/* ============================================
   Design System — Math Adventure 2.0
   ============================================ */

:root {
    /* Color Palette — Warm, playful but mature */
    --color-bg-start: #0f0c29;
    --color-bg-mid: #302b63;
    --color-bg-end: #24243e;

    --color-primary: #ff6b9d;
    --color-primary-dark: #e0557f;
    --color-secondary: #c084fc;
    --color-secondary-dark: #a855f7;
    --color-accent: #fbbf24;
    --color-accent-glow: rgba(251, 191, 36, 0.3);

    --color-success: #34d399;
    --color-success-dark: #059669;
    --color-error: #f87171;
    --color-error-dark: #dc2626;

    --color-text: #f1f5f9;
    --color-text-muted: #94a3b8;
    --color-card-bg: rgba(255, 255, 255, 0.08);
    --color-card-border: rgba(255, 255, 255, 0.12);
    --color-glass: rgba(255, 255, 255, 0.05);

    /* Shadows */
    --shadow-card: 0 8px 32px rgba(0, 0, 0, 0.3);
    --shadow-btn: 0 4px 14px rgba(0, 0, 0, 0.25);
    --shadow-glow-primary: 0 0 20px rgba(255, 107, 157, 0.4);
    --shadow-glow-secondary: 0 0 20px rgba(192, 132, 252, 0.4);

    /* Radius */
    --radius-xl: 28px;
    --radius-lg: 20px;
    --radius-md: 14px;
    --radius-sm: 10px;
    --radius-round: 50px;

    /* Transitions */
    --ease-out-back: cubic-bezier(0.34, 1.56, 0.64, 1);
    --ease-spring: cubic-bezier(0.175, 0.885, 0.32, 1.275);
}

/* ============================================
   Reset & Base
   ============================================ */

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

body {
    font-family: 'Outfit', 'Microsoft JhengHei', 'PingFang TC', sans-serif;
    background: linear-gradient(135deg, var(--color-bg-start), var(--color-bg-mid), var(--color-bg-end));
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    color: var(--color-text);
    overflow: hidden;
    position: relative;
}

/* ============================================
   Floating Background Shapes
   ============================================ */

.bg-shapes {
    position: fixed;
    inset: 0;
    pointer-events: none;
    z-index: 0;
    overflow: hidden;
}

.shape {
    position: absolute;
    border-radius: 50%;
    opacity: 0.08;
    filter: blur(60px);
}

.shape-1 {
    width: 300px;
    height: 300px;
    background: var(--color-primary);
    top: -50px;
    left: -50px;
    animation: float-1 18s ease-in-out infinite;
}

.shape-2 {
    width: 250px;
    height: 250px;
    background: var(--color-secondary);
    top: 60%;
    right: -80px;
    animation: float-2 22s ease-in-out infinite;
}

.shape-3 {
    width: 200px;
    height: 200px;
    background: var(--color-accent);
    bottom: -40px;
    left: 30%;
    animation: float-3 20s ease-in-out infinite;
}

.shape-4 {
    width: 150px;
    height: 150px;
    background: var(--color-success);
    top: 20%;
    right: 10%;
    animation: float-1 25s ease-in-out infinite reverse;
}

.shape-5 {
    width: 180px;
    height: 180px;
    background: var(--color-primary);
    bottom: 20%;
    left: -40px;
    animation: float-2 16s ease-in-out infinite reverse;
}

@keyframes float-1 {
    0%, 100% { transform: translate(0, 0) scale(1); }
    33% { transform: translate(30px, -30px) scale(1.1); }
    66% { transform: translate(-20px, 20px) scale(0.9); }
}

@keyframes float-2 {
    0%, 100% { transform: translate(0, 0) rotate(0deg); }
    50% { transform: translate(-40px, -30px) rotate(180deg); }
}

@keyframes float-3 {
    0%, 100% { transform: translate(0, 0); }
    25% { transform: translate(20px, -40px); }
    75% { transform: translate(-30px, 10px); }
}

/* ============================================
   App Container
   ============================================ */

#app {
    width: 100%;
    max-width: 480px;
    padding: 20px;
    position: relative;
    z-index: 1;
}

/* ============================================
   Card (Glass Morphism)
   ============================================ */

.card {
    background: var(--color-card-bg);
    backdrop-filter: blur(24px);
    -webkit-backdrop-filter: blur(24px);
    border: 1px solid var(--color-card-border);
    border-radius: var(--radius-xl);
    padding: 32px 28px;
    box-shadow: var(--shadow-card);
    min-height: 520px;
    display: flex;
    flex-direction: column;
    justify-content: center;
    position: relative;
    overflow: hidden;
}

.card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 1px;
    background: linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.2), transparent);
}

/* ============================================
   Screens
   ============================================ */

.screen {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 20px;
    width: 100%;
    animation: fadeIn 0.4s ease-out;
}

.hidden {
    display: none !important;
}

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

/* ============================================
   Start Screen
   ============================================ */

.logo-area {
    text-align: center;
    margin-bottom: 8px;
}

.logo-icon {
    font-size: 3.5rem;
    margin-bottom: 8px;
    animation: bounce-in 0.6s var(--ease-out-back);
}

@keyframes bounce-in {
    from { transform: scale(0); opacity: 0; }
    to { transform: scale(1); opacity: 1; }
}

.title {
    font-family: 'Fredoka', 'Microsoft JhengHei', 'PingFang TC', sans-serif;
    font-size: 2.4rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--color-primary), var(--color-secondary));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    margin-bottom: 4px;
}

.subtitle {
    font-size: 1rem;
    color: var(--color-text-muted);
    font-weight: 400;
}

/* Difficulty Selector */
.difficulty-selector {
    display: flex;
    flex-direction: column;
    gap: 12px;
    width: 100%;
}

.diff-btn {
    display: flex;
    align-items: center;
    gap: 14px;
    width: 100%;
    padding: 16px 20px;
    background: var(--color-glass);
    border: 1.5px solid var(--color-card-border);
    border-radius: var(--radius-lg);
    color: var(--color-text);
    cursor: pointer;
    transition: all 0.3s var(--ease-spring);
    text-align: left;
    position: relative;
    overflow: hidden;
}

.diff-btn::before {
    content: '';
    position: absolute;
    inset: 0;
    background: linear-gradient(135deg, transparent, rgba(255, 255, 255, 0.03));
    opacity: 0;
    transition: opacity 0.3s;
}

.diff-btn:hover {
    border-color: var(--color-primary);
    transform: translateX(6px);
    box-shadow: var(--shadow-glow-primary);
}

.diff-btn:hover::before {
    opacity: 1;
}

.diff-btn:active {
    transform: translateX(6px) scale(0.98);
}

.diff-stars {
    font-size: 1.2rem;
    flex-shrink: 0;
    width: 60px;
    text-align: center;
}

.diff-label {
    font-family: 'Fredoka', sans-serif;
    font-size: 1.2rem;
    font-weight: 600;
    min-width: 50px;
}

.diff-desc {
    font-size: 0.85rem;
    color: var(--color-text-muted);
    margin-left: auto;
}

/* Difficulty-specific hover colors */
.diff-btn[data-difficulty="easy"]:hover {
    border-color: var(--color-success);
    box-shadow: 0 0 20px rgba(52, 211, 153, 0.3);
}

.diff-btn[data-difficulty="medium"]:hover {
    border-color: var(--color-accent);
    box-shadow: 0 0 20px var(--color-accent-glow);
}

.diff-btn[data-difficulty="hard"]:hover {
    border-color: var(--color-error);
    box-shadow: 0 0 20px rgba(248, 113, 113, 0.3);
}

/* ============================================
   Game Screen
   ============================================ */

.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: 100%;
    margin-bottom: 8px;
}

.header-info {
    display: flex;
    gap: 10px;
}

.score-pill,
.progress-pill {
    background: var(--color-glass);
    border: 1px solid var(--color-card-border);
    padding: 6px 14px;
    border-radius: var(--radius-round);
    font-weight: 700;
    font-size: 0.9rem;
    color: var(--color-text);
}

.score-pill .label {
    color: var(--color-text-muted);
    font-size: 0.8rem;
    margin-right: 4px;
    font-weight: 400;
}

.icon-btn {
    background: var(--color-glass);
    border: 1px solid var(--color-card-border);
    cursor: pointer;
    font-size: 1.3rem;
    padding: 8px 12px;
    border-radius: 50%;
    transition: all 0.25s var(--ease-spring);
    color: var(--color-text);
}

.icon-btn:hover {
    background: rgba(255, 255, 255, 0.12);
    transform: scale(1.1);
}

.diff-badge {
    background: var(--color-glass);
    border: 1px solid var(--color-card-border);
    padding: 6px 12px;
    border-radius: var(--radius-round);
    font-size: 0.85rem;
}

/* Progress Bar */
.progress-bar-track {
    width: 100%;
    height: 4px;
    background: var(--color-glass);
    border-radius: 2px;
    margin-bottom: 16px;
    overflow: hidden;
}

.progress-bar-fill {
    height: 100%;
    width: 10%;
    background: linear-gradient(90deg, var(--color-primary), var(--color-secondary));
    border-radius: 2px;
    transition: width 0.5s var(--ease-spring);
}

/* Question Area */
.question-area {
    text-align: center;
    margin: 12px 0 24px;
}

.question {
    font-family: 'Fredoka', cursive;
    font-size: 3.8rem;
    font-weight: 600;
    color: var(--color-text);
    animation: pop-in 0.35s var(--ease-out-back);
}

@keyframes pop-in {
    from { transform: scale(0.8); opacity: 0; }
    to { transform: scale(1); opacity: 1; }
}

/* Streak Indicator */
.streak-indicator {
    display: inline-flex;
    align-items: center;
    gap: 4px;
    background: linear-gradient(135deg, rgba(251, 191, 36, 0.15), rgba(251, 191, 36, 0.05));
    border: 1px solid rgba(251, 191, 36, 0.3);
    padding: 4px 14px;
    border-radius: var(--radius-round);
    font-size: 0.85rem;
    font-weight: 600;
    color: var(--color-accent);
    margin-top: 8px;
    animation: fadeIn 0.3s ease;
}

.streak-fire {
    font-size: 1rem;
}

/* Options Grid */
.options-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 14px;
    width: 100%;
}

.option-btn {
    background: var(--color-glass);
    border: 2px solid var(--color-card-border);
    border-radius: var(--radius-md);
    padding: 18px;
    font-family: 'Fredoka', 'Microsoft JhengHei', 'PingFang TC', sans-serif;
    font-size: 2rem;
    font-weight: 500;
    color: var(--color-text);
    cursor: pointer;
    transition: all 0.25s var(--ease-spring);
    position: relative;
    overflow: hidden;
}

.option-btn::before {
    content: '';
    position: absolute;
    inset: 0;
    background: linear-gradient(135deg, transparent, rgba(255, 255, 255, 0.05));
    opacity: 0;
    transition: opacity 0.3s;
}

.option-btn:hover {
    border-color: var(--color-secondary);
    transform: translateY(-3px);
    box-shadow: var(--shadow-glow-secondary);
}

.option-btn:hover::before {
    opacity: 1;
}

.option-btn:active {
    transform: translateY(1px);
    box-shadow: none;
}

.option-btn.correct {
    background: linear-gradient(135deg, var(--color-success), var(--color-success-dark));
    color: white;
    border-color: var(--color-success);
    box-shadow: 0 0 20px rgba(52, 211, 153, 0.4);
    animation: correct-pop 0.4s var(--ease-out-back);
}

@keyframes correct-pop {
    0% { transform: scale(1); }
    50% { transform: scale(1.08); }
    100% { transform: scale(1); }
}

.option-btn.wrong {
    background: linear-gradient(135deg, var(--color-error), var(--color-error-dark));
    color: white;
    border-color: var(--color-error);
    box-shadow: 0 0 15px rgba(248, 113, 113, 0.3);
    animation: shake 0.5s;
}

@keyframes shake {
    0%, 100% { transform: translateX(0); }
    20% { transform: translateX(-6px); }
    40% { transform: translateX(6px); }
    60% { transform: translateX(-4px); }
    80% { transform: translateX(4px); }
}

/* ============================================
   Result Screen
   ============================================ */

.result-content {
    text-align: center;
    width: 100%;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 20px;
}

.result-stars {
    font-size: 2.5rem;
    animation: stars-bounce 0.8s var(--ease-out-back);
}

@keyframes stars-bounce {
    from { transform: scale(0) rotate(-30deg); opacity: 0; }
    to { transform: scale(1) rotate(0deg); opacity: 1; }
}

.result-title {
    font-family: 'Fredoka', sans-serif;
    font-size: 1.8rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--color-primary), var(--color-accent));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.result-stats {
    display: flex;
    gap: 12px;
    width: 100%;
    justify-content: center;
}

.stat-card {
    background: var(--color-glass);
    border: 1px solid var(--color-card-border);
    border-radius: var(--radius-md);
    padding: 16px 12px;
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 4px;
}

.stat-value {
    font-family: 'Fredoka', sans-serif;
    font-size: 1.8rem;
    font-weight: 700;
    color: var(--color-accent);
}

.stat-label {
    font-size: 0.8rem;
    color: var(--color-text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.result-actions {
    display: flex;
    flex-direction: column;
    gap: 10px;
    width: 100%;
    max-width: 280px;
}

/* ============================================
   Buttons
   ============================================ */

.btn {
    font-family: 'Fredoka', 'Microsoft JhengHei', 'PingFang TC', sans-serif;
    font-size: 1.2rem;
    font-weight: 600;
    padding: 14px 36px;
    border: none;
    border-radius: var(--radius-round);
    cursor: pointer;
    transition: all 0.25s var(--ease-spring);
    width: 100%;
    position: relative;
    overflow: hidden;
}

.btn::before {
    content: '';
    position: absolute;
    inset: 0;
    background: linear-gradient(135deg, rgba(255, 255, 255, 0.15), transparent);
    opacity: 0;
    transition: opacity 0.3s;
}

.btn:hover::before {
    opacity: 1;
}

.btn:active {
    transform: scale(0.97);
}

.primary-btn {
    background: linear-gradient(135deg, var(--color-primary), var(--color-secondary));
    color: white;
    box-shadow: var(--shadow-glow-primary);
}

.primary-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 0 30px rgba(255, 107, 157, 0.5);
}

.secondary-btn {
    background: var(--color-glass);
    border: 1.5px solid var(--color-card-border);
    color: var(--color-text);
}

.secondary-btn:hover {
    border-color: var(--color-text-muted);
    transform: translateY(-2px);
}

/* ============================================
   Confetti Canvas
   ============================================ */

#confetti-canvas {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    pointer-events: none;
    z-index: 100;
}

/* ============================================
   Mobile Responsiveness
   ============================================ */

@media (max-width: 600px) {
    #app {
        padding: 6px;
        height: 100vh;
        height: 100dvh;
        max-width: 100%;
        display: flex;
        align-items: stretch;
    }

    .card {
        height: 100%;
        border-radius: var(--radius-lg);
        padding: 20px 16px;
        min-height: auto;
    }

    .logo-icon {
        font-size: 2.5rem;
    }

    .title {
        font-size: 2rem;
    }

    .question {
        font-size: 4rem;
        margin: 6px 0 16px;
    }

    .options-grid {
        gap: 10px;
        flex-grow: 1;
        align-content: center;
    }

    .option-btn {
        padding: 14px;
        font-size: 2.2rem;
        display: flex;
        align-items: center;
        justify-content: center;
    }

    .header {
        margin-bottom: 4px;
    }

    .btn {
        width: 100%;
        max-width: 100%;
        padding: 16px;
        font-size: 1.2rem;
    }

    .result-actions {
        max-width: 100%;
    }

    .stat-value {
        font-size: 1.5rem;
    }

    .diff-btn {
        padding: 14px 16px;
    }
}

/* Very small screens */
@media (max-height: 650px) {
    .logo-icon {
        font-size: 2rem;
        margin-bottom: 2px;
    }

    .title {
        font-size: 1.7rem;
    }

    .subtitle {
        font-size: 0.85rem;
        margin-bottom: 0;
    }

    .logo-area {
        margin-bottom: 0;
    }

    .diff-btn {
        padding: 12px 16px;
    }

    .question {
        font-size: 3.2rem;
    }

    .option-btn {
        padding: 10px;
        font-size: 1.8rem;
    }
}