04Step Path Line That Splits Based on the Chosen Answer

<main class="stage">
<div class="bm is-demo" id="bm">
  <svg class="bm__svg" id="bmSvg" viewBox="0 0 100 100" role="img" aria-label="Survey branching path">
    <path class="bm__trunk" d="M50 12 L50 42" />
    <path class="bm__branch bm__branch--a" id="pathA" d="M50 42 C50 58 50 68 50 86" />
    <path class="bm__branch bm__branch--b" id="pathB" d="M50 42 C50 58 50 68 50 86" />
    <circle class="bm__dot bm__dot--start" cx="50" cy="12" r="6" />
    <circle class="bm__dot bm__dot--a" id="dotA" cx="20" cy="90" r="6" />
    <circle class="bm__dot bm__dot--b" id="dotB" cx="80" cy="90" r="6" />
  </svg>
  <div class="bm__labels">
    <span class="bm__labelStart">Question</span>
    <span class="bm__labelA">On-Premise</span>
    <span class="bm__labelB">Subscription</span>
  </div>
  <p class="bm__status" id="bmStatus" aria-live="polite">Pick an answer to split the path</p>
  <div class="bm__controls">
    <button type="button" class="bm__btn" id="bmA">Choose On-Premise</button>
    <button type="button" class="bm__btn" id="bmB">Choose Subscription</button>
    <button type="button" class="bm__btn bm__btn--ghost" id="bmReset">Reset</button>
  </div>
</div>
</main>

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

// 핵심: 분기 경로는 CSS 트랜지션이 아니라 JS 가 매 프레임 SVG path `d` 의 제어점 x좌표를
// lerp(fromX, targetX, easeOutCubic(t)) 로 계산해 다시 쓴 값이다 — 직선(50)에서 좌우(20/80)로
// 좌표 자체가 보간되며 형태가 바뀐다.

$duration: $dur-quick;
$easing: $ease-out;
$color: $stage-ink;

@include stage($stage-paper);

.bm {
  --bm-progress: 0;
  display: grid;
  gap: $sp-1;
  justify-items: center;
  width: min(85vw, 260px);
  border-radius: $r-sm;
  background: radial-gradient(140px 110px at 50% 62%, rgba(10, 79, 232, var(--bm-progress)) 0%, rgba(10, 79, 232, 0) 72%);
}

.bm__svg {
  width: 150px;
  aspect-ratio: 1 / 1;
  overflow: visible;
  margin-top: $sp-2;
}

.bm__trunk {
  fill: none;
  stroke: rgba(23, 20, 26, .3);
  stroke-width: 2.4;
  stroke-linecap: round;
}

.bm__branch {
  fill: none;
  stroke: rgba(23, 20, 26, .22);
  stroke-width: 2.4;
  stroke-linecap: round;
}

.bm__branch.is-chosen { stroke: $subject-blue-deep; stroke-width: 3; }
.bm__branch.is-dim { stroke: rgba(23, 20, 26, .12); }

.bm__dot { fill: $stage-ink; }
.bm__dot--start { fill: $stage-ink; }
.bm__dot--a, .bm__dot--b { fill: $stage-paper; stroke: rgba(23, 20, 26, .3); stroke-width: 2; }
.bm__dot--a.is-chosen, .bm__dot--b.is-chosen { fill: $subject-blue-deep; stroke: $subject-blue-deep; }

.bm__labels {
  position: relative;
  width: 150px;
  height: 0;
}

.bm__labelStart, .bm__labelA, .bm__labelB {
  position: absolute;
  transform: translate(-50%, -140%);
  font-size: 10px;
  font-weight: 700;
  color: rgba(23, 20, 26, .7);
  white-space: nowrap;
}

.bm__labelStart { left: 50%; top: -100%; }
.bm__labelA { left: 20%; top: -12%; }
.bm__labelB { left: 80%; top: -12%; }

.bm__status {
  margin: 0;
  font-size: 11px;
  color: rgba(23, 20, 26, .78);
  font-weight: 600;
  min-height: 14px;
  text-align: center;
}

.bm__controls { display: flex; gap: $sp-1; flex-wrap: wrap; justify-content: center; }

.bm__btn {
  border: 1px solid rgba(23, 20, 26, .28);
  background: transparent;
  color: $stage-ink;
  border-radius: $r-control;
  padding: $sp-1 $sp-2;
  font-size: 10px;
  font-weight: 600;
  cursor: pointer;
}

.bm__btn--ghost { border-style: dashed; color: rgba(23, 20, 26, .6); }

.bm__btn:focus-visible {
  outline: 2px solid $subject-blue-deep;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .bm { width: min(94vw, 240px); }
  .bm__svg { width: 108px; margin-top: $sp-1; }
  .bm__labels { width: 108px; }
  .bm__labelStart, .bm__labelA, .bm__labelB { font-size: 9px; }
}

@media (prefers-reduced-motion: reduce) {
  .bm__branch { transition: none; }
}

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);
}

.bm {
  --bm-progress: 0;
  display: grid;
  gap: 4px;
  justify-items: center;
  width: min(85vw, 260px);
  border-radius: 8px;
  background: radial-gradient(140px 110px at 50% 62%, rgba(10, 79, 232, var(--bm-progress)) 0%, rgba(10, 79, 232, 0) 72%);
}

.bm__svg {
  width: 150px;
  aspect-ratio: 1/1;
  overflow: visible;
  margin-top: 8px;
}

.bm__trunk {
  fill: none;
  stroke: rgba(23, 20, 26, 0.3);
  stroke-width: 2.4;
  stroke-linecap: round;
}

.bm__branch {
  fill: none;
  stroke: rgba(23, 20, 26, 0.22);
  stroke-width: 2.4;
  stroke-linecap: round;
}

.bm__branch.is-chosen {
  stroke: #0a4fe8;
  stroke-width: 3;
}

.bm__branch.is-dim {
  stroke: rgba(23, 20, 26, 0.12);
}

.bm__dot {
  fill: #17141a;
}

.bm__dot--start {
  fill: #17141a;
}

.bm__dot--a, .bm__dot--b {
  fill: #ffffff;
  stroke: rgba(23, 20, 26, 0.3);
  stroke-width: 2;
}

.bm__dot--a.is-chosen, .bm__dot--b.is-chosen {
  fill: #0a4fe8;
  stroke: #0a4fe8;
}

.bm__labels {
  position: relative;
  width: 150px;
  height: 0;
}

.bm__labelStart, .bm__labelA, .bm__labelB {
  position: absolute;
  transform: translate(-50%, -140%);
  font-size: 10px;
  font-weight: 700;
  color: rgba(23, 20, 26, 0.7);
  white-space: nowrap;
}

.bm__labelStart {
  left: 50%;
  top: -100%;
}

.bm__labelA {
  left: 20%;
  top: -12%;
}

.bm__labelB {
  left: 80%;
  top: -12%;
}

.bm__status {
  margin: 0;
  font-size: 11px;
  color: rgba(23, 20, 26, 0.78);
  font-weight: 600;
  min-height: 14px;
  text-align: center;
}

.bm__controls {
  display: flex;
  gap: 4px;
  flex-wrap: wrap;
  justify-content: center;
}

.bm__btn {
  border: 1px solid rgba(23, 20, 26, 0.28);
  background: transparent;
  color: #17141a;
  border-radius: 12px;
  padding: 4px 8px;
  font-size: 10px;
  font-weight: 600;
  cursor: pointer;
}

.bm__btn--ghost {
  border-style: dashed;
  color: rgba(23, 20, 26, 0.6);
}

.bm__btn:focus-visible {
  outline: 2px solid #0a4fe8;
  outline-offset: 2px;
}

@media (max-width: 340px) {
  .bm {
    width: min(94vw, 240px);
  }
  .bm__svg {
    width: 108px;
    margin-top: 4px;
  }
  .bm__labels {
    width: 108px;
  }
  .bm__labelStart, .bm__labelA, .bm__labelB {
    font-size: 9px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bm__branch {
    transition: none;
  }
}

(function () {
  var STEP_BRANCH_MORPH_MS = 420;
  var CENTER_X = 50;
  var TARGET_A_X = 20;
  var TARGET_B_X = 80;
  var MAX_OFFSET = TARGET_B_X - CENTER_X; // 표시 레이어 정규화 기준(로직 무관)

  var root = document.getElementById('bm');
  var status = document.getElementById('bmStatus');
  var pathA = document.getElementById('pathA');
  var pathB = document.getElementById('pathB');
  var dotA = document.getElementById('dotA');
  var dotB = document.getElementById('dotB');
  var btnA = document.getElementById('bmA');
  var btnB = document.getElementById('bmB');
  var btnReset = document.getElementById('bmReset');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  var choice = null; // null | 'a' | 'b'
  var progress = { a: 0, b: 0 }; // 각 경로의 중심 이탈률(0~1) — 굵기·글로우·배경 톤이 여기서 파생

  function clamp(v, lo, hi) { return Math.max(lo, Math.min(hi, v)); }
  function lerp(a, b, t) { return a + (b - a) * t; }
  function easeOutCubic(t) { return 1 - Math.pow(1 - t, 3); }

  function buildPath(midX, endX) {
    return 'M50 42 C50 58 ' + midX.toFixed(2) + ' 68 ' + endX.toFixed(2) + ' 86';
  }

  function paintProgress() {
    root.style.setProperty('--bm-progress', Math.max(progress.a, progress.b).toFixed(3));
  }

  function applyMorph(key, pathEl, targetX, fromX) {
    if (reduce) {
      pathEl.setAttribute('d', buildPath(targetX, targetX));
      progress[key] = clamp(Math.abs(targetX - CENTER_X) / MAX_OFFSET, 0, 1);
      paintProgress();
      return;
    }
    var start = null;
    function step(now) {
      if (start === null) start = now;
      var t = clamp((now - start) / STEP_BRANCH_MORPH_MS, 0, 1);
      var eased = easeOutCubic(t);
      var x = lerp(fromX, targetX, eased);
      pathEl.setAttribute('d', buildPath(x, x));
      var travelled = clamp(Math.abs(x - CENTER_X) / MAX_OFFSET, 0, 1);
      progress[key] = travelled;
      pathEl.style.strokeWidth = (2.4 + travelled * 2.6).toFixed(2);
      pathEl.style.filter = travelled > 0.04
        ? 'drop-shadow(0 0 ' + (travelled * 5.5).toFixed(1) + 'px rgba(10, 79, 232, ' + (travelled * 0.55).toFixed(2) + '))'
        : 'none';
      paintProgress();
      if (t < 1) requestAnimationFrame(step);
    }
    requestAnimationFrame(step);
  }

  function currentX(pathEl) {
    var d = pathEl.getAttribute('d');
    var m = /C50 58 ([\-0-9.]+) 68/.exec(d);
    return m ? parseFloat(m[1]) : CENTER_X;
  }

  function setChoice(next) {
    choice = next;
    var aTarget = next === 'a' ? TARGET_A_X : CENTER_X;
    var bTarget = next === 'b' ? TARGET_B_X : CENTER_X;
    applyMorph('a', pathA, aTarget, currentX(pathA));
    applyMorph('b', pathB, bTarget, currentX(pathB));
    pathA.classList.toggle('is-chosen', next === 'a');
    pathA.classList.toggle('is-dim', next === 'b');
    pathB.classList.toggle('is-chosen', next === 'b');
    pathB.classList.toggle('is-dim', next === 'a');
    dotA.classList.toggle('is-chosen', next === 'a');
    dotB.classList.toggle('is-chosen', next === 'b');
    status.textContent = next === null ? 'Pick an answer to split the path'
      : (next === 'a' ? 'Split into the on-premise path' : 'Split into the subscription path');
  }

  btnA.addEventListener('click', function () { root.classList.remove('is-demo'); setChoice(choice === 'a' ? null : 'a'); });
  btnB.addEventListener('click', function () { root.classList.remove('is-demo'); setChoice(choice === 'b' ? null : 'b'); });
  btnReset.addEventListener('click', function () { root.classList.remove('is-demo'); setChoice(null); });
  root.addEventListener('keydown', function (e) {
    if (e.key === 'ArrowRight') { root.classList.remove('is-demo'); setChoice('b'); e.preventDefault(); }
    else if (e.key === 'ArrowLeft') { root.classList.remove('is-demo'); setChoice('a'); e.preventDefault(); }
  });

  var DEMO_SCHEDULE = [
    { t: 0.20, c: 'a' },
    { t: 0.70, c: 'b' },
    { t: 1.20, c: 'a' },
    { t: 1.70, c: 'b' }
  ];
  var startTime = null;
  var demoIdx = -1;
  function tick(now) {
    if (startTime === null) startTime = now;
    var t = (now - startTime) / 1000 % 2;
    if (root.classList.contains('is-demo')) {
      if (t < DEMO_SCHEDULE[0].t && demoIdx !== -1) demoIdx = -1;
      for (var i = 0; i < DEMO_SCHEDULE.length; i++) {
        if (t >= DEMO_SCHEDULE[i].t && demoIdx < i) { setChoice(DEMO_SCHEDULE[i].c); demoIdx = i; }
      }
    }
    requestAnimationFrame(tick);
  }

  requestAnimationFrame(tick);
})();

// StepBranchPathMorph — vanilla 판(04-step-precision-branch-path-morph) 의 React 포트.
// 값 하나: 선택에 따라 갈라지는 경로는 CSS 트랜지션이 아니라 매 프레임 SVG path `d` 의
// 제어점 x좌표를 lerp(fromX, targetX, easeOutCubic(t)) 로 다시 써서 얻는다 — 좌표 자체가 보간된다.
import { useEffect, useRef, useState } from "react";
import styles from "./StepBranchPathMorph.module.scss";

type Box<T> = { current: T };
type Choice = "a" | "b" | null;

const STEP_BRANCH_MORPH_MS = 420;
const CENTER_X = 50;
const TARGET_A_X = 20;
const TARGET_B_X = 80;
const MAX_OFFSET = TARGET_B_X - CENTER_X; // 표시 레이어 정규화 기준(로직 무관)

function clamp(v: number, lo: number, hi: number): number {
  return Math.max(lo, Math.min(hi, v));
}
function lerp(a: number, b: number, t: number): number {
  return a + (b - a) * t;
}
function easeOutCubic(t: number): number {
  return 1 - Math.pow(1 - t, 3);
}
function buildPath(midX: number, endX: number): string {
  return "M50 42 C50 58 " + midX.toFixed(2) + " 68 " + endX.toFixed(2) + " 86";
}
function currentX(pathEl: SVGPathElement | null): number {
  if (!pathEl) return CENTER_X;
  const d = pathEl.getAttribute("d") || "";
  const m = /C50 58 ([\-0-9.]+) 68/.exec(d);
  return m ? parseFloat(m[1]) : CENTER_X;
}

export default function StepBranchPathMorph() {
  const [choice, setChoiceState] = useState<Choice>(null);
  const [reduce, setReduce] = useState(false);
  const choiceRef = useRef<Choice>(null) as Box<Choice>;
  const demoRef = useRef<boolean>(true) as Box<boolean>;
  const rootRef = useRef<HTMLDivElement | null>(null);
  const pathARef = useRef<SVGPathElement | null>(null);
  const pathBRef = useRef<SVGPathElement | null>(null);
  const dotARef = useRef<SVGCircleElement | null>(null);
  const dotBRef = useRef<SVGCircleElement | null>(null);
  const frameRef = useRef<number>(0) as Box<number>;
  const progressRef = useRef<{ a: number; b: number }>({ a: 0, b: 0 }) as Box<{ a: number; b: number }>;

  useEffect(() => {
    setReduce(window.matchMedia("(prefers-reduced-motion: reduce)").matches);
  }, []);

  function paintProgress(): void {
    const root = rootRef.current;
    if (!root) return;
    root.style.setProperty("--bm-progress", Math.max(progressRef.current.a, progressRef.current.b).toFixed(3));
  }

  function applyMorph(key: "a" | "b", pathEl: SVGPathElement | null, targetX: number, fromX: number): void {
    if (!pathEl) return;
    if (reduce) {
      pathEl.setAttribute("d", buildPath(targetX, targetX));
      progressRef.current[key] = clamp(Math.abs(targetX - CENTER_X) / MAX_OFFSET, 0, 1);
      paintProgress();
      return;
    }
    let start = 0;
    function step(now: number): void {
      if (start === 0) start = now;
      const t = clamp((now - start) / STEP_BRANCH_MORPH_MS, 0, 1);
      const eased = easeOutCubic(t);
      const x = lerp(fromX, targetX, eased);
      if (pathEl) {
        pathEl.setAttribute("d", buildPath(x, x));
        const travelled = clamp(Math.abs(x - CENTER_X) / MAX_OFFSET, 0, 1);
        progressRef.current[key] = travelled;
        pathEl.style.strokeWidth = (2.4 + travelled * 2.6).toFixed(2);
        pathEl.style.filter = travelled > 0.04
          ? "drop-shadow(0 0 " + (travelled * 5.5).toFixed(1) + "px rgba(10, 79, 232, " + (travelled * 0.55).toFixed(2) + "))"
          : "none";
        paintProgress();
      }
      if (t < 1) requestAnimationFrame(step);
    }
    requestAnimationFrame(step);
  }

  function setChoice(next: Choice): void {
    choiceRef.current = next;
    setChoiceState(next);
    const aTarget = next === "a" ? TARGET_A_X : CENTER_X;
    const bTarget = next === "b" ? TARGET_B_X : CENTER_X;
    applyMorph("a", pathARef.current, aTarget, currentX(pathARef.current));
    applyMorph("b", pathBRef.current, bTarget, currentX(pathBRef.current));
  }

  function stopDemo(): void { demoRef.current = false; }

  useEffect(() => {
    let startTime = 0;
    let demoIdx = -1;
    const schedule: Array<{ t: number; c: Choice }> = [
      { t: 0.20, c: "a" },
      { t: 0.70, c: "b" },
      { t: 1.20, c: "a" },
      { t: 1.70, c: "b" }
    ];
    function tick(now: number): void {
      if (startTime === 0) startTime = now;
      const t = ((now - startTime) / 1000) % 2;
      if (demoRef.current) {
        if (t < schedule[0].t && demoIdx !== -1) demoIdx = -1;
        for (let i = 0; i < schedule.length; i++) {
          if (t >= schedule[i].t && demoIdx < i) { setChoice(schedule[i].c); demoIdx = i; }
        }
      }
      frameRef.current = requestAnimationFrame(tick);
    }
    frameRef.current = requestAnimationFrame(tick);
    return () => cancelAnimationFrame(frameRef.current);
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [reduce]);

  function onKeyDown(e: { key: string; preventDefault: () => void }): void {
    if (e.key === "ArrowRight") { stopDemo(); setChoice("b"); e.preventDefault(); }
    else if (e.key === "ArrowLeft") { stopDemo(); setChoice("a"); e.preventDefault(); }
  }

  const statusText = choice === null
    ? "답을 고르면 경로가 갈라집니다"
    : choice === "a" ? "설치형 경로로 갈라졌습니다" : "구독형 경로로 갈라졌습니다";

  return (
    <main className={styles.stage}>
      <div className={styles.bm} ref={rootRef} onKeyDown={onKeyDown}>
        <svg className={styles.svg} viewBox="0 0 100 100" role="img" aria-label="설문 분기 경로">
          <path className={styles.trunk} d="M50 12 L50 42" />
          <path
            className={[styles.branch, choice === "a" ? styles.branchChosen : "", choice === "b" ? styles.branchDim : ""].join(" ")}
            ref={pathARef}
            d="M50 42 C50 58 50 68 50 86"
          />
          <path
            className={[styles.branch, choice === "b" ? styles.branchChosen : "", choice === "a" ? styles.branchDim : ""].join(" ")}
            ref={pathBRef}
            d="M50 42 C50 58 50 68 50 86"
          />
          <circle className={styles.dotStart} cx={50} cy={12} r={6} />
          <circle className={[styles.dot, choice === "a" ? styles.dotChosen : ""].join(" ")} ref={dotARef} cx={20} cy={90} r={6} />
          <circle className={[styles.dot, choice === "b" ? styles.dotChosen : ""].join(" ")} ref={dotBRef} cx={80} cy={90} r={6} />
        </svg>
        <div className={styles.labels}>
          <span className={styles.labelStart}>질문</span>
          <span className={styles.labelA}>설치형</span>
          <span className={styles.labelB}>구독형</span>
        </div>
        <p className={styles.status} aria-live="polite">{statusText}</p>
        <div className={styles.controls}>
          <button type="button" className={styles.btn} onClick={() => { stopDemo(); setChoice(choice === "a" ? null : "a"); }}>설치형 선택</button>
          <button type="button" className={styles.btn} onClick={() => { stopDemo(); setChoice(choice === "b" ? null : "b"); }}>구독형 선택</button>
          <button type="button" className={[styles.btn, styles.btnGhost].join(" ")} onClick={() => { stopDemo(); setChoice(null); }}>초기화</button>
        </div>
      </div>
    </main>
  );
}