04Letters that follow — a text train

<main class="stage">
<div class="tt is-demo" id="tt">
  <div class="tt__field" role="img" aria-label="The letters of HELLO queue up behind the cursor, one beat apart">
    <span class="tt__ch">H</span>
    <span class="tt__ch">E</span>
    <span class="tt__ch">L</span>
    <span class="tt__ch">L</span>
    <span class="tt__ch">O</span>
  </div>
  <p class="tt__hint">Move the mouse and the letters follow in a line</p>
</div>
</main>

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

// 따라 다니는 글씨들 — 첫 글자는 커서를, 뒤 글자는 앞 글자를 lerp 로 쫓게 한다(계수 0.22 → 0.07).
// 격자 미리보기(.is-demo)는 같은 물결 경로를 글자마다 animation-delay 로 한 박자씩 어긋나게 해
// 기차가 밀려오는 모양을 보여 준다.

$duration: $dur-loop;
$easing: $ease-spring;
$color: $subject-ink;

@include stage($stage-paper);

.tt { display: grid; justify-items: center; gap: $sp-3; width: 100%; }

.tt__field {
  position: relative;
  width: 100%; height: clamp(110px, 32vh, 160px);
  border: 3px solid $color; border-radius: $r-card;
  background: $subject-cream;
}

.tt__ch {
  position: absolute; left: 0; top: 0;
  width: 28px; height: 40px;
  color: $color;
  font-weight: 800; font-size: 28px; line-height: 40px;
  text-align: center;
  will-change: transform;
}
// 글자마다 색을 조금씩 달리해 줄이 얼마나 길어지는지 보기 쉽게 한다
.tt__ch:nth-child(1) { color: $color; }
.tt__ch:nth-child(2) { color: rgba($color, .84); }
.tt__ch:nth-child(3) { color: rgba($color, .72); }
.tt__ch:nth-child(4) { color: rgba($color, .72); }
.tt__ch:nth-child(5) { color: rgba($color, .84); }

.tt__hint { margin: 0; color: $color; font-weight: 700; font-size: clamp(9px, 2.4vw, 11px); }

// ── 미리보기 루프: 큰 물결을 글자가 차례로 지나간다. delay 로 한 박자씩 밀린다.
//    같은 경로를 쓰므로 글자마다 시작점(left/top)을 비틀어 덩리로 겹치지 않게 한다
.tt.is-demo .tt__ch {
  animation-name: ttWave;
  animation-duration: $duration;
  animation-timing-function: $easing;
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
}
.tt.is-demo .tt__ch:nth-child(1) { animation-delay: 0ms; }
.tt.is-demo .tt__ch:nth-child(2) { animation-delay: 110ms; left: 24px; top: 12px; }
.tt.is-demo .tt__ch:nth-child(3) { animation-delay: 220ms; left: 48px; top: 24px; }
.tt.is-demo .tt__ch:nth-child(4) { animation-delay: 330ms; left: 72px; top: 4px; }
.tt.is-demo .tt__ch:nth-child(5) { animation-delay: 440ms; left: 104px; top: 22px; }

@keyframes ttWave {
  0%       { transform: translate3d(36px, 96px, 0); }
  38%, 46% { transform: translate3d(196px, 30px, 0); }
  60%, 68% { transform: translate3d(120px, 72px, 0); }
  86%, 100% { transform: translate3d(36px, 96px, 0); }
}

@media (max-width: 340px) {
  .tt { gap: $sp-2; }
  .tt__field { height: clamp(96px, 28vh, 130px); }
  .tt__ch { width: 22px; height: 32px; font-size: 22px; line-height: 32px; }
  .tt.is-demo .tt__ch { animation-name: ttWaveNarrow; }
}

@keyframes ttWaveNarrow {
  0%       { transform: translate3d(18px, 76px, 0); }
  38%, 46% { transform: translate3d(140px, 26px, 0); }
  60%, 68% { transform: translate3d(88px, 58px, 0); }
  86%, 100% { transform: translate3d(18px, 76px, 0); }
}

@media (prefers-reduced-motion: reduce) {
  .tt.is-demo .tt__ch { animation: none !important; }
  // reduce 에서는 글자 다섯 개가 가로로 늘어선 정지 상태를 보인다
  .tt.is-demo .tt__ch:nth-child(1) { transform: translate3d(64px, 56px, 0) !important; }
  .tt.is-demo .tt__ch:nth-child(2) { transform: translate3d(100px, 56px, 0) !important; }
  .tt.is-demo .tt__ch:nth-child(3) { transform: translate3d(136px, 56px, 0) !important; }
  .tt.is-demo .tt__ch:nth-child(4) { transform: translate3d(172px, 56px, 0) !important; }
  .tt.is-demo .tt__ch:nth-child(5) { transform: translate3d(208px, 56px, 0) !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);
}

.tt {
  display: grid;
  justify-items: center;
  gap: 12px;
  width: 100%;
}

.tt__field {
  position: relative;
  width: 100%;
  height: clamp(110px, 32vh, 160px);
  border: 3px solid #17141a;
  border-radius: 16px;
  background: #fff7e6;
}

.tt__ch {
  position: absolute;
  left: 0;
  top: 0;
  width: 28px;
  height: 40px;
  color: #17141a;
  font-weight: 800;
  font-size: 28px;
  line-height: 40px;
  text-align: center;
  will-change: transform;
}

.tt__ch:nth-child(1) {
  color: #17141a;
}

.tt__ch:nth-child(2) {
  color: rgba(23, 20, 26, 0.84);
}

.tt__ch:nth-child(3) {
  color: rgba(23, 20, 26, 0.72);
}

.tt__ch:nth-child(4) {
  color: rgba(23, 20, 26, 0.72);
}

.tt__ch:nth-child(5) {
  color: rgba(23, 20, 26, 0.84);
}

.tt__hint {
  margin: 0;
  color: #17141a;
  font-weight: 700;
  font-size: clamp(9px, 2.4vw, 11px);
}

.tt.is-demo .tt__ch {
  animation-name: ttWave;
  animation-duration: 2s;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
  animation-iteration-count: infinite;
  animation-fill-mode: backwards;
}

.tt.is-demo .tt__ch:nth-child(1) {
  animation-delay: 0ms;
}

.tt.is-demo .tt__ch:nth-child(2) {
  animation-delay: 110ms;
  left: 24px;
  top: 12px;
}

.tt.is-demo .tt__ch:nth-child(3) {
  animation-delay: 220ms;
  left: 48px;
  top: 24px;
}

.tt.is-demo .tt__ch:nth-child(4) {
  animation-delay: 330ms;
  left: 72px;
  top: 4px;
}

.tt.is-demo .tt__ch:nth-child(5) {
  animation-delay: 440ms;
  left: 104px;
  top: 22px;
}

@keyframes ttWave {
  0% {
    transform: translate3d(36px, 96px, 0);
  }
  38%, 46% {
    transform: translate3d(196px, 30px, 0);
  }
  60%, 68% {
    transform: translate3d(120px, 72px, 0);
  }
  86%, 100% {
    transform: translate3d(36px, 96px, 0);
  }
}
@media (max-width: 340px) {
  .tt {
    gap: 8px;
  }
  .tt__field {
    height: clamp(96px, 28vh, 130px);
  }
  .tt__ch {
    width: 22px;
    height: 32px;
    font-size: 22px;
    line-height: 32px;
  }
  .tt.is-demo .tt__ch {
    animation-name: ttWaveNarrow;
  }
}
@keyframes ttWaveNarrow {
  0% {
    transform: translate3d(18px, 76px, 0);
  }
  38%, 46% {
    transform: translate3d(140px, 26px, 0);
  }
  60%, 68% {
    transform: translate3d(88px, 58px, 0);
  }
  86%, 100% {
    transform: translate3d(18px, 76px, 0);
  }
}
@media (prefers-reduced-motion: reduce) {
  .tt.is-demo .tt__ch {
    animation: none !important;
  }
  .tt.is-demo .tt__ch:nth-child(1) {
    transform: translate3d(64px, 56px, 0) !important;
  }
  .tt.is-demo .tt__ch:nth-child(2) {
    transform: translate3d(100px, 56px, 0) !important;
  }
  .tt.is-demo .tt__ch:nth-child(3) {
    transform: translate3d(136px, 56px, 0) !important;
  }
  .tt.is-demo .tt__ch:nth-child(4) {
    transform: translate3d(172px, 56px, 0) !important;
  }
  .tt.is-demo .tt__ch:nth-child(5) {
    transform: translate3d(208px, 56px, 0) !important;
  }
}

var root = document.getElementById('tt');
var field = root.querySelector('.tt__field');
var chars = Array.prototype.slice.call(root.querySelectorAll('.tt__ch'));

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

function lerp(a, b, k) { return a + (b - a) * k; }

var target = { x: 0, y: 0 };
var spots = [];

function home() {
  // 글자들은 처음에 가로로 늘어서 있다 — .is-demo 를 뗀 정적 상태의 모습
  var r = field.getBoundingClientRect();
  var gap = Math.min(44, (r.width - 60) / (chars.length - 1));
  var startX = r.width / 2 - gap * (chars.length - 1) / 2;
  target = { x: r.width / 2, y: r.height / 2 };
  spots = [];
  for (var i = 0; i < chars.length; i++) {
    spots.push({ x: startX + gap * i, y: r.height / 2 + 26 });
  }
}

function setTarget(cx, cy) {
  var r = field.getBoundingClientRect();
  target.x = cx - r.left;
  target.y = cy - r.top;
}

function draw() {
  for (var i = 0; i < chars.length; i++) {
    chars[i].style.transform = 'translate3d(' + (spots[i].x - 14) + 'px,' + (spots[i].y - 20) + 'px,0)';
  }
}

// 첫 글자는 커서를, 나머지는 앞 글자를 쫓는다 — 계수가 뒤로 갈수록 작아져 기차처럼 밀린다
var follow = [0.22, 0.16, 0.12, 0.09, 0.07];

function tick() {
  spots[0].x = lerp(spots[0].x, target.x, follow[0]);
  spots[0].y = lerp(spots[0].y, target.y - 34, follow[0]);
  for (var i = 1; i < spots.length; i++) {
    spots[i].x = lerp(spots[i].x, spots[i - 1].x, follow[i]);
    spots[i].y = lerp(spots[i].y, spots[i - 1].y + 8, follow[i]);
  }
  draw();
  requestAnimationFrame(tick);
}

window.addEventListener('pointermove', function (e) {
  root.classList.remove('is-demo');
  setTarget(e.clientX, e.clientY);
});
window.addEventListener('pointerdown', function (e) {
  root.classList.remove('is-demo');
  setTarget(e.clientX, e.clientY);
});

home();
draw();
if (!reduce) {
  requestAnimationFrame(tick);
} else {
  // reduce 에서는 글자가 늘어선 자리에 그대로 머문다
  window.addEventListener('pointermove', function (e) {
    setTarget(e.clientX, e.clientY);
    for (var i = 0; i < spots.length; i++) {
      spots[i].x = target.x - i * 34;
      spots[i].y = target.y;
    }
    draw();
  });
}

// TextTrain — vanilla 판(run/079/demo/04-follow-text-train/index.html) 의 React 포트.
// 첫 글자는 커서를, 뒤 글자는 앞 글자를 lerp 로 쫓게 한다(계수 0.22 → 0.07) — 기차처럼 한 박자씩 밀려온다.
// 자리 계산·줄간격·reduce 동작은 vanilla 와 같은 값이다 (다시 타이핑하지 않는다).
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./TextTrain.module.scss";

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

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
// useRef 는 T | null 을 돌려준다. 처음부터 값이 있고 절대 비지 않는 상자는 이렇게 좁혀 쓴다
type Box<T> = { current: T };
// 전역 DOM 타입에 기대지 않는다 — 포인터 이벤트에서 실제로 읽는 것만 적는다
type PtrEvt = { clientX: number; clientY: number };

const FIELD_LABEL = "안녕하세요 글자가 커서를 쫓아 한 박자씩 밀려오는 그림";
const HINT = "마우스를 움직이면 글자들이 줄지어 따라옵니다";
const CHARS = ["안", "녕", "하", "세", "요"];
// 첫 글자는 커서를, 나머지는 앞 글자를 쫓는다 — 계수가 뒤로 갈수록 작아져 기차처럼 밀린다
const FOLLOW = [0.22, 0.16, 0.12, 0.09, 0.07];

// lerp: 매 프레임 남은 거리의 일부만 더 가는 선형 보간
function lerp(a: number, b: number, k: number): number {
  return a + (b - a) * k;
}

export default function TextTrain({
  duration = 2000,
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#17141a",
}: TextTrainProps) {
  const fieldRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement>;
  const charRef = useRef<(HTMLSpanElement | null)[]>(new Array(CHARS.length).fill(null)) as Box<(HTMLSpanElement | null)[]>;
  const [demo, setDemo] = useState(true);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  function setChar(index: number) {
    return (el: HTMLSpanElement | null): void => {
      charRef.current[index] = el;
    };
  }

  useEffect(() => {
    const field = fieldRef.current;
    if (!field) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    const target = { x: 0, y: 0 };
    let spots = CHARS.map(() => ({ x: 0, y: 0 }));

    function home(): void {
      // 글자들은 처음에 가로로 늘어서 있다 — .is-demo 를 뗀 정적 상태의 모습
      const r = field.getBoundingClientRect();
      const gap = Math.min(44, (r.width - 60) / (CHARS.length - 1));
      const startX = r.width / 2 - gap * (CHARS.length - 1) / 2;
      target.x = r.width / 2;
      target.y = r.height / 2;
      spots = CHARS.map((_, i) => ({ x: startX + gap * i, y: r.height / 2 + 26 }));
    }

    function setTarget(cx: number, cy: number): void {
      const r = field.getBoundingClientRect();
      target.x = cx - r.left;
      target.y = cy - r.top;
    }

    function draw(): void {
      for (let i = 0; i < spots.length; i++) {
        const ch = charRef.current[i];
        if (ch) ch.style.transform = "translate3d(" + (spots[i].x - 14) + "px," + (spots[i].y - 20) + "px,0)";
      }
    }

    function tick(): void {
      spots[0].x = lerp(spots[0].x, target.x, FOLLOW[0]);
      spots[0].y = lerp(spots[0].y, target.y - 34, FOLLOW[0]);
      for (let i = 1; i < spots.length; i++) {
        spots[i].x = lerp(spots[i].x, spots[i - 1].x, FOLLOW[i]);
        spots[i].y = lerp(spots[i].y, spots[i - 1].y + 8, FOLLOW[i]);
      }
      draw();
      raf = requestAnimationFrame(tick);
    }

    function onMove(event: PtrEvt): void {
      setDemo(false);
      setTarget(event.clientX, event.clientY);
      if (reduce) {
        // reduce 에서는 글자가 커서 뒤로 가로줄로 바로 붙는다
        for (let i = 0; i < spots.length; i++) {
          spots[i].x = target.x - i * 34;
          spots[i].y = target.y;
        }
        draw();
      }
    }
    function onDown(event: PtrEvt): void {
      setDemo(false);
      setTarget(event.clientX, event.clientY);
    }

    window.addEventListener("pointermove", onMove);
    window.addEventListener("pointerdown", onDown);

    let raf = 0;
    home();
    draw();
    if (!reduce) raf = requestAnimationFrame(tick);
    return () => {
      window.removeEventListener("pointermove", onMove);
      window.removeEventListener("pointerdown", onDown);
      cancelAnimationFrame(raf);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root + (demo ? " " + styles.isDemo : "")} style={vars}>
        <div className={styles.field} ref={fieldRef} role="img" aria-label={FIELD_LABEL}>
          {CHARS.map((ch, i) => (
            <span className={styles.ch} key={i} ref={setChar(i)}>{ch}</span>
          ))}
        </div>
        <p className={styles.hint}>{HINT}</p>
      </div>
    </main>
  );
}