/* ===================================================
   Physics 356 — ML/AI Lesson Site
   style.css
   =================================================== */

/* --- Google Fonts --- */
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500&display=swap');

/* --- CSS Variables --- */
:root {
  --usafa-blue:     #003087;
  --usafa-gold:     #c5a900;
  --usafa-light:    #e8eef7;
  --accent:         #1565c0;
  --accent-hover:   #0d47a1;
  --text-primary:   #1a1a2e;
  --text-secondary: #4a5568;
  --bg-page:        #f7f8fc;
  --bg-card:        #ffffff;
  --bg-code:        #1e1e2e;
  --border:         #d1d9e6;
  --success:        #2e7d32;
  --warning:        #e65100;
  --radius:         8px;
  --shadow-sm:      0 1px 3px rgba(0,0,0,.08);
  --shadow-md:      0 4px 12px rgba(0,0,0,.10);
  --shadow-lg:      0 8px 24px rgba(0,0,0,.13);
  --max-width:      900px;
  --font-body:      'Inter', system-ui, sans-serif;
  --font-mono:      'JetBrains Mono', 'Fira Code', monospace;
}

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

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

body {
  font-family: var(--font-body);
  background: var(--bg-page);
  color: var(--text-primary);
  line-height: 1.7;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; color: var(--accent-hover); }

img { max-width: 100%; height: auto; }

/* --- Site Header / Nav --- */
.site-header {
  background: var(--usafa-blue);
  color: #fff;
  padding: 0;
  position: sticky;
  top: 0;
  z-index: 100;
  box-shadow: var(--shadow-md);
}

.site-header .header-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.75rem 1.5rem;
  gap: 1rem;
}

.site-logo {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  text-decoration: none;
  color: #fff;
}

.site-logo .logo-badge {
  background: var(--usafa-gold);
  color: var(--usafa-blue);
  font-weight: 700;
  font-size: 0.75rem;
  padding: 0.2rem 0.5rem;
  border-radius: 4px;
  letter-spacing: 0.05em;
  text-transform: uppercase;
}

.site-logo .logo-text {
  font-weight: 600;
  font-size: 1rem;
}

.site-nav {
  display: flex;
  gap: 0.25rem;
  flex-wrap: wrap;
}

.site-nav a {
  color: rgba(255,255,255,0.85);
  font-size: 0.82rem;
  font-weight: 500;
  padding: 0.35rem 0.6rem;
  border-radius: 4px;
  transition: background 0.15s, color 0.15s;
  text-decoration: none;
  white-space: nowrap;
}

.site-nav a:hover,
.site-nav a.active {
  background: rgba(255,255,255,0.15);
  color: #fff;
  text-decoration: none;
}

/* --- Page Layout --- */
.page-wrapper {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 2rem 1.5rem 4rem;
}

/* --- Lesson Header Banner --- */
.lesson-banner {
  background: linear-gradient(135deg, var(--usafa-blue) 0%, var(--accent) 100%);
  color: #fff;
  border-radius: var(--radius);
  padding: 2rem 2.5rem;
  margin-bottom: 2rem;
  position: relative;
  overflow: hidden;
}

.lesson-banner::after {
  content: '';
  position: absolute;
  right: -2rem;
  top: -2rem;
  width: 200px;
  height: 200px;
  border-radius: 50%;
  background: rgba(255,255,255,0.05);
}

.lesson-banner .lesson-meta {
  font-size: 0.78rem;
  font-weight: 600;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  opacity: 0.8;
  margin-bottom: 0.4rem;
}

.lesson-banner h1 {
  font-size: 2rem;
  font-weight: 700;
  line-height: 1.2;
  margin-bottom: 0.5rem;
}

.lesson-banner .reading-time {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  background: rgba(255,255,255,0.15);
  padding: 0.2rem 0.7rem;
  border-radius: 20px;
  font-size: 0.8rem;
  margin-top: 0.5rem;
}

/* --- Cards & Sections --- */
.card {
  background: var(--bg-card);
  border-radius: var(--radius);
  box-shadow: var(--shadow-sm);
  border: 1px solid var(--border);
  padding: 1.5rem 2rem;
  margin-bottom: 1.75rem;
}

.card h2 {
  font-size: 1.25rem;
  font-weight: 700;
  color: var(--usafa-blue);
  margin-bottom: 1rem;
  padding-bottom: 0.5rem;
  border-bottom: 2px solid var(--usafa-light);
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.card h2 .section-icon {
  font-size: 1.1rem;
}

.card h3 {
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--text-primary);
  margin: 1.25rem 0 0.5rem;
}

.card p { margin-bottom: 0.9rem; color: var(--text-secondary); }
.card p:last-child { margin-bottom: 0; }

/* --- Objectives Box --- */
.objectives-box {
  background: var(--usafa-light);
  border-left: 4px solid var(--usafa-blue);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: 1.25rem 1.5rem;
  margin-bottom: 1.75rem;
}

.objectives-box h2 {
  font-size: 1rem;
  font-weight: 700;
  color: var(--usafa-blue);
  margin-bottom: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

.objectives-box ul {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}

.objectives-box ul li {
  display: flex;
  align-items: flex-start;
  gap: 0.6rem;
  color: var(--text-primary);
  font-size: 0.95rem;
}

.objectives-box ul li::before {
  content: '✓';
  color: var(--usafa-blue);
  font-weight: 700;
  flex-shrink: 0;
  margin-top: 0.05rem;
}

/* --- Notation Box --- */
.notation-box {
  background: #fff8e1;
  border: 1px solid #ffe082;
  border-radius: var(--radius);
  padding: 1.25rem 1.5rem;
  margin-bottom: 1.75rem;
}

.notation-box h3 {
  font-size: 0.9rem;
  font-weight: 700;
  color: #b45309;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: 0.75rem;
}

.notation-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 0.4rem 1.5rem;
}

.notation-item {
  display: flex;
  align-items: center;
  gap: 0.25rem;
  font-size: 0.9rem;
}

.notation-item .sym {
  font-weight: 600;
  min-width: 5rem;
  flex-shrink: 0;
  color: var(--text-primary);
}

.notation-item .desc {
  color: var(--text-secondary);
  font-size: 0.85rem;
}

/* --- Math Display --- */
.math-block {
  background: var(--usafa-light);
  border-radius: var(--radius);
  padding: 1rem 1.5rem;
  margin: 1rem 0;
  overflow-x: auto;
  text-align: center;
}

/* --- Code Blocks --- */
pre {
  background: var(--bg-code);
  border-radius: var(--radius);
  padding: 1.25rem 1.5rem;
  overflow-x: auto;
  margin: 1rem 0;
  font-family: var(--font-mono);
  font-size: 0.85rem;
  line-height: 1.6;
  position: relative;
}

pre .code-lang {
  position: absolute;
  top: 0.5rem;
  right: 0.75rem;
  background: rgba(255,255,255,0.1);
  color: rgba(255,255,255,0.6);
  font-size: 0.7rem;
  padding: 0.15rem 0.5rem;
  border-radius: 4px;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

code {
  font-family: var(--font-mono);
  font-size: 0.875em;
  background: var(--usafa-light);
  padding: 0.1em 0.35em;
  border-radius: 3px;
  color: var(--usafa-blue);
}

pre code {
  background: transparent;
  padding: 0;
  color: #cdd6f4;
  font-size: 1em;
}

/* --- YouTube Embed --- */
.video-container {
  position: relative;
  padding-bottom: 56.25%;
  height: 0;
  overflow: hidden;
  border-radius: var(--radius);
  margin: 1rem 0;
  background: #000;
}

.video-container iframe {
  position: absolute;
  top: 0; left: 0;
  width: 100%; height: 100%;
  border: none;
}

.video-caption {
  font-size: 0.82rem;
  color: var(--text-secondary);
  text-align: center;
  margin-top: 0.4rem;
  font-style: italic;
}

/* Video link card (fallback when embedding is disabled) */
.video-link-card {
  display: flex;
  align-items: center;
  gap: 1rem;
  padding: 1rem 1.25rem;
  margin: 1rem 0;
  background: var(--usafa-light);
  border: 1px solid #c8d4e8;
  border-radius: var(--radius);
  text-decoration: none;
  color: var(--text-primary);
  transition: background 0.2s, border-color 0.2s;
}
.video-link-card:hover {
  background: #dce6f5;
  border-color: var(--accent);
}
.video-link-thumb {
  flex-shrink: 0;
  width: 3rem;
  height: 3rem;
  background: var(--usafa-blue);
  color: #fff;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.2rem;
}
.video-link-title {
  font-weight: 600;
  font-size: 0.95rem;
  margin-bottom: 0.2rem;
}
.video-link-channel {
  font-size: 0.82rem;
  color: var(--text-secondary);
}

/* --- Summary Table --- */
.summary-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.9rem;
}

.summary-table th {
  background: var(--usafa-blue);
  color: #fff;
  padding: 0.6rem 1rem;
  text-align: left;
  font-weight: 600;
}

.summary-table td {
  padding: 0.6rem 1rem;
  border-bottom: 1px solid var(--border);
  color: var(--text-secondary);
  vertical-align: top;
}

.summary-table tr:nth-child(even) td {
  background: var(--usafa-light);
}

.summary-table tr:last-child td { border-bottom: none; }

/* --- Lesson Navigation --- */
.lesson-nav {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  margin-top: 2.5rem;
  flex-wrap: wrap;
}

.lesson-nav a {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 0.75rem 1.25rem;
  color: var(--accent);
  font-weight: 600;
  font-size: 0.9rem;
  transition: box-shadow 0.15s, transform 0.1s;
  text-decoration: none;
  flex: 1;
  min-width: 160px;
}

.lesson-nav a:hover {
  box-shadow: var(--shadow-md);
  transform: translateY(-1px);
  text-decoration: none;
}

.lesson-nav a.next { justify-content: flex-end; text-align: right; }
.lesson-nav a.prev { justify-content: flex-start; }

.lesson-nav .nav-label { font-size: 0.72rem; font-weight: 400; opacity: 0.7; }

/* --- HW Link Button --- */
.hw-button {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--usafa-gold);
  color: var(--usafa-blue);
  font-weight: 700;
  padding: 0.6rem 1.25rem;
  border-radius: var(--radius);
  font-size: 0.9rem;
  margin-top: 0.5rem;
  transition: opacity 0.15s;
  text-decoration: none;
}

.hw-button:hover { opacity: 0.85; text-decoration: none; color: var(--usafa-blue); }

/* --- Interactive Demo --- */
.demo-container {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.5rem;
  margin: 1rem 0;
}

.demo-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  align-items: center;
  margin-bottom: 1rem;
}

.demo-controls label {
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--text-primary);
}

.demo-controls input[type="range"] {
  width: 180px;
  accent-color: var(--usafa-blue);
}

.demo-controls .value-display {
  background: var(--usafa-light);
  border-radius: 4px;
  padding: 0.2rem 0.6rem;
  font-family: var(--font-mono);
  font-size: 0.85rem;
  color: var(--usafa-blue);
  font-weight: 600;
  min-width: 3.5rem;
  text-align: center;
}

.demo-controls button {
  background: var(--usafa-blue);
  color: #fff;
  border: none;
  border-radius: var(--radius);
  padding: 0.45rem 1rem;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: opacity 0.15s;
}

.demo-controls button:hover { opacity: 0.85; }

#demo-chart-container {
  position: relative;
  height: 300px;
}

.demo-info {
  margin-top: 0.75rem;
  font-size: 0.85rem;
  color: var(--text-secondary);
  display: flex;
  gap: 1.5rem;
  flex-wrap: wrap;
}

.demo-info span strong { color: var(--text-primary); }

/* --- Index / Home Page --- */
.hero {
  background: linear-gradient(135deg, var(--usafa-blue) 0%, #1565c0 100%);
  color: #fff;
  padding: 3rem 2.5rem;
  border-radius: var(--radius);
  margin-bottom: 2rem;
  text-align: center;
}

.hero .course-code {
  font-size: 0.85rem;
  font-weight: 600;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  opacity: 0.75;
  margin-bottom: 0.5rem;
}

.hero h1 {
  font-size: 2.2rem;
  font-weight: 700;
  margin-bottom: 0.75rem;
  line-height: 1.2;
}

.hero p {
  font-size: 1.05rem;
  opacity: 0.85;
  max-width: 600px;
  margin: 0 auto 1.25rem;
}

.lesson-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: 1.25rem;
  margin-bottom: 2rem;
}

.lesson-card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.5rem;
  transition: box-shadow 0.15s, transform 0.1s;
  text-decoration: none;
  color: var(--text-primary);
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.lesson-card:hover {
  box-shadow: var(--shadow-md);
  transform: translateY(-2px);
  text-decoration: none;
}

.lesson-card .lesson-num {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--usafa-gold);
  background: var(--usafa-blue);
  display: inline-block;
  padding: 0.15rem 0.5rem;
  border-radius: 3px;
  width: fit-content;
}

.lesson-card h3 {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--text-primary);
  line-height: 1.3;
}

.lesson-card p {
  font-size: 0.85rem;
  color: var(--text-secondary);
  line-height: 1.5;
}

.lesson-card .card-footer {
  margin-top: auto;
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
}

.tag {
  font-size: 0.72rem;
  background: var(--usafa-light);
  color: var(--usafa-blue);
  padding: 0.15rem 0.5rem;
  border-radius: 20px;
  font-weight: 500;
}

.tag.hw { background: #fff8e1; color: #b45309; }

/* --- Alert / callout boxes --- */
.callout {
  border-radius: var(--radius);
  padding: 1rem 1.25rem;
  margin: 1rem 0;
  border-left: 4px solid;
  font-size: 0.92rem;
}

.callout.info {
  background: #e3f2fd;
  border-color: #1565c0;
  color: #0d47a1;
}

.callout.warning {
  background: #fff8e1;
  border-color: #f9a825;
  color: #6d4c00;
}

.callout.tip {
  background: #e8f5e9;
  border-color: #2e7d32;
  color: #1b5e20;
}

.callout strong { font-weight: 700; }

/* --- References --- */
.references ol {
  padding-left: 1.5rem;
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}

.references li {
  font-size: 0.88rem;
  color: var(--text-secondary);
  line-height: 1.5;
}

/* --- HW Pages --- */
.hw-header {
  background: linear-gradient(135deg, #1a237e, #283593);
  color: #fff;
  border-radius: var(--radius);
  padding: 2rem 2.5rem;
  margin-bottom: 2rem;
}

.hw-header .hw-label {
  font-size: 0.78rem;
  font-weight: 700;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--usafa-gold);
  margin-bottom: 0.4rem;
}

.hw-header h1 { font-size: 1.8rem; font-weight: 700; margin-bottom: 0.5rem; }

.hw-header .hw-meta {
  display: flex;
  gap: 1.25rem;
  font-size: 0.85rem;
  opacity: 0.8;
  flex-wrap: wrap;
}

.problem {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.5rem 2rem;
  margin-bottom: 1.5rem;
}

.problem h3 {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--usafa-blue);
  margin-bottom: 1rem;
  padding-bottom: 0.4rem;
  border-bottom: 1px solid var(--border);
}

.problem ol {
  padding-left: 1.5rem;
  display: flex;
  flex-direction: column;
  gap: 0.6rem;
}

.problem li { color: var(--text-secondary); font-size: 0.95rem; }
.problem li strong { color: var(--text-primary); }

/* --- Footer --- */
.site-footer {
  background: var(--usafa-blue);
  color: rgba(255,255,255,0.65);
  text-align: center;
  padding: 1.25rem;
  font-size: 0.82rem;
  margin-top: 3rem;
}

.site-footer a { color: rgba(255,255,255,0.8); }

/* --- Responsive --- */
@media (max-width: 640px) {
  .site-header .header-inner { flex-direction: column; padding: 0.75rem 1rem; }
  .site-nav { justify-content: center; }
  .lesson-banner h1 { font-size: 1.5rem; }
  .lesson-banner { padding: 1.5rem; }
  .card { padding: 1.25rem; }
  .card h2 { font-size: 1.1rem; }
  .hero h1 { font-size: 1.6rem; }
  .demo-controls { flex-direction: column; align-items: flex-start; }
  .demo-controls input[type="range"] { width: 100%; }
  .lesson-nav { flex-direction: column; }
}

/* --- Print --- */
@media print {
  .site-header, .site-footer, .lesson-nav, .demo-container { display: none; }
  .card { box-shadow: none; border: 1px solid #ccc; break-inside: avoid; }
  body { background: #fff; }
}