04과거 위치 이력을 잇고 겹칠수록 밝아지는 꼬리와 빛무리

<main class="stage">
<div class="sc is-demo" id="scRoot">
  <canvas class="sc__canvas" id="scCanvas" width="480" height="300" tabindex="0" aria-label="과거 위치 이력을 잇고 겹칠수록 밝아지는 꼬리와 빛무리"></canvas>
  <p class="sc__status" id="scStatus" aria-live="polite">꼬리 이력을 가산 합성으로 겹친다 — 겹친 곳이 흰빛으로 번진다</p>
  <button class="sc__btn" id="scBtn" type="button">멈춤</button>
  <p class="sc__caption">과거 위치 이력을 잇고 겹칠수록 밝아지는 꼬리와 빛무리</p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-ink);

.sc {
  position: relative;
  width: min(92vw, calc(160vh - 12.8vw));
  aspect-ratio: 480 / 300;
  margin: 0 auto;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
  background: $subject-ink;
}

.sc__canvas {
  display: block;
  width: 100%;
  height: 100%;
  touch-action: manipulation;
}

.sc__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, .85);
  outline-offset: -6px;
}

.sc__status {
  position: absolute;
  left: $sp-3;
  top: $sp-3;
  max-width: 62%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.sc__btn {
  position: absolute;
  right: $sp-3;
  bottom: $sp-3;
  margin: 0;
  padding: $sp-1 $sp-3;
  border: 0;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
}

.sc__btn:focus-visible {
  outline: 2px solid #ffffff;
  outline-offset: 2px;
}

.sc__caption {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  max-width: 68%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .sc__caption, .sc__btn { font-size: 10px; padding: $sp-1 $sp-2; }
  .sc__status { font-size: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  .sc { cursor: default; }
}

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);
}

.sc {
  position: relative;
  width: min(92vw, 160vh - 12.8vw);
  aspect-ratio: 480/300;
  margin: 0 auto;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  background: #17141a;
}

.sc__canvas {
  display: block;
  width: 100%;
  height: 100%;
  touch-action: manipulation;
}

.sc__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, 0.85);
  outline-offset: -6px;
}

.sc__status {
  position: absolute;
  left: 12px;
  top: 12px;
  max-width: 62%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.sc__btn {
  position: absolute;
  right: 12px;
  bottom: 12px;
  margin: 0;
  padding: 4px 12px;
  border: 0;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
}

.sc__btn:focus-visible {
  outline: 2px solid #ffffff;
  outline-offset: 2px;
}

.sc__caption {
  position: absolute;
  left: 12px;
  bottom: 12px;
  max-width: 68%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .sc__caption, .sc__btn {
    font-size: 10px;
    padding: 4px 8px;
  }
  .sc__status {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sc {
    cursor: default;
  }
}

(function () {
    const W = 480;
    const H = 300;
    const PI2 = Math.PI * 2;
    const FIXED_DT = 16;
    const REDUCE_STEP_MS = 900;
    const KEY_STEP = 14;
    const IDLE_TICKS = 150;
    const BTN_PAUSE = '멈춤';
    const BTN_PLAY = '재생';
    const BTN_NEXT = '다음 장면';
    // 파스텔 알갱이 팔레트와 밤하늘 두 끝 색
    const PINK = [255, 170, 196];
    const MINT = [150, 226, 196];
    const LILAC = [196, 176, 255];
    const SKY_BLUE = [150, 214, 255];
    const BUTTER = [255, 230, 150];
    const CREAM = [255, 247, 230];
    const PALETTE = [PINK, MINT, LILAC, SKY_BLUE, BUTTER];
    const SKY_TOP = [18, 16, 32];
    const SKY_BOT = [46, 40, 76];
    function clamp(v, lo, hi) { return v < lo ? lo : (v > hi ? hi : v); }
    function lerp(a, b, k) { return a + (b - a) * k; }
    function smooth(x) { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
    // 시드 고정 난수 — 같은 시드면 언제나 같은 수열 (선형 합동 방식)
    function lcg(seed) {
        let s = seed >>> 0;
        return function () {
            s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
            return s / 4294967296;
        };
    }
    // 정수 하나와 시드로 0 이상 1 미만의 값을 만드는 해시 (같은 입력은 늘 같은 값)
    function hash1(i, seed) {
        let h = (Math.imul(i | 0, 374761393) + Math.imul(seed | 0, 668265263)) | 0;
        h = Math.imul(h ^ (h >>> 13), 1274126177);
        h ^= h >>> 16;
        return (h >>> 0) / 4294967296;
    }
    function hash2(x, y, seed) {
        return hash1((Math.imul(x | 0, 73856093) ^ Math.imul(y | 0, 19349663)) | 0, seed);
    }
    // 1차원 값 노이즈 — 정수점 해시를 smooth 로 이어 붙인다
    function noise1(x, seed) {
        const xi = Math.floor(x);
        return lerp(hash1(xi, seed), hash1(xi + 1, seed), smooth(x - xi));
    }
    // 2차원 값 노이즈
    function vnoise(x, y, seed) {
        const xi = Math.floor(x);
        const yi = Math.floor(y);
        const fx = smooth(x - xi);
        const fy = smooth(y - yi);
        const a = lerp(hash2(xi, yi, seed), hash2(xi + 1, yi, seed), fx);
        const b = lerp(hash2(xi, yi + 1, seed), hash2(xi + 1, yi + 1, seed), fx);
        return lerp(a, b, fy);
    }
    function rgba(c, a) {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
    }
    function mixRGB(a, b, k) {
        return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)];
    }
    // 밤하늘 바탕 — 위가 어둡고 아래가 보랏빛
    function drawSky(ctx, height) {
        const g = ctx.createLinearGradient(0, 0, 0, height);
        g.addColorStop(0, rgba(SKY_TOP, 1));
        g.addColorStop(1, rgba(SKY_BOT, 1));
        ctx.fillStyle = g;
        ctx.fillRect(0, 0, W, height);
    }
    function start(root, canvas, statusEl, btnEl, make) {
        const ctx0 = canvas.getContext('2d');
        if (!ctx0) {
            return function () { return; };
        }
        const ctx = ctx0;
        const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
        const inp = { x: W / 2, y: H / 2, idle: 99999, vx: 0, vy: 0, dpr: 1 };
        let lastPx = -1;
        let lastPy = -1;
        // 화면 배율이 바뀌어도 캔버스 픽셀 수를 다시 맞춘다
        function fit() {
            const dpr = Math.min(window.devicePixelRatio || 1, 2);
            inp.dpr = dpr;
            canvas.width = Math.round(W * dpr);
            canvas.height = Math.round(H * dpr);
            ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
        }
        fit();
        const scene = make(inp, reduce);
        let paused = false;
        let rafId = 0;
        let lastTs = -1;
        let accMs = 0;
        let lastLabel = '';
        const cleaners = [];
        function listen(target, type, fn) {
            target.addEventListener(type, fn);
            cleaners.push(function () { target.removeEventListener(type, fn); });
        }
        function show() {
            ctx.setTransform(inp.dpr, 0, 0, inp.dpr, 0, 0);
            scene.draw(ctx);
            const t = scene.label();
            if (t !== lastLabel) {
                lastLabel = t;
                statusEl.textContent = t;
            }
        }
        function tickOnce() {
            inp.idle += 1;
            inp.vx *= 0.9;
            inp.vy *= 0.9;
            scene.step();
        }
        function advance(ms) {
            const n = Math.round(ms / FIXED_DT);
            for (let i = 0; i < n; i += 1) {
                tickOnce();
            }
        }
        function refresh() {
            if (reduce) {
                advance(FIXED_DT * 6);
                show();
            }
        }
        function toLocal(e) {
            const r = canvas.getBoundingClientRect();
            const nx = clamp((e.clientX - r.left) * W / r.width, 0, W);
            const ny = clamp((e.clientY - r.top) * H / r.height, 0, H);
            if (lastPx >= 0) {
                inp.vx = lerp(inp.vx, nx - lastPx, 0.5);
                inp.vy = lerp(inp.vy, ny - lastPy, 0.5);
            }
            lastPx = nx;
            lastPy = ny;
            inp.x = nx;
            inp.y = ny;
            inp.idle = 0;
        }
        function tick(ts) {
            if (lastTs < 0) {
                lastTs = ts;
            }
            const frameDt = Math.min(ts - lastTs, 160);
            lastTs = ts;
            if (!paused) {
                accMs += frameDt;
                while (accMs >= FIXED_DT) {
                    tickOnce();
                    accMs -= FIXED_DT;
                }
                show();
            }
            rafId = requestAnimationFrame(tick);
        }
        btnEl.textContent = reduce ? BTN_NEXT : BTN_PAUSE;
        if (reduce) {
            root.classList.remove('is-demo');
        }
        show();
        listen(btnEl, 'click', function () {
            if (reduce) {
                advance(REDUCE_STEP_MS);
                show();
                return;
            }
            paused = !paused;
            if (paused) {
                root.classList.remove('is-demo');
            }
            else {
                root.classList.add('is-demo');
            }
            btnEl.textContent = paused ? BTN_PLAY : BTN_PAUSE;
        });
        listen(canvas, 'pointermove', function (e) {
            toLocal(e);
            refresh();
        });
        listen(canvas, 'pointerdown', function (e) {
            lastPx = -1;
            toLocal(e);
            scene.tap();
            refresh();
        });
        listen(canvas, 'keydown', function (e) {
            const k = e.key;
            let dx = 0;
            let dy = 0;
            if (k === 'Enter' || k === ' ') {
                e.preventDefault();
                scene.tap();
                refresh();
                return;
            }
            if (k === 'ArrowLeft') {
                dx = -KEY_STEP;
            }
            else if (k === 'ArrowRight') {
                dx = KEY_STEP;
            }
            else if (k === 'ArrowUp') {
                dy = -KEY_STEP;
            }
            else if (k === 'ArrowDown') {
                dy = KEY_STEP;
            }
            else {
                return;
            }
            e.preventDefault();
            inp.x = clamp(inp.x + dx, 0, W);
            inp.y = clamp(inp.y + dy, 0, H);
            inp.vx = dx * 0.5;
            inp.vy = dy * 0.5;
            inp.idle = 0;
            refresh();
        });
        listen(window, 'resize', function () {
            fit();
            show();
        });
        if (!reduce) {
            rafId = requestAnimationFrame(tick);
        }
        return function () {
            cancelAnimationFrame(rafId);
            for (let i = 0; i < cleaners.length; i += 1) {
                cleaners[i]();
            }
        };
    }
    const TRAIL_LEN = 16;
    const GLOW_BLUR = 9;
    const FADE_ALPHA = 0.86;
    const HEAD_R = 2.1;
    const BURST_GAP = 44;
    const SPARK_N = 56;
    const SPARK_DRAG = 0.972;
    const SPARK_GRAVITY = 0.045;
    const PREWARM_STEPS = 38;
    const TXT_AUTO = '꼬리 이력을 가산 합성으로 겹친다 — 겹친 곳이 흰빛으로 번진다';
    const TXT_TAP = '누른 자리에서 빛무리가 터진다';
    // 위치 이력 한 줄에서 i 번째 점의 알파 — 머리에서 멀수록 FADE_ALPHA 를 거듭 곱한다
    function trailAlpha(back) {
        return Math.pow(FADE_ALPHA, back);
    }
    function makeScene(inp, reduce) {
        const rng = lcg(4004);
        let sparks = [];
        let gap = 2;
        let colorIdx = 0;
        const glows = [];
        function glowFor(idx) {
            if (glows[idx]) {
                return glows[idx];
            }
            const size = GLOW_BLUR * 4;
            const cv = document.createElement('canvas');
            cv.width = size;
            cv.height = size;
            const c = cv.getContext('2d');
            if (c) {
                const g = c.createRadialGradient(size / 2, size / 2, 0, size / 2, size / 2, size / 2);
                g.addColorStop(0, rgba(CREAM, 0.9));
                g.addColorStop(0.25, rgba(PALETTE[idx], 0.45));
                g.addColorStop(1, rgba(PALETTE[idx], 0));
                c.fillStyle = g;
                c.fillRect(0, 0, size, size);
            }
            glows[idx] = cv;
            return cv;
        }
        function burst(x, y) {
            const hue = colorIdx % PALETTE.length;
            colorIdx += 1;
            for (let i = 0; i < SPARK_N; i += 1) {
                const a = rng() * PI2;
                const sp = 2.2 + rng() * 2.4;
                sparks.push({ x: x, y: y, vx: Math.cos(a) * sp, vy: Math.sin(a) * sp, age: 0, life: 70 + rng() * 26, hist: [], hue: i % 4 === 0 ? (hue + 1) % PALETTE.length : hue });
            }
            if (sparks.length > 330) {
                sparks.splice(0, sparks.length - 330);
            }
        }
        const scene = {
            step() {
                gap -= 1;
                if (gap <= 0) {
                    burst(90 + rng() * 300, 80 + rng() * 70);
                    gap = BURST_GAP;
                }
                const alive = [];
                for (let i = 0; i < sparks.length; i += 1) {
                    const s = sparks[i];
                    s.age += 1;
                    s.hist.push(s.x, s.y);
                    if (s.hist.length > TRAIL_LEN * 2) {
                        s.hist.splice(0, 2);
                    }
                    s.vx *= SPARK_DRAG;
                    s.vy = s.vy * SPARK_DRAG + SPARK_GRAVITY;
                    s.x += s.vx;
                    s.y += s.vy;
                    if (s.age < s.life) {
                        alive.push(s);
                    }
                }
                sparks = alive;
            },
            draw(ctx) {
                drawSky(ctx, H);
                // 이 항목의 핵심 — 합성 방식을 가산으로 바꾸면 겹친 곳의 색이 더해져 하얗게 번진다
                ctx.globalCompositeOperation = 'lighter';
                ctx.lineCap = 'round';
                for (let i = 0; i < sparks.length; i += 1) {
                    const s = sparks[i];
                    const life = Math.pow(1 - s.age / s.life, 0.7);
                    const n = s.hist.length / 2;
                    for (let k = 1; k < n; k += 1) {
                        const a = trailAlpha(n - k) * life;
                        ctx.strokeStyle = rgba(PALETTE[s.hue], a * 0.85);
                        ctx.lineWidth = lerp(0.6, HEAD_R * 1.3, k / n);
                        ctx.beginPath();
                        ctx.moveTo(s.hist[(k - 1) * 2], s.hist[(k - 1) * 2 + 1]);
                        ctx.lineTo(s.hist[k * 2], s.hist[k * 2 + 1]);
                        ctx.stroke();
                    }
                    const r = GLOW_BLUR * (0.6 + 0.4 * life);
                    ctx.globalAlpha = life * (reduce ? 0.6 : 0.85);
                    ctx.drawImage(glowFor(s.hue), s.x - r, s.y - r, r * 2, r * 2);
                    ctx.globalAlpha = 1;
                    ctx.fillStyle = rgba(CREAM, life);
                    ctx.beginPath();
                    ctx.arc(s.x, s.y, HEAD_R * (0.5 + 0.5 * life), 0, PI2);
                    ctx.fill();
                }
                ctx.globalCompositeOperation = 'source-over';
            },
            label() {
                return inp.idle < IDLE_TICKS ? TXT_TAP : TXT_AUTO;
            },
            tap() {
                burst(inp.x, inp.y);
            }
        };
        for (let i = 0; i < PREWARM_STEPS; i += 1) {
            scene.step();
        }
        return scene;
    }
    start(document.getElementById('scRoot'), document.getElementById('scCanvas'), document.getElementById('scStatus'), document.getElementById('scBtn'), makeScene);
})();

import { useEffect, useRef } from "react";
import styles from "./FwAdditiveTrailHistoryGlow.module.scss";

const W = 480;
const H = 300;
const PI2 = Math.PI * 2;
const FIXED_DT = 16;
const REDUCE_STEP_MS = 900;
const KEY_STEP = 14;
const IDLE_TICKS = 150;
const BTN_PAUSE = '멈춤';
const BTN_PLAY = '재생';
const BTN_NEXT = '다음 장면';

type RGB = [number, number, number];
interface Input { x: number; y: number; idle: number; vx: number; vy: number; dpr: number; }
interface Scene {
    step(): void;
    draw(ctx: CanvasRenderingContext2D): void;
    label(): string;
    tap(): void;
}
type Maker = (inp: Input, reduce: boolean) => Scene;

// 파스텔 알갱이 팔레트와 밤하늘 두 끝 색
const PINK: RGB = [255, 170, 196];
const MINT: RGB = [150, 226, 196];
const LILAC: RGB = [196, 176, 255];
const SKY_BLUE: RGB = [150, 214, 255];
const BUTTER: RGB = [255, 230, 150];
const CREAM: RGB = [255, 247, 230];
const PALETTE: RGB[] = [PINK, MINT, LILAC, SKY_BLUE, BUTTER];
const SKY_TOP: RGB = [18, 16, 32];
const SKY_BOT: RGB = [46, 40, 76];

function clamp(v: number, lo: number, hi: number): number { return v < lo ? lo : (v > hi ? hi : v); }
function lerp(a: number, b: number, k: number): number { return a + (b - a) * k; }
function smooth(x: number): number { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
// 시드 고정 난수 — 같은 시드면 언제나 같은 수열 (선형 합동 방식)
function lcg(seed: number): () => number {
    let s = seed >>> 0;
    return function (): number {
        s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
        return s / 4294967296;
    };
}
// 정수 하나와 시드로 0 이상 1 미만의 값을 만드는 해시 (같은 입력은 늘 같은 값)
function hash1(i: number, seed: number): number {
    let h = (Math.imul(i | 0, 374761393) + Math.imul(seed | 0, 668265263)) | 0;
    h = Math.imul(h ^ (h >>> 13), 1274126177);
    h ^= h >>> 16;
    return (h >>> 0) / 4294967296;
}
function hash2(x: number, y: number, seed: number): number {
    return hash1((Math.imul(x | 0, 73856093) ^ Math.imul(y | 0, 19349663)) | 0, seed);
}
// 1차원 값 노이즈 — 정수점 해시를 smooth 로 이어 붙인다
function noise1(x: number, seed: number): number {
    const xi = Math.floor(x);
    return lerp(hash1(xi, seed), hash1(xi + 1, seed), smooth(x - xi));
}
// 2차원 값 노이즈
function vnoise(x: number, y: number, seed: number): number {
    const xi = Math.floor(x);
    const yi = Math.floor(y);
    const fx = smooth(x - xi);
    const fy = smooth(y - yi);
    const a = lerp(hash2(xi, yi, seed), hash2(xi + 1, yi, seed), fx);
    const b = lerp(hash2(xi, yi + 1, seed), hash2(xi + 1, yi + 1, seed), fx);
    return lerp(a, b, fy);
}
function rgba(c: RGB, a: number): string {
    return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
}
function mixRGB(a: RGB, b: RGB, k: number): RGB {
    return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)];
}
// 밤하늘 바탕 — 위가 어둡고 아래가 보랏빛
function drawSky(ctx: CanvasRenderingContext2D, height: number): void {
    const g = ctx.createLinearGradient(0, 0, 0, height);
    g.addColorStop(0, rgba(SKY_TOP, 1));
    g.addColorStop(1, rgba(SKY_BOT, 1));
    ctx.fillStyle = g;
    ctx.fillRect(0, 0, W, height);
}

function start(root: HTMLElement, canvas: HTMLCanvasElement, statusEl: HTMLElement, btnEl: HTMLElement, make: Maker): () => void {
    const ctx0 = canvas.getContext('2d');
    if (!ctx0) {
        return function (): void { return; };
    }
    const ctx: CanvasRenderingContext2D = ctx0;
    const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
    const inp: Input = { x: W / 2, y: H / 2, idle: 99999, vx: 0, vy: 0, dpr: 1 };
    let lastPx = -1;
    let lastPy = -1;
    // 화면 배율이 바뀌어도 캔버스 픽셀 수를 다시 맞춘다
    function fit(): void {
        const dpr = Math.min(window.devicePixelRatio || 1, 2);
        inp.dpr = dpr;
        canvas.width = Math.round(W * dpr);
        canvas.height = Math.round(H * dpr);
        ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    }
    fit();
    const scene = make(inp, reduce);
    let paused = false;
    let rafId = 0;
    let lastTs = -1;
    let accMs = 0;
    let lastLabel = '';
    const cleaners: Array<() => void> = [];
    function listen(target: EventTarget, type: string, fn: (e: any) => void): void {
        target.addEventListener(type, fn);
        cleaners.push(function (): void { target.removeEventListener(type, fn); });
    }
    function show(): void {
        ctx.setTransform(inp.dpr, 0, 0, inp.dpr, 0, 0);
        scene.draw(ctx);
        const t = scene.label();
        if (t !== lastLabel) {
            lastLabel = t;
            statusEl.textContent = t;
        }
    }
    function tickOnce(): void {
        inp.idle += 1;
        inp.vx *= 0.9;
        inp.vy *= 0.9;
        scene.step();
    }
    function advance(ms: number): void {
        const n = Math.round(ms / FIXED_DT);
        for (let i = 0; i < n; i += 1) {
            tickOnce();
        }
    }
    function refresh(): void {
        if (reduce) {
            advance(FIXED_DT * 6);
            show();
        }
    }
    function toLocal(e: PointerEvent): void {
        const r = canvas.getBoundingClientRect();
        const nx = clamp((e.clientX - r.left) * W / r.width, 0, W);
        const ny = clamp((e.clientY - r.top) * H / r.height, 0, H);
        if (lastPx >= 0) {
            inp.vx = lerp(inp.vx, nx - lastPx, 0.5);
            inp.vy = lerp(inp.vy, ny - lastPy, 0.5);
        }
        lastPx = nx;
        lastPy = ny;
        inp.x = nx;
        inp.y = ny;
        inp.idle = 0;
    }
    function tick(ts: number): void {
        if (lastTs < 0) {
            lastTs = ts;
        }
        const frameDt = Math.min(ts - lastTs, 160);
        lastTs = ts;
        if (!paused) {
            accMs += frameDt;
            while (accMs >= FIXED_DT) {
                tickOnce();
                accMs -= FIXED_DT;
            }
            show();
        }
        rafId = requestAnimationFrame(tick);
    }
    btnEl.textContent = reduce ? BTN_NEXT : BTN_PAUSE;
    if (reduce) {
        root.classList.remove('is-demo');
    }
    show();
    listen(btnEl, 'click', function (): void {
        if (reduce) {
            advance(REDUCE_STEP_MS);
            show();
            return;
        }
        paused = !paused;
        if (paused) {
            root.classList.remove('is-demo');
        }
        else {
            root.classList.add('is-demo');
        }
        btnEl.textContent = paused ? BTN_PLAY : BTN_PAUSE;
    });
    listen(canvas, 'pointermove', function (e: PointerEvent): void {
        toLocal(e);
        refresh();
    });
    listen(canvas, 'pointerdown', function (e: PointerEvent): void {
        lastPx = -1;
        toLocal(e);
        scene.tap();
        refresh();
    });
    listen(canvas, 'keydown', function (e: KeyboardEvent): void {
        const k = e.key;
        let dx = 0;
        let dy = 0;
        if (k === 'Enter' || k === ' ') {
            e.preventDefault();
            scene.tap();
            refresh();
            return;
        }
        if (k === 'ArrowLeft') {
            dx = -KEY_STEP;
        }
        else if (k === 'ArrowRight') {
            dx = KEY_STEP;
        }
        else if (k === 'ArrowUp') {
            dy = -KEY_STEP;
        }
        else if (k === 'ArrowDown') {
            dy = KEY_STEP;
        }
        else {
            return;
        }
        e.preventDefault();
        inp.x = clamp(inp.x + dx, 0, W);
        inp.y = clamp(inp.y + dy, 0, H);
        inp.vx = dx * 0.5;
        inp.vy = dy * 0.5;
        inp.idle = 0;
        refresh();
    });
    listen(window, 'resize', function (): void {
        fit();
        show();
    });
    if (!reduce) {
        rafId = requestAnimationFrame(tick);
    }
    return function (): void {
        cancelAnimationFrame(rafId);
        for (let i = 0; i < cleaners.length; i += 1) {
            cleaners[i]();
        }
    };
}

const TRAIL_LEN = 16;
const GLOW_BLUR = 9;
const FADE_ALPHA = 0.86;
const HEAD_R = 2.1;
const BURST_GAP = 44;
const SPARK_N = 56;
const SPARK_DRAG = 0.972;
const SPARK_GRAVITY = 0.045;
const PREWARM_STEPS = 38;
const TXT_AUTO = '꼬리 이력을 가산 합성으로 겹친다 — 겹친 곳이 흰빛으로 번진다';
const TXT_TAP = '누른 자리에서 빛무리가 터진다';

interface Spark { x: number; y: number; vx: number; vy: number; age: number; life: number; hist: number[]; hue: number; }

// 위치 이력 한 줄에서 i 번째 점의 알파 — 머리에서 멀수록 FADE_ALPHA 를 거듭 곱한다
function trailAlpha(back: number): number {
    return Math.pow(FADE_ALPHA, back);
}

function makeScene(inp: Input, reduce: boolean): Scene {
    const rng = lcg(4004);
    let sparks: Spark[] = [];
    let gap = 2;
    let colorIdx = 0;
    const glows: HTMLCanvasElement[] = [];
    function glowFor(idx: number): HTMLCanvasElement {
        if (glows[idx]) {
            return glows[idx];
        }
        const size = GLOW_BLUR * 4;
        const cv = document.createElement('canvas');
        cv.width = size;
        cv.height = size;
        const c = cv.getContext('2d');
        if (c) {
            const g = c.createRadialGradient(size / 2, size / 2, 0, size / 2, size / 2, size / 2);
            g.addColorStop(0, rgba(CREAM, 0.9));
            g.addColorStop(0.25, rgba(PALETTE[idx], 0.45));
            g.addColorStop(1, rgba(PALETTE[idx], 0));
            c.fillStyle = g;
            c.fillRect(0, 0, size, size);
        }
        glows[idx] = cv;
        return cv;
    }
    function burst(x: number, y: number): void {
        const hue = colorIdx % PALETTE.length;
        colorIdx += 1;
        for (let i = 0; i < SPARK_N; i += 1) {
            const a = rng() * PI2;
            const sp = 2.2 + rng() * 2.4;
            sparks.push({ x: x, y: y, vx: Math.cos(a) * sp, vy: Math.sin(a) * sp, age: 0, life: 70 + rng() * 26, hist: [], hue: i % 4 === 0 ? (hue + 1) % PALETTE.length : hue });
        }
        if (sparks.length > 330) {
            sparks.splice(0, sparks.length - 330);
        }
    }
    const scene: Scene = {
        step(): void {
            gap -= 1;
            if (gap <= 0) {
                burst(90 + rng() * 300, 80 + rng() * 70);
                gap = BURST_GAP;
            }
            const alive: Spark[] = [];
            for (let i = 0; i < sparks.length; i += 1) {
                const s = sparks[i];
                s.age += 1;
                s.hist.push(s.x, s.y);
                if (s.hist.length > TRAIL_LEN * 2) {
                    s.hist.splice(0, 2);
                }
                s.vx *= SPARK_DRAG;
                s.vy = s.vy * SPARK_DRAG + SPARK_GRAVITY;
                s.x += s.vx;
                s.y += s.vy;
                if (s.age < s.life) {
                    alive.push(s);
                }
            }
            sparks = alive;
        },
        draw(ctx: CanvasRenderingContext2D): void {
            drawSky(ctx, H);
            // 이 항목의 핵심 — 합성 방식을 가산으로 바꾸면 겹친 곳의 색이 더해져 하얗게 번진다
            ctx.globalCompositeOperation = 'lighter';
            ctx.lineCap = 'round';
            for (let i = 0; i < sparks.length; i += 1) {
                const s = sparks[i];
                const life = Math.pow(1 - s.age / s.life, 0.7);
                const n = s.hist.length / 2;
                for (let k = 1; k < n; k += 1) {
                    const a = trailAlpha(n - k) * life;
                    ctx.strokeStyle = rgba(PALETTE[s.hue], a * 0.85);
                    ctx.lineWidth = lerp(0.6, HEAD_R * 1.3, k / n);
                    ctx.beginPath();
                    ctx.moveTo(s.hist[(k - 1) * 2], s.hist[(k - 1) * 2 + 1]);
                    ctx.lineTo(s.hist[k * 2], s.hist[k * 2 + 1]);
                    ctx.stroke();
                }
                const r = GLOW_BLUR * (0.6 + 0.4 * life);
                ctx.globalAlpha = life * (reduce ? 0.6 : 0.85);
                ctx.drawImage(glowFor(s.hue), s.x - r, s.y - r, r * 2, r * 2);
                ctx.globalAlpha = 1;
                ctx.fillStyle = rgba(CREAM, life);
                ctx.beginPath();
                ctx.arc(s.x, s.y, HEAD_R * (0.5 + 0.5 * life), 0, PI2);
                ctx.fill();
            }
            ctx.globalCompositeOperation = 'source-over';
        },
        label(): string {
            return inp.idle < IDLE_TICKS ? TXT_TAP : TXT_AUTO;
        },
        tap(): void {
            burst(inp.x, inp.y);
        }
    };
    for (let i = 0; i < PREWARM_STEPS; i += 1) {
        scene.step();
    }
    return scene;
}

export default function FwAdditiveTrailHistoryGlow() {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const canvasRef = useRef<HTMLCanvasElement | null>(null);
  const statusRef = useRef<HTMLParagraphElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    const canvas = canvasRef.current;
    const statusEl = statusRef.current;
    const btnEl = btnRef.current;
    if (!root || !canvas || !statusEl || !btnEl) return;
    return start(root, canvas, statusEl, btnEl, makeScene);
  }, []);

  return (
    <main className={styles.stage}>
      <div className={`${styles.sc} is-demo`} ref={rootRef}>
        <canvas className={styles.sc__canvas} ref={canvasRef} width={480} height={300} tabIndex={0} aria-label="과거 위치 이력을 잇고 겹칠수록 밝아지는 꼬리와 빛무리" />
        <p className={styles.sc__status} ref={statusRef} aria-live="polite">꼬리 이력을 가산 합성으로 겹친다 — 겹친 곳이 흰빛으로 번진다</p>
        <button className={styles.sc__btn} ref={btnRef} type="button">멈춤</button>
        <p className={styles.sc__caption}>과거 위치 이력을 잇고 겹칠수록 밝아지는 꼬리와 빛무리</p>
      </div>
    </main>
  );
}