01Length threshold fill — the strength value steps up each time the input's character-count value crosses a set boundary value

<main class="stage">
<div class="pw is-demo" id="pw">
  <label class="pw__field">
    <input class="pw__input" id="pwInput" type="text" autocomplete="off" spellcheck="false" value="">
  </label>
  <div class="pw__track" id="pwTrack">
    <span class="pw__tick" style="--tick-at:25%"></span>
    <span class="pw__tick" style="--tick-at:50%"></span>
    <span class="pw__tick" style="--tick-at:75%"></span>
    <div class="pw__fill" id="pwFill"></div>
  </div>
  <div class="pw__readout">
    <span class="pw__label" id="pwLabel">Boundary 0/4</span>
    <span class="pw__len">Length <b id="pwLen">0</b></span>
  </div>
</div>
</main>

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

// 계산 겹: 글자 수 값이 4개 경계(4·8·12·16)를 넘을 때마다 강도 단계(0~4)가 오르고,
// 그 단계에 맞는 토큰 색으로 막대와 글자색이 함께 바뀐다 — 트윈은 채움 비율(scaleX)뿐이다.
@include stage($stage-ink);

.pw {
  width: 100%;
  max-width: 320px;
  display: grid;
  gap: $sp-4;
}

.pw__field { display: block; }

.pw__input {
  width: 100%;
  box-sizing: border-box;
  padding: $sp-3 $sp-4;
  border-radius: $r-control;
  border: 0;
  background: rgba($stage-paper, .12);
  color: $subject-cream;
  font: 500 16px/1.3 -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  letter-spacing: .04em;
  -webkit-text-security: disc; // 마스킹: type=text 유지 + 브라우저 저장 프롬프트 회피

  &:focus { outline: 2px solid $subject-blue-light; outline-offset: 2px; }
}

.pw__track {
  position: relative;
  height: $sp-3;
  border-radius: $r-sm;
  background: rgba($stage-paper, .14);
  overflow: hidden;
}

.pw__fill {
  position: absolute;
  inset: 0;
  border-radius: $r-sm;
  background: $error;
  transform: scaleX(0);
  transform-origin: left center; // 채움 막대 — 시작 변에서 자란다
  transition: transform $dur-base $ease-out, background-color $dur-base $ease-out;
}

.pw--s1 .pw__fill { background: $error; }
.pw--s2 .pw__fill { background: $subject-orange-deep; }
.pw--s3 .pw__fill { background: $subject-blue; }
.pw--s4 .pw__fill { background: $subject-mint; }

.pw__tick {
  position: absolute;
  top: 0;
  left: var(--tick-at);
  width: 2px;
  height: 100%;
  background: rgba($stage-ink, .35);
  z-index: 1;
}

.pw__readout {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font: 600 13px/1.2 -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  color: $stage-paper;
}

.pw__len { color: rgba($stage-paper, .72); font-weight: 500; }
.pw__len b { font-variant-numeric: tabular-nums; }

@media (max-width: 340px) {
  .pw { max-width: 260px; }
  .pw__input { padding: $sp-2 $sp-3; font-size: 14px; }
  .pw__readout { font-size: 12px; }
}

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

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);
}

.pw {
  width: 100%;
  max-width: 320px;
  display: grid;
  gap: 16px;
}

.pw__field {
  display: block;
}

.pw__input {
  width: 100%;
  box-sizing: border-box;
  padding: 12px 16px;
  border-radius: 12px;
  border: 0;
  background: rgba(255, 255, 255, 0.12);
  color: #fff7e6;
  font: 500 16px/1.3 -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  letter-spacing: 0.04em;
  -webkit-text-security: disc;
}
.pw__input:focus {
  outline: 2px solid #5b8cf8;
  outline-offset: 2px;
}

.pw__track {
  position: relative;
  height: 12px;
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.14);
  overflow: hidden;
}

.pw__fill {
  position: absolute;
  inset: 0;
  border-radius: 8px;
  background: #c2361a;
  transform: scaleX(0);
  transform-origin: left center;
  transition: transform 300ms ease-out, background-color 300ms ease-out;
}

.pw--s1 .pw__fill {
  background: #c2361a;
}

.pw--s2 .pw__fill {
  background: #c93000;
}

.pw--s3 .pw__fill {
  background: #2f6df6;
}

.pw--s4 .pw__fill {
  background: #4cd4a6;
}

.pw__tick {
  position: absolute;
  top: 0;
  left: var(--tick-at);
  width: 2px;
  height: 100%;
  background: rgba(23, 20, 26, 0.35);
  z-index: 1;
}

.pw__readout {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font: 600 13px/1.2 -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  color: #ffffff;
}

.pw__len {
  color: rgba(255, 255, 255, 0.72);
  font-weight: 500;
}

.pw__len b {
  font-variant-numeric: tabular-nums;
}

@media (max-width: 340px) {
  .pw {
    max-width: 260px;
  }
  .pw__input {
    padding: 8px 12px;
    font-size: 14px;
  }
  .pw__readout {
    font-size: 12px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .pw__fill {
    transition: none;
  }
}

(function () {
  var root = document.getElementById('pw');
  var input = document.getElementById('pwInput');
  var fill = document.getElementById('pwFill');
  var label = document.getElementById('pwLabel');
  var lenEl = document.getElementById('pwLen');

  // 계산 겹: Length 값이 이 Boundary 값들을 넘을 때마다 강도 Value(0~4)이 한 단계씩 오른다.
  var THRESHOLDS = [4, 8, 12, 16];
  var STAGE_CLASS = ['pw--s0', 'pw--s1', 'pw--s2', 'pw--s3', 'pw--s4'];

  function stageOf(len) {
    var s = 0;
    for (var i = 0; i < THRESHOLDS.length; i++) { if (len >= THRESHOLDS[i]) s = i + 1; }
    return s;
  }

  function paint(value) {
    var len = value.length;
    var stage = stageOf(len);
    var ratio = stage / THRESHOLDS.length;
    fill.style.transform = 'scaleX(' + ratio + ')';
    STAGE_CLASS.forEach(function (c) { root.classList.remove(c); });
    root.classList.add(STAGE_CLASS[stage]);
    label.textContent = 'Boundary ' + stage + '/4';
    lenEl.textContent = String(len);
  }

  input.addEventListener('input', function () {
    root.classList.remove('is-demo');
    stopAutoplay();
    paint(input.value);
  });

  paint(input.value);

  // 자동 루프: 시드 고정 문자열 배열로 글자 수를 늘렸다 줄이며 네 경계를 오간다.
  var SEQ = ['', 'ab', 'abcd', 'abcdef', 'abcdefgh', 'abcdefghij', 'abcdefghijkl',
             'abcdefghijklmn', 'abcdefghijklmnop', 'abcdefghijklmn', 'abcdefghij',
             'abcdefgh', 'abcd', ''];
  var si = 0;
  var autoTimer = null;
  function autoTick() { input.value = SEQ[si]; paint(SEQ[si]); si = (si + 1) % SEQ.length; }
  function stopAutoplay() { if (autoTimer) { clearInterval(autoTimer); autoTimer = null; } }
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (!reduce) { autoTimer = setInterval(autoTick, 320); autoTick(); }
})();

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

type Box<T> = { current: T };

const THRESHOLDS = [4, 8, 12, 16];
const STAGE_CLASS = ["pw--s0", "pw--s1", "pw--s2", "pw--s3", "pw--s4"];

export default function ThresholdBar() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement | null>;
  const inputRef = useRef(null) as unknown as Box<HTMLInputElement | null>;
  const fillRef = useRef(null) as unknown as Box<HTMLDivElement | null>;
  const labelRef = useRef(null) as unknown as Box<HTMLSpanElement | null>;
  const lenRef = useRef(null) as unknown as Box<HTMLElement | null>;

  useEffect(() => {
    const root = rootRef.current;
    const input = inputRef.current;
    const fill = fillRef.current;
    const label = labelRef.current;
    const lenEl = lenRef.current;
    if (!root || !input || !fill || !label || !lenEl) return;

    // 계산 겹: 글자 수 값이 4개 경계(4·8·12·16)를 넘을 때마다 강도 단계(0~4)가 오른다.
    function stageOf(len: number) {
      let s = 0;
      for (let i = 0; i < THRESHOLDS.length; i++) { if (len >= THRESHOLDS[i]) s = i + 1; }
      return s;
    }

    function paint(value: string) {
      const len = value.length;
      const stage = stageOf(len);
      const ratio = stage / THRESHOLDS.length;
      fill!.style.transform = "scaleX(" + ratio + ")";
      STAGE_CLASS.forEach((c) => root!.classList.remove(styles[c]));
      root!.classList.add(styles[STAGE_CLASS[stage]]);
      label!.textContent = "경계 " + stage + "/4";
      lenEl!.textContent = String(len);
    }

    function onInput() {
      root!.classList.remove(styles["is-demo"]);
      stopAutoplay();
      paint(input!.value);
    }
    input.addEventListener("input", onInput);
    paint(input.value);

    // 자동 루프: 시드 고정 문자열 배열로 글자 수를 늘렸다 줄이며 네 경계를 오간다.
    const SEQ = ["", "ab", "abcd", "abcdef", "abcdefgh", "abcdefghij", "abcdefghijkl",
      "abcdefghijklmn", "abcdefghijklmnop", "abcdefghijklmn", "abcdefghij",
      "abcdefgh", "abcd", ""];
    let si = 0;
    let autoTimer: number | null = null;
    function autoTick() { input!.value = SEQ[si]; paint(SEQ[si]); si = (si + 1) % SEQ.length; }
    function stopAutoplay() { if (autoTimer !== null) { window.clearInterval(autoTimer); autoTimer = null; } }
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (!reduce) { autoTimer = window.setInterval(autoTick, 320); autoTick(); }

    return () => {
      stopAutoplay();
      input.removeEventListener("input", onInput);
    };
  }, []);

  return (
    <div ref={rootRef} className={`${styles.stage} ${styles.pw} ${styles["is-demo"]} ${styles["pw--s0"]}`}>
      <label className={styles.field}>
        <input ref={inputRef} className={styles.input} type="text" autoComplete="off" spellCheck={false} defaultValue="" />
      </label>
      <div className={styles.track}>
        <span className={styles.tick} style={{ left: "25%" }} />
        <span className={styles.tick} style={{ left: "50%" }} />
        <span className={styles.tick} style={{ left: "75%" }} />
        <div ref={fillRef} className={styles.fill} />
      </div>
      <div className={styles.readout}>
        <span ref={labelRef} className={styles.label}>경계 0/4</span>
        <span className={styles.len}>글자 수 <b ref={lenRef}>0</b></span>
      </div>
    </div>
  );
}