/* ============================================
   Landing Page Styles
   Understanding Machine Learning Study Guide
   ============================================ */

html {
  scroll-behavior: smooth;
}

/* --- Hero Section --- */

.hero {
  background: linear-gradient(135deg, #1e3a8a 0%, #0d9488 100%);
  color: #fff;
  text-align: center;
  padding: 80px 24px;
}

.hero__title {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 3rem;
  font-weight: 700;
  margin: 0 0 12px;
  letter-spacing: -0.02em;
}

.hero__subtitle {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 1.35rem;
  font-weight: 400;
  margin: 0 0 8px;
  opacity: 0.92;
}

.hero__author {
  font-size: 1.1rem;
  margin: 0 0 4px;
  opacity: 0.85;
}

.hero__meta {
  font-size: 0.95rem;
  margin: 0 0 36px;
  opacity: 0.7;
}

.hero__actions {
  display: flex;
  gap: 16px;
  justify-content: center;
  flex-wrap: wrap;
}

.btn {
  display: inline-block;
  padding: 14px 32px;
  border-radius: 8px;
  font-size: 1rem;
  font-weight: 600;
  text-decoration: none;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
  cursor: pointer;
  border: none;
}

.btn:hover {
  transform: translateY(-2px);
}

.btn--primary {
  background: #fff;
  color: #1e3a8a;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
}

.btn--primary:hover {
  box-shadow: 0 6px 20px rgba(0, 0, 0, 0.2);
}

.btn--secondary {
  background: rgba(255, 255, 255, 0.15);
  color: #fff;
  border: 2px solid rgba(255, 255, 255, 0.5);
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
}

.btn--secondary:hover {
  background: rgba(255, 255, 255, 0.25);
  box-shadow: 0 6px 20px rgba(0, 0, 0, 0.15);
}

/* --- Sections --- */

.section {
  padding: 64px 24px;
  max-width: 1200px;
  margin: 0 auto;
}

.section__heading {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 2rem;
  font-weight: 700;
  text-align: center;
  margin: 0 0 8px;
  color: #1e293b;
}

.section__description {
  text-align: center;
  color: #64748b;
  margin: 0 0 40px;
  font-size: 1.05rem;
}

.section__heading::after {
  content: "";
  display: block;
  width: 48px;
  height: 3px;
  background: linear-gradient(90deg, #1e3a8a, #0d9488);
  margin: 12px auto 0;
  border-radius: 2px;
}

/* --- Grid --- */

.grid,
.toc__grid,
.simulators__grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 24px;
}

/* --- Section containers --- */

.toc,
.simulators {
  padding: 64px 24px;
  max-width: 1200px;
  margin: 0 auto;
}

.toc__heading,
.simulators__heading {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 2rem;
  font-weight: 700;
  text-align: center;
  margin: 0 0 32px;
  color: #1e293b;
}

/* --- Part Header Cards --- */

.part-header {
  grid-column: 1 / -1;
  background: linear-gradient(135deg, #1e3a8a 0%, #2563eb 100%);
  color: #fff;
  border-radius: 10px;
  padding: 20px 28px;
  margin-top: 8px;
}

.part-header__label {
  font-size: 0.8rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  opacity: 0.75;
}

.part-header__title {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 1.3rem;
  font-weight: 700;
  margin: 4px 0 0;
}

.part-card {
  grid-column: 1 / -1;
  background: linear-gradient(135deg, #1e3a8a 0%, #2563eb 100%);
  color: #fff;
  border-radius: 10px;
  padding: 24px 28px;
  margin-top: 16px;
}

.part-card:first-child {
  margin-top: 0;
}

.part-card__number {
  font-size: 0.8rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  opacity: 0.75;
  margin: 0 0 4px;
}

.part-card__title {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 1.4rem;
  font-weight: 700;
  margin: 0 0 6px;
}

.part-card__desc {
  font-size: 0.95rem;
  opacity: 0.85;
  margin: 0;
  line-height: 1.5;
}

/* Part-specific gradients */
.part-card--1 { background: linear-gradient(135deg, #1e3a8a 0%, #0ea5e9 100%); }
.part-card--2 { background: linear-gradient(135deg, #92400e 0%, #f59e0b 100%); }
.part-card--3 { background: linear-gradient(135deg, #064e3b 0%, #10b981 100%); }
.part-card--4 { background: linear-gradient(135deg, #4c1d95 0%, #8b5cf6 100%); }

/* --- Chapter Cards --- */

.chapter-card {
  background: #fff;
  border-radius: 10px;
  box-shadow: 0 1px 4px rgba(0, 0, 0, 0.08), 0 2px 8px rgba(0, 0, 0, 0.04);
  padding: 24px 24px 24px 20px;
  border-left: 4px solid transparent;
  text-decoration: none;
  color: inherit;
  display: block;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.chapter-card:hover {
  transform: translateY(-4px);
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1), 0 8px 24px rgba(0, 0, 0, 0.08);
}

.chapter-card__number {
  font-size: 0.8rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin: 0 0 4px;
  opacity: 0.6;
}

.chapter-card__title {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 1.2rem;
  font-weight: 700;
  margin: 0 0 8px;
  color: #1e293b;
}

.chapter-card__desc {
  font-size: 0.92rem;
  color: #64748b;
  margin: 0 0 12px;
  line-height: 1.5;
}

.chapter-card__pages {
  font-size: 0.8rem;
  color: #94a3b8;
  font-weight: 500;
}

/* Chapter border colors — Ch 1: standalone */
.chapter-card--1  { border-left-color: #1e3a8a; }

/* Chapter border colors — Part I: Foundations (Ch 2-8): blues/teals */
.chapter-card--2  { border-left-color: #3b82f6; }
.chapter-card--3  { border-left-color: #0ea5e9; }
.chapter-card--4  { border-left-color: #06b6d4; }
.chapter-card--5  { border-left-color: #0d9488; }
.chapter-card--6  { border-left-color: #14b8a6; }
.chapter-card--7  { border-left-color: #2dd4bf; }
.chapter-card--8  { border-left-color: #5eead4; }

/* Chapter border colors — Part II: From Theory to Algorithms (Ch 9-20): warm colors */
.chapter-card--9  { border-left-color: #f59e0b; }
.chapter-card--10 { border-left-color: #f97316; }
.chapter-card--11 { border-left-color: #ef4444; }
.chapter-card--12 { border-left-color: #ec4899; }
.chapter-card--13 { border-left-color: #d946ef; }
.chapter-card--14 { border-left-color: #a855f7; }
.chapter-card--15 { border-left-color: #8b5cf6; }
.chapter-card--16 { border-left-color: #6366f1; }
.chapter-card--17 { border-left-color: #3b82f6; }
.chapter-card--18 { border-left-color: #0ea5e9; }
.chapter-card--19 { border-left-color: #06b6d4; }
.chapter-card--20 { border-left-color: #0d9488; }

/* Chapter border colors — Part III: Additional Learning Models (Ch 21-25): greens */
.chapter-card--21 { border-left-color: #10b981; }
.chapter-card--22 { border-left-color: #059669; }
.chapter-card--23 { border-left-color: #047857; }
.chapter-card--24 { border-left-color: #065f46; }
.chapter-card--25 { border-left-color: #064e3b; }

/* Chapter border colors — Part IV: Advanced Theory (Ch 26-31): purples/roses */
.chapter-card--26 { border-left-color: #8b5cf6; }
.chapter-card--27 { border-left-color: #7c3aed; }
.chapter-card--28 { border-left-color: #6d28d9; }
.chapter-card--29 { border-left-color: #a855f7; }
.chapter-card--30 { border-left-color: #d946ef; }
.chapter-card--31 { border-left-color: #ec4899; }

/* --- Simulator Cards --- */

.simulator-card {
  background: #fff;
  border-radius: 10px;
  box-shadow: 0 1px 4px rgba(0, 0, 0, 0.08), 0 2px 8px rgba(0, 0, 0, 0.04);
  padding: 28px 24px;
  border-top: 4px solid transparent;
  text-decoration: none;
  color: inherit;
  display: block;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.simulator-card:hover {
  transform: scale(1.02);
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1), 0 8px 24px rgba(0, 0, 0, 0.08);
}

.simulator-card__icon {
  font-size: 2rem;
  margin: 0 0 12px;
  display: block;
}

.simulator-card__title {
  font-family: Georgia, "Times New Roman", serif;
  font-size: 1.15rem;
  font-weight: 700;
  margin: 0 0 8px;
  color: #1e293b;
}

.simulator-card__desc {
  font-size: 0.92rem;
  color: #64748b;
  line-height: 1.5;
  margin: 0;
}

/* Simulator border colors */
.simulator-card--blue    { border-top-color: #3b82f6; }
.simulator-card--amber   { border-top-color: #f59e0b; }
.simulator-card--purple  { border-top-color: #8b5cf6; }
.simulator-card--green   { border-top-color: #10b981; }
.simulator-card--rose    { border-top-color: #f43f5e; }
.simulator-card--indigo  { border-top-color: #6366f1; }

/* --- Footer --- */

.footer {
  text-align: center;
  padding: 32px 24px;
  color: #94a3b8;
  font-size: 0.9rem;
  border-top: 1px solid #e2e8f0;
}

/* --- Responsive --- */

@media (max-width: 640px) {
  .hero {
    padding: 56px 20px;
  }

  .hero__title {
    font-size: 2rem;
  }

  .hero__subtitle {
    font-size: 1.1rem;
  }

  .grid,
  .toc__grid,
  .simulators__grid {
    grid-template-columns: 1fr;
  }

  .toc,
  .simulators,
  .section {
    padding: 48px 16px;
  }

  .toc__heading,
  .simulators__heading,
  .section__heading {
    font-size: 1.6rem;
  }

  .part-card {
    padding: 20px 22px;
  }

  .part-card__title {
    font-size: 1.2rem;
  }
}