07Rip current — a swirling offshore patch

<main class="stage">
<div class="rp" id="rp">
  <p class="rp__hint">
    <svg class="rp__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M16 120c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".25"/><path d="M16 168c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".15"/><path d="M228.92 49.09a112 112 0 1 0 14 84.91a8 8 0 0 0-15.5-4a96 96 0 1 1-12-72.91A8 8 0 0 1 216 71.63V40a8 8 0 0 1-8-8h-31.63a8 8 0 0 1-5.65-13.09ZM64 104a12 12 0 1 1 12 12a12 12 0 0 1-12-12m40 64a12 12 0 1 1 12 12a12 12 0 0 1-12-12m72-16a16 16 0 1 1 16 16a16 16 0 0 1-16-16" opacity=".9"/></g></svg>
    <span>Layering localized rotation onto the wave field makes a swirling patch — hover to send the vortex core there</span>
  </p>
  <div class="rp__field" id="field" role="application" aria-label="Rip current playground" tabindex="0">
    <canvas class="rp__cv" id="cv"></canvas>
    <b class="rp__badge" aria-hidden="true"><span class="rp__badgeNum" id="swirlVal">swirl 1.0</span><span class="rp__badgeUnit" id="windVal">wind 1.0</span></b>
  </div>
</div>
</main>

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

// 파도 소용돌이 — 파도장 위에 국소 회전(와류)을 얹는 이안류 패치.
// 움직임은 전부 캔버스 rAF 루프 안에 있다. @keyframes 를 두지 않는다(077 방식).
// 색은 전부 토큰 또는 mix() 로컬 변수(BD26) — 헥스 리터럴 금지.
// 캔버스가 쓰는 색은 커스텀 프로퍼티로 내려보낸다 — 값은 이 파일 한 곳에만 둔다.

$rp-crest:  mix($subject-mint, $stage-paper, 72%);
$rp-crest2: mix($subject-sky,  $stage-paper, 66%);
$rp-trough: mix($subject-blue-deep, $stage-paper, 52%);
$rp-trough2: mix($subject-blue, $stage-paper, 58%);
$rp-foam:   mix($subject-cream, $stage-paper, 88%);   // 와류 중심 거품 — 밝은 크림

$duration: $dur-loop;
$easing: $ease-spring;
$color: $rp-foam;

@include stage($stage-paper);

.rp {
  --crest:   #{$rp-crest};
  --crest-2: #{$rp-crest2};
  --trough:  #{$rp-trough};
  --trough-2: #{$rp-trough2};
  --foam: #{$rp-foam};

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

.rp__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: $subject-ink;
}
.rp__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $subject-blue; }

.rp__field {
  position: relative; height: 208px;
  border-radius: $r-card; overflow: hidden;
  background: $rp-trough2;
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .16);
  cursor: crosshair; touch-action: none;
}
.rp__field:focus-visible { outline: 2px solid $subject-blue; outline-offset: 2px; }

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

// 와류 세기·바람 세기를 화면에도 적어 둔다 — frame() 이 갱신한다
.rp__badge {
  position: absolute; right: $sp-2; top: $sp-2;
  display: flex; align-items: baseline; gap: $sp-1;
  padding: $sp-1 $sp-2; border-radius: $r-pill;
  background: rgba($subject-ink, .78);
  color: $subject-cream; font-weight: 700; font-size: 11px;
}
.rp__badgeNum { font-size: 13px; color: $stage-yellow; }
.rp__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .rp { gap: $sp-1; }
  .rp__field { height: 124px; }
  .rp__badge { font-size: 10px; padding: 0 $sp-1; }
  .rp__badgeNum { font-size: 11px; }
  .rp__hint { font-size: 10px; }
}

// reduce — 루프가 아예 안 돈다. 소용돌이가 살아 있는 정적 한 장이 남으므로
// 끌 수 있다는 신호(커서)만 거둔다.
@media (prefers-reduced-motion: reduce) {
  .rp.is-still .rp__field { cursor: default !important; }
  .rp.is-still .rp__badge { opacity: .92 !important; }
}

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

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

.rp {
  --crest: rgb(49.4588235294%, 87.8588235294%, 74.8705882353%);
  --crest-2: rgb(62.4705882353%, 85.7647058824%, 100%);
  --trough: rgb(50.0392156863%, 64.1098039216%, 95.3098039216%);
  --trough-2: rgb(52.6901960784%, 66.7921568627%, 97.9529411765%);
  --foam: rgb(100%, 97.2392156863%, 91.3725490196%);
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.rp__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: #17141a;
}

.rp__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #2f6df6;
}

.rp__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: rgb(52.6901960784%, 66.7921568627%, 97.9529411765%);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: crosshair;
  touch-action: none;
}

.rp__field:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

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

.rp__badge {
  position: absolute;
  right: 8px;
  top: 8px;
  display: flex;
  align-items: baseline;
  gap: 4px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.78);
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
}

.rp__badgeNum {
  font-size: 13px;
  color: #ffd23f;
}

.rp__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .rp {
    gap: 4px;
  }
  .rp__field {
    height: 124px;
  }
  .rp__badge {
    font-size: 10px;
    padding: 0 4px;
  }
  .rp__badgeNum {
    font-size: 11px;
  }
  .rp__hint {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .rp.is-still .rp__field {
    cursor: default !important;
  }
  .rp.is-still .rp__badge {
    opacity: 0.92 !important;
  }
}

// 파도 소용돌이(이안류 패치) — 01 과 같은 뿌리(정현파 6개 mulberry32(7))의 파도장 위에
// 국소 와류를 한 겹 얹는다. swirl 유도체 접근: 각 셀의 좌표를 와류 중심 기준 극좌표로 바꿔
// 위상에 -각도(swirl)와 반경 비례 감쇠(ramp)를 더한다. 위상이 비틀리면 정현파의 마루·골이
// 와류 주변에서 나선처럼 감기며 "파도가 뭉쳐 바깥으로 빠져나가는" 이안류의 인상을 준다.
// 와류 중심은 자동 모드에서 리사주로 천천히 옮겨 다니고, 포인터가 있으면 그 위치를 따라간다.
// 와류 주변 밝기(foam 농도)는 반경 가우시안으로 부풀려 중심이 물빛보다 밝게 뜨게 한다.
var root = document.getElementById('rp');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var swirlVal = document.getElementById('swirlVal');
var windVal = document.getElementById('windVal');
var ctx = cv.getContext('2d');

var css = getComputedStyle(root);
function v(name) { return css.getPropertyValue(name).trim(); }
var CREST = [v('--crest'), v('--crest-2')];   // 마루 색 램프 — 민트 계열
var TROUGH = [v('--trough'), v('--trough-2')]; // 골 색 램프 — 파랑 계열
var FOAM = v('--foam');                        // 와류 중심 foam — 밝은 크림

var COLS = 110;   // 파도장 가로 칸 수(01 과 동일)
var STEP = 3;     // 격자 한 칸이 렌더 버퍼에서 차지하는 픽셀
var NW = 6;       // 겹칠 정현파 개수(공통 물리)
var SWIRL = 2.6;  // 와류 세기 — 위상을 이만큼 비튼다(라디안 스케일)
var VORTEX_R = 30;// 와류 반경(격자 칸 수) — 이 안에서 회전이 감쇠하며 살아 있다

function mulberry32(a) {
  return function () {
    a |= 0; a = (a + 0x6D2B79F5) | 0;
    var t = Math.imul(a ^ (a >>> 15), 1 | a);
    var u = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((u ^ (u >>> 14)) >>> 0) / 4294967296;
  };
}
var rng = mulberry32(7);

var WAVES = [];
(function () {
  var lam = 34;
  for (var i = 0; i < NW; i++) {
    var ang = rng() * Math.PI * 2;
    WAVES.push({
      lam: lam,
      freq: (Math.PI * 2) / lam,
      dirX: Math.cos(ang), dirY: Math.sin(ang),
      amp: 0.58 / (i * 0.62 + 1),
      spd: 0.55 + rng() * 0.25,
      ph: rng() * Math.PI * 2
    });
    lam *= 0.55;
  }
})();
for (var w = 0; w < NW; w++) {
  WAVES[w].om = WAVES[w].spd * 3.6 * Math.sqrt(1 / WAVES[w].lam) * 6;
}

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var W = 0, H = 0, dpr = 1, ROWS = 0;
var hgt = null;                          // 파도장 높이(위상 비틀림 반영)
var foam = null;                         // 와류 foam 농도(0..1)
var buf = null, bctx = null, out = null;
var RW = 0, RH = 0;
var raf = 0, auto = true, nowSec = 0;
var wind = 1.0, windT = 1.0;             // wind 세기(실효 진폭)
var windAng = 0.5, windAngT = 0.5;       // wind 방향
var vx = 0.5, vy = 0.42;                 // 와류 중심(판 비율 0..1) 현재
var vxT = 0.5, vyT = 0.42;               // 목표(포인터 또는 자동 리사주)
var swirl = 1.0, swirlT = 1.0;           // 와류 세기 배율
var lastPX = -1, lastPY = -1, lastPT = 0;

function hex2rgb(s) {
  var pm = /^\s*rgb\(\s*([\d.]+)%\s*,\s*([\d.]+)%\s*,\s*([\d.]+)%\s*\)\s*$/.exec(s);
  if (pm) return [Math.round(pm[1] * 2.55), Math.round(pm[2] * 2.55), Math.round(pm[3] * 2.55)];
  s = s.replace('#', '');
  if (s.length === 3) s = s[0] + s[0] + s[1] + s[1] + s[2] + s[2];
  var n = parseInt(s, 16);
  return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
}
var C0 = hex2rgb(CREST[0]), C1 = hex2rgb(CREST[1]);
var T0 = hex2rgb(TROUGH[0]), T1 = hex2rgb(TROUGH[1]);
var FC = hex2rgb(FOAM);

function fit() {
  var r = field.getBoundingClientRect();
  dpr = window.devicePixelRatio || 1;
  W = r.width; H = r.height;
  cv.width = Math.round(W * dpr);
  cv.height = Math.round(H * dpr);
  ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
  ROWS = Math.max(40, Math.round(COLS * H / W));
  var n = COLS * ROWS;
  hgt = new Float32Array(n);
  foam = new Float32Array(n);
  RW = COLS * STEP; RH = ROWS * STEP;
  buf = document.createElement('canvas');
  buf.width = RW; buf.height = RH;
  bctx = buf.getContext('2d');
  out = bctx.createImageData(RW, RH);
}

// 파도장 + 와류 — height(x,y,t) = Σ amp·sin(위상 + 비틀림(각도·반경))
// 비틀림 twist = SWIRL·세기·(1 - r/R)·θ  … θ 는 중심 기준 방위각, r 은 반경.
// 반경이 R 을 넘으면 twist=0 — 와류가 국소 패치로만 살게.
function fieldcalc(t, wstr, wang, cx, cy, sw) {
  var i = 0;
  for (var y = 0; y < ROWS; y++) {
    for (var x = 0; x < COLS; x++, i++) {
      var dxv = x - cx, dyv = y - cy;
      var r = Math.sqrt(dxv * dxv + dyv * dyv);
      var ang = Math.atan2(dyv, dxv);
      var fall = r < VORTEX_R ? 1 - r / VORTEX_R : 0;
      fall *= fall;                              // 감쇠를 가속 — 중심만 또렷하게
      var twist = SWIRL * sw * fall * ang;
      var h = 0;
      for (var w2 = 0; w2 < NW; w2++) {
        var WV = WAVES[w2];
        var da = wang * 0.5;
        var dx = WV.dirX * Math.cos(da) - WV.dirY * Math.sin(da);
        var dy = WV.dirX * Math.sin(da) + WV.dirY * Math.cos(da);
        var ph = (x * dx + y * dy) * WV.freq * 0.62 + t * WV.om + WV.ph;
        h += WV.amp * wstr * Math.sin(ph + twist);
      }
      hgt[i] = h;
      // foam 농도 — 와류 중심 가우시안 + 파도 마루 겹침. 진폭이 뭉치는 자리가 밝다
      var foamv = fall * 0.85 + Math.max(0, h) * 0.5;
      if (foamv > 1) foamv = 1;
      foam[i] = foamv;
    }
  }
}

// 렌더 — 높이→색 램프(골 파랑 → 마루 민트) 뒤 foam 농도만큼 밝은 크림으로 당긴다.
// 와류가 옮겨 다니면 밝은 패치가 파도를 헤치고 다니는 게 눈에 띄어 누적면적이 크게 잡힌다.
function render() {
  var o = out.data;
  for (var y = 0; y < ROWS; y++) {
    var row = y * COLS;
    for (var x = 0; x < COLS; x++) {
      var i = row + x;
      var h = hgt[i];
      var u = h * 1.24 + 0.5;
      if (u < 0) u = 0; else if (u > 1) u = 1;
      var r, g, b;
      if (u > 0.5) {
        var k = (u - 0.5) * 2;
        r = C1[0] + (C0[0] - C1[0]) * k;
        g = C1[1] + (C0[1] - C1[1]) * k;
        b = C1[2] + (C0[2] - C1[2]) * k;
      } else {
        var k2 = u * 2;
        r = T0[0] + (T1[0] - T0[0]) * k2;
        g = T0[1] + (T1[1] - T0[1]) * k2;
        b = T0[2] + (T1[2] - T0[2]) * k2;
      }
      var f = foam[i];
      if (f > 0.02) {
        r = r + (FC[0] - r) * f * 0.8;
        g = g + (FC[1] - g) * f * 0.8;
        b = b + (FC[2] - b) * f * 0.8;
      }
      for (var sy = 0; sy < STEP; sy++) {
        var base = ((y * STEP + sy) * RW + x * STEP) * 4;
        for (var sx = 0; sx < STEP; sx++, base += 4) {
          o[base] = r;
          o[base + 1] = g;
          o[base + 2] = b;
          o[base + 3] = 255;
        }
      }
    }
  }
  bctx.putImageData(out, 0, 0);
  ctx.drawImage(buf, 0, 0, W, H);
}

function frame(now) {
  nowSec = now / 1000;
  if (auto) {
    var th = nowSec * 0.31;
    windT = 1.05 + Math.sin(th * 0.9 + 0.4) * 0.55 + Math.sin(th * 2.3) * 0.12;
    windAngT = 0.5 + Math.sin(th * 0.42) * 0.85;
    // 와류 중심이 판 안을 천천히 배회 — 두 주파수 리사주로 예측 불가능하게
    vxT = 0.5 + Math.sin(th * 0.37 + 1.2) * 0.3;
    vyT = 0.44 + Math.sin(th * 0.53 + 2.1) * 0.26;
    swirlT = 1.0 + Math.sin(th * 0.7) * 0.35;
  }
  wind += (windT - wind) * 0.05;
  windAng += (windAngT - windAng) * 0.05;
  vx += (vxT - vx) * 0.045;
  vy += (vyT - vy) * 0.045;
  swirl += (swirlT - swirl) * 0.04;
  fieldcalc(nowSec, wind, windAng, vx * COLS, vy * ROWS, swirl);
  render();
  swirlVal.textContent = 'swirl ' + swirl.toFixed(1);
  windVal.textContent = 'wind ' + wind.toFixed(2);
  raf = requestAnimationFrame(frame);
}

// 정지판 — 루프 없이 시드가 정한 한 시각의 와류를 계산해 소용돌이가 살아 있는 정적 한 장
function still() {
  fieldcalc(7.3, 1.35, 0.5, 0.46 * COLS, 0.42 * ROWS, 1.2);
  render();
}

function localXY(e) {
  var r = field.getBoundingClientRect();
  return [e.clientX - r.left, e.clientY - r.top];
}

field.addEventListener('pointermove', function (e) {
  var p = localXY(e);
  var t = performance.now();
  // 포인터 = 와류 중심 목표. 이동 벡터는 바람으로도 실측(01 방식)
  vxT = Math.max(0.04, Math.min(0.96, p[0] / W));
  vyT = Math.max(0.04, Math.min(0.96, p[1] / H));
  if (lastPX >= 0 && t > lastPT) {
    var dxp = p[0] - lastPX, dyp = p[1] - lastPY;
    var sp2 = Math.sqrt(dxp * dxp + dyp * dyp);
    if (sp2 > 0.5) {
      windT = Math.max(0.35, Math.min(2.1, 0.9 + sp2 * 0.02));
      windAngT = Math.atan2(dyp, dxp) * 0.16;
    }
  }
  lastPX = p[0]; lastPY = p[1]; lastPT = t;
  auto = false;
});
field.addEventListener('pointerleave', function () {
  auto = true;
  lastPX = -1;
});
field.addEventListener('keydown', function (e) {
  var d = { ArrowLeft: [-1, 0], ArrowRight: [1, 0], ArrowUp: [0, -1], ArrowDown: [0, 1] }[e.key];
  if (!d) return;
  e.preventDefault();
  auto = false;
  vxT = Math.max(0.05, Math.min(0.95, vxT + d[0] * 0.06));
  vyT = Math.max(0.05, Math.min(0.95, vyT + d[1] * 0.06));
  swirlT = Math.max(0.3, Math.min(1.9, swirlT + (d[1] < 0 ? 0.12 : -0.12)));
  if (reduce) { fieldcalc(nowSec || 3.1, windT, windAngT, vxT * COLS, vyT * ROWS, swirlT); render(); }
});
window.addEventListener('resize', function () { fit(); if (reduce) still(); });

fit();
if (reduce) {
  root.classList.add('is-still');
  still();
} else {
  raf = requestAnimationFrame(frame);
}

// RipCurrent — vanilla 판(run/313/demo/07-ocean-rip-current/index.html) 의 React 포트.
// 01(GerstnerSum)과 같은 뿌리(정현파 6개 mulberry32(7))의 파도장에 국소 와류(회전 유도체)를
// 한 겹 얹는다 — 셀 좌표를 와류 중심 기준 극좌표로 바꿔 위상에 각도·반경 감쇠를 더한다.
// 와류 중심은 자동 리사주로 배회하거나 포인터를 따라간다. 움직임은 rAF 루프가 전부.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./RipCurrent.module.scss";

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

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type PointerMotionEvent = { clientX: number; clientY: number };

const COLS = 110;
const STEP = 3;
const NW = 6;
const SWIRL = 2.6;
const VORTEX_R = 30;

function mulberry32(a: number): () => number {
  return function () {
    a |= 0; a = (a + 0x6D2B79F5) | 0;
    const t = Math.imul(a ^ (a >>> 15), 1 | a);
    const u = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((u ^ (u >>> 14)) >>> 0) / 4294967296;
  };
}

type Wave = {
  lam: number; freq: number; dirX: number; dirY: number;
  amp: number; spd: number; ph: number; om: number;
};

function buildWaves(): Wave[] {
  const rng = mulberry32(7);
  const out: Wave[] = [];
  let lam = 34;
  for (let i = 0; i < NW; i++) {
    const ang = rng() * Math.PI * 2;
    const wv: Wave = {
      lam,
      freq: (Math.PI * 2) / lam,
      dirX: Math.cos(ang),
      dirY: Math.sin(ang),
      amp: 0.58 / (i * 0.62 + 1),
      spd: 0.55 + rng() * 0.25,
      ph: rng() * Math.PI * 2,
      om: 0,
    };
    wv.om = wv.spd * 3.6 * Math.sqrt(1 / wv.lam) * 6;
    out.push(wv);
    lam *= 0.55;
  }
  return out;
}

function hex2rgb(s: string): [number, number, number] {
  const pm = /^\s*rgb\(\s*([\d.]+)%\s*,\s*([\d.]+)%\s*,\s*([\d.]+)%\s*\)\s*$/.exec(s);
  if (pm) return [Math.round(+pm[1] * 2.55), Math.round(+pm[2] * 2.55), Math.round(+pm[3] * 2.55)];
  let t = s.replace("#", "");
  if (t.length === 3) t = t[0] + t[0] + t[1] + t[1] + t[2] + t[2];
  const n = parseInt(t, 16);
  return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
}

export default function RipCurrent({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#f4ead2",
}: RipCurrentProps) {
  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 swirlValRef = useRef<HTMLSpanElement | null>(null);
  const windValRef = useRef<HTMLSpanElement | null>(null);
  const wavesRef = useRef<Wave[] | null>(null);
  const sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const rafRef = useRef(0) as Box<number>;
  const stillRef = useRef(false) as Box<boolean>;
  type St = {
    rows: number; rw: number; rh: number;
    hgt: Float32Array | null;
    foam: Float32Array | null;
    out: ImageData | null;
    bctx: CanvasRenderingContext2D | null;
    buf: HTMLCanvasElement | null;
    auto: boolean;
    wind: number; windT: number;
    windAng: number; windAngT: number;
    vx: number; vy: number; vxT: number; vyT: number;
    swirl: number; swirlT: number;
    lastPX: number; lastPY: number; lastPT: number;
    c0: [number, number, number] | null;
    c1: [number, number, number] | null;
    t0: [number, number, number] | null;
    t1: [number, number, number] | null;
    fc: [number, number, number] | null;
  };
  const stateRef = useRef({
    rows: 0, rw: 0, rh: 0,
    hgt: null as Float32Array | null,
    foam: null as Float32Array | null,
    out: null as ImageData | null,
    bctx: null as CanvasRenderingContext2D | null,
    buf: null as HTMLCanvasElement | null,
    auto: true,
    wind: 1.0, windT: 1.0,
    windAng: 0.5, windAngT: 0.5,
    vx: 0.5, vy: 0.42, vxT: 0.5, vyT: 0.42,
    swirl: 1.0, swirlT: 1.0,
    lastPX: -1, lastPY: -1, lastPT: 0,
    c0: null as [number, number, number] | null,
    c1: null as [number, number, number] | null,
    t0: null as [number, number, number] | null,
    t1: null as [number, number, number] | null,
    fc: null as [number, number, number] | null,
  } as St) as Box<St>;
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

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

  function fit(): void {
    const field = fieldRef.current, cv = cvRef.current;
    if (!field || !cv) return;
    const r = field.getBoundingClientRect();
    const dpr = window.devicePixelRatio || 1;
    sizeRef.current.w = r.width;
    sizeRef.current.h = r.height;
    cv.width = Math.round(r.width * dpr);
    cv.height = Math.round(r.height * dpr);
    const ctx = ctxRef.current;
    if (ctx) ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    const st = stateRef.current;
    st.rows = Math.max(40, Math.round(COLS * r.height / r.width));
    const n = COLS * st.rows;
    st.hgt = new Float32Array(n);
    st.foam = new Float32Array(n);
    st.rw = COLS * STEP;
    st.rh = st.rows * STEP;
    const buf = document.createElement("canvas");
    buf.width = st.rw; buf.height = st.rh;
    st.buf = buf;
    st.bctx = buf.getContext("2d");
    if (st.bctx) st.out = st.bctx.createImageData(st.rw, st.rh);
  }

  function fieldcalc(t: number, wstr: number, wang: number, cx: number, cy: number, sw: number): void {
    const st = stateRef.current;
    const WV = wavesRef.current;
    if (!st.hgt || !st.foam || !WV) return;
    let i = 0;
    for (let y = 0; y < st.rows; y++) {
      for (let x = 0; x < COLS; x++, i++) {
        const dxv = x - cx, dyv = y - cy;
        const r = Math.sqrt(dxv * dxv + dyv * dyv);
        const ang = Math.atan2(dyv, dxv);
        let fall = r < VORTEX_R ? 1 - r / VORTEX_R : 0;
        fall *= fall;
        const twist = SWIRL * sw * fall * ang;
        let h = 0;
        for (let k = 0; k < NW; k++) {
          const wv = WV[k];
          const da = wang * 0.5;
          const dx = wv.dirX * Math.cos(da) - wv.dirY * Math.sin(da);
          const dy = wv.dirX * Math.sin(da) + wv.dirY * Math.cos(da);
          const ph = (x * dx + y * dy) * wv.freq * 0.62 + t * wv.om + wv.ph;
          h += wv.amp * wstr * Math.sin(ph + twist);
        }
        st.hgt[i] = h;
        let foamv = fall * 0.85 + Math.max(0, h) * 0.5;
        if (foamv > 1) foamv = 1;
        st.foam[i] = foamv;
      }
    }
  }

  function render(): void {
    const st = stateRef.current;
    const ctx = ctxRef.current;
    if (!st.out || !st.bctx || !st.buf || !ctx || !st.hgt || !st.foam) return;
    if (!st.c0 || !st.c1 || !st.t0 || !st.t1 || !st.fc) return;
    const o = st.out.data;
    const C0 = st.c0, C1 = st.c1, T0 = st.t0, T1 = st.t1, FC = st.fc;
    for (let y = 0; y < st.rows; y++) {
      const row = y * COLS;
      for (let x = 0; x < COLS; x++) {
        const i = row + x;
        let u = st.hgt[i] * 1.24 + 0.5;
        if (u < 0) u = 0; else if (u > 1) u = 1;
        let r: number, g: number, b: number;
        if (u > 0.5) {
          const k = (u - 0.5) * 2;
          r = C1[0] + (C0[0] - C1[0]) * k;
          g = C1[1] + (C0[1] - C1[1]) * k;
          b = C1[2] + (C0[2] - C1[2]) * k;
        } else {
          const k2 = u * 2;
          r = T0[0] + (T1[0] - T0[0]) * k2;
          g = T0[1] + (T1[1] - T0[1]) * k2;
          b = T0[2] + (T1[2] - T0[2]) * k2;
        }
        const f = st.foam[i];
        if (f > 0.02) {
          r = r + (FC[0] - r) * f * 0.8;
          g = g + (FC[1] - g) * f * 0.8;
          b = b + (FC[2] - b) * f * 0.8;
        }
        for (let sy = 0; sy < STEP; sy++) {
          let base = ((y * STEP + sy) * st.rw + x * STEP) * 4;
          for (let sx = 0; sx < STEP; sx++, base += 4) {
            o[base] = r;
            o[base + 1] = g;
            o[base + 2] = b;
            o[base + 3] = 255;
          }
        }
      }
    }
    st.bctx.putImageData(st.out, 0, 0);
    ctx.drawImage(st.buf, 0, 0, sizeRef.current.w, sizeRef.current.h);
  }

  function frame(now: number): void {
    const st = stateRef.current;
    const sec = now / 1000;
    if (st.auto) {
      const th = sec * 0.31;
      st.windT = 1.05 + Math.sin(th * 0.9 + 0.4) * 0.55 + Math.sin(th * 2.3) * 0.12;
      st.windAngT = 0.5 + Math.sin(th * 0.42) * 0.85;
      st.vxT = 0.5 + Math.sin(th * 0.37 + 1.2) * 0.3;
      st.vyT = 0.44 + Math.sin(th * 0.53 + 2.1) * 0.26;
      st.swirlT = 1.0 + Math.sin(th * 0.7) * 0.35;
    }
    st.wind += (st.windT - st.wind) * 0.05;
    st.windAng += (st.windAngT - st.windAng) * 0.05;
    st.vx += (st.vxT - st.vx) * 0.045;
    st.vy += (st.vyT - st.vy) * 0.045;
    st.swirl += (st.swirlT - st.swirl) * 0.04;
    fieldcalc(sec, st.wind, st.windAng, st.vx * COLS, st.vy * st.rows, st.swirl);
    render();
    if (swirlValRef.current) swirlValRef.current.textContent = "회전 " + st.swirl.toFixed(1);
    if (windValRef.current) windValRef.current.textContent = "바람 " + st.wind.toFixed(2);
    rafRef.current = requestAnimationFrame(frame);
  }

  function onPointerMove(e: PointerMotionEvent): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    const px = e.clientX - r.left, py = e.clientY - r.top;
    const t = performance.now();
    const st = stateRef.current;
    st.vxT = Math.max(0.04, Math.min(0.96, px / sizeRef.current.w));
    st.vyT = Math.max(0.04, Math.min(0.96, py / sizeRef.current.h));
    if (st.lastPX >= 0 && t > st.lastPT) {
      const vx = px - st.lastPX, vy = py - st.lastPY;
      const sp = Math.sqrt(vx * vx + vy * vy);
      if (sp > 0.5) {
        st.windT = Math.max(0.35, Math.min(2.1, 0.9 + sp * 0.02));
        st.windAngT = Math.atan2(vy, vx) * 0.16;
      }
    }
    st.lastPX = px; st.lastPY = py; st.lastPT = t;
    st.auto = false;
  }

  function onPointerLeave(): void {
    stateRef.current.auto = true;
    stateRef.current.lastPX = -1;
  }

  function onResize(): void {
    fit();
    if (reduced()) {
      const st = stateRef.current;
      fieldcalc(7.3, 1.35, 0.5, 0.46 * COLS, 0.42 * st.rows, 1.2);
      render();
    }
  }

  useEffect(() => {
    const cv = cvRef.current;
    const root = rootRef.current;
    if (!cv || !root) return;
    ctxRef.current = cv.getContext("2d");
    if (!ctxRef.current) return;
    wavesRef.current = buildWaves();
    const cs = getComputedStyle(root);
    const tok = (name: string) => cs.getPropertyValue(name).trim();
    const st = stateRef.current;
    st.c0 = hex2rgb(tok("--crest"));
    st.c1 = hex2rgb(tok("--crest-2"));
    st.t0 = hex2rgb(tok("--trough"));
    st.t1 = hex2rgb(tok("--trough-2"));
    st.fc = hex2rgb(tok("--foam"));
    fit();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillRef.current = true;
      fieldcalc(7.3, 1.35, 0.5, 0.46 * COLS, 0.42 * st.rows, 1.2);
      render();
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  const cls = [styles.root, stillRef.current ? styles.isStill : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls} ref={rootRef} style={vars}>
        <p className={styles.hint}>
          <svg className={styles.ico} viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M16 120c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".25"/><path d="M16 168c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".15"/><path d="M228.92 49.09a112 112 0 1 0 14 84.91a8 8 0 0 0-15.5-4a96 96 0 1 1-12-72.91A8 8 0 0 1 216 71.63V40a8 8 0 0 1-8-8h-31.63a8 8 0 0 1-5.65-13.09ZM64 104a12 12 0 1 1 12 12a12 12 0 0 1-12-12m40 64a12 12 0 1 1 12 12a12 12 0 0 1-12-12m72-16a16 16 0 1 1 16 16a16 16 0 0 1-16-16" opacity=".9"/></g></svg>
          <span>파도장에 국소 회전을 한 겹 얹어 소용돌이를 만듭니다 — 마우스를 대면 회전 중심이 그리로 갑니다</span>
        </p>
        <div className={styles.field} ref={fieldRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <canvas className={styles.cv} ref={cvRef}></canvas>
          <b className={styles.badge} aria-hidden="true"><span className={styles.badgeNum} ref={swirlValRef}>회전 1.0</span><span className={styles.badgeUnit} ref={windValRef}>바람 1.0</span></b>
        </div>
      </div>
    </main>
  );
}