/* ============================================
   ONLINE CGPA CALCULATOR - MOBILE FIRST CSS
   ============================================ */

/* CSS Custom Properties */
:root {
    --bg-primary: #0a0a0a;
    --bg-secondary: #111111;
    --bg-card: #141414;
    --bg-input: #1a1a1a;
    --bg-hover: #1f1f1f;
    --border-color: #2a2a2a;
    --border-light: #333333;
    --text-primary: #ffffff;
    --text-secondary: #a0a0a0;
    --text-muted: #666666;
    --accent: #ffffff;
    --accent-hover: #e0e0e0;
    --success: #22c55e;
    --error: #ef4444;
    --ring-bg: #1f1f1f;
    --ring-fill: #ffffff;

    --font-main: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --radius-xl: 24px;

    --shadow-sm: 0 1px 2px rgba(0,0,0,0.3);
    --shadow-md: 0 4px 12px rgba(0,0,0,0.4);
    --shadow-lg: 0 8px 30px rgba(0,0,0,0.5);

    --transition-fast: 150ms ease;
    --transition-base: 250ms ease;
    --transition-slow: 350ms ease;
}

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

html {
    scroll-behavior: smooth;
    -webkit-text-size-adjust: 100%;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

body {
    font-family: var(--font-main);
    background-color: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
    overflow-x: hidden;
}

img, svg {
    display: block;
    max-width: 100%;
}

button, input {
    font-family: inherit;
    font-size: inherit;
    color: inherit;
}

button {
    cursor: pointer;
    border: none;
    background: none;
}

a {
    color: inherit;
    text-decoration: none;
}

ul, ol {
    list-style: none;
}

/* Screen Reader Only */
.sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    white-space: nowrap;
    border: 0;
}

/* Container - Mobile First */
.container {
    width: 100%;
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 1rem;
}

/* ============================================
   HEADER
   ============================================ */
.site-header {
    position: sticky;
    top: 0;
    z-index: 100;
    background: rgba(10, 10, 10, 0.85);
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--border-color);
}

.header-container {
    display: flex;
    align-items: center;
    justify-content: space-between;
    height: 56px;
    gap: 1rem;
}

.logo-text {
    font-size: 0.875rem;
    font-weight: 700;
    color: var(--text-primary);
    white-space: nowrap;
    letter-spacing: -0.02em;
}

/* Mobile Menu Toggle */
.mobile-menu-toggle {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 40px;
    height: 40px;
    padding: 8px;
    border-radius: var(--radius-sm);
    transition: background var(--transition-fast);
}

.mobile-menu-toggle:hover,
.mobile-menu-toggle[aria-expanded="true"] {
    background: var(--bg-hover);
}

.hamburger {
    display: block;
    width: 20px;
    height: 2px;
    background: var(--text-primary);
    position: relative;
    transition: background var(--transition-fast);
}

.hamburger::before,
.hamburger::after {
    content: '';
    position: absolute;
    left: 0;
    width: 20px;
    height: 2px;
    background: var(--text-primary);
    transition: transform var(--transition-fast), top var(--transition-fast);
}

.hamburger::before { top: -6px; }
.hamburger::after { top: 6px; }

.mobile-menu-toggle[aria-expanded="true"] .hamburger {
    background: transparent;
}

.mobile-menu-toggle[aria-expanded="true"] .hamburger::before {
    top: 0;
    transform: rotate(45deg);
}

.mobile-menu-toggle[aria-expanded="true"] .hamburger::after {
    top: 0;
    transform: rotate(-45deg);
}

/* Navigation */
.main-nav {
    position: relative;
}

.nav-list {
    display: none;
    position: absolute;
    top: calc(100% + 8px);
    right: 0;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 0.5rem;
    min-width: 160px;
    box-shadow: var(--shadow-lg);
    z-index: 99;
}

.nav-list.open {
    display: block;
    animation: slideDown 200ms ease forwards;
}

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

.nav-link {
    display: block;
    padding: 0.625rem 1rem;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--text-secondary);
    border-radius: var(--radius-sm);
    transition: all var(--transition-fast);
    white-space: nowrap;
}

.nav-link:hover,
.nav-link.active {
    color: var(--text-primary);
    background: var(--bg-hover);
}

/* Header Search */
.header-search {
    display: none;
}

.search-input {
    width: 100%;
    padding: 0.5rem 0.875rem;
    background: var(--bg-input);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    font-size: 0.875rem;
    color: var(--text-primary);
    transition: border-color var(--transition-fast);
}

.search-input::placeholder {
    color: var(--text-muted);
}

.search-input:focus {
    outline: none;
    border-color: var(--border-light);
}

/* ============================================
   HERO / CALCULATOR
   ============================================ */
.hero-section {
    padding: 2rem 0 3rem;
}

.hero-header {
    text-align: center;
    margin-bottom: 2rem;
}

.hero-title {
    font-size: 1.75rem;
    font-weight: 800;
    letter-spacing: -0.03em;
    text-transform: uppercase;
    margin-bottom: 0.5rem;
    line-height: 1.1;
}

.hero-subtitle {
    font-size: 0.875rem;
    color: var(--text-secondary);
    max-width: 400px;
    margin: 0 auto;
}

/* Method Toggle */
.method-toggle {
    display: flex;
    justify-content: center;
    gap: 0.25rem;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 0.25rem;
    max-width: 320px;
    margin: 0 auto 1.5rem;
}

.toggle-btn {
    flex: 1;
    padding: 0.625rem 0.75rem;
    font-size: 0.75rem;
    font-weight: 600;
    color: var(--text-muted);
    background: transparent;
    border-radius: var(--radius-sm);
    transition: all var(--transition-fast);
    text-align: center;
    line-height: 1.3;
}

.toggle-btn:hover {
    color: var(--text-secondary);
}

.toggle-btn.active {
    background: var(--text-primary);
    color: var(--bg-primary);
    box-shadow: var(--shadow-sm);
}

/* Calculator Layout */
.calculator-layout {
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
}

/* Semesters Panel */
.semesters-panel {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.semesters-grid {
    display: grid;
    grid-template-columns: 1fr;
    gap: 1rem;
}

/* Semester Card */
.semester-card {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 1rem;
    position: relative;
    transition: border-color var(--transition-fast), transform var(--transition-fast);
    animation: fadeInUp 300ms ease forwards;
}

@keyframes fadeInUp {
    from { opacity: 0; transform: translateY(12px); }
    to { opacity: 1; transform: translateY(0); }
}

.semester-card:hover {
    border-color: var(--border-light);
}

.semester-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: 1rem;
}

.semester-title {
    font-size: 0.9375rem;
    font-weight: 700;
}

.btn-remove-semester {
    width: 28px;
    height: 28px;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: var(--radius-sm);
    color: var(--text-muted);
    transition: all var(--transition-fast);
    font-size: 1.125rem;
    line-height: 1;
}

.btn-remove-semester:hover {
    color: var(--error);
    background: rgba(239, 68, 68, 0.1);
}

.semester-fields {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 0.75rem;
}

.field-group {
    display: flex;
    flex-direction: column;
    gap: 0.375rem;
}

.field-label {
    font-size: 0.6875rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-muted);
}

.field-input {
    width: 100%;
    padding: 0.625rem 0.75rem;
    background: var(--bg-input);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    font-size: 0.9375rem;
    font-weight: 500;
    color: var(--text-primary);
    transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.field-input::placeholder {
    color: var(--text-muted);
}

.field-input:focus {
    outline: none;
    border-color: var(--border-light);
    box-shadow: 0 0 0 3px rgba(255, 255, 255, 0.05);
}

.field-input.error {
    border-color: var(--error) !important;
    box-shadow: 0 0 0 3px rgba(239, 68, 68, 0.15) !important;
}

/* Add Semester Button */
.btn-add-semester {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    width: 100%;
    padding: 0.875rem;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    font-size: 0.875rem;
    font-weight: 600;
    color: var(--text-primary);
    text-transform: uppercase;
    letter-spacing: 0.03em;
    transition: all var(--transition-fast);
}

.btn-add-semester:hover {
    background: var(--bg-hover);
    border-color: var(--border-light);
}

.plus-icon {
    font-size: 1.25rem;
    font-weight: 400;
    line-height: 1;
}

/* Result Panel */
.result-panel {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 1.25rem;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 1.25rem;
    position: sticky;
    top: 72px;
}

.result-heading {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.9375rem;
    font-weight: 600;
    color: var(--text-secondary);
    align-self: flex-start;
}

.result-icon {
    color: var(--text-primary);
}

/* Progress Ring */
.progress-ring-container {
    position: relative;
    width: 140px;
    height: 140px;
}

.progress-ring {
    transform: rotate(-90deg);
    width: 140px;
    height: 140px;
}

.progress-ring-bg {
    fill: none;
    stroke: var(--ring-bg);
    stroke-width: 10;
    stroke-linecap: round;
}

.progress-ring-fill {
    fill: none;
    stroke: var(--ring-fill);
    stroke-width: 10;
    stroke-linecap: round;
    transition: stroke-dashoffset 800ms cubic-bezier(0.4, 0, 0.2, 1);
}

.cgpa-value-wrapper {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    text-align: center;
}

.cgpa-value {
    display: block;
    font-size: 2.25rem;
    font-weight: 800;
    line-height: 1;
    letter-spacing: -0.03em;
}

.cgpa-outof {
    display: block;
    font-size: 0.6875rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--text-muted);
    margin-top: 0.25rem;
}

/* Calculate Button */
.btn-calculate {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    width: 100%;
    padding: 0.875rem 1.5rem;
    background: var(--text-primary);
    color: var(--bg-primary);
    border-radius: var(--radius-sm);
    font-size: 0.9375rem;
    font-weight: 600;
    transition: all var(--transition-fast);
}

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

.btn-calculate:active {
    transform: translateY(0);
}

/* Stats Grid */
.stats-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 0.75rem;
    width: 100%;
}

.stat-card {
    background: var(--bg-input);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    padding: 0.875rem;
    text-align: center;
}

.stat-value {
    display: block;
    font-size: 1.5rem;
    font-weight: 800;
    line-height: 1;
    letter-spacing: -0.02em;
}

.stat-label {
    display: block;
    font-size: 0.625rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--text-muted);
    margin-top: 0.375rem;
}

/* ============================================
   FORMULA SECTION
   ============================================ */
.formula-section {
    padding: 1rem 0 2rem;
}

.formula-card {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 1rem 1.25rem;
    display: flex;
    flex-direction: column;
    gap: 0.75rem;
}

.formula-header {
    display: flex;
    align-items: center;
    gap: 0.75rem;
}

.formula-icon {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--text-secondary);
    line-height: 1;
}

.formula-title {
    font-size: 0.75rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--text-secondary);
}

.formula-body {
    background: var(--bg-input);
    border-radius: var(--radius-sm);
    padding: 0.75rem 1rem;
    overflow-x: auto;
}

.formula-code {
    font-family: 'SF Mono', Monaco, 'Cascadia Code', monospace;
    font-size: 0.8125rem;
    color: var(--text-primary);
    white-space: nowrap;
}

/* ============================================
   CALCULATORS SECTION
   ============================================ */
.calculators-section {
    padding: 2rem 0;
}

.section-title {
    font-size: 1.25rem;
    font-weight: 800;
    text-transform: uppercase;
    letter-spacing: -0.02em;
    text-align: center;
    margin-bottom: 1.5rem;
}

.calculators-grid {
    display: grid;
    grid-template-columns: 1fr;
    gap: 1rem;
}

.calculator-card {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: 1.25rem;
    transition: border-color var(--transition-fast), transform var(--transition-fast);
}

.calculator-card:hover {
    border-color: var(--border-light);
    transform: translateY(-2px);
}

.calc-card-icon {
    width: 40px;
    height: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--bg-input);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    font-size: 1.125rem;
    margin-bottom: 1rem;
}

.calc-card-title {
    font-size: 1rem;
    font-weight: 700;
    margin-bottom: 0.375rem;
}

.calc-card-desc {
    font-size: 0.8125rem;
    color: var(--text-secondary);
    line-height: 1.5;
    margin-bottom: 1rem;
}

.calc-card-link {
    display: inline-flex;
    align-items: center;
    gap: 0.375rem;
    font-size: 0.75rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-primary);
    transition: gap var(--transition-fast);
}

.calc-card-link:hover {
    gap: 0.625rem;
}

.view-all-wrapper {
    text-align: center;
    margin-top: 1.5rem;
}

.btn-view-all {
    display: inline-block;
    padding: 0.625rem 1.5rem;
    background: transparent;
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    font-size: 0.75rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-secondary);
    transition: all var(--transition-fast);
}

.btn-view-all:hover {
    border-color: var(--border-light);
    color: var(--text-primary);
    background: var(--bg-hover);
}

/* ============================================
   UNDERSTANDING SECTION
   ============================================ */
.understanding-section {
    padding: 2rem 0;
    border-top: 1px solid var(--border-color);
}

.understanding-layout {
    display: flex;
    flex-direction: column;
    gap: 2rem;
}

.understanding-header {
    flex-shrink: 0;
}

.understanding-title {
    text-align: left;
    margin-bottom: 0.75rem;
    line-height: 1.2;
}

.understanding-lead {
    font-size: 0.875rem;
    color: var(--text-secondary);
}

.understanding-content {
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
}

.info-block {
    padding-bottom: 1.5rem;
    border-bottom: 1px solid var(--border-color);
}

.info-block:last-child {
    border-bottom: none;
    padding-bottom: 0;
}

.info-block-title {
    font-size: 1rem;
    font-weight: 700;
    margin-bottom: 0.5rem;
}

.info-block-text {
    font-size: 0.875rem;
    color: var(--text-secondary);
    line-height: 1.6;
}

/* ============================================
   FAQ SECTION
   ============================================ */
.faq-section {
    padding: 2rem 0 3rem;
}

.faq-section-title {
    margin-bottom: 1.5rem;
}

.faq-list {
    display: flex;
    flex-direction: column;
    gap: 0.75rem;
    max-width: 640px;
    margin: 0 auto;
}

.faq-item {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    overflow: hidden;
    transition: border-color var(--transition-fast);
}

.faq-item:hover {
    border-color: var(--border-light);
}

.faq-item[open] {
    border-color: var(--border-light);
}

.faq-question {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    width: 100%;
    padding: 1rem 1.25rem;
    font-size: 0.9375rem;
    font-weight: 500;
    color: var(--text-primary);
    cursor: pointer;
    list-style: none;
    transition: background var(--transition-fast);
}

.faq-question:hover {
    background: var(--bg-hover);
}

.faq-question::-webkit-details-marker {
    display: none;
}

.faq-chevron {
    flex-shrink: 0;
    color: var(--text-muted);
    transition: transform var(--transition-base);
}

.faq-item[open] .faq-chevron {
    transform: rotate(180deg);
}

.faq-answer {
    padding: 0.75rem 1.25rem 1.25rem;
    font-size: 0.875rem;
    color: var(--text-secondary);
    line-height: 1.6;
    animation: fadeIn 200ms ease forwards;
}

.faq-item[open] .faq-question {
    padding-bottom: 0.5rem;
}

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

/* ============================================
   FOOTER
   ============================================ */
.site-footer {
    border-top: 1px solid var(--border-color);
    padding: 2.5rem 0;
}

.footer-container {
    display: grid;
    grid-template-columns: 1fr;
    gap: 2rem;
}

.footer-logo {
    display: block;
    font-size: 1rem;
    font-weight: 700;
    margin-bottom: 0.5rem;
}

.footer-copy {
    font-size: 0.8125rem;
    color: var(--text-muted);
    line-height: 1.5;
    max-width: 280px;
}

.footer-nav-title {
    font-size: 0.6875rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--text-muted);
    margin-bottom: 0.75rem;
}

.footer-nav-list {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.footer-nav-list a {
    font-size: 0.875rem;
    color: var(--text-secondary);
    transition: color var(--transition-fast);
}

.footer-nav-list a:hover {
    color: var(--text-primary);
}

/* ============================================
   TABLET (768px+)
   ============================================ */
@media (min-width: 768px) {
    .container {
        padding: 0 1.5rem;
    }

    .header-container {
        height: 64px;
    }

    .mobile-menu-toggle {
        display: none;
    }

    .nav-list {
        display: flex !important;
        position: static;
        background: transparent;
        border: none;
        padding: 0;
        min-width: auto;
        box-shadow: none;
        gap: 0.25rem;
    }

    .nav-link {
        padding: 0.5rem 0.875rem;
    }

    .header-search {
        display: block;
        width: 160px;
    }

    .hero-title {
        font-size: 2.5rem;
    }

    .hero-subtitle {
        font-size: 1rem;
    }

    .method-toggle {
        max-width: 380px;
    }

    .toggle-btn {
        font-size: 0.8125rem;
        padding: 0.75rem 1rem;
    }

    .semesters-grid {
        grid-template-columns: repeat(2, 1fr);
    }

    .calculators-grid {
        grid-template-columns: repeat(3, 1fr);
    }

    .understanding-layout {
        flex-direction: row;
        gap: 3rem;
    }

    .understanding-header {
        width: 35%;
        position: sticky;
        top: 80px;
        align-self: flex-start;
    }

    .understanding-content {
        flex: 1;
    }

    .footer-container {
        grid-template-columns: 2fr 1fr 1fr;
        gap: 3rem;
    }
}

/* ============================================
   DESKTOP (1024px+)
   ============================================ */
@media (min-width: 1024px) {
    .container {
        padding: 0 2rem;
    }

    .header-search {
        width: 200px;
    }

    .hero-section {
        padding: 3rem 0 4rem;
    }

    .hero-title {
        font-size: 3rem;
    }

    .calculator-layout {
        flex-direction: row;
        align-items: flex-start;
        gap: 2rem;
    }

    .semesters-panel {
        flex: 1;
    }

    .semesters-grid {
        grid-template-columns: repeat(2, 1fr);
    }

    .result-panel {
        width: 320px;
        flex-shrink: 0;
        padding: 1.5rem;
    }

    .progress-ring-container {
        width: 160px;
        height: 160px;
    }

    .progress-ring {
        width: 160px;
        height: 160px;
    }

    .cgpa-value {
        font-size: 2.75rem;
    }

    .formula-card {
        flex-direction: row;
        align-items: center;
        justify-content: space-between;
        padding: 1rem 1.5rem;
    }

    .formula-body {
        padding: 0.625rem 1.25rem;
    }

    .section-title {
        font-size: 1.5rem;
        margin-bottom: 2rem;
    }

    .understanding-title {
        font-size: 1.75rem;
    }
}

/* ============================================
   LARGE DESKTOP (1280px+)
   ============================================ */
@media (min-width: 1280px) {
    .hero-title {
        font-size: 3.5rem;
    }

    .semesters-grid {
        grid-template-columns: repeat(2, 1fr);
    }
}

/* ============================================
   REDUCED MOTION
   ============================================ */
@media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
    }

    html {
        scroll-behavior: auto;
    }
}

/* ============================================
   FOCUS VISIBLE
   ============================================ */
:focus-visible {
    outline: 2px solid var(--text-primary);
    outline-offset: 2px;
    border-radius: var(--radius-sm);
}

button:focus-visible,
a:focus-visible,
input:focus-visible,
summary:focus-visible {
    outline: 2px solid var(--text-primary);
    outline-offset: 2px;
}

/* Remove default focus when focus-visible is supported */
:focus:not(:focus-visible) {
    outline: none;
}