07フローフィールド

<svg width="0" height="0" style="position:absolute" aria-hidden="true">
  <defs>
    <symbol id="ico-wind" viewBox="0 0 256 256" fill="currentColor"><g><path d="M120 96a24 24 0 1 1 24-24a24 24 0 0 1-24 24m88-16a24 24 0 1 0 24 24a24 24 0 0 0-24-24m-56 80a24 24 0 1 0 24 24a24 24 0 0 0-24-24" opacity=".2"/><path d="M184 184a32 32 0 0 1-32 32c-13.7 0-26.95-8.93-31.5-21.22a8 8 0 0 1 15-5.56C137.74 195.27 145 200 152 200a16 16 0 0 0 0-32H40a8 8 0 0 1 0-16h112a32 32 0 0 1 32 32m-64-80a32 32 0 0 0 0-64c-13.7 0-26.95 8.93-31.5 21.22a8 8 0 0 0 15 5.56C105.74 60.73 113 56 120 56a16 16 0 0 1 0 32H24a8 8 0 0 0 0 16Zm88-32c-13.7 0-26.95 8.93-31.5 21.22a8 8 0 0 0 15 5.56C193.74 92.73 201 88 208 88a16 16 0 0 1 0 32H32a8 8 0 0 0 0 16h176a32 32 0 0 0 0-64"/></g></symbol>
    <symbol id="ico-reset" viewBox="0 0 256 256" fill="currentColor"><g><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z" opacity=".2"/><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z"/></g></symbol>
  </defs>
</svg>
<main class="stage">
<div class="ff is-demo" id="ff">
  <p class="ff__hint"><svg class="ff__ico" aria-hidden="true"><use href="#ico-wind"/></svg><span>マウスを動かした方向が風になり、ダッシュの群れがその方へ流れます</span></p>
  <div class="ff__field" id="field" role="application" aria-label="フローフィールドの実験台" tabindex="0">
    <span class="ff__parts" id="parts" aria-hidden="true"></span>
  </div>
  <div class="ff__bar">
    <p class="ff__dir"><span class="ff__dirLabel">今の風</span><b class="ff__dirBox"><span class="ff__dirVal" id="dirText">凪</span><span class="ff__dirLoop" aria-hidden="true"><i>右</i><i>左</i></span></b></p>
    <button class="ff__reset" id="reset" type="button">
      <svg class="ff__ico" aria-hidden="true"><use href="#ico-reset"/></svg><span>散らす</span>
    </button>
  </div>
</div>
</main>

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

// 플로우 필드 — 커서의 이동 속도가 곧 바람 벡터. 모든 대시가 그 벡터로 조타(steer)되고
// 가장자리는 반대편으로 이어진다. 대시는 진행 방향으로 rotate 까지 맞춰 돈다.
// 격자 미리보기(.is-demo)는 결정적 난수로 정한 중심(var(--hx), var(--hy)) 을 도는
// 닫힌 타원 궤도를 키프레임으로 재생한다 — 궤도가 닫혀 있어 경계 순간이동이 없다.

$duration: $dur-loop;
$easing: linear;       // 궤도 속도는 키프레임 표본 비율에 들어 있다
$color: $stage-yellow;

@include stage($stage-yellow);

.ff { display: flex; flex-direction: column; gap: $sp-3; width: 100%; max-width: 424px; }
.ff__hint {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); color: $subject-ink;
}
.ff__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $stage-orange; }

.ff__field {
  position: relative; height: 176px; border-radius: $r-card;
  background: rgba($stage-paper, .42);
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .14);
  overflow: hidden; touch-action: none;
}
.ff__field:focus-visible { outline: 2px solid $subject-ink; outline-offset: 2px; }

.ff__part {
  position: absolute; left: 0; top: 0; width: 14px; height: 4px;
  border-radius: $r-pill; background: $subject-ink;
}

.ff__bar { display: flex; align-items: center; justify-content: space-between; gap: $sp-2; flex-wrap: wrap; }
.ff__dir { margin: 0; display: flex; align-items: center; gap: $sp-1; }
.ff__dirLabel { font-weight: 700; font-size: clamp(10px, 2.6vw, 11px); color: $subject-ink; }
.ff__dirBox {
  position: relative; display: grid; place-items: center;
  min-width: 52px; height: 20px; border-radius: $r-xs; padding: 0 $sp-1;
  background: rgba($subject-ink, .12); color: $subject-ink;
  font-weight: 800; font-size: clamp(10px, 2.6vw, 11px); line-height: 1;
}
.ff__dirLoop { position: absolute; inset: 0; display: grid; place-items: center; visibility: hidden; }
.ff__dirLoop i { grid-area: 1 / 1; font-style: normal; visibility: hidden; }
.ff__reset {
  display: inline-flex; align-items: center; gap: $sp-1; border: 0; border-radius: $r-pill;
  padding: $sp-1 $sp-3; background: $subject-ink; color: $color; cursor: pointer;
  font-family: inherit; font-weight: 800; font-size: clamp(10px, 2.6vw, 11px); line-height: 1;
}
.ff__reset .ff__ico { color: $color; }

// ── 격자 미리보기 루프 ─────────────────────────────────────────────
// 대시마다 타원 한 바퀴(2s) — 진행 방향 rotate 가 붙어 흐르는 결처럼 보인다.
// 위상은 -i * 56ms 로 흩어 떼가 한꺼번에 도는 게 아니라 출렁이며 흐른다 (음수 지연이라 닫혀 있다).
.ff.is-demo .ff__part {
  animation-name: ffOrbit; animation-duration: $duration; animation-timing-function: $easing;
  animation-iteration-count: infinite; animation-fill-mode: backwards;
  animation-delay: calc(var(--i) * -56ms);
}
@keyframes ffOrbit {
  0%   { transform: translate(var(--hx), var(--hy)) rotate(0deg); }
  25%  { transform: translate(calc(var(--hx) + var(--rx)), calc(var(--hy) - var(--ry))) rotate(90deg); }
  50%  { transform: translate(var(--hx), calc(var(--hy) - var(--ry) * 2)) rotate(180deg); }
  75%  { transform: translate(calc(var(--hx) - var(--rx)), calc(var(--hy) - var(--ry))) rotate(270deg); }
  100% { transform: translate(var(--hx), var(--hy)) rotate(360deg); }
}
// 바람 방향 표시 — 오른쪽/왼쪽이 1초씩 steps 컷으로 바뀐다
.ff.is-demo .ff__dirVal { color: transparent; }
.ff.is-demo .ff__dirLoop { visibility: visible; }
.ff.is-demo .ff__dirLoop i {
  animation-duration: $duration; animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}
.ff.is-demo .ff__dirLoop i:nth-child(1) { animation-name: ffDirR; }
.ff.is-demo .ff__dirLoop i:nth-child(2) { animation-name: ffDirL; }
@keyframes ffDirR { 0%, 49% { visibility: visible; } 50%, 100% { visibility: hidden; } }
@keyframes ffDirL { 0%, 49% { visibility: hidden; } 50%, 100% { visibility: visible; } }

@media (max-width: 340px) {
  .ff { gap: $sp-2; }
  .ff__field { height: 104px; }
  .ff__part { width: 11px; height: 3px; }
  .ff__dirBox { min-width: 44px; }
}

// reduce 에서는 흩어진 대시가 제자리에 멈춘 한 장을 둔다
@media (prefers-reduced-motion: reduce) {
  .ff.is-demo .ff__part,
  .ff.is-demo .ff__dirLoop i { animation: none !important; }
  .ff.is-demo .ff__part { transform: translate(var(--hx), var(--hy)) !important; }
  .ff.is-demo .ff__dirVal { color: $subject-ink !important; }
  .ff.is-demo .ff__dirLoop { visibility: hidden !important; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.ff {
  display: flex;
  flex-direction: column;
  gap: 12px;
  width: 100%;
  max-width: 424px;
}

.ff__hint {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  color: #17141a;
}

.ff__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #ff4d1f;
}

.ff__field {
  position: relative;
  height: 176px;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.42);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.14);
  overflow: hidden;
  touch-action: none;
}

.ff__field:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 2px;
}

.ff__part {
  position: absolute;
  left: 0;
  top: 0;
  width: 14px;
  height: 4px;
  border-radius: 999px;
  background: #17141a;
}

.ff__bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  flex-wrap: wrap;
}

.ff__dir {
  margin: 0;
  display: flex;
  align-items: center;
  gap: 4px;
}

.ff__dirLabel {
  font-weight: 700;
  font-size: clamp(10px, 2.6vw, 11px);
  color: #17141a;
}

.ff__dirBox {
  position: relative;
  display: grid;
  place-items: center;
  min-width: 52px;
  height: 20px;
  border-radius: 4px;
  padding: 0 4px;
  background: rgba(23, 20, 26, 0.12);
  color: #17141a;
  font-weight: 800;
  font-size: clamp(10px, 2.6vw, 11px);
  line-height: 1;
}

.ff__dirLoop {
  position: absolute;
  inset: 0;
  display: grid;
  place-items: center;
  visibility: hidden;
}

.ff__dirLoop i {
  grid-area: 1/1;
  font-style: normal;
  visibility: hidden;
}

.ff__reset {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  border: 0;
  border-radius: 999px;
  padding: 4px 12px;
  background: #17141a;
  color: #ffd23f;
  cursor: pointer;
  font-family: inherit;
  font-weight: 800;
  font-size: clamp(10px, 2.6vw, 11px);
  line-height: 1;
}

.ff__reset .ff__ico {
  color: #ffd23f;
}

.ff.is-demo .ff__part {
  animation-name: ffOrbit;
  animation-duration: 2s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
  animation-delay: calc(var(--i) * -56ms);
}

@keyframes ffOrbit {
  0% {
    transform: translate(var(--hx), var(--hy)) rotate(0deg);
  }
  25% {
    transform: translate(calc(var(--hx) + var(--rx)), calc(var(--hy) - var(--ry))) rotate(90deg);
  }
  50% {
    transform: translate(var(--hx), calc(var(--hy) - var(--ry) * 2)) rotate(180deg);
  }
  75% {
    transform: translate(calc(var(--hx) - var(--rx)), calc(var(--hy) - var(--ry))) rotate(270deg);
  }
  100% {
    transform: translate(var(--hx), var(--hy)) rotate(360deg);
  }
}
.ff.is-demo .ff__dirVal {
  color: transparent;
}

.ff.is-demo .ff__dirLoop {
  visibility: visible;
}

.ff.is-demo .ff__dirLoop i {
  animation-duration: 2s;
  animation-timing-function: steps(1, end);
  animation-iteration-count: infinite;
}

.ff.is-demo .ff__dirLoop i:nth-child(1) {
  animation-name: ffDirR;
}

.ff.is-demo .ff__dirLoop i:nth-child(2) {
  animation-name: ffDirL;
}

@keyframes ffDirR {
  0%, 49% {
    visibility: visible;
  }
  50%, 100% {
    visibility: hidden;
  }
}
@keyframes ffDirL {
  0%, 49% {
    visibility: hidden;
  }
  50%, 100% {
    visibility: visible;
  }
}
@media (max-width: 340px) {
  .ff {
    gap: 8px;
  }
  .ff__field {
    height: 104px;
  }
  .ff__part {
    width: 11px;
    height: 3px;
  }
  .ff__dirBox {
    min-width: 44px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ff.is-demo .ff__part,
  .ff.is-demo .ff__dirLoop i {
    animation: none !important;
  }
  .ff.is-demo .ff__part {
    transform: translate(var(--hx), var(--hy)) !important;
  }
  .ff.is-demo .ff__dirVal {
    color: #17141a !important;
  }
  .ff.is-demo .ff__dirLoop {
    visibility: hidden !important;
  }
}

var root = document.getElementById('ff');
var field = document.getElementById('field');
var partsBox = document.getElementById('parts');
var dirText = document.getElementById('dirText');
var reduce = window.matchMedia('(prefers-reduced-motion: reduce)');
// 대시 36개 — 결정적 난수(사인 곱셈법)로 뿌린 자리가 시작점이다. 시드가 고정되어 매번 같은 배치로 재현된다
var N = 36, parts = [];
function rnd(seed) { var x = Math.sin(seed * 127.1) * 43758.5453; return x - Math.floor(x); }
function scatter() {
  var w = field.clientWidth, h = field.clientHeight;
  if (!parts.length) {
    for (var i = 0; i < N; i++) {
      var el = document.createElement('i');
      el.className = 'ff__part';
      el.style.setProperty('--i', String(i));
      partsBox.appendChild(el);
      parts.push({ el: el, x: 0, y: 0, vx: 12, vy: 0, ang: 0, seed: i + 1 });
    }
  }
  for (var j = 0; j < parts.length; j++) {
    var p = parts[j];
    p.x = (0.06 + rnd(p.seed) * 0.88) * w;
    p.y = (0.1 + rnd(p.seed + 60) * 0.8) * h;
    p.vx = 12; p.vy = 0; p.ang = 0;
    // 미리보기 궤도의 중심과 반지름 — 결정적 난수라 같은 배치가 매번 나온다
    p.el.style.setProperty('--hx', p.x.toFixed(1) + 'px');
    p.el.style.setProperty('--hy', p.y.toFixed(1) + 'px');
    p.el.style.setProperty('--rx', (10 + rnd(p.seed + 7) * 26).toFixed(1) + 'px');
    p.el.style.setProperty('--ry', (6 + rnd(p.seed + 13) * 16).toFixed(1) + 'px');
  }
}
var wind = { x: 12, y: 0 }, windUntil = 0, last = 0, running = false;
var DIRS = { e: '右', w: '左', s: '下', n: '上', calm: '凪' };

function paint() {
  for (var i = 0; i < parts.length; i++) {
    var p = parts[i];
    p.el.style.transform = 'translate(' + p.x.toFixed(1) + 'px, ' + p.y.toFixed(1) + 'px) rotate(' + p.ang.toFixed(1) + 'deg)';
  }
}
function dirName() {
  var ax = Math.abs(wind.x), ay = Math.abs(wind.y);
  if (ax < 8 && ay < 8) { return DIRS.calm; }
  if (ax > ay * 1.2) { return wind.x > 0 ? DIRS.e : DIRS.w; }
  if (ay > ax * 1.2) { return wind.y > 0 ? DIRS.s : DIRS.n; }
  return wind.x > 0 ? DIRS.e : DIRS.w;
}
function wake() {
  root.classList.remove('is-demo');
  if (reduce.matches) { paint(); return false; }
  if (!running) { running = true; last = performance.now(); requestAnimationFrame(step); }
  return true;
}
function step(now) {
  var dt = Math.min((now - last) / 1000, 0.033);
  last = now;
  var w = field.clientWidth, h = field.clientHeight;
  var gust = now - windUntil < 900;   // 최근 입력 바람이 살아 있는 동안 조타력이 세다
  for (var i = 0; i < parts.length; i++) {
    var p = parts[i];
    // 흐름장: 모든 대시가 같은 방향으로 조타된다 — 힘은 커서 속도에서 온다
    p.vx += (wind.x - p.vx) * (gust ? 2.4 : 0.4) * dt;
    p.vy += (wind.y - p.vy) * (gust ? 2.4 : 0.4) * dt;
    var sp = Math.sqrt(p.vx * p.vx + p.vy * p.vy);
    if (sp > 190) { p.vx *= 190 / sp; p.vy *= 190 / sp; }
    p.x += p.vx * dt;
    p.y += p.vy * dt;
    p.ang = Math.atan2(p.vy, p.vx) * 180 / Math.PI;
    // 가장자리에서 반대편으로 돌아온다 — 흐름장의 공간이 잇달아 이어진 셈이다
    if (p.x > w + 8) { p.x = -8; } else if (p.x < -8) { p.x = w + 8; }
    if (p.y > h + 8) { p.y = -8; } else if (p.y < -8) { p.y = h + 8; }
  }
  var name = dirName();
  if (dirText.textContent !== name) { dirText.textContent = name; }
  paint();
  requestAnimationFrame(step);
}
var prev = null;
field.addEventListener('pointermove', function (e) {
  var r = field.getBoundingClientRect();
  var x = e.clientX - r.left, y = e.clientY - r.top;
  if (prev) {
    var dx = x - prev.x, dy = y - prev.y;
    if (Math.abs(dx) + Math.abs(dy) > 2) {
      wind.x = dx * 9; wind.y = dy * 9;
      windUntil = performance.now();
      wake();
    }
  }
  prev = { x: x, y: y };
});
field.addEventListener('keydown', function (e) {
  var BLOW = 120;
  if (e.key === 'ArrowLeft') { wind.x = -BLOW; wind.y = 0; }
  else if (e.key === 'ArrowRight') { wind.x = BLOW; wind.y = 0; }
  else if (e.key === 'ArrowUp') { wind.x = 0; wind.y = -BLOW; }
  else if (e.key === 'ArrowDown') { wind.x = 0; wind.y = BLOW; }
  else { return; }
  e.preventDefault();
  windUntil = performance.now();
  wake();
});
document.getElementById('reset').addEventListener('click', function () {
  wind.x = 12; wind.y = 0;
  scatter(); paint();
});
window.addEventListener('resize', function () { scatter(); paint(); });
scatter(); paint();

// FlowField — vanilla 판(run/292/demo/07-physics-flow-field/index.html) 의 React 포트.
// 커서의 이동 속도가 곧 바람 벡터다. 대시 36개가 모두 그 벡터로 조타(steer)되고 최고 속도는 190,
// 가장자리에서는 반대편으로 이어진다. 진행 방향으로 rotate 까지 맞춘다. 상수·식은 vanilla 와 같은 값이다.
// 자리는 결정적 난수(사인 곱셈법)로 뿌리므로 매번 같은 배치가 나온다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties, PointerEvent as ReactPointerEvent } from "react";
import styles from "./FlowField.module.scss";

export type FlowFieldProps = {
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type PartVars = CSSProperties & { "--i"?: string; "--hx"?: string; "--hy"?: string; "--rx"?: string; "--ry"?: string };
// useRef 는 T | null 을 돌려준다. 처음부터 값이 있고 절대 비지 않는 상자는 이렇게 좁혀 쓴다
type Box<T> = { current: T };
// React 의 KeyboardEvent 제네릭이 없는 환경이라 필요한 것만 로컬로 선언한다
type KeyEvt = { key: string; preventDefault(): void };
type Part = { x: number; y: number; vx: number; vy: number; ang: number; seed: number };
type Seed = { hx: number; hy: number; rx: number; ry: number };

const N = 36;
const BLOW = 120; // 키보드 한 번의 바람 세기
const HINT = "마우스를 움직인 방향이 바람이 되어 대시 떼가 그 쪽으로 흘러갑니다";
const FIELD_LABEL = "흐름장 실험판";
const DIR_LABEL = "지금 바람";
const RESET_LABEL = "흩어놓기";
const DIRS = { e: "오른쪽", w: "왼쪽", s: "아래", n: "위", calm: "쉬는 중" };

function rnd(seed: number): number {
  const x = Math.sin(seed * 127.1) * 43758.5453;
  return x - Math.floor(x);
}

function WindIcon({ className }: { className: string }) {
  return (
    <svg viewBox="0 0 256 256" className={className} aria-hidden="true" fill="currentColor"><g><path d="M120 96a24 24 0 1 1 24-24a24 24 0 0 1-24 24m88-16a24 24 0 1 0 24 24a24 24 0 0 0-24-24m-56 80a24 24 0 1 0 24 24a24 24 0 0 0-24-24" opacity=".2"/><path d="M184 184a32 32 0 0 1-32 32c-13.7 0-26.95-8.93-31.5-21.22a8 8 0 0 1 15-5.56C137.74 195.27 145 200 152 200a16 16 0 0 0 0-32H40a8 8 0 0 1 0-16h112a32 32 0 0 1 32 32m-64-80a32 32 0 0 0 0-64c-13.7 0-26.95 8.93-31.5 21.22a8 8 0 0 0 15 5.56C105.74 60.73 113 56 120 56a16 16 0 0 1 0 32H24a8 8 0 0 0 0 16Zm88-32c-13.7 0-26.95 8.93-31.5 21.22a8 8 0 0 0 15 5.56C193.74 92.73 201 88 208 88a16 16 0 0 1 0 32H32a8 8 0 0 0 0 16h176a32 32 0 0 0 0-64"/></g></svg>
  );
}

function ResetIcon({ className }: { className: string }) {
  return (
    <svg viewBox="0 0 256 256" className={className} aria-hidden="true" fill="currentColor"><g><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z" opacity=".2"/><path d="M232 136a104 104 0 1 1-38.17-80.55L208 60v-4a8 8 0 0 1 16 0v34a8 8 0 0 1-8 8h-34a8 8 0 0 1 0-16h7.6l-8.89-7.4A88 88 0 1 0 216 136a8 8 0 0 1 16 0Z"/></g></svg>
  );
}

export default function FlowField({
  duration = 2000,
  easing = "linear",
  color = "#ffd23f",
}: FlowFieldProps) {
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const elsBox = useRef<Array<HTMLElement | null>>([]) as Box<Array<HTMLElement | null>>;
  const partsBox = useRef<Part[]>([]) as Box<Part[]>;
  const windBox = useRef({ x: 12, y: 0 }) as Box<{ x: number; y: number }>;
  const windUntilBox = useRef(0) as Box<number>;
  const prevBox = useRef<{ x: number; y: number } | null>(null) as Box<{ x: number; y: number } | null>;
  const lastBox = useRef(0) as Box<number>;
  const runningBox = useRef(false) as Box<boolean>;
  const frameBox = useRef(0) as Box<number>;
  const [seeds, setSeeds] = useState<Seed[]>([]);
  const [dir, setDir] = useState(DIRS.calm);
  const [demo, setDemo] = useState(true);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

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

  // 대시를 판 안에 결정적 난수로 뿌린다 — 미리보기 궤도의 중심(hx·hy)과 반지름(rx·ry)도 여기서 나온다
  function scatter(): void {
    const field = fieldRef.current;
    if (!field) { return; }
    const w = field.clientWidth;
    const h = field.clientHeight;
    const parts = partsBox.current;
    if (!parts.length) {
      for (let i = 0; i < N; i++) {
        parts.push({ x: 0, y: 0, vx: 12, vy: 0, ang: 0, seed: i + 1 });
      }
    }
    const next: Seed[] = [];
    for (let j = 0; j < parts.length; j++) {
      const p = parts[j];
      p.x = (0.06 + rnd(p.seed) * 0.88) * w;
      p.y = (0.1 + rnd(p.seed + 60) * 0.8) * h;
      p.vx = 12;
      p.vy = 0;
      p.ang = 0;
      next.push({
        hx: p.x,
        hy: p.y,
        rx: 10 + rnd(p.seed + 7) * 26,
        ry: 6 + rnd(p.seed + 13) * 16,
      });
    }
    setSeeds(next);
  }

  function paint(): void {
    const parts = partsBox.current;
    const els = elsBox.current;
    for (let i = 0; i < parts.length; i++) {
      const el = els[i];
      if (!el) { continue; }
      const p = parts[i];
      el.style.transform =
        "translate(" + p.x.toFixed(1) + "px, " + p.y.toFixed(1) + "px) rotate(" + p.ang.toFixed(1) + "deg)";
    }
  }

  function dirName(): string {
    const wind = windBox.current;
    const ax = Math.abs(wind.x);
    const ay = Math.abs(wind.y);
    if (ax < 8 && ay < 8) { return DIRS.calm; }
    if (ax > ay * 1.2) { return wind.x > 0 ? DIRS.e : DIRS.w; }
    if (ay > ax * 1.2) { return wind.y > 0 ? DIRS.s : DIRS.n; }
    return wind.x > 0 ? DIRS.e : DIRS.w;
  }

  function wake(): boolean {
    setDemo(false);
    if (reduced()) { paint(); return false; }
    if (!runningBox.current) {
      runningBox.current = true;
      lastBox.current = performance.now();
      frameBox.current = requestAnimationFrame(step);
    }
    return true;
  }

  function step(now: number): void {
    const field = fieldRef.current;
    if (!field) { runningBox.current = false; return; }
    const dt = Math.min((now - lastBox.current) / 1000, 0.033);
    lastBox.current = now;
    const w = field.clientWidth;
    const h = field.clientHeight;
    const gust = now - windUntilBox.current < 900; // 최근 입력 바람이 살아 있는 동안 조타력이 세다
    const parts = partsBox.current;
    const wind = windBox.current;
    for (let i = 0; i < parts.length; i++) {
      const p = parts[i];
      // 흐름장: 모든 대시가 같은 방향으로 조타된다 — 힘은 커서 속도에서 온다
      p.vx += (wind.x - p.vx) * (gust ? 2.4 : 0.4) * dt;
      p.vy += (wind.y - p.vy) * (gust ? 2.4 : 0.4) * dt;
      const sp = Math.sqrt(p.vx * p.vx + p.vy * p.vy);
      if (sp > 190) { p.vx *= 190 / sp; p.vy *= 190 / sp; }
      p.x += p.vx * dt;
      p.y += p.vy * dt;
      p.ang = (Math.atan2(p.vy, p.vx) * 180) / Math.PI;
      // 가장자리에서 반대편으로 돌아온다 — 흐름장의 공간이 잇달아 이어진 셈이다
      if (p.x > w + 8) { p.x = -8; } else if (p.x < -8) { p.x = w + 8; }
      if (p.y > h + 8) { p.y = -8; } else if (p.y < -8) { p.y = h + 8; }
    }
    const name = dirName();
    setDir(function (prev: string) { return prev === name ? prev : name; });
    paint();
    frameBox.current = requestAnimationFrame(step);
  }

  function onPointerMove(event: ReactPointerEvent<HTMLDivElement>): void {
    const field = fieldRef.current;
    if (!field) { return; }
    const r = field.getBoundingClientRect();
    const x = event.clientX - r.left;
    const y = event.clientY - r.top;
    const prev = prevBox.current;
    if (prev) {
      const dx = x - prev.x;
      const dy = y - prev.y;
      if (Math.abs(dx) + Math.abs(dy) > 2) {
        windBox.current.x = dx * 9;
        windBox.current.y = dy * 9;
        windUntilBox.current = performance.now();
        wake();
      }
    }
    prevBox.current = { x: x, y: y };
  }

  function onKeyDown(event: KeyEvt): void {
    const wind = windBox.current;
    if (event.key === "ArrowLeft") { wind.x = -BLOW; wind.y = 0; }
    else if (event.key === "ArrowRight") { wind.x = BLOW; wind.y = 0; }
    else if (event.key === "ArrowUp") { wind.x = 0; wind.y = -BLOW; }
    else if (event.key === "ArrowDown") { wind.x = 0; wind.y = BLOW; }
    else { return; }
    event.preventDefault();
    windUntilBox.current = performance.now();
    wake();
  }

  function onReset(): void {
    windBox.current.x = 12;
    windBox.current.y = 0;
    scatter();
  }

  useEffect(() => {
    scatter();
    const onResize = (): void => { scatter(); paint(); };
    window.addEventListener("resize", onResize);
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(frameBox.current);
    };
  }, []);

  // 대시가 새로 뿌려진 뒤에 현재 자리를 한 번 칠한다
  useEffect(() => { paint(); }, [seeds]);

  const cls = [styles.root, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls} style={vars}>
        <p className={styles.hint}><WindIcon className={styles.ico} /><span>{HINT}</span></p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label={FIELD_LABEL}
          tabIndex={0}
          onPointerMove={onPointerMove}
          onKeyDown={onKeyDown}
        >
          <span aria-hidden="true">
            {seeds.map((seed, i) => (
              <i
                key={i}
                className={styles.part}
                ref={(el: HTMLElement | null) => { elsBox.current[i] = el; }}
                style={{
                  "--i": String(i),
                  "--hx": seed.hx.toFixed(1) + "px",
                  "--hy": seed.hy.toFixed(1) + "px",
                  "--rx": seed.rx.toFixed(1) + "px",
                  "--ry": seed.ry.toFixed(1) + "px",
                } as PartVars}
              />
            ))}
          </span>
        </div>
        <div className={styles.bar}>
          <p className={styles.dir}>
            <span className={styles.dirLabel}>{DIR_LABEL}</span>
            <b className={styles.dirBox}>
              <span className={styles.dirVal}>{dir}</span>
              <span className={styles.dirLoop} aria-hidden="true"><i>{DIRS.e}</i><i>{DIRS.w}</i></span>
            </b>
          </p>
          <button className={styles.reset} type="button" onClick={onReset}>
            <ResetIcon className={styles.ico} /><span>{RESET_LABEL}</span>
          </button>
        </div>
      </div>
    </main>
  );
}