09Underline-animated link buttons

<main class="stage">
  <div class="link-group">
    <button class="link-btn is-demo" type="button">
      <span class="link-btn__label">Learn more</span>
    </button>
    <input class="input-static" type="text" placeholder="Email address" readonly>
  </div>
</main>

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

$duration: $dur-loop;                              // 밑줄이 가운데서 자랐다 사그라드는 주기
$easing: $ease-spring;
$color: $subject-blue;                             // 밑줄·포커스 링 색

@include stage($stage-ink);

.link-group {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: center;
  gap: $sp-6;
}
.link-btn {
  position: relative;
  border: 0;
  background: none;
  padding: $sp-2 0;
  cursor: pointer;
  color: $subject-cream;
}
.link-btn__label {
  font-weight: 800;
  font-size: clamp(18px, 5vw, 26px);
  line-height: 1;
}
// 밑줄이 가운데(transform-origin: center)에서 시작해 양쪽으로 자랐다 사그라든다
.link-btn::after {
  content: '';
  position: absolute;
  left: 0; right: 0; bottom: 0;
  height: 2px;
  background: $color;
  transform-origin: center;
  transform: scaleX(0);
}
.link-btn.is-demo::after {
  animation: underline-pulse $duration $easing infinite;
}
@keyframes underline-pulse {
  0%, 10%   { transform: scaleX(0); }
  50%       { transform: scaleX(1); }
  90%, 100% { transform: scaleX(0); }
}
.link-btn:focus-visible {
  outline: 2px solid $color;
  outline-offset: $sp-1;
}

// 버튼 라벨 글자 낱개 — index.html 말미 스크립트가 텍스트를 이 span 으로 쪼개고
// 결정론 해시로 --nx0·--ny0·--nr0(출발점) · --wx·--wy·--wr(힘장 경유점) 을 심는다. (형태2차 한 겹, 077 품질기준선)
.char {
  display: inline-block;
  animation: char-flow $duration $easing infinite;
}
.char--space { width: .28em; }

@keyframes char-flow {
  0%, 4%    { transform: translate(var(--nx0, 0), var(--ny0, 16px)) rotate(var(--nr0, 0deg)) scale(.55); opacity: 0; }
  16%       { transform: translate(var(--wx, 0), var(--wy, 4px)) rotate(var(--wr, 0deg)) scale(1.08); opacity: 1; }
  30%, 78%  { transform: translate(0, 0) rotate(0deg) scale(1); opacity: 1; }
  96%, 100% { transform: translate(var(--nx0, 0), var(--ny0, 16px)) rotate(var(--nr0, 0deg)) scale(.55); opacity: 0; }
}

.input-static {
  border: 1px solid rgba($subject-cream, .3);
  border-radius: $r-control;
  background: transparent;
  color: $subject-cream;
  padding: $sp-2 $sp-3;
  font-weight: 500;
  font-size: clamp(12px, 3vw, 14px);
  line-height: 1;
}
.input-static::placeholder {
  color: rgba($subject-cream, .5);
}

@media (prefers-reduced-motion: reduce) {
  .link-btn.is-demo::after { animation: none; transform: scaleX(0); }
  .char { animation: none; transform: none; opacity: 1; }
}

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);
}

.link-group {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: center;
  gap: 24px;
}

.link-btn {
  position: relative;
  border: 0;
  background: none;
  padding: 8px 0;
  cursor: pointer;
  color: #fff7e6;
}

.link-btn__label {
  font-weight: 800;
  font-size: clamp(18px, 5vw, 26px);
  line-height: 1;
}

.link-btn::after {
  content: "";
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 2px;
  background: #2f6df6;
  transform-origin: center;
  transform: scaleX(0);
}

.link-btn.is-demo::after {
  animation: underline-pulse 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@keyframes underline-pulse {
  0%, 10% {
    transform: scaleX(0);
  }
  50% {
    transform: scaleX(1);
  }
  90%, 100% {
    transform: scaleX(0);
  }
}
.link-btn:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 4px;
}

.char {
  display: inline-block;
  animation: char-flow 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.char--space {
  width: 0.28em;
}

@keyframes char-flow {
  0%, 4% {
    transform: translate(var(--nx0, 0), var(--ny0, 16px)) rotate(var(--nr0, 0deg)) scale(0.55);
    opacity: 0;
  }
  16% {
    transform: translate(var(--wx, 0), var(--wy, 4px)) rotate(var(--wr, 0deg)) scale(1.08);
    opacity: 1;
  }
  30%, 78% {
    transform: translate(0, 0) rotate(0deg) scale(1);
    opacity: 1;
  }
  96%, 100% {
    transform: translate(var(--nx0, 0), var(--ny0, 16px)) rotate(var(--nr0, 0deg)) scale(0.55);
    opacity: 0;
  }
}
.input-static {
  border: 1px solid rgba(255, 247, 230, 0.3);
  border-radius: 12px;
  background: transparent;
  color: #fff7e6;
  padding: 8px 12px;
  font-weight: 500;
  font-size: clamp(12px, 3vw, 14px);
  line-height: 1;
}

.input-static::placeholder {
  color: rgba(255, 247, 230, 0.5);
}

@media (prefers-reduced-motion: reduce) {
  .link-btn.is-demo::after {
    animation: none;
    transform: scaleX(0);
  }
  .char {
    animation: none;
    transform: none;
    opacity: 1;
  }
}

// 버튼 라벨 글자별 노이즈 오프셋 + 벡터필드 진입 (형태2차) — 라벨을 span 으로 쪼개고
// 결정론 해시로 출발점/경유점 변수만 심는다. 프레임은 style.scss 의 char-flow 키프레임이 we build..
// 기존 underline-pulse 와 겹쳐 — 글자가 흩어졌다 힘장을 따라 자리 잡고, 그 아래 밑줄이 양쪽으로 자란다.
(function () {
  function hash(n) {           // 정수 → 0..1 결정론 난수 (sin 해시)
    var s = Math.sin(n * 127.1 + 311.7) * 43758.5453;
    return s - Math.floor(s);
  }
  document.querySelectorAll('.link-btn .link-btn__label').forEach(function (label) {
    var text = label.textContent;
    label.textContent = '';
    for (var c = 0; c < text.length; c++) {
      var ch = document.createElement('span');
      if (text[c] === ' ') { ch.className = 'char char--space'; }
      else {
        ch.className = 'char';
        ch.textContent = text[c];
        var s = c * 13 + 5;
        var h1 = hash(s), h2 = hash(s + 1), h3 = hash(s + 2), h4 = hash(s + 3);
        var ang = h1 * Math.PI * 2;
        var rad = 12 + h2 * 22;
        var x0 = Math.cos(ang) * rad, y0 = 16 + h3 * 14;
        var rot0 = (h4 - .5) * 30;
        var wx = x0 - 0.9 * y0 * 0.55, wy = y0 + 0.9 * x0 * 0.55;
        var wr = rot0 * 0.4;
        ch.style.setProperty('--nx0', x0.toFixed(1) + 'px');
        ch.style.setProperty('--ny0', y0.toFixed(1) + 'px');
        ch.style.setProperty('--nr0', rot0.toFixed(1) + 'deg');
        ch.style.setProperty('--wx', (wx * .6).toFixed(1) + 'px');
        ch.style.setProperty('--wy', (wy * .18 + 3).toFixed(1) + 'px');
        ch.style.setProperty('--wr', wr.toFixed(1) + 'deg');
        ch.style.animationDelay = (-0.045 * c).toFixed(3) + 's';
      }
      label.appendChild(ch);
    }
  });
})();

import type { CSSProperties } from "react";
import styles from "./KineticButtonsInputs.module.scss";

export type KineticButtonsInputsProps = {
  label?: string;
  placeholder?: string;
  /** 밑줄이 가운데서 자랐다 사그라드는 주기(기본 2000ms) */
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type CharVars = CSSProperties & {
  "--nx0"?: string; "--ny0"?: string; "--nr0"?: string;
  "--wx"?: string; "--wy"?: string; "--wr"?: string;
};

/** 정수 → 0..1 결정론 난수 (sin 해시 — vanilla 09 와 동일 계열) */
function hash(n: number): number {
  const s = Math.sin(n * 127.1 + 311.7) * 43758.5453;
  return s - Math.floor(s);
}

/** 한 글자 = 출발점(노이즈) + 힘장 경유점 변수 묶음. vanilla index.html 스크립트와 같은 식. */
function charVars(c: number): CharVars {
  const s = c * 13 + 5;
  const h1 = hash(s), h2 = hash(s + 1), h3 = hash(s + 2), h4 = hash(s + 3);
  const ang = h1 * Math.PI * 2;
  const rad = 12 + h2 * 22;
  const x0 = Math.cos(ang) * rad, y0 = 16 + h3 * 14;
  const rot0 = (h4 - .5) * 30;
  // 회전 힘장 v(p) = ω × p 를 출발점에서 dt 적분 — v = ω*(-y, x), dt=0.55, ω=0.9
  const wx = x0 - 0.9 * y0 * 0.55, wy = y0 + 0.9 * x0 * 0.55;
  const wr = rot0 * 0.4;
  return {
    "--nx0": `${x0.toFixed(1)}px`,
    "--ny0": `${y0.toFixed(1)}px`,
    "--nr0": `${rot0.toFixed(1)}deg`,
    "--wx": `${(wx * .6).toFixed(1)}px`,
    "--wy": `${(wy * .18 + 3).toFixed(1)}px`,
    "--wr": `${wr.toFixed(1)}deg`,
    animationDelay: `${(-0.045 * c).toFixed(3)}s`,
  };
}

export default function KineticButtonsInputs({
  label = "Learn more",
  placeholder = "Email address",
  duration = 2000,
  easing = "cubic-bezier(.2,.8,.2,1)",
  color = "#2f6df6",
}: KineticButtonsInputsProps) {
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  return (
    <div className={styles.stage} style={vars}>
      <div className={styles.group}>
        <button type="button" className={styles.linkBtn}>
          <span className={styles.label}>
            {label.split("").map((ch, c) =>
              ch === " " ? (
                <span key={c} className={styles.charSpace} />
              ) : (
                <span key={c} className={styles.char} style={charVars(c)}>{ch}</span>
              )
            )}
          </span>
          <span className={styles.underline} aria-hidden="true" />
        </button>
        <input className={styles.input} type="text" placeholder={placeholder} readOnly />
      </div>
    </div>
  );
}