0; yy < rows; yy++) {
      set(leftWall[yy], yy, WALL);
      set(rightWall[yy], yy, WALL);
    }
    // 위칸 가득 채우기 — mid - neckBand 위쪽 내부 전부 모래
    var fillBottom = Math.floor(mid - neckBand);
    for (var y2 = 0; y2 < fillBottom; y2++) {
      for (var x2 = leftWall[y2] + 1; x2 < rightWall[y2]; x2++) {
        set(x2, y2, SAND);
      }
    }
  }

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

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

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

  var tick = 0;
  var cycleTick = 0;            // 뒤집기 후 경과 틱
  var CYCLE_TICKS = 150;        // 한 뒤집기 주기(틱) — 위칸이 다시 채워지는 시간 포함
  var lastMove = 0;             // 마지막 모래 이동 틱(정지 감지)
  function update() {
    tick++;
    cycleTick++;
    var dir = (tick % 2 === 0) ? 1 : -1;
    var moved = false;
    for (var y = rows - 2; y >= 0; y--) {
      if (dir > 0) {
        for (var x = 0; x < cols; x++) { if (updateSand(x, y, dir)) moved = true; }
      } else {
        for (var x2 = cols - 1; x2 >= 0; x2--) { if (updateSand(x2, y, dir)) moved = true; }
      }
    }
    if (moved) lastMove = tick;
    // 뒤집기 — 낙하가 멎은 뒤 잠깐 멈춤 보여주고 위칸을 다시 채운다(무한 타이머)
    if (!moved && tick - lastMove > 12 && cycleTick > 40) {
      grid = new Uint8Array(cols * rows);
      buildHourglass();
      cycleTick = 0;
      lastMove = tick;
    }
  }

  function draw(t) {
    var W = cv.width, H = cv.height;
    var wave = Math.sin(t * 0.00055) * 0.5 + 0.5;
    var cx = W * (0.5 + 0.3 * Math.sin(t * 0.00042));
    var cy = H * (0.5 + 0.3 * Math.cos(t * 0.00036));
    var maxD = Math.hypot(W, H) * 0.6;
    var grad = ctx.createRadialGradient(cx, cy, 0, cx, cy, maxD);
    grad.addColorStop(0, rgba(COL.glow, 0.10 + 0.05 * wave));
    grad.addColorStop(1, rgba(COL.night, 0));
    ctx.fillStyle = rgba(COL.night, 1);
    ctx.fillRect(0, 0, W, H);
    ctx.fillStyle = grad;
    ctx.fillRect(0, 0, W, H);
    for (var y = 0; y < rows; y++) {
      for (var x = 0; x < cols; x++) {
        var m = grid[idx(x, y)];
        if (m === EMPTY) continue;
        var px = x * CELL, py = y * CELL;
        if (m === WALL) {
          ctx.fillStyle = rgba(COL.mist, 0.42);
        } else {
          ctx.fillStyle = rgba(((x + y) % 3 === 0) ? COL.sandA : COL.sandB, 1);
        }
        ctx.fillRect(px, py, CELL, CELL);
      }
    }
  }

  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));
    resetGrid();
  }

  function stillFrame() {
    for (var i = 0; i < 220; i++) update();
    draw(3600);
    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); }
})();

// HourglassTimer — vanilla 판(run/318/demo/08-sand-hourglass-timer/index.html) 의 React 포트.
// 위 원뿔 + 아래 역원뿔 벽(목 폭 neck 칸)을 계단선으로 그려 두고 위칸을 모래로 가득 채우면,
// 기본 낙하 + 대각 미끄짐 규칙만으로 위가 비고 아래가 차는 모래시계 리듬이 나온다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./HourglassTimer.module.scss";

export type HourglassTimerProps = {
  duration?: string;
  easing?: string;
  color?: string;
  neck?: number;     // 목 폭(칸)
  slideP?: number;   // 사면 대각 미끄짐 확률(0~1)
  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; mist: RGB; ice: RGB; sandA: RGB; sandB: RGB; glow: RGB };

const FIELD_LABEL = "모래시계 셀 오토마타 데모";
const EMPTY = 0, SAND = 1, WALL = 3;

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 HourglassTimer({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#b9a0ff",
  neck = 3,
  slideP = 1.0,
  cellPx = 4,
}: HourglassTimerProps) {
  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 neckRef = useRef(neck) as Box<number>;
  const slideRef = useRef(slideP) as Box<number>;
  const randRef = useRef(makePRNG(0x318a08)) as Box<() => number>;
  const tickRef = useRef(0) as Box<number>;
  const cycleRef = useRef(0) as Box<number>;
  const lastMoveRef = useRef(0) as Box<number>;
  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 rgba(c: RGB, a: number): string { return "rgba(" + c[0] + "," + c[1] + "," + c[2] + "," + a + ")"; }

  function get(x: number, y: number): number {
    const { cols, rows } = dimRef.current;
    if (x < 0 || x >= cols || y < 0 || y >= rows) return WALL;
    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 buildHourglass(): void {
    const { cols, rows } = dimRef.current;
    const cx = cols / 2;
    const neckHalf = neckRef.current / 2;
    const edgeMargin = Math.max(1, Math.round(cols * 0.04));
    const wideHalf = cols / 2 - edgeMargin;
    const neckBand = Math.max(2, Math.round(rows * 0.05));
    const mid = rows / 2;
    const leftWall = new Int16Array(rows);
    const rightWall = new Int16Array(rows);
    for (let y = 0; y < rows; y++) {
      let half: number;
      if (Math.abs(y - mid) <= neckBand) {
        half = neckHalf;
      } else if (y < mid) {
        const f = 1 - (mid - neckBand - y) / Math.max(1, mid - neckBand);
        half = neckHalf + (wideHalf - neckHalf) * Math.max(0, Math.min(1, 1 - f));
      } else {
        const f2 = (y - mid - neckBand) / Math.max(1, rows - 1 - mid - neckBand);
        half = neckHalf + (wideHalf - neckHalf) * Math.max(0, Math.min(1, f2));
      }
      leftWall[y] = Math.round(cx - half);
      rightWall[y] = Math.round(cx + half);
    }
    for (let y = 0; y < rows; y++) {
      set(leftWall[y], y, WALL);
      set(rightWall[y], y, WALL);
    }
    const fillBottom = Math.floor(mid - neckBand);
    for (let y = 0; y < fillBottom; y++) {
      for (let x = leftWall[y] + 1; x < rightWall[y]; x++) set(x, y, SAND);
    }
  }

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

  function updateSand(x: number, y: number, dir: number): boolean {
    const rand = randRef.current;
    if (get(x, y) !== SAND) return false;
    if (moveInto(x, y, x, y + 1)) return true;
    if (rand() > slideRef.current) return false;
    const first = dir > 0 ? -1 : 1;
    if (moveInto(x, y, x + first, y + 1)) return true;
    return moveInto(x, y, x - first, y + 1);
  }

  function update(): void {
    const { cols, rows } = dimRef.current;
    tickRef.current++;
    cycleRef.current++;
    const dir = tickRef.current % 2 === 0 ? 1 : -1;
    let moved = false;
    for (let y = rows - 2; y >= 0; y--) {
      if (dir > 0) {
        for (let x = 0; x < cols; x++) { if (updateSand(x, y, dir)) moved = true; }
      } else {
        for (let x = cols - 1; x >= 0; x--) { if (updateSand(x, y, dir)) moved = true; }
      }
    }
    if (moved) lastMoveRef.current = tickRef.current;
    // 뒤집기 — 낙하가 멎은 뒤 잠깐 멈춤 보여주고 위칸을 다시 채운다(무한 타이머)
    if (!moved && tickRef.current - lastMoveRef.current > 12 && cycleRef.current > 40) {
      gridRef.current = new Uint8Array(cols * rows);
      buildHourglass();
      cycleRef.current = 0;
      lastMoveRef.current = tickRef.current;
    }
  }

  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;
    const wave = Math.sin(t * 0.00055) * 0.5 + 0.5;
    const cx = W * (0.5 + 0.3 * Math.sin(t * 0.00042));
    const cy = H * (0.5 + 0.3 * Math.cos(t * 0.00036));
    const maxD = Math.hypot(W, H) * 0.6;
    const grad = ctx.createRadialGradient(cx, cy, 0, cx, cy, maxD);
    grad.addColorStop(0, rgba(col.glow, 0.10 + 0.05 * wave));
    grad.addColorStop(1, rgba(col.night, 0));
    ctx.fillStyle = rgba(col.night, 1);
    ctx.fillRect(0, 0, W, H);
    ctx.fillStyle = grad;
    ctx.fillRect(0, 0, W, H);
    const grid = gridRef.current;
    for (let y = 0; y < rows; y++) {
      for (let x = 0; x < cols; x++) {
        const m = grid[y * cols + x];
        if (m === EMPTY) continue;
        const px = x * CELL, py = y * CELL;
        if (m === WALL) ctx.fillStyle = rgba(col.mist, 0.42);
        else ctx.fillStyle = rgba((x + y) % 3 === 0 ? col.sandA : col.sandB, 1);
        ctx.fillRect(px, py, CELL, CELL);
      }
    }
  }

  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);
    buildHourglass();
  }

  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;
    neckRef.current = neck;
    slideRef.current = slideP;
    randRef.current = makePRNG(0x318a08);
    tickRef.current = 0;
    cycleRef.current = 0;
    lastMoveRef.current = 0;
    colRef.current = {
      night: tokRGB(root, "--sand-night"),
      mist: tokRGB(root, "--sand-mist"),
      ice: tokRGB(root, "--sand-ice"),
      sandA: tokRGB(root, "--sand-grain"),
      sandB: tokRGB(root, "--sand-gold"),
      glow: tokRGB(root, "--sand-glow"),
    };
    fit();
    function onResize(): void { last = 0; acc = 0; fit(); }
    window.addEventListener("resize", onResize);
    if (reduced()) {
      for (let i = 0; i < 220; i++) update();
      draw(3600);
      setStill(true);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [neck, slideP, 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>좁은 목을 통과해 위칸이 비고 아래칸이 찹니다</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}>{neck}</b>칸 목 폭 · 모래시계 낙하</span>
          <span className={styles.metaDim}>canvas · fillRect · requestAnimationFrame</span>
        </div>
      </div>
    </main>
  );
}