<div class="wrap">
  <h2>Product List</h2>
  <button class="filter-btn" id="filterBtn">
    <svg class="funnel-icon" id="funnelIcon" viewBox="0 0 64 64" width="20" height="20">
      <path id="funnelPath" d="" fill="none" stroke="currentColor" stroke-width="4.5" stroke-linejoin="round" stroke-linecap="round"></path>
    </svg>
    <span>Filters</span>
    <span class="badge" id="badge">0</span>
  </button>

  <div class="panel" id="panel">
    <label class="opt"><input type="checkbox" data-tag="in-stock"> In stock</label>
    <label class="opt"><input type="checkbox" data-tag="on-sale"> On sale</label>
    <label class="opt"><input type="checkbox" data-tag="new"> New arrivals</label>
  </div>
</div>

* { box-sizing: border-box; }
body { font-family: system-ui, sans-serif; background: #f8fafc; display: flex; justify-content: center; padding: 60px 20px; }

.wrap { width: 100%; max-width: 300px; }
h2 { font-size: 18px; font-weight: 800; color: #0f172a; margin: 0 0 16px; }

.filter-btn {
  position: relative; display: inline-flex; align-items: center; gap: 8px;
  padding: 10px 16px; border-radius: 9px; border: 1.5px solid #e2e8f0;
  background: #fff; color: #334155; font-size: 13.5px; font-weight: 700;
  font-family: inherit; cursor: pointer; transition: border-color 0.2s, color 0.2s;
}
.filter-btn.active { border-color: #6366f1; color: #6366f1; }
.filter-btn:hover { border-color: #cbd5e1; }
.filter-btn.active:hover { border-color: #4f46e5; }

.funnel-icon { flex-shrink: 0; }

.badge {
  min-width: 18px; height: 18px; padding: 0 5px; border-radius: 999px;
  background: #6366f1; color: #fff; font-size: 10.5px; font-weight: 800;
  display: flex; align-items: center; justify-content: center;
  transform: scale(0); transition: transform 0.25s cubic-bezier(0.34,1.56,0.64,1);
}
.badge.show { transform: scale(1); }

.panel {
  margin-top: 12px; padding: 14px; border-radius: 10px;
  background: #fff; border: 1px solid #e2e8f0;
  display: flex; flex-direction: column; gap: 10px;
}
.opt { display: flex; align-items: center; gap: 8px; font-size: 13px; color: #475569; cursor: pointer; }
.opt input { accent-color: #6366f1; width: 15px; height: 15px; }

/* Two 10-point closed outlines (matched vertex count) in a 0-64 viewBox.
   restPts draws a relaxed, wide-mouthed funnel (no filters applied).
   activePts draws the same funnel pinched narrower with a shorter stem,
   reading as "actively filtering". Both are interpolated per-frame so the
   funnel genuinely narrows rather than crossfading between two icons. */
const restPts = [
  [8, 12], [56, 12], [56, 18], [38, 34],
  [38, 50], [26, 50], [26, 34], [8, 18],
  [8, 15], [8, 12],
];
const activePts = [
  [14, 14], [50, 14], [50, 18], [36, 30],
  [36, 52], [28, 52], [28, 30], [14, 18],
  [14, 16], [14, 14],
];

function pointsToPath(pts) {
  return pts.map((p, i) => (i === 0 ? 'M' : 'L') + p[0].toFixed(2) + ',' + p[1].toFixed(2)).join(' ');
}
function lerpPath(a, b, t) {
  const pts = a.map((p, i) => [p[0] + (b[i][0] - p[0]) * t, p[1] + (b[i][1] - p[1]) * t]);
  return pointsToPath(pts);
}

const pathEl = document.getElementById('funnelPath');
const btn = document.getElementById('filterBtn');
const badge = document.getElementById('badge');
const panel = document.getElementById('panel');
const checkboxes = panel.querySelectorAll('input[type="checkbox"]');
pathEl.setAttribute('d', pointsToPath(restPts));

let rafId = null;
function morphTo(target, duration) {
  if (rafId) cancelAnimationFrame(rafId);
  const start = performance.now();
  // Always interpolate between the two canonical states so repeated
  // toggles never drift from the intended shapes.
  const startState = target === activePts ? restPts : activePts;
  function frame(now) {
    const t = Math.min(1, (now - start) / duration);
    const eased = t * (2 - t); // ease-out quad
    pathEl.setAttribute('d', lerpPath(startState, target, eased));
    if (t < 1) rafId = requestAnimationFrame(frame);
  }
  rafId = requestAnimationFrame(frame);
}

function updateState() {
  const count = Array.from(checkboxes).filter(c => c.checked).length;
  const isActive = count > 0;
  btn.classList.toggle('active', isActive);
  badge.textContent = String(count);
  badge.classList.toggle('show', isActive);
  morphTo(isActive ? activePts : restPts, 320);
}

btn.addEventListener('click', () => {
  panel.classList.toggle('open');
  panel.style.display = panel.classList.contains('open') || panel.style.display === 'none' ? (panel.style.display === 'flex' ? 'none' : 'flex') : 'flex';
});
panel.style.display = 'flex';

checkboxes.forEach(cb => cb.addEventListener('change', updateState));