03Direction reverse — the particle flow's direction value flips the instant the scroll-direction value switches from down to up

<main class="stage">
<div class="sp" id="sp">
  <p class="sp__hint">
    <svg class="sp__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M224 128a96 96 0 1 1-96-96a96 96 0 0 1 96 96" opacity=".2"/><path d="M232 128a104 104 0 0 1-208 0c0-41 23.81-78.36 60.66-95.27a8 8 0 0 1 6.68 14.54C60.15 61.59 40 93.27 40 128a88 88 0 0 0 176 0c0-34.73-20.15-66.41-51.34-80.73a8 8 0 0 1 6.68-14.54C208.19 49.64 232 87 232 128"/></g></svg>
    <span>Scroll up or down and the crisscrossing grain flips direction.</span>
  </p>
  <div class="sp__viewport" id="viewport" tabindex="0" role="region" aria-label="Particle direction scroll area">
    <canvas class="sp__cv" id="cv" aria-hidden="true"></canvas>
    <b class="sp__val" id="val" aria-hidden="true">↓</b>
    <div class="sp__track" aria-hidden="true"></div>
  </div>
</div>
</main>

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

// 변형 3개는 이 위 세 변수만 바꾼다. 계산 겹(방향 부호장)은 index.html 의 step() 에 있다.
$duration: 2s;
$easing:   $ease-out;
$color:    $stage-ink;

@include stage($stage-orange);

.sp {
  --particle-color: #{$color};
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-2;
  width: min(400px, 92vw);
}

.sp__hint {
  display: flex;
  align-items: center;
  gap: $sp-2;
  margin: 0;
  color: rgba($stage-ink, .78);
  font-size: 12px;
  font-weight: 500;
  line-height: 1.3;
}

.sp__ico {
  width: 16px;
  height: 16px;
  flex: none;
  color: $stage-ink;
}

.sp__viewport {
  position: relative;
  width: 100%;
  height: 176px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba($stage-ink, .07);
  touch-action: pan-y;
  scrollbar-width: thin;
}

.sp__cv {
  position: sticky;
  top: 0;
  left: 0;
  display: block;
  width: 100%;
  height: 176px;
}

.sp__track {
  height: 320%;
}

.sp__val {
  position: absolute;
  top: $sp-2;
  right: $sp-2;
  padding: $sp-1 $sp-2;
  border-radius: $r-pill;
  background: rgba($stage-ink, .5);
  color: $subject-cream;
  font-size: 12px;
  font-weight: 700;
  line-height: 1.6;
  pointer-events: none;
  transition: opacity $duration $easing;
}

@media (max-width: 340px) {
  .sp__hint { font-size: 11px; }
  .sp__viewport, .sp__cv { height: 120px; }
}

@media (prefers-reduced-motion: reduce) {
  .sp__viewport { scroll-behavior: auto; }
  .sp__val { transition: none; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ff4d1f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sp {
  --particle-color: #17141a;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  width: min(400px, 92vw);
}

.sp__hint {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: 0;
  color: rgba(23, 20, 26, 0.78);
  font-size: 12px;
  font-weight: 500;
  line-height: 1.3;
}

.sp__ico {
  width: 16px;
  height: 16px;
  flex: none;
  color: #17141a;
}

.sp__viewport {
  position: relative;
  width: 100%;
  height: 176px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.07);
  touch-action: pan-y;
  scrollbar-width: thin;
}

.sp__cv {
  position: sticky;
  top: 0;
  left: 0;
  display: block;
  width: 100%;
  height: 176px;
}

.sp__track {
  height: 320%;
}

.sp__val {
  position: absolute;
  top: 8px;
  right: 8px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.5);
  color: #fff7e6;
  font-size: 12px;
  font-weight: 700;
  line-height: 1.6;
  pointer-events: none;
  transition: opacity 2s ease-out;
}

@media (max-width: 340px) {
  .sp__hint {
    font-size: 11px;
  }
  .sp__viewport, .sp__cv {
    height: 120px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sp__viewport {
    scroll-behavior: auto;
  }
  .sp__val {
    transition: none;
  }
}

(function () {
  var root = document.getElementById('sp');
  var viewport = document.getElementById('viewport');
  var cv = document.getElementById('cv');
  var ctx = cv.getContext('2d');
  var val = document.getElementById('val');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var isDemo = true;

  function makeLcg(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rng = makeLcg(7331);

  var css = getComputedStyle(root);
  var PARTICLE_COLOR = css.getPropertyValue('--particle-color').trim() || '#17141a';

  // 계산 겹: 방향 부호장(direction-sign field) — 파티클은 레인마다 고정된 부호(lane sign)를 갖고,
  // 실제 가로 속도는 그 부호에 스크롤 방향 부호를 곱한 값이다. 방향이 바뀌면 레인의 엇갈림이 통째로 뒤집힌다.
  var LANES = 5;
  var LANE_SPEED = 46;   // 레인 가로 속도 크기 (px/s)
  var RISE_SPEED = 20;   // 기준 상승 속도 (px/s)
  var COUNT = 40;

  var W = 0, H = 0, dpr = 1;
  var particles = [];

  function fit() {
    var r = viewport.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);
  }

  function laneSign(lane) { return (lane % 2 === 0) ? 1 : -1; }

  function spawnAt(p, first) {
    var lane = Math.floor(rng() * LANES);
    p.lane = lane;
    p.x = (lane + 0.5) / LANES * W + (rng() - 0.5) * (W / LANES) * 0.6;
    p.y = first ? rng() * H : H + rng() * 20;
    p.r = 1.4 + rng() * 2.2;
    p.alpha = 0.4 + rng() * 0.45;
  }

  function initParticles() {
    particles = [];
    for (var i = 0; i < COUNT; i++) { var p = {}; spawnAt(p, true); particles.push(p); }
  }

  var scrollPos = 0, scrollVel = 0, scrollDir = 0, dirSign = 1, idleMs = 0, lastTop = 0, lastT = 0;
  function sampleScroll(now) {
    var top = viewport.scrollTop;
    var max = Math.max(1, viewport.scrollHeight - viewport.clientHeight);
    scrollPos = top / max;
    var dt = lastT ? Math.max(1, now - lastT) : 16;
    var v = (top - lastTop) / dt * 1000;
    scrollVel += (v - scrollVel) * 0.3;
    if (Math.abs(scrollVel) > 4) { scrollDir = scrollVel > 0 ? 1 : -1; dirSign = scrollDir; idleMs = 0; }
    else { scrollDir = 0; idleMs += dt; } // dirSign 은 마지막 방향을 유지한다 — 정지해도 결이 사라지지 않는다
    lastTop = top; lastT = now;
  }
  viewport.addEventListener('scroll', function () {
    sampleScroll(performance.now());
    if (reduce) paintStatic();
  });

  function human() { if (isDemo) { root.classList.remove('is-demo'); isDemo = false; } }
  ['wheel', 'touchstart', 'pointerdown'].forEach(function (evt) {
    viewport.addEventListener(evt, human, { passive: true });
  });

  function step(dtSec) {
    for (var i = 0; i < particles.length; i++) {
      var p = particles[i];
      p.y -= RISE_SPEED * dtSec;
      p.x += laneSign(p.lane) * dirSign * LANE_SPEED * dtSec;
      var laneMin = p.lane / LANES * W, laneMax = (p.lane + 1) / LANES * W;
      if (p.x < laneMin - 6) p.x = laneMin - 6; else if (p.x > laneMax + 6) p.x = laneMax + 6;
      if (p.y < -8) spawnAt(p, false);
    }
  }

  function paint() {
    ctx.clearRect(0, 0, W, H);
    ctx.fillStyle = PARTICLE_COLOR;
    for (var i = 0; i < particles.length; i++) {
      var p = particles[i];
      ctx.globalAlpha = p.alpha;
      ctx.beginPath(); ctx.arc(p.x, p.y, p.r, 0, Math.PI * 2); ctx.fill();
    }
    ctx.globalAlpha = 1;
  }

  function updateVal() { val.textContent = dirSign >= 0 ? '↓' : '↑'; }

  function paintStatic() {
    ctx.clearRect(0, 0, W, H);
    ctx.fillStyle = PARTICLE_COLOR;
    for (var i = 0; i < particles.length; i++) {
      var p = particles[i];
      var laneMid = (p.lane + 0.5) / LANES * W;
      var offset = laneSign(p.lane) * dirSign * scrollPos * (W / LANES) * 0.35;
      ctx.globalAlpha = p.alpha;
      ctx.beginPath(); ctx.arc(laneMid + offset, p.y, p.r, 0, Math.PI * 2); ctx.fill();
    }
    ctx.globalAlpha = 1;
    updateVal();
  }

  var lastFrameT = 0;
  function frame(now) {
    if (!lastFrameT) lastFrameT = now;
    var dtSec = Math.min(0.05, (now - lastFrameT) / 1000);
    lastFrameT = now;
    step(dtSec);
    paint();
    updateVal();
    requestAnimationFrame(frame);
  }

  var DRIVE_PERIOD_MS = 2000;
  var driveStart = 0;
  function triangle(frac) { return frac < 0.5 ? frac * 2 : 2 - frac * 2; }
  function driveTick(now) {
    if (!driveStart) driveStart = now;
    if (isDemo && !reduce) {
      var frac = ((now - driveStart) % DRIVE_PERIOD_MS) / DRIVE_PERIOD_MS;
      var w = triangle(frac);
      var max = Math.max(1, viewport.scrollHeight - viewport.clientHeight);
      viewport.scrollTop = w * max;
    }
    requestAnimationFrame(driveTick);
  }

  window.addEventListener('resize', fit);
  fit();
  initParticles();

  if (reduce) {
    root.classList.add('is-still');
    sampleScroll(performance.now());
    paintStatic();
  } else {
    requestAnimationFrame(frame);
    requestAnimationFrame(driveTick);
  }
})();

// DirectionReverse — vanilla 판(run/370/demo/03-scroll-particle-direction-reverse/index.html) 의 React 포트.
// 계산 겹: 방향 부호장(direction-sign field) — 파티클은 레인마다 고정된 부호(lane sign)를 갖고,
// 실제 가로 속도는 그 부호에 스크롤 방향 부호를 곱한 값이다. 방향이 바뀌면 레인의 엇갈림이 통째로 뒤집힌다.
import { useEffect, useRef } from "react";
import styles from "./DirectionReverse.module.scss";

type Box<T> = { current: T };
interface Particle { lane: number; x: number; y: number; r: number; alpha: number; }

export default function DirectionReverse() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const viewportRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const canvasRef = useRef(null) as unknown as Box<HTMLCanvasElement>;
  const valRef = useRef(null) as unknown as Box<HTMLElement>;
  const rafFrameRef = useRef(0) as unknown as Box<number>;
  const rafDriveRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const root = rootRef.current;
    const viewport = viewportRef.current;
    const canvas = canvasRef.current;
    const val = valRef.current;
    if (!root || !viewport || !canvas || !val) return;
    const ctx = canvas.getContext("2d");
    if (!ctx) return;

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let isDemo = true;

    function makeLcg(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rng = makeLcg(7331);

    const css = getComputedStyle(root);
    const PARTICLE_COLOR = css.getPropertyValue("--particle-color").trim() || "#17141a";

    const LANES = 5;
    const LANE_SPEED = 46;
    const RISE_SPEED = 20;
    const COUNT = 40;

    let W = 0, H = 0, dpr = 1;
    let particles: Particle[] = [];

    function fit() {
      const r = viewport!.getBoundingClientRect();
      dpr = window.devicePixelRatio || 1;
      W = r.width; H = r.height;
      canvas!.width = Math.round(W * dpr);
      canvas!.height = Math.round(H * dpr);
      ctx!.setTransform(dpr, 0, 0, dpr, 0, 0);
    }

    function laneSign(lane: number) { return lane % 2 === 0 ? 1 : -1; }

    function spawnAt(p: Particle, first: boolean) {
      const lane = Math.floor(rng() * LANES);
      p.lane = lane;
      p.x = (lane + 0.5) / LANES * W + (rng() - 0.5) * (W / LANES) * 0.6;
      p.y = first ? rng() * H : H + rng() * 20;
      p.r = 1.4 + rng() * 2.2;
      p.alpha = 0.4 + rng() * 0.45;
    }

    function initParticles() {
      particles = [];
      for (let i = 0; i < COUNT; i++) {
        const p: Particle = { lane: 0, x: 0, y: 0, r: 0, alpha: 0 };
        spawnAt(p, true);
        particles.push(p);
      }
    }

    let scrollPos = 0, scrollVel = 0, dirSign = 1, idleMs = 0, lastTop = 0, lastT = 0;
    function sampleScroll(now: number) {
      const top = viewport!.scrollTop;
      const max = Math.max(1, viewport!.scrollHeight - viewport!.clientHeight);
      scrollPos = top / max;
      const dt = lastT ? Math.max(1, now - lastT) : 16;
      const v = (top - lastTop) / dt * 1000;
      scrollVel += (v - scrollVel) * 0.3;
      // dirSign 은 마지막 방향을 유지한다 — 정지해도 결이 사라지지 않는다
      if (Math.abs(scrollVel) > 4) { dirSign = scrollVel > 0 ? 1 : -1; idleMs = 0; } else { idleMs += dt; }
      lastTop = top; lastT = now;
    }

    function onScroll() {
      sampleScroll(performance.now());
      if (reduce) paintStatic();
    }
    viewport.addEventListener("scroll", onScroll);

    function human() { if (isDemo) { isDemo = false; } }
    viewport.addEventListener("wheel", human, { passive: true });
    viewport.addEventListener("touchstart", human, { passive: true });
    viewport.addEventListener("pointerdown", human, { passive: true });

    function step(dtSec: number) {
      for (let i = 0; i < particles.length; i++) {
        const p = particles[i];
        p.y -= RISE_SPEED * dtSec;
        p.x += laneSign(p.lane) * dirSign * LANE_SPEED * dtSec;
        const laneMin = p.lane / LANES * W, laneMax = (p.lane + 1) / LANES * W;
        if (p.x < laneMin - 6) p.x = laneMin - 6; else if (p.x > laneMax + 6) p.x = laneMax + 6;
        if (p.y < -8) spawnAt(p, false);
      }
    }

    function paint() {
      ctx!.clearRect(0, 0, W, H);
      ctx!.fillStyle = PARTICLE_COLOR;
      for (let i = 0; i < particles.length; i++) {
        const p = particles[i];
        ctx!.globalAlpha = p.alpha;
        ctx!.beginPath(); ctx!.arc(p.x, p.y, p.r, 0, Math.PI * 2); ctx!.fill();
      }
      ctx!.globalAlpha = 1;
    }

    function updateVal() { val!.textContent = dirSign >= 0 ? "↓" : "↑"; }

    function paintStatic() {
      ctx!.clearRect(0, 0, W, H);
      ctx!.fillStyle = PARTICLE_COLOR;
      for (let i = 0; i < particles.length; i++) {
        const p = particles[i];
        const laneMid = (p.lane + 0.5) / LANES * W;
        const offset = laneSign(p.lane) * dirSign * scrollPos * (W / LANES) * 0.35;
        ctx!.globalAlpha = p.alpha;
        ctx!.beginPath(); ctx!.arc(laneMid + offset, p.y, p.r, 0, Math.PI * 2); ctx!.fill();
      }
      ctx!.globalAlpha = 1;
      updateVal();
    }

    let lastFrameT = 0;
    function frame(now: number) {
      if (!lastFrameT) lastFrameT = now;
      const dtSec = Math.min(0.05, (now - lastFrameT) / 1000);
      lastFrameT = now;
      step(dtSec);
      paint();
      updateVal();
      rafFrameRef.current = requestAnimationFrame(frame);
    }

    const DRIVE_PERIOD_MS = 2000;
    let driveStart = 0;
    function triangle(frac: number) { return frac < 0.5 ? frac * 2 : 2 - frac * 2; }
    function driveTick(now: number) {
      if (!driveStart) driveStart = now;
      if (isDemo && !reduce) {
        const frac = ((now - driveStart) % DRIVE_PERIOD_MS) / DRIVE_PERIOD_MS;
        const w = triangle(frac);
        const max = Math.max(1, viewport!.scrollHeight - viewport!.clientHeight);
        viewport!.scrollTop = w * max;
      }
      rafDriveRef.current = requestAnimationFrame(driveTick);
    }

    function onResize() { fit(); }
    window.addEventListener("resize", onResize);
    fit();
    initParticles();

    if (reduce) {
      sampleScroll(performance.now());
      paintStatic();
    } else {
      rafFrameRef.current = requestAnimationFrame(frame);
      rafDriveRef.current = requestAnimationFrame(driveTick);
    }

    return () => {
      window.removeEventListener("resize", onResize);
      viewport.removeEventListener("scroll", onScroll);
      viewport.removeEventListener("wheel", human);
      viewport.removeEventListener("touchstart", human);
      viewport.removeEventListener("pointerdown", human);
      cancelAnimationFrame(rafFrameRef.current);
      cancelAnimationFrame(rafDriveRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root} ref={rootRef}>
        <p className={styles.hint}>
          <svg className={styles.icon} viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M224 128a96 96 0 1 1-96-96a96 96 0 0 1 96 96" opacity=".2" /><path d="M232 128a104 104 0 0 1-208 0c0-41 23.81-78.36 60.66-95.27a8 8 0 0 1 6.68 14.54C60.15 61.59 40 93.27 40 128a88 88 0 0 0 176 0c0-34.73-20.15-66.41-51.34-80.73a8 8 0 0 1 6.68-14.54C208.19 49.64 232 87 232 128" /></g></svg>
          <span>올리고 내리는 방향에 따라 엇갈리는 결이 뒤집힙니다</span>
        </p>
        <div className={styles.viewport} ref={viewportRef} tabIndex={0} role="region" aria-label="파티클 방향 스크롤 영역">
          <canvas className={styles.canvas} ref={canvasRef} aria-hidden="true" />
          <b className={styles.val} ref={valRef} aria-hidden="true">↓</b>
          <div className={styles.track} aria-hidden="true" />
        </div>
      </div>
    </main>
  );
}