/* ─── CSS Custom Properties ──────────────────────────────────────────────── */
:root {
  --color-primary: #2563eb;
  --color-primary-dark: #1d4ed8;
  --color-primary-light: #dbeafe;
  --color-bg: #f8fafc;
  --color-card: #ffffff;
  --color-text: #1e293b;
  --color-text-muted: #64748b;
  --color-border: #e2e8f0;
  --color-border-strong: #cbd5e1;
  --color-warning-bg: #fefce8;
  --color-warning-border: #eab308;
  --color-success: #16a34a;
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.08), 0 1px 2px rgba(0,0,0,0.05);
  --shadow-md: 0 4px 6px -1px rgba(0,0,0,0.07), 0 2px 4px -1px rgba(0,0,0,0.05);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.08), 0 4px 6px -2px rgba(0,0,0,0.04);
}

/* ─── Reset & Base ───────────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html { font-size: 16px; scroll-behavior: smooth; }

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  background: var(--color-bg);
  color: var(--color-text);
  line-height: 1.6;
  min-height: 100vh;
}

/* ─── Layout ─────────────────────────────────────────────────────────────── */
.container {
  max-width: 800px;
  margin: 0 auto;
  padding: 0 20px;
}

/* ─── Site Header (index page) ───────────────────────────────────────────── */
.site-header {
  background: var(--color-primary);
  color: white;
  padding: 10px 0 8px;
}

.site-header h1 {
  font-size: 3.2rem;
  font-weight: 800;
  letter-spacing: -0.5px;
  margin-bottom: 0px;
}

.site-header p {
  font-size: 1.1rem;
  opacity: 0.88;
}

.share-bar {
  display: flex;
  justify-content: flex-end;
  padding-top: 10px;
}

.share-btn {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  background: var(--color-primary);
  color: white;
  border: 1px solid var(--color-primary);
  border-radius: 999px;
  padding: 6px 16px;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, color 0.15s, box-shadow 0.15s;
  box-shadow: var(--shadow-sm);
  white-space: nowrap;
}

.share-btn:hover {
  background: white;
  color: var(--color-primary);
  box-shadow: var(--shadow-md);
}

.brand-name {
  font-family: 'Raleway', sans-serif;
  font-weight: 300;
  letter-spacing: 2px;
}

.brand-tld {
  font-family: 'Raleway', sans-serif;
  font-weight: 400;
  font-size: 0.55em;
  color: #93c5fd;
  letter-spacing: 1px;
  vertical-align: super;
}

/* ─── Site Nav (formula pages) ───────────────────────────────────────────── */
.site-nav {
  background: var(--color-card);
  border-bottom: 1px solid var(--color-border);
  padding: 14px 20px;
}

.nav-back {
  color: var(--color-primary);
  text-decoration: none;
  font-weight: 600;
  font-size: 0.95rem;
  display: inline-flex;
  align-items: center;
  gap: 4px;
  transition: color 0.15s;
}

.nav-back:hover { color: var(--color-primary-dark); }

/* ─── Category Sections (index) ──────────────────────────────────────────── */
.category-section {
  margin: 48px 0;
}

.category-heading {
  font-size: 1.35rem;
  font-weight: 700;
  color: #2563eb;
  margin-bottom: 20px;
  padding-bottom: 10px;
  border-bottom: 2px solid var(--color-border);
}

/* ─── Formula Grid (index) ───────────────────────────────────────────────── */
.formula-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 16px;
}

/* ─── Formula Card (index) ───────────────────────────────────────────────── */
.formula-card {
  background: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 20px;
  text-decoration: none;
  color: inherit;
  display: block;
  transition: box-shadow 0.2s, transform 0.15s, border-color 0.2s;
  box-shadow: var(--shadow-sm);
}

.formula-card:hover {
  box-shadow: var(--shadow-lg);
  transform: translateY(-2px);
  border-color: var(--color-primary-light);
}

.formula-card-header {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 14px;
}

.formula-card-header h3 {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--color-text);
}

.formula-preview {
  background: var(--color-bg);
  border-radius: var(--radius-sm);
  padding: 10px 12px;
  text-align: center;
  font-size: 1rem;
  min-height: 44px;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
}

/* ─── Category Badge ─────────────────────────────────────────────────────── */
.category-badge {
  display: inline-block;
  padding: 3px 10px;
  border-radius: 999px;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.03em;
  text-transform: uppercase;
  white-space: nowrap;
  flex-shrink: 0;
}

.category-badge[data-category="Pre-Algebra"],
.category-badge.pre-algebra {
  background: #dcfce7;
  color: #15803d;
}

.category-badge[data-category="Algebra 1"],
.category-badge.algebra-1 {
  background: var(--color-primary-light);
  color: var(--color-primary-dark);
}

.category-badge[data-category="Calculus"],
.category-badge.calculus {
  background: #fae8ff;
  color: #7e22ce;
}

.category-badge[data-category="Finance"],
.category-badge.finance {
  background: #fef9c3;
  color: #854d0e;
}

/* ─── Formula Page Header ────────────────────────────────────────────────── */
.formula-header {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 14px;
  margin: 40px 0 24px;
}

.formula-name {
  font-size: 2rem;
  font-weight: 800;
  letter-spacing: -0.5px;
  color: var(--color-text);
}

/* ─── Formula Display Block ──────────────────────────────────────────────── */
.formula-display {
  background: var(--color-card);
  border: 2px solid var(--color-primary);
  border-radius: var(--radius-lg);
  padding: 28px 24px;
  text-align: center;
  margin-bottom: 36px;
  box-shadow: var(--shadow-md);
  font-size: 1.4em;
  overflow-x: auto;
}

/* ─── Sub-formula section ────────────────────────────────────────────────── */
.sub-formulas {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 16px;
  margin-bottom: 36px;
}

.sub-formula-card {
  background: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 16px 20px;
  box-shadow: var(--shadow-sm);
}

.sub-formula-card h3 {
  font-size: 0.95rem;
  font-weight: 700;
  color: var(--color-text-muted);
  text-transform: uppercase;
  letter-spacing: 0.04em;
  margin-bottom: 12px;
}

.sub-formula-row {
  display: flex;
  align-items: center;
  gap: 10px;
  margin-bottom: 8px;
}

.sub-formula-label {
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--color-text-muted);
  min-width: 80px;
}

.sub-formula-math {
  font-size: 1em;
  overflow-x: auto;
}

/* ─── Variables Section ──────────────────────────────────────────────────── */
.variables-section {
  margin-bottom: 36px;
}

.variables-section h2,
.when-to-use h2,
.example-section h2,
.calculator-section h2 {
  font-size: 1.25rem;
  font-weight: 700;
  margin-bottom: 16px;
  color: var(--color-text);
}

.variable-list {
  background: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  overflow: hidden;
  box-shadow: var(--shadow-sm);
}

.variable-row {
  display: grid;
  grid-template-columns: 90px 1fr;
  align-items: center;
  gap: 16px;
  padding: 12px 20px;
  border-bottom: 1px solid var(--color-border);
}

.variable-row:last-child { border-bottom: none; }
.variable-row:nth-child(even) { background: #f8fafc; }

.var-symbol {
  font-size: 1.1em;
  font-weight: 600;
  text-align: center;
  color: var(--color-primary);
}

.var-meaning {
  font-size: 0.95rem;
  color: var(--color-text);
}

/* ─── When to Use ────────────────────────────────────────────────────────── */
.when-to-use {
  margin-bottom: 36px;
}

.when-to-use-box {
  background: var(--color-warning-bg);
  border-left: 4px solid var(--color-warning-border);
  border-radius: 0 var(--radius-md) var(--radius-md) 0;
  padding: 16px 20px;
  font-size: 0.97rem;
  line-height: 1.65;
  color: #78350f;
}

/* ─── Example / Steps Section ────────────────────────────────────────────── */
.example-section {
  margin-bottom: 36px;
}

.problem-statement {
  background: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 14px 18px;
  margin-bottom: 20px;
  font-size: 1rem;
  box-shadow: var(--shadow-sm);
}

.steps-container {
  display: flex;
  flex-direction: column;
  gap: 12px;
  margin-bottom: 24px;
}

/* ─── Step Cards ─────────────────────────────────────────────────────────── */
.step {
  display: none;
  background: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 20px;
  box-shadow: var(--shadow-sm);
  animation: fadeInStep 0.35s ease forwards;
}

.step.visible {
  display: block;
}

@keyframes fadeInStep {
  from { opacity: 0; transform: translateY(8px); }
  to   { opacity: 1; transform: translateY(0); }
}

.step-label {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.07em;
  color: var(--color-primary);
  margin-bottom: 6px;
}

.step-title {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--color-text);
  margin-bottom: 8px;
}

.step-explanation {
  font-size: 0.95rem;
  color: var(--color-text-muted);
  margin-bottom: 14px;
  line-height: 1.6;
}

.step-math {
  background: var(--color-bg);
  border-radius: var(--radius-sm);
  padding: 12px;
  text-align: center;
  font-size: 1.1em;
  overflow-x: auto;
}

/* ─── Step Controls ──────────────────────────────────────────────────────── */
.step-controls {
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
}

.step-counter {
  font-size: 0.9rem;
  color: var(--color-text-muted);
  font-variant-numeric: tabular-nums;
}

.next-btn {
  background: var(--color-primary);
  color: white;
  border: none;
  border-radius: var(--radius-md);
  padding: 13px 28px;
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  min-height: 48px;
  transition: background 0.15s, transform 0.1s, box-shadow 0.15s;
  box-shadow: var(--shadow-sm);
  letter-spacing: 0.01em;
}

.next-btn:hover:not(:disabled) {
  background: var(--color-primary-dark);
  transform: translateY(-1px);
  box-shadow: var(--shadow-md);
}

.next-btn:active:not(:disabled) { transform: translateY(0); }

.next-btn:disabled {
  background: var(--color-text-muted);
  cursor: not-allowed;
  box-shadow: none;
}

/* ─── Calculator Section ─────────────────────────────────────────────────── */
.calculator-section {
  background: var(--color-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 28px;
  margin-bottom: 48px;
  box-shadow: var(--shadow-sm);
}

.calc-form {
  display: flex;
  flex-direction: column;
  gap: 14px;
}

.calc-field {
  display: flex;
  flex-direction: column;
  gap: 5px;
}

.calc-field label {
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--color-text);
}

.calc-field input,
.calc-field select {
  padding: 10px 14px;
  border: 1px solid var(--color-border-strong);
  border-radius: var(--radius-sm);
  font-size: 1rem;
  color: var(--color-text);
  background: var(--color-bg);
  transition: border-color 0.15s, box-shadow 0.15s;
  min-height: 44px;
  width: 100%;
}

.calc-field input:focus,
.calc-field select:focus {
  outline: none;
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(37,99,235,0.12);
  background: white;
}

.calc-inputs-group {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 12px;
}

.calc-submit-btn {
  background: var(--color-primary);
  color: white;
  border: none;
  border-radius: var(--radius-md);
  padding: 13px 28px;
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  min-height: 48px;
  transition: background 0.15s, transform 0.1s;
  align-self: flex-start;
  margin-top: 4px;
}

.calc-submit-btn:hover {
  background: var(--color-primary-dark);
  transform: translateY(-1px);
}

.calc-result {
  margin-top: 4px;
  padding: 16px;
  background: #f0fdf4;
  border: 1px solid #bbf7d0;
  border-radius: var(--radius-md);
  font-size: 0.97rem;
  color: #14532d;
  white-space: pre-wrap;
  font-family: 'Menlo', 'Monaco', 'Consolas', monospace;
  display: none;
  line-height: 1.7;
}

.calc-result.visible { display: block; }

.calc-result.error {
  background: #fef2f2;
  border-color: #fecaca;
  color: #991b1b;
}

/* Shape-specific input groups hidden by default */
.shape-inputs { display: none; }
.shape-inputs.active { display: contents; }

/* ─── No Calculator Notice ───────────────────────────────────────────────── */
.no-calculator-notice {
  background: var(--color-bg);
  border: 1px dashed var(--color-border-strong);
  border-radius: var(--radius-md);
  padding: 16px 20px;
  font-size: 0.95rem;
  color: var(--color-text-muted);
  margin-bottom: 48px;
}

/* ─── Utility ─────────────────────────────────────────────────────────────── */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  white-space: nowrap;
}

/* ─── Mobile Responsive ──────────────────────────────────────────────────── */
@media (max-width: 640px) {
  .site-header { padding: 32px 0 28px; }
  .site-header h1 { font-size: 2.8rem; }
  .share-bar { padding-top: 18px; margin-top: 12px; }

  .formula-grid { grid-template-columns: 1fr; }

  .formula-name { font-size: 1.5rem; }
  .formula-display { font-size: 1.15em; padding: 20px 14px; }

  .formula-header { gap: 10px; margin: 28px 0 18px; }

  .variable-row { grid-template-columns: 70px 1fr; gap: 10px; }

  .step-controls { flex-direction: column; align-items: flex-start; }
  .next-btn { width: 100%; text-align: center; }

  .calc-inputs-group { grid-template-columns: 1fr; }
  .calc-submit-btn { width: 100%; }

  .calculator-section { padding: 20px 16px; }

  .sub-formulas { grid-template-columns: 1fr; }

  .container { padding: 0 14px; }
}

@media (max-width: 400px) {
  .formula-card-header { flex-direction: column; }
  .category-badge { align-self: flex-start; }
}