:root {
  --bg: #f4f6f8;
  --card: #ffffff;
  --ink: #1a2733;
  --muted: #5c6b7a;
  --accent: #00558c;
  --accent-dark: #003e66;
  --warn-bg: #fff6e5;
  --warn-border: #e0a800;
  --border: #d6dde3;
  --good: #2e7d32;
  --bad: #c62828;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  background: var(--bg);
  color: var(--ink);
  line-height: 1.5;
}

header {
  background: var(--accent);
  color: #fff;
  padding: 1.5rem 1rem;
  border-bottom: 4px solid var(--accent-dark);
}
.header-inner { max-width: 1100px; margin: 0 auto; }
header h1 { margin: 0; font-size: 1.6rem; }
.subtitle { margin: 0.25rem 0 0; opacity: 0.9; font-size: 0.95rem; }

.tabs {
  display: flex;
  gap: 0;
  margin: 1rem 0 -1.5rem;
  position: relative;
  z-index: 2;
}
.tab-btn {
  background: rgba(255, 255, 255, 0.1);
  color: rgba(255, 255, 255, 0.85);
  border: none;
  border-bottom: 3px solid transparent;
  padding: 0.65rem 1.2rem;
  font-size: 0.95rem;
  font-weight: 500;
  cursor: pointer;
  border-radius: 6px 6px 0 0;
  transition: background 0.15s, color 0.15s, border-color 0.15s;
}
.tab-btn:hover { background: rgba(255, 255, 255, 0.18); color: #fff; }
.tab-btn.active {
  background: var(--bg);
  color: var(--accent-dark);
  border-bottom-color: var(--bg);
}

.tab-panel { display: none; }
.tab-panel.active { display: block; }

main {
  max-width: 1100px;
  margin: 1.5rem auto;
  padding: 0 1rem 3rem;
}

.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.25rem 1.25rem 1.5rem;
  margin-bottom: 1.25rem;
  box-shadow: 0 1px 2px rgba(0,0,0,0.04);
}
.card h2 {
  margin: 0 0 0.5rem;
  font-size: 1.15rem;
  color: var(--accent-dark);
  border-bottom: 1px solid var(--border);
  padding-bottom: 0.4rem;
}
.card.warning {
  background: var(--warn-bg);
  border-left: 5px solid var(--warn-border);
}
.card.warning h2 { color: #8a6500; border-bottom-color: #e9d28b; }

.muted { color: var(--muted); }
.small { font-size: 0.85rem; }

ul { margin: 0.5rem 0 0 1.2rem; padding: 0; }
li { margin-bottom: 0.4rem; }

label { display: block; margin: 0.4rem 0; }
.check {
  display: flex;
  align-items: flex-start;
  gap: 0.5rem;
  margin: 0.35rem 0;
  cursor: pointer;
}
.check input { margin-top: 0.25rem; }

input[type="number"], input[type="text"] {
  font-size: 1rem;
  padding: 0.4rem 0.5rem;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: #fff;
  width: 100%;
  max-width: 160px;
}
.input-suffix {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  margin-top: 0.25rem;
}
.input-suffix span { color: var(--muted); font-size: 0.9rem; }

.row { display: flex; flex-wrap: wrap; gap: 1rem 1.5rem; align-items: flex-end; }
.row label { display: flex; flex-direction: column; font-size: 0.9rem; color: var(--muted); }

.btn-primary, .btn-secondary, .btn-danger {
  font-size: 0.95rem;
  padding: 0.55rem 1rem;
  border-radius: 4px;
  border: 1px solid transparent;
  cursor: pointer;
  font-weight: 500;
}
.btn-primary {
  background: var(--accent);
  color: #fff;
}
.btn-primary:hover { background: var(--accent-dark); }
.btn-secondary {
  background: #fff;
  color: var(--accent);
  border-color: var(--accent);
}
.btn-secondary:hover { background: #eef5fa; }
.btn-danger {
  background: #fff;
  color: var(--bad);
  border-color: #f0c2c2;
  font-size: 0.8rem;
  padding: 0.25rem 0.6rem;
}
.btn-danger:hover { background: #fdecec; }

.drain, .scenario {
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1rem;
  margin-bottom: 0.9rem;
  background: #fbfcfd;
}
.drain-head, .scenario-head {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 0.5rem;
}
.drain-head input[type="text"] { max-width: 200px; }

.fittings-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
  gap: 0.5rem 0.75rem;
  margin-top: 0.5rem;
}
.fittings-grid label { display: flex; flex-direction: column; font-size: 0.85rem; color: var(--muted); margin: 0; }
.fittings-grid input { max-width: 100%; }

.fittings-key {
  background: #eef5fa;
  border-left: 3px solid var(--accent);
  padding: 0.5rem 0.75rem;
  border-radius: 4px;
  font-size: 0.85rem;
  margin-bottom: 0.75rem;
}

.eq-len-display {
  margin-top: 0.5rem;
  padding: 0.5rem 0.75rem;
  background: #eef9f0;
  border-left: 3px solid var(--good);
  border-radius: 4px;
  font-size: 0.9rem;
}
.eq-len-display strong { color: var(--good); }

.scenario-drains { margin-top: 0.6rem; }
.scenario-drain-row {
  display: grid;
  grid-template-columns: 90px 1fr 160px 120px;
  align-items: center;
  gap: 0.5rem;
  padding: 0.35rem 0;
  border-top: 1px dashed var(--border);
  font-size: 0.9rem;
}
.scenario-drain-row:first-child { border-top: none; }
.scenario-drain-row label { display: inline-flex; align-items: center; gap: 0.4rem; margin: 0; }

#results {
  margin: 1rem 0;
}
.result-table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 0.75rem;
  font-size: 0.92rem;
}
.result-table th, .result-table td {
  border: 1px solid var(--border);
  padding: 0.5rem 0.6rem;
  text-align: right;
}
.result-table th { background: #eef5fa; text-align: center; color: var(--accent-dark); }
.result-table td:first-child, .result-table th:first-child { text-align: left; }

.error-banner {
  background: #fdecec;
  border-left: 4px solid var(--bad);
  color: var(--bad);
  padding: 0.6rem 0.8rem;
  border-radius: 4px;
  margin-top: 0.75rem;
  font-size: 0.9rem;
}

.chart-wrap {
  margin-top: 1rem;
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 4px;
  padding: 0.75rem;
  height: 460px;
}

footer {
  margin-top: 2rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  font-size: 0.85rem;
  color: var(--muted);
}
footer p { margin: 0.4rem 0; }

/* ---------- Physics writeup ---------- */
.writeup {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 2.5rem 3rem;
  max-width: 820px;
  margin: 0 auto;
  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.04);
  font-size: 1rem;
  line-height: 1.65;
  color: #1a2733;
}
.writeup-header {
  text-align: center;
  margin-bottom: 2.5rem;
  padding-bottom: 1.5rem;
  border-bottom: 2px solid var(--accent);
}
.writeup-header h1 {
  font-size: 1.65rem;
  margin: 0 0 0.5rem;
  color: var(--accent-dark);
  line-height: 1.3;
}
.byline {
  margin: 0.75rem 0 0.25rem;
  font-size: 1.05rem;
  color: var(--ink);
}
.writeup-subtitle {
  margin: 0.25rem 0 0;
  font-style: italic;
  color: var(--muted);
  font-size: 0.95rem;
}
.writeup section {
  margin-bottom: 1.8rem;
}
.writeup h2 {
  font-size: 1.2rem;
  color: var(--accent-dark);
  border-bottom: 1px solid var(--border);
  padding-bottom: 0.3rem;
  margin: 1.6rem 0 0.75rem;
}
.writeup h3 {
  font-size: 1.05rem;
  color: var(--accent-dark);
  margin: 1.2rem 0 0.4rem;
}
.writeup p { margin: 0.5rem 0; text-align: justify; }
.writeup ul, .writeup ol { margin: 0.5rem 0 0.5rem 1.5rem; }
.writeup li { margin-bottom: 0.3rem; }
.writeup-table {
  width: 100%;
  border-collapse: collapse;
  margin: 1rem auto;
  font-size: 0.95rem;
}
.writeup-table th,
.writeup-table td {
  border: 1px solid var(--border);
  padding: 0.5rem 0.75rem;
  text-align: left;
}
.writeup-table th {
  background: #eef5fa;
  color: var(--accent-dark);
  font-weight: 600;
}
.writeup-table tr.total-row td {
  background: #f4f8fc;
  border-top: 2px solid var(--accent);
}
.writeup mjx-container[display="true"] {
  margin: 0.8rem 0 !important;
}

@media (max-width: 720px) {
  .writeup { padding: 1.5rem 1.2rem; }
  .writeup p { text-align: left; }
}

@media print {
  header { background: #fff; color: #000; border-bottom: 2px solid #000; }
  .tabs { display: none; }
  .btn-primary, .btn-secondary, .btn-danger { display: none; }
  .card { box-shadow: none; page-break-inside: avoid; }
  .tab-panel { display: block !important; }
  .writeup { box-shadow: none; border: none; padding: 0; }
}