/* ============================================
   DACS ML Study Guide - Clean Design System
   Mobile-First, Definition & Math Focused
   ============================================ */

:root {
    --primary: #2563eb;
    --primary-dark: #1d4ed8;
    --success: #10b981;
    --warning: #f59e0b;
    --danger: #ef4444;
    --bg: #ffffff;
    --bg-alt: #f8fafc;
    --text: #1e293b;
    --text-light: #64748b;
    --text-muted: #94a3b8;
    --border: #e2e8f0;
    --radius: 12px;
    --shadow: 0 1px 3px rgba(0,0,0,0.08);
}

* { margin: 0; padding: 0; box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background: var(--bg-alt);
    color: var(--text);
    line-height: 1.6;
    font-size: 16px;
}

/* Typography */
h1 { font-size: 1.75rem; font-weight: 700; margin-bottom: 0.5rem; }
h2 { font-size: 1.35rem; font-weight: 600; margin-bottom: 0.5rem; }
h3 { font-size: 1.1rem; font-weight: 600; margin-bottom: 0.5rem; }
p { margin-bottom: 1rem; }
a { color: var(--primary); text-decoration: none; }
a:hover { text-decoration: underline; }

/* Container */
.container {
    max-width: 800px;
    margin: 0 auto;
    padding: 1rem;
}

@media (min-width: 640px) {
    .container { padding: 1.5rem; }
    h1 { font-size: 2rem; }
}

/* ============================================
   Navigation - Simple & Clean
   ============================================ */
.nav {
    background: var(--bg);
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav-inner {
    max-width: 800px;
    margin: 0 auto;
    padding: 0.75rem 1rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.nav-brand {
    font-weight: 700;
    color: var(--text);
    font-size: 1rem;
}

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

.nav-links a {
    padding: 0.5rem 0.75rem;
    color: var(--text-light);
    font-size: 0.875rem;
    border-radius: 6px;
}

.nav-links a:hover, .nav-links a.active {
    background: var(--bg-alt);
    color: var(--text);
    text-decoration: none;
}

.nav-toggle {
    display: none;
    background: none;
    border: none;
    font-size: 1.5rem;
    cursor: pointer;
}

@media (max-width: 600px) {
    .nav-toggle { display: block; }
    .nav-links {
        display: none;
        position: absolute;
        top: 100%;
        left: 0;
        right: 0;
        background: var(--bg);
        border-bottom: 1px solid var(--border);
        flex-direction: column;
        padding: 0.5rem;
    }
    .nav-links.show { display: flex; }
}

/* ============================================
   Buttons
   ============================================ */
.btn {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.75rem 1.25rem;
    background: var(--primary);
    color: white;
    border: none;
    border-radius: 8px;
    font-size: 0.95rem;
    font-weight: 500;
    cursor: pointer;
    text-decoration: none;
    transition: background 0.2s;
}

.btn:hover {
    background: var(--primary-dark);
    text-decoration: none;
}

.btn-outline {
    background: transparent;
    border: 2px solid var(--border);
    color: var(--text);
}

.btn-outline:hover {
    border-color: var(--primary);
    color: var(--primary);
    background: transparent;
}

.btn-sm {
    padding: 0.5rem 1rem;
    font-size: 0.85rem;
}

.btn-block { width: 100%; justify-content: center; }

/* ============================================
   Cards
   ============================================ */
.card {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.25rem;
    margin-bottom: 1rem;
}

/* ============================================
   DEFINITION CARDS - Primary Component
   ============================================ */
.def {
    background: var(--bg);
    border-left: 4px solid var(--primary);
    padding: 1rem 1.25rem;
    margin-bottom: 1rem;
    border-radius: 0 8px 8px 0;
    box-shadow: var(--shadow);
}

.def-term {
    font-weight: 700;
    color: var(--primary);
    font-size: 1.05rem;
    margin-bottom: 0.25rem;
}

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

.def-example {
    margin-top: 0.75rem;
    padding-top: 0.75rem;
    border-top: 1px dashed var(--border);
    font-size: 0.85rem;
    color: var(--text-light);
}

/* Definition grid */
.def-grid {
    display: grid;
    gap: 0.75rem;
}

@media (min-width: 640px) {
    .def-grid { grid-template-columns: repeat(2, 1fr); }
}

/* ============================================
   FORMULA CARDS - Math Emphasis
   ============================================ */
.formula {
    background: linear-gradient(135deg, #1e3a5f, #2563eb);
    color: white;
    border-radius: var(--radius);
    padding: 1.25rem;
    margin-bottom: 1rem;
    text-align: center;
}

.formula-label {
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 1px;
    opacity: 0.8;
    margin-bottom: 0.5rem;
}

.formula-math {
    font-size: 1.1rem;
    padding: 0.75rem;
    background: rgba(255,255,255,0.1);
    border-radius: 8px;
    margin-bottom: 0.5rem;
}

.formula-note {
    font-size: 0.8rem;
    opacity: 0.85;
}

/* Formula grid */
.formula-grid {
    display: grid;
    gap: 0.75rem;
}

@media (min-width: 640px) {
    .formula-grid { grid-template-columns: repeat(2, 1fr); }
}

/* Inline formula */
.math {
    display: inline-block;
    background: #f1f5f9;
    padding: 0.15rem 0.5rem;
    border-radius: 4px;
    font-family: 'Cambria Math', Georgia, serif;
}

/* ============================================
   Step Progress - Guided Navigation
   ============================================ */
.steps {
    display: flex;
    justify-content: space-between;
    padding: 1rem 0;
    margin-bottom: 1rem;
}

.step {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.25rem;
    flex: 1;
    text-decoration: none;
    color: var(--text-muted);
}

.step-num {
    width: 32px;
    height: 32px;
    border-radius: 50%;
    background: var(--bg);
    border: 2px solid var(--border);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.85rem;
    font-weight: 600;
    position: relative;
    z-index: 1;
}

.step-label {
    font-size: 0.7rem;
    text-align: center;
    display: none;
}

@media (min-width: 500px) {
    .step-label { display: block; }
}

.step.active .step-num {
    background: var(--primary);
    border-color: var(--primary);
    color: white;
}

.step.done .step-num {
    background: var(--success);
    border-color: var(--success);
    color: white;
}

/* ============================================
   Topic Navigation
   ============================================ */
.topic-nav {
    display: flex;
    justify-content: space-between;
    gap: 1rem;
    padding: 1rem;
    background: var(--bg-alt);
    border-radius: var(--radius);
    margin: 1.5rem 0;
}

.topic-nav a {
    padding: 0.75rem 1rem;
    background: var(--bg);
    border-radius: 8px;
    color: var(--text-light);
    font-size: 0.85rem;
}

.topic-nav a:hover {
    color: var(--primary);
    text-decoration: none;
}

/* ============================================
   Quiz
   ============================================ */
.quiz-q {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.25rem;
    margin-bottom: 1rem;
}

.quiz-q-num {
    font-size: 0.7rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 1px;
}

.quiz-q-text {
    font-size: 1.05rem;
    font-weight: 500;
    margin: 0.5rem 0 1rem;
}

.quiz-opts {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.quiz-opt {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    padding: 0.875rem 1rem;
    background: var(--bg-alt);
    border: 2px solid transparent;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.2s;
}

.quiz-opt:hover { background: #f1f5f9; }
.quiz-opt.selected { border-color: var(--primary); background: #eff6ff; }
.quiz-opt.correct { border-color: var(--success); background: #ecfdf5; }
.quiz-opt.wrong { border-color: var(--danger); background: #fef2f2; }

.quiz-opt input { display: none; }

.quiz-opt-letter {
    width: 26px;
    height: 26px;
    border-radius: 50%;
    background: var(--border);
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 600;
    font-size: 0.8rem;
}

.quiz-opt.selected .quiz-opt-letter { background: var(--primary); color: white; }
.quiz-opt.correct .quiz-opt-letter { background: var(--success); color: white; }
.quiz-opt.wrong .quiz-opt-letter { background: var(--danger); color: white; }

/* ============================================
   Tips & Alerts
   ============================================ */
.tip {
    padding: 1rem;
    border-radius: 8px;
    margin-bottom: 1rem;
    border-left: 4px solid;
}

.tip-title {
    font-weight: 600;
    margin-bottom: 0.25rem;
}

.tip-info { background: #eff6ff; border-color: var(--primary); }
.tip-warn { background: #fffbeb; border-color: var(--warning); }
.tip-ok { background: #ecfdf5; border-color: var(--success); }
.tip-bad { background: #fef2f2; border-color: var(--danger); }

/* ============================================
   Tables
   ============================================ */
.table-wrap {
    overflow-x: auto;
    margin-bottom: 1rem;
}

table {
    width: 100%;
    border-collapse: collapse;
    font-size: 0.9rem;
}

th, td {
    padding: 0.75rem;
    text-align: left;
    border-bottom: 1px solid var(--border);
}

th {
    background: var(--bg-alt);
    font-weight: 600;
    font-size: 0.8rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    color: var(--text-light);
}

/* ============================================
   Hero
   ============================================ */
.hero {
    background: linear-gradient(135deg, #1e3a5f, #2563eb);
    color: white;
    padding: 2rem 1.5rem;
    border-radius: var(--radius);
    text-align: center;
    margin-bottom: 1.5rem;
}

.hero h1 { color: white; }
.hero p { opacity: 0.9; margin-bottom: 0; }

/* ============================================
   Section
   ============================================ */
.section {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.5rem;
    margin-bottom: 1.5rem;
}

.section h2 {
    color: var(--primary);
    padding-bottom: 0.5rem;
    border-bottom: 2px solid var(--border);
    margin-bottom: 1rem;
}

/* ============================================
   Badges
   ============================================ */
.badge {
    display: inline-block;
    padding: 0.2rem 0.5rem;
    font-size: 0.7rem;
    font-weight: 600;
    border-radius: 4px;
    text-transform: uppercase;
}

.badge-primary { background: #eff6ff; color: var(--primary); }
.badge-success { background: #ecfdf5; color: var(--success); }
.badge-warn { background: #fffbeb; color: #b45309; }
.badge-danger { background: #fef2f2; color: var(--danger); }

/* ============================================
   Grid
   ============================================ */
.grid { display: grid; gap: 1rem; }
.grid-2 { grid-template-columns: repeat(auto-fit, minmax(250px, 1fr)); }

/* ============================================
   Utilities
   ============================================ */
.text-center { text-align: center; }
.text-muted { color: var(--text-light); }
.text-sm { font-size: 0.875rem; }
.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.mb-1 { margin-bottom: 0.5rem; }
.mb-2 { margin-bottom: 1rem; }

/* Lists */
ul, ol { margin: 1rem 0; padding-left: 1.5rem; }
li { margin: 0.35rem 0; }

/* Hidden */
.hidden { display: none !important; }

/* ============================================
   Topic Cards - Home Page
   ============================================ */
.topic-card {
    display: block;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.25rem;
    text-decoration: none;
    color: inherit;
    transition: all 0.2s;
}

.topic-card:hover {
    border-color: var(--primary);
    box-shadow: 0 4px 12px rgba(0,0,0,0.08);
    text-decoration: none;
}

.topic-card-num {
    width: 28px;
    height: 28px;
    border-radius: 50%;
    background: var(--primary);
    color: white;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    font-weight: 600;
    font-size: 0.85rem;
    margin-bottom: 0.5rem;
}

.topic-card h3 {
    color: var(--text);
    margin-bottom: 0.25rem;
}

.topic-card p {
    color: var(--text-light);
    font-size: 0.85rem;
    margin-bottom: 0;
}

/* ============================================
   SVG Responsiveness
   ============================================ */
svg {
    max-width: 100%;
    height: auto;
}

/* ============================================
   Mobile Responsive (Small Screens)
   ============================================ */
@media (max-width: 480px) {
    /* Reduce padding */
    .hero { padding: 1.5rem 1rem; }
    .section { padding: 1rem; }
    .card { padding: 1rem; }
    .def { padding: 0.75rem 1rem; }
    .formula { padding: 1rem; }

    /* Scale typography */
    h1 { font-size: 1.4rem; }
    h2 { font-size: 1.15rem; }
    h3 { font-size: 1rem; }

    /* Stack navigation buttons vertically */
    .topic-nav {
        flex-direction: column;
        gap: 0.5rem;
    }
    .topic-nav a {
        width: 100%;
        text-align: center;
    }

    /* Grid adjustments */
    .grid-2 {
        grid-template-columns: 1fr;
    }

    /* Smaller table padding */
    th, td { padding: 0.5rem; }

    /* Quiz options */
    .quiz-opt { padding: 0.75rem; }
}

/* ============================================
   Very Small Screens (< 400px)
   ============================================ */
@media (max-width: 400px) {
    /* Container */
    .container { padding: 0.5rem; }

    /* Even smaller padding */
    .hero { padding: 1rem 0.75rem; }
    .section { padding: 0.75rem; }
    .card { padding: 0.75rem; }
    .def { padding: 0.5rem 0.75rem; }
    .formula { padding: 0.75rem; }

    /* Smaller typography */
    h1 { font-size: 1.25rem; }
    h2 { font-size: 1.1rem; }
    h3 { font-size: 0.95rem; }
    body { font-size: 14px; }

    /* Navigation - top nav */
    .nav-inner { padding: 0.5rem; }
    .nav-brand { font-size: 0.85rem; }
    .nav-toggle { font-size: 1.25rem; }

    /* Steps - wrap to 2x2 grid */
    .steps {
        display: grid;
        grid-template-columns: repeat(2, 1fr);
        gap: 0.5rem;
        justify-items: center;
    }
    .step { width: 100%; }
    .step-num { width: 28px; height: 28px; font-size: 0.75rem; }

    /* Formula grid - single column */
    .formula-grid { grid-template-columns: 1fr; }
    .formula { padding: 0.75rem; }
    .formula-math { font-size: 0.9rem; padding: 0.5rem; }
    .formula-label { font-size: 0.65rem; }
    .formula-note { font-size: 0.7rem; }

    /* Buttons */
    .btn { padding: 0.5rem 0.75rem; font-size: 0.85rem; }
    .topic-nav { padding: 0.75rem; }
    .topic-nav a { padding: 0.5rem 0.75rem; font-size: 0.8rem; }

    /* Topic cards */
    .topic-card { padding: 1rem; }
    .topic-card h3 { font-size: 0.95rem; }
    .topic-card p { font-size: 0.8rem; }

    /* Tables */
    th, td { padding: 0.35rem; font-size: 0.8rem; }

    /* Ensure no overflow */
    * { word-wrap: break-word; overflow-wrap: break-word; }
}

/* ============================================
   Print
   ============================================ */
@media print {
    .nav, .topic-nav { display: none; }
    body { background: white; }
    .section { border: none; box-shadow: none; }
}