/* ============================================
   CSS Variables & Theme
   ============================================ */

:root {
    /* Colors */
    --color-primary: #3b82f6;
    --color-primary-dark: #2563eb;
    --color-primary-light: #60a5fa;

    --color-easy: #10b981;
    --color-medium: #f59e0b;
    --color-hard: #ef4444;

    --color-text-primary: #1f2937;
    --color-text-secondary: #6b7280;
    --color-text-tertiary: #9ca3af;

    --color-bg-primary: #ffffff;
    --color-bg-secondary: #f9fafb;
    --color-bg-tertiary: #f3f4f6;

    --color-border: #e5e7eb;
    --color-border-dark: #d1d5db;

    /* Typography */
    --font-display: 'Outfit', system-ui, -apple-system, sans-serif;
    --font-body: 'DM Sans', system-ui, -apple-system, sans-serif;

    --font-size-xs: 0.75rem;
    --font-size-sm: 0.875rem;
    --font-size-base: 1rem;
    --font-size-lg: 1.125rem;
    --font-size-xl: 1.25rem;
    --font-size-2xl: 1.5rem;
    --font-size-3xl: 2rem;

    /* Spacing */
    --spacing-1: 0.25rem;
    --spacing-2: 0.5rem;
    --spacing-3: 0.75rem;
    --spacing-4: 1rem;
    --spacing-5: 1.25rem;
    --spacing-6: 1.5rem;
    --spacing-8: 2rem;
    --spacing-10: 2.5rem;
    --spacing-12: 3rem;
    --spacing-16: 4rem;

    /* Shadows */
    --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
    --shadow-base: 0 1px 3px 0 rgba(0, 0, 0, 0.1), 0 1px 2px -1px rgba(0, 0, 0, 0.1);
    --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -2px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -4px rgba(0, 0, 0, 0.1);

    /* Border Radius */
    --radius-sm: 0.375rem;
    --radius-base: 0.5rem;
    --radius-lg: 0.75rem;
    --radius-full: 9999px;

    /* Transitions */
    --transition-fast: 150ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-base: 200ms cubic-bezier(0.4, 0, 0.2, 1);
    --transition-slow: 300ms cubic-bezier(0.4, 0, 0.2, 1);
}

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

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

html {
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

body {
    font-family: var(--font-body);
    font-size: var(--font-size-base);
    line-height: 1.6;
    color: var(--color-text-primary);
    background-color: var(--color-bg-secondary);
}

/* ============================================
   Layout
   ============================================ */

.container {
    width: 100%;
    max-width: 1400px;
    margin: 0 auto;
    padding-left: var(--spacing-4);
    padding-right: var(--spacing-4);
}

/* ============================================
   Header
   ============================================ */

.header {
    background-color: var(--color-bg-primary);
    border-bottom: 1px solid var(--color-border);
    position: sticky;
    top: 0;
    z-index: 100;
    backdrop-filter: blur(10px);
    background-color: rgba(255, 255, 255, 0.95);
}

.header-content {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: var(--spacing-6);
    padding: var(--spacing-5) 0;
}

.logo {
    font-family: var(--font-display);
    font-size: var(--font-size-2xl);
    font-weight: 700;
    color: var(--color-text-primary);
    letter-spacing: -0.02em;
}

.search-wrapper {
    position: relative;
    flex: 1;
    max-width: 500px;
}

.search-icon {
    position: absolute;
    left: var(--spacing-4);
    top: 50%;
    transform: translateY(-50%);
    color: var(--color-text-tertiary);
    pointer-events: none;
}

.search-input {
    width: 100%;
    padding: var(--spacing-3) var(--spacing-4) var(--spacing-3) var(--spacing-10);
    font-family: var(--font-body);
    font-size: var(--font-size-base);
    color: var(--color-text-primary);
    background-color: var(--color-bg-secondary);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-lg);
    transition: all var(--transition-base);
    outline: none;
}

.search-input:focus {
    background-color: var(--color-bg-primary);
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
}

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

/* ============================================
   Main Content
   ============================================ */

.main {
    padding: var(--spacing-8) 0;
}

/* ============================================
   Filter Section
   ============================================ */

.filter-section {
    margin-bottom: var(--spacing-6);
}

.filters {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: var(--spacing-4);
    margin-bottom: var(--spacing-4);
}

.filter-group {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-2);
}

.filter-label {
    font-size: var(--font-size-xs);
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--color-text-secondary);
}

.filter-select {
    padding: var(--spacing-3) var(--spacing-4);
    font-family: var(--font-body);
    font-size: var(--font-size-sm);
    color: var(--color-text-primary);
    background-color: var(--color-bg-primary);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-base);
    cursor: pointer;
    transition: all var(--transition-base);
    outline: none;
    appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg width='12' height='8' viewBox='0 0 12 8' fill='none' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath d='M1 1.5L6 6.5L11 1.5' stroke='%236b7280' stroke-width='1.5' stroke-linecap='round' stroke-linejoin='round'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right var(--spacing-4) center;
    padding-right: var(--spacing-10);
}

.filter-select:hover {
    border-color: var(--color-border-dark);
}

.filter-select:focus {
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
}

/* Active Filters */
.active-filters {
    display: flex;
    flex-wrap: wrap;
    gap: var(--spacing-2);
    min-height: var(--spacing-8);
    align-items: center;
}

.filter-chip {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-2);
    padding: var(--spacing-2) var(--spacing-3);
    font-size: var(--font-size-sm);
    font-weight: 500;
    color: var(--color-primary);
    background-color: rgba(59, 130, 246, 0.1);
    border: 1px solid rgba(59, 130, 246, 0.2);
    border-radius: var(--radius-full);
    transition: all var(--transition-base);
}

.filter-chip-remove {
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0;
    background: none;
    border: none;
    color: var(--color-primary);
    cursor: pointer;
    opacity: 0.7;
    transition: opacity var(--transition-fast);
}

.filter-chip-remove:hover {
    opacity: 1;
}

.clear-all-btn {
    padding: var(--spacing-2) var(--spacing-4);
    font-family: var(--font-body);
    font-size: var(--font-size-sm);
    font-weight: 600;
    color: var(--color-text-secondary);
    background-color: transparent;
    border: 1px solid var(--color-border);
    border-radius: var(--radius-full);
    cursor: pointer;
    transition: all var(--transition-base);
}

.clear-all-btn:hover {
    color: var(--color-text-primary);
    border-color: var(--color-border-dark);
    background-color: var(--color-bg-tertiary);
}

/* ============================================
   Stats Section
   ============================================ */

.stats-section {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: var(--spacing-4);
    margin-bottom: var(--spacing-6);
    padding-bottom: var(--spacing-4);
    border-bottom: 1px solid var(--color-border);
}

.stats-text {
    font-size: var(--font-size-sm);
    font-weight: 500;
    color: var(--color-text-secondary);
    letter-spacing: 0.02em;
}

/* View Toggle */
.view-toggle {
    display: flex;
    gap: var(--spacing-1);
    background-color: var(--color-bg-tertiary);
    padding: var(--spacing-1);
    border-radius: var(--radius-base);
}

.view-toggle-btn {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 36px;
    height: 36px;
    padding: 0;
    background-color: transparent;
    border: none;
    border-radius: var(--radius-sm);
    color: var(--color-text-tertiary);
    cursor: pointer;
    transition: all var(--transition-base);
}

.view-toggle-btn:hover {
    color: var(--color-text-primary);
    background-color: rgba(0, 0, 0, 0.05);
}

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

/* ============================================
   Problem Grid
   ============================================ */

.problem-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: var(--spacing-5);
}

/* List View */
.problem-grid.list-view {
    grid-template-columns: 1fr;
    gap: var(--spacing-3);
}

/* Loading State */
.loading-state {
    grid-column: 1 / -1;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: var(--spacing-16) var(--spacing-4);
    gap: var(--spacing-4);
}

.loading-spinner {
    width: 40px;
    height: 40px;
    border: 3px solid var(--color-bg-tertiary);
    border-top-color: var(--color-primary);
    border-radius: 50%;
    animation: spin 1s linear infinite;
}

@keyframes spin {
    to { transform: rotate(360deg); }
}

.loading-state p {
    color: var(--color-text-tertiary);
    font-size: var(--font-size-sm);
}

/* Empty State */
.empty-state {
    grid-column: 1 / -1;
    text-align: center;
    padding: var(--spacing-16) var(--spacing-4);
}

.empty-state-icon {
    width: 64px;
    height: 64px;
    margin: 0 auto var(--spacing-4);
    color: var(--color-text-tertiary);
}

.empty-state h3 {
    font-family: var(--font-display);
    font-size: var(--font-size-xl);
    font-weight: 600;
    color: var(--color-text-primary);
    margin-bottom: var(--spacing-2);
}

.empty-state p {
    font-size: var(--font-size-base);
    color: var(--color-text-secondary);
}

/* ============================================
   Problem Card
   ============================================ */

.problem-card {
    display: flex;
    flex-direction: column;
    background-color: var(--color-bg-primary);
    border: 1px solid var(--color-border);
    border-radius: var(--radius-lg);
    padding: var(--spacing-5);
    transition: all var(--transition-base);
    position: relative;
    overflow: hidden;
}

.problem-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 3px;
    background: linear-gradient(90deg, var(--color-primary), var(--color-primary-light));
    opacity: 0;
    transition: opacity var(--transition-base);
}

.problem-card:hover {
    border-color: var(--color-border-dark);
    box-shadow: var(--shadow-lg);
    transform: translateY(-2px);
}

.problem-card:hover::before {
    opacity: 1;
}

/* List View Card Layout */
.list-view .problem-card {
    flex-direction: row;
    align-items: center;
    gap: var(--spacing-5);
    padding: var(--spacing-4);
}

.list-view .problem-card:hover {
    transform: translateX(4px);
}

.list-view .problem-header {
    flex: 1;
    margin-bottom: 0;
}

.list-view .problem-title {
    font-size: var(--font-size-base);
    margin-bottom: var(--spacing-2);
    -webkit-line-clamp: 1;
}

.list-view .problem-body {
    flex-shrink: 0;
    margin-bottom: 0;
    min-width: 150px;
}

.list-view .problem-footer {
    flex-shrink: 0;
    border-top: none;
    padding-top: 0;
    gap: var(--spacing-4);
}

.list-view .platform-tag {
    min-width: 80px;
}

.problem-header {
    margin-bottom: var(--spacing-4);
}

.problem-title {
    font-family: var(--font-display);
    font-size: var(--font-size-lg);
    font-weight: 600;
    color: var(--color-text-primary);
    line-height: 1.4;
    margin-bottom: var(--spacing-3);
    display: -webkit-box;
    -webkit-line-clamp: 2;
    -webkit-box-orient: vertical;
    overflow: hidden;
}

.problem-meta {
    display: flex;
    align-items: center;
    gap: var(--spacing-2);
    flex-wrap: wrap;
}

.difficulty-badge {
    display: inline-flex;
    align-items: center;
    padding: var(--spacing-1) var(--spacing-3);
    font-size: var(--font-size-xs);
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    border-radius: var(--radius-sm);
}

.difficulty-badge.easy {
    color: var(--color-easy);
    background-color: rgba(16, 185, 129, 0.1);
}

.difficulty-badge.medium {
    color: var(--color-medium);
    background-color: rgba(245, 158, 11, 0.1);
}

.difficulty-badge.hard {
    color: var(--color-hard);
    background-color: rgba(239, 68, 68, 0.1);
}

.difficulty-badge.unknown {
    color: var(--color-text-tertiary);
    background-color: var(--color-bg-tertiary);
}

.problem-topic {
    font-size: var(--font-size-sm);
    color: var(--color-text-secondary);
    font-weight: 500;
}

.problem-body {
    flex: 1;
    margin-bottom: var(--spacing-4);
}

.problem-pattern {
    display: inline-flex;
    align-items: center;
    padding: var(--spacing-2) var(--spacing-3);
    font-size: var(--font-size-xs);
    font-weight: 500;
    color: var(--color-text-secondary);
    background-color: var(--color-bg-secondary);
    border-radius: var(--radius-sm);
    border: 1px solid var(--color-border);
}

.problem-footer {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding-top: var(--spacing-4);
    border-top: 1px solid var(--color-border);
}

.platform-tag {
    font-size: var(--font-size-xs);
    font-weight: 500;
    color: var(--color-text-tertiary);
}

.problem-link {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-2);
    padding: var(--spacing-2) var(--spacing-4);
    font-size: var(--font-size-sm);
    font-weight: 600;
    color: var(--color-primary);
    background-color: rgba(59, 130, 246, 0.05);
    border: 1px solid rgba(59, 130, 246, 0.2);
    border-radius: var(--radius-base);
    text-decoration: none;
    transition: all var(--transition-base);
}

.problem-link:hover {
    color: var(--color-primary-dark);
    background-color: rgba(59, 130, 246, 0.1);
    border-color: rgba(59, 130, 246, 0.3);
    transform: translateX(2px);
}

.problem-link svg {
    width: 14px;
    height: 14px;
}

/* ============================================
   Responsive Design
   ============================================ */

@media (max-width: 640px) {
    .header-content {
        flex-direction: column;
        gap: var(--spacing-4);
    }

    .search-wrapper {
        max-width: 100%;
    }

    .filters {
        grid-template-columns: 1fr;
    }

    .problem-grid {
        grid-template-columns: 1fr;
    }

    /* Stack list view on mobile for better readability */
    .list-view .problem-card {
        flex-direction: column;
        align-items: flex-start;
    }

    .list-view .problem-header {
        width: 100%;
    }

    .list-view .problem-body {
        width: 100%;
        min-width: auto;
    }

    .list-view .problem-footer {
        width: 100%;
    }
}

@media (min-width: 641px) and (max-width: 1024px) {
    .problem-grid {
        grid-template-columns: repeat(2, 1fr);
    }
}

@media (min-width: 1025px) {
    .container {
        padding-left: var(--spacing-6);
        padding-right: var(--spacing-6);
    }

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

@media (min-width: 1400px) {
    .problem-grid {
        grid-template-columns: repeat(4, 1fr);
    }
}