/* ===== CSS Variables & Global Styles ===== */
:root {
    /* Color Palette */
    --bg-color: #fdfbf7;
    --text-color: #333333;
    --primary-color: #4a5568;
    --accent-color: #718096;
    --border-color: #e2e8f0;
    --highlight-bg: #fff9c4;
    --code-bg: #f7fafc;
    --mastered-color: #48bb78;
    --review-color: #f6ad55;
    --confused-color: #f56565;
    
    /* Typography */
    --font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, 'Noto Sans', sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji';
    --font-size-base: 16px;
    --line-height-base: 1.8;
    
    /* Spacing */
    --spacing-xs: 0.25rem;
    --spacing-sm: 0.5rem;
    --spacing-md: 1rem;
    --spacing-lg: 1.5rem;
    --spacing-xl: 2rem;
    --spacing-2xl: 3rem;
    
    /* Transitions */
    --transition-fast: 0.2s ease;
    --transition-normal: 0.3s ease;
    --transition-slow: 0.4s ease;
    
    /* Shadows */
    --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.1);
    --shadow-md: 0 4px 8px rgba(0, 0, 0, 0.15);
    --shadow-lg: 0 8px 16px rgba(0, 0, 0, 0.2);
    
    /* Border Radius */
    --radius-sm: 4px;
    --radius-md: 8px;
    --radius-lg: 12px;
    --radius-full: 50%;
}

/* ===== Base Styles ===== */
* {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

body {
    font-family: var(--font-family);
    font-size: var(--font-size-base);
    line-height: var(--line-height-base);
    margin: 0;
    padding: 0;
    background-color: var(--bg-color);
    color: var(--text-color);
    overflow-x: hidden;
}

/* ===== Layout Components ===== */
.container {
    display: flex;
    min-height: 100vh;
}

/* Language Switch Button */
.language-switch-container {
    position: fixed;
    top: 20px;
    right: 20px;
    z-index: 2000;
}

.language-switch-btn {
    background: var(--primary-color);
    color: white;
    padding: 10px 18px;
    border-radius: var(--radius-md);
    border: none;
    cursor: pointer;
    font-weight: bold;
    box-shadow: var(--shadow-sm);
    transition: all var(--transition-fast);
    font-size: 0.9rem;
}

.language-switch-btn:hover {
    background: var(--accent-color);
    transform: translateY(-1px);
    box-shadow: var(--shadow-md);
}

.language-switch-btn:focus {
    outline: 2px solid var(--accent-color);
    outline-offset: 2px;
}

/* ===== Sidebar Navigation ===== */
.sidebar {
    width: 280px;
    position: fixed;
    top: 0;
    left: 0;
    height: 100vh;
    background-color: var(--primary-color);
    color: white;
    padding: var(--spacing-xl);
    overflow-y: auto;
    z-index: 1000;
    transform: translateX(-100%);
    transition: transform var(--transition-slow);
    box-shadow: var(--shadow-lg);
}

.sidebar.open {
    transform: translateX(0);
}

.sidebar-header h2 {
    font-size: 1.5em;
    font-weight: bold;
    margin: 0 0 var(--spacing-xl) 0;
    text-align: center;
    color: white;
}

.sidebar nav ul {
    list-style: none;
    padding: 0;
    margin: 0;
}

.sidebar nav li a {
    display: block;
    color: white;
    text-decoration: none;
    padding: 12px 15px;
    border-radius: var(--radius-md);
    margin-bottom: 5px;
    transition: all var(--transition-normal);
    font-weight: 500;
    position: relative;
}

.sidebar nav li a:hover {
    background: linear-gradient(135deg, #48bb78 0%, #38a169 100%);
    transform: translateX(3px);
    box-shadow: 0 3px 12px rgba(72, 187, 120, 0.3);
}

.sidebar nav li a.active {
    background-color: var(--accent-color);
}

/* Main chapter titles */
.sidebar nav li a[href="#analogue-electronics"],
.sidebar nav li a[href="#digital-electronics"] {
    background: linear-gradient(135deg, rgba(255,255,255,0.15) 0%, rgba(255,255,255,0.08) 100%);
    border: 1px solid rgba(255,255,255,0.2);
    font-weight: 700;
    font-size: 1.1em;
    margin: 10px 0;
    padding: 15px;
    box-shadow: 0 2px 10px rgba(0,0,0,0.2);
    text-transform: uppercase;
    letter-spacing: 1px;
}

.sidebar nav li a[href="#analogue-electronics"]:hover,
.sidebar nav li a[href="#digital-electronics"]:hover {
    background: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
    transform: translateX(8px);
    box-shadow: 0 6px 20px rgba(240, 147, 251, 0.4);
}

.sidebar nav .sub-menu {
    padding-left: 20px;
}

.sidebar nav .sub-menu li a {
    font-size: 0.9em;
    padding: 8px 15px;
}

/* ===== Sidebar Trigger Strip ===== */
.sidebar-trigger-strip {
    position: fixed;
    top: 0;
    left: 0;
    width: 25px;
    height: 100vh;
    z-index: 999;
    background: linear-gradient(90deg, rgba(74, 85, 104, 0.1) 0%, rgba(74, 85, 104, 0.05) 70%, transparent 100%);
    cursor: pointer;
    transition: all var(--transition-normal);
    background-image: 
        radial-gradient(circle, rgba(74, 85, 104, 0.3) 1px, transparent 1px),
        radial-gradient(circle, rgba(74, 85, 104, 0.2) 1px, transparent 1px);
    background-size: 8px 20px, 8px 20px;
    background-position: 8px 10px, 8px 30px;
    background-repeat: repeat-y;
}

.sidebar-trigger-strip:hover {
    width: 30px;
    background: linear-gradient(90deg, rgba(74, 85, 104, 0.2) 0%, rgba(74, 85, 104, 0.1) 70%, transparent 100%);
}

.sidebar.open ~ .sidebar-trigger-strip {
    opacity: 0;
    pointer-events: none;
}

/* ===== Main Content Area ===== */
.main-content {
    margin-left: 0;
    width: 100%;
    padding: var(--spacing-2xl);
    transition: margin-left var(--transition-slow);
    min-height: 100vh;
}

.content-container {
    width: 100%;
    max-width: 1200px;
    margin: 0 auto;
}

.page-header {
    text-align: center;
    margin-bottom: var(--spacing-2xl);
}

.main-content h1 {
    font-size: 2.8em;
    color: var(--primary-color);
    border-bottom: 3px solid var(--border-color);
    padding-bottom: 15px;
    margin: 0;
    background: linear-gradient(135deg, var(--primary-color) 0%, var(--accent-color) 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    line-height: 1.2;
}

.main-content h2 {
    font-size: 2.2em;
    color: var(--primary-color);
    margin-top: 60px;
    padding: 20px 0 15px 0;
    border-bottom: 2px solid var(--accent-color);
    position: relative;
    font-weight: 600;
}

.main-content h2::before {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    width: 4px;
    height: 100%;
    background: linear-gradient(180deg, var(--accent-color) 0%, transparent 100%);
    border-radius: 2px;
}

.main-content h3 {
    font-size: 1.6em;
    color: var(--primary-color);
    margin-top: 40px;
    padding: 12px 0 8px 0;
    border-left: 3px solid var(--accent-color);
    padding-left: 15px;
    background: linear-gradient(90deg, rgba(113, 128, 150, 0.05) 0%, transparent 50%);
    font-weight: 600;
}

.main-content h4 {
    color: var(--accent-color);
    font-size: 1.3em;
    margin-top: 30px;
    margin-bottom: 15px;
    font-weight: 600;
    padding-bottom: 5px;
    border-bottom: 1px dotted var(--accent-color);
}

/* ===== Content Presentation ===== */
strong, .highlight {
    background-color: var(--highlight-bg);
    padding: 2px 6px;
    border-radius: var(--radius-sm);
    font-weight: bold;
    border-left: 2px solid var(--accent-color);
    padding-left: 8px;
}

code, .formula-block {
    background-color: var(--code-bg);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 20px;
    display: block;
    overflow-x: auto;
    margin: 20px 0;
    font-family: 'Courier New', Courier, monospace;
    font-size: 1.1em;
    box-shadow: var(--shadow-sm);
}

table {
    width: 100%;
    border-collapse: collapse;
    margin: 25px 0;
    box-shadow: var(--shadow-sm);
    border-radius: var(--radius-md);
    overflow: hidden;
}

th, td {
    border: 1px solid var(--border-color);
    padding: 12px;
    text-align: left;
}

th {
    background-color: var(--code-bg);
    font-weight: 600;
}

/* ===== Interactive Components ===== */
.collapsible {
    cursor: pointer;
    display: inline-flex;
    align-items: center;
    font-weight: bold;
    color: var(--accent-color);
    border-bottom: 2px dotted var(--accent-color);
    transition: color var(--transition-fast);
}

.collapsible:hover {
    color: var(--primary-color);
}

.collapsible-icon {
    margin-right: 8px;
    transition: transform var(--transition-normal);
}

.collapsible-content {
    display: none;
    padding: 10px 0 10px 25px;
    border-left: 3px solid var(--border-color);
    margin-left: 5px;
    margin-top: 5px;
    animation: slideDown 0.3s ease;
}

@keyframes slideDown {
    from {
        opacity: 0;
        transform: translateY(-10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* ===== Mastery Controls ===== */
.mastery-controls {
    display: inline-flex;
    gap: 10px;
    margin-left: 15px;
    vertical-align: middle;
}

.mastery-icon {
    cursor: pointer;
    font-size: 1.4em;
    opacity: 0.3;
    transition: all var(--transition-normal);
    border-radius: var(--radius-full);
    padding: 2px;
}

.mastery-icon:hover {
    transform: scale(1.2);
    opacity: 0.7;
}

.mastery-icon.selected {
    opacity: 1;
    transform: scale(1.1);
}

/* ===== Quiz Components ===== */
.quiz-button {
    display: inline-block;
    margin-top: 30px;
    padding: 12px 25px;
    background-color: var(--accent-color);
    color: white;
    border: none;
    border-radius: var(--radius-md);
    cursor: pointer;
    font-size: 1em;
    font-weight: 500;
    transition: all var(--transition-normal);
    box-shadow: var(--shadow-sm);
}

.quiz-button:hover {
    background-color: var(--primary-color);
    transform: translateY(-1px);
    box-shadow: var(--shadow-md);
}

.quiz-container {
    display: none;
    margin-top: 20px;
    padding: 20px;
    background-color: rgba(247, 250, 252, 0.3);
    border: 1px solid rgba(226, 232, 240, 0.5);
    border-radius: var(--radius-md);
    position: relative;
    box-shadow: var(--shadow-sm);
}

.quiz-item {
    margin-bottom: 20px;
    padding: 15px;
    background: white;
    border-radius: var(--radius-sm);
    border-left: 3px solid var(--accent-color);
}

.quiz-answer {
    display: none;
    margin-top: 10px;
    padding-left: 15px;
    border-left: 3px solid var(--accent-color);
    animation: slideDown 0.3s ease;
}

.show-answer-btn {
    margin-left: 10px;
    padding: 5px 10px;
    border: 1px solid var(--accent-color);
    background: none;
    color: var(--accent-color);
    cursor: pointer;
    border-radius: var(--radius-sm);
    transition: all var(--transition-fast);
    font-size: 0.9em;
}

.show-answer-btn:hover {
    background-color: var(--accent-color);
    color: white;
}

.quiz-refresh-btn {
    position: absolute;
    top: 15px;
    left: 15px;
    width: 32px;
    height: 32px;
    border-radius: var(--radius-full);
    background: var(--accent-color);
    color: white;
    border: none;
    cursor: pointer;
    font-size: 1.1em;
    transition: all var(--transition-normal);
    display: flex;
    align-items: center;
    justify-content: center;
    box-shadow: var(--shadow-sm);
}

.quiz-refresh-btn:hover {
    background-color: var(--primary-color);
    transform: rotate(180deg);
    box-shadow: var(--shadow-md);
}

/* ===== Section Navigation ===== */
.section-nav-button {
    display: block;
    margin: 40px auto 20px auto;
    padding: 15px 30px;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    color: white;
    border: none;
    border-radius: var(--radius-lg);
    cursor: pointer;
    font-size: 1.1em;
    font-weight: 600;
    transition: all var(--transition-normal);
    box-shadow: 0 4px 15px rgba(102, 126, 234, 0.3);
    min-width: 250px;
    text-decoration: none;
    text-align: center;
}

.section-nav-button:hover {
    background: linear-gradient(135deg, #764ba2 0%, #667eea 100%);
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(102, 126, 234, 0.4);
}

.section-nav-button:active {
    transform: translateY(0);
}

/* ===== Section Containers ===== */
.section-container {
    display: none;
}

.section-container.active {
    display: block;
    animation: fadeIn 0.5s ease;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* ===== Section Dividers ===== */
.section-divider {
    margin: 60px 0 40px 0;
    text-align: center;
    position: relative;
}

.section-divider::before {
    content: '';
    position: absolute;
    top: 50%;
    left: 0;
    right: 0;
    height: 2px;
    background: linear-gradient(90deg, transparent 0%, var(--border-color) 20%, var(--accent-color) 50%, var(--border-color) 80%, transparent 100%);
}

.section-divider::after {
    content: '◆ ◆ ◆';
    background-color: var(--bg-color);
    color: var(--accent-color);
    padding: 0 20px;
    font-size: 1.2em;
    position: relative;
    z-index: 1;
}

/* ===== Back to Top Button ===== */
.back-to-top {
    position: fixed;
    bottom: 30px;
    right: 30px;
    background-color: var(--primary-color);
    color: white;
    width: 50px;
    height: 50px;
    border-radius: var(--radius-full);
    text-align: center;
    line-height: 50px;
    font-size: 24px;
    text-decoration: none;
    opacity: 0;
    visibility: hidden;
    transition: all var(--transition-normal);
    z-index: 1001;
    box-shadow: var(--shadow-md);
    display: flex;
    align-items: center;
    justify-content: center;
}

.back-to-top.show {
    opacity: 1;
    visibility: visible;
}

.back-to-top:hover {
    background-color: var(--accent-color);
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
}

/* ===== Responsive Design ===== */
@media (max-width: 768px) {
    .main-content {
        padding: 20px;
    }
    
    .main-content h1 {
        font-size: 2em;
    }
    
    .main-content h2 {
        font-size: 1.8em;
    }
    
    .main-content h3 {
        font-size: 1.4em;
    }
    
    .sidebar {
        width: 100%;
        max-width: 300px;
    }
    
    .language-switch-container {
        top: 10px;
        right: 10px;
    }
    
    .language-switch-btn {
        padding: 8px 12px;
        font-size: 0.8rem;
    }
}

@media (min-width: 769px) {
    .sidebar.open ~ .main-content {
        margin-left: 280px;
    }
}

/* ===== Print Styles ===== */
@media print {
    .sidebar,
    .sidebar-trigger-strip,
    .language-switch-container,
    .back-to-top,
    .quiz-button,
    .section-nav-button {
        display: none !important;
    }
    
    .main-content {
        margin-left: 0 !important;
        padding: 0 !important;
    }
    
    .main-content h1,
    .main-content h2,
    .main-content h3 {
        page-break-after: avoid;
    }
    
    .section-container {
        display: block !important;
    }
}

/* ===== Focus Styles for Accessibility ===== */
*:focus {
    outline: 2px solid var(--accent-color);
    outline-offset: 2px;
}

/* ===== Loading States ===== */
.loading {
    opacity: 0.6;
    pointer-events: none;
}

.loading::after {
    content: '';
    position: absolute;
    top: 50%;
    left: 50%;
    width: 20px;
    height: 20px;
    margin: -10px 0 0 -10px;
    border: 2px solid var(--accent-color);
    border-top: 2px solid transparent;
    border-radius: var(--radius-full);
    animation: spin 1s linear infinite;
}

@keyframes spin {
    0% { transform: rotate(0deg); }
    100% { transform: rotate(360deg); }
}