/* --- 1. Global Styles and Reset --- */
:root {
    --primary-color: #3f51b5; /* Indigo */
    --primary-dark: #303f9f;
    --secondary-color: #ff9800; /* Orange */
    --background-color: #f4f7f6; /* Light Grey/Blue */
    --container-bg: #ffffff;
    --text-color: #333;
    --border-color: #ccc;
    --error-color: #f44336; /* Red */
    --success-color: #4caf50; /* Green */
    --hover-bg: #e0e0e0;
    --cell-bg-fixed: #eee;
    --cell-bg-active: #fff9c4; /* Light yellow for selection */
    --box-border-thickness: 3px;
}

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

body {
    font-family: 'Poppins', sans-serif;
    background-color: var(--background-color);
    color: var(--text-color);
    line-height: 1.6;
    display: flex;
    flex-direction: column; /* Changed to column to accommodate navbar */
    justify-content: flex-start; /* Align content to the top */
    align-items: center;
    min-height: 100vh;
    padding: 0; /* Removed padding from body, added to sections */
}

#app {
    width: 100%;
    max-width: 1000px;
    background-color: var(--container-bg);
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
    border-radius: 12px;
    overflow: hidden;
    padding: 30px;
    margin: 20px; /* Add margin to keep it off the edges */
}

/* --- 1.5. Navigation Bar Styles (UPDATED) --- */
#navbar {
    width: 100%;
    background-color: var(--primary-color);
    padding: 10px 0;
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

.nav-container {
    max-width: 1000px;
    margin: 0 auto;
    padding: 0 20px;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.logo a {
    color: white;
    font-size: 1.8em;
    font-weight: 700;
    text-decoration: none;
    letter-spacing: 0.5px;
}

.nav-links {
    list-style: none;
    display: flex;
    align-items: center;
    gap: 20px;
}

.nav-links a {
    color: white;
    text-decoration: none;
    font-weight: 400;
    padding: 5px 10px;
    border-radius: 4px;
    transition: background-color 0.2s;
}

.nav-links a:hover {
    background-color: var(--primary-dark);
}

/* --- 2. Page Transitions and Layout (Flexbox) --- */
.page {
    display: none;
    padding: 20px 0;
    transition: opacity 0.3s ease-in-out;
}

.page.active {
    display: block;
    opacity: 1;
}

.container {
    max-width: 800px;
    margin: 0 auto;
    display: flex;
    flex-direction: column;
    align-items: center;
    text-align: center;
}

h1, h2 {
    color: var(--primary-dark);
    margin-bottom: 15px;
    font-weight: 700;
}

p.description {
    margin-bottom: 30px;
    color: #666;
    font-size: 1.1em;
}

/* --- 3. Buttons and Inputs --- */
.btn {
    padding: 12px 25px;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    font-size: 1em;
    font-weight: 600;
    transition: background-color 0.3s ease, transform 0.1s ease;
    margin: 8px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.btn-primary {
    background-color: var(--primary-color);
    color: white;
}

.btn-primary:hover {
    background-color: var(--primary-dark);
    transform: translateY(-2px);
}

.btn-secondary {
    background-color: var(--secondary-color);
    color: white;
}

.btn-secondary:hover {
    background-color: #f57c00; /* Darker orange */
    transform: translateY(-2px);
}

.btn-action {
    background-color: var(--success-color);
    color: white;
}

.btn-action:hover {
    background-color: #388e3c;
    transform: translateY(-2px);
}

.btn-link {
    background: none;
    color: var(--primary-color);
    padding: 5px;
    margin-top: 20px;
    font-size: 0.9em;
    text-transform: none;
}

.input-field {
    padding: 8px 15px;
    border: 1px solid var(--border-color);
    border-radius: 6px;
    font-size: 1em;
    color: var(--text-color);
}

.home-actions, .game-buttons {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    margin-bottom: 30px;
}

.difficulty-select {
    margin-top: 20px;
    display: flex;
    align-items: center;
    gap: 10px;
}

/* --- 4. Sudoku Grid Layout (CSS Grid) --- */
.game-container {
    max-width: 600px; /* Max width for the grid area */
}

.game-controls {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: 100%;
    margin-bottom: 20px;
}

.timer-display {
    font-size: 1.5em;
    font-weight: 600;
    color: var(--primary-color);
}

.sudoku-grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    grid-template-rows: repeat(9, 1fr);
    width: 100%;
    aspect-ratio: 1 / 1; /* Ensures a perfect square */
    border: var(--box-border-thickness) solid var(--text-color);
    margin: 0 auto;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.05);
}

/* Cell Styling */
.cell {
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: clamp(1em, 3vw, 1.8em);
    border: 1px solid var(--border-color);
    outline: none;
    text-align: center;
    cursor: pointer;
    transition: background-color 0.1s;
    user-select: none;
}

/* Grid Box Borders (3x3) */
.cell:nth-child(3n) {
    border-right-width: var(--box-border-thickness);
}

.cell:nth-child(27n) {
    border-bottom-width: var(--box-border-thickness);
}

/* Correct border reset for the last column/row */
.cell:nth-child(9n) {
    border-right-width: 1px;
}
.cell:nth-last-child(-n+9) {
    border-bottom-width: 1px;
}

/* Specific 3x3 box borders based on row/column index */
/* (Grid Indexing is easier for 9x9 layout) */
.cell:nth-child(9n+1):nth-child(3n+1) ~ .cell:nth-child(9n+3),
.cell:nth-child(9n+4):nth-child(3n+1) ~ .cell:nth-child(9n+6),
.cell:nth-child(9n+7):nth-child(3n+1) ~ .cell:nth-child(9n+9) {
    border-right-width: var(--box-border-thickness);
}
.cell:nth-child(9n+9) { border-right-width: 1px; }

/* Thick horizontal borders for the 3x3 boxes */
.cell:nth-child(n) {
    /* Thick border for the 3rd, 6th, and 9th row */
    border-bottom-width: 1px;
}
.cell:nth-child(n + 19):nth-child(-n + 27),
.cell:nth-child(n + 46):nth-child(-n + 54) {
    border-bottom-width: var(--box-border-thickness);
}

/* Fixed/Pre-filled Cell Style */
.cell.fixed {
    background-color: var(--cell-bg-fixed);
    color: var(--text-color);
    font-weight: 700;
    cursor: default;
}

/* User Input Cell Style */
.cell:not(.fixed) {
    color: var(--primary-dark);
    font-weight: 600;
}

/* Highlight Styles */
.cell.active-cell {
    background-color: var(--cell-bg-active) !important;
}

.cell.highlight {
    background-color: #e3f2fd; /* Light blue for row/col/box highlight */
}

.cell.error {
    background-color: #ffcdd2; /* Light red for wrong input */
    color: var(--error-color);
}

/* --- 5. How to Play Page --- */
.rules-section {
    width: 100%;
    text-align: left;
    margin-top: 20px;
    max-width: 600px;
}

.rules-section ul {
    list-style: none;
    padding-left: 0;
}

.rules-section li {
    background-color: #f0f4ff;
    padding: 15px;
    margin-bottom: 10px;
    border-left: 4px solid var(--primary-color);
    border-radius: 4px;
}

.rule-title {
    color: var(--primary-dark);
    font-weight: 700;
    display: block;
    margin-bottom: 5px;
}

/* Example Grid for How to Play */
.example-grid-container p {
    margin-bottom: 10px;
}

.example-grid {
    display: grid;
    grid-template-columns: repeat(9, 1fr);
    width: 270px; /* Small example grid */
    height: 270px;
    margin: 20px auto;
    border: 3px solid #333;
}

.example-grid .cell {
    border: 1px solid #999;
    font-size: 1.2em;
    width: 100%;
    height: 100%;
}

.example-grid .fixed-example {
    background-color: #ddd;
    font-weight: bold;
}

.example-grid .highlight-box {
    /* Style for the visual 3x3 rule illustration */
    border: 2px solid var(--error-color) !important;
    background-color: #ffe0b2; /* Light orange */
}

/* --- 6. Mobile Optimization (Media Queries) --- */
@media (max-width: 768px) {
    #app {
        padding: 15px;
        box-shadow: none;
        border-radius: 0;
        margin: 0;
    }

    body {
        padding: 0;
    }

    .nav-container {
        flex-direction: column;
        text-align: center;
    }

    .nav-links {
        margin-top: 10px;
        flex-wrap: wrap;
        justify-content: center;
        gap: 10px;
    }

    .btn {
        width: 100%;
        margin: 5px 0;
    }

    .home-actions, .game-buttons {
        flex-direction: column;
        width: 100%;
    }

    .difficulty-select, .difficulty-options {
        width: 100%;
        justify-content: space-between;
    }

    .timer-display {
        font-size: 1.2em;
    }
}

/* Mobile Keypad (Bonus) */
.mobile-keypad {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 5px;
    width: 100%;
    max-width: 450px;
    margin: 20px auto 10px;
}

.mobile-keypad.hidden {
    display: none;
}

.keypad-btn {
    padding: 15px 0;
    background-color: #f0f0f0;
    border: 1px solid #ddd;
    border-radius: 6px;
    font-size: 1.2em;
    font-weight: 600;
    cursor: pointer;
    transition: background-color 0.1s;
}

.keypad-btn:hover {
    background-color: var(--hover-bg);
}

.clear-btn {
    grid-column: span 5;
    background-color: var(--error-color);
    color: white;
}
.clear-btn:hover {
    background-color: #c62828;
}

@media (min-width: 769px) {
    /* Hide keypad on desktop */
    .mobile-keypad {
        display: none !important;
    }
}
body.dark-theme {
    --background-color: #121212;
    --container-bg: #1e1e1e;
    --text-color: #e0e0e0;
    --border-color: #444;
    --cell-bg-fixed: #333;
    --cell-bg-active: #3d5afe33; /* Soft indigo for dark mode selection */
}

#history-list li {
    display: flex;
    justify-content: space-between;
    padding: 10px;
    border-bottom: 1px solid var(--border-color);
}
.how-to-video {
    width: 100%;
    max-width: 700px;
    margin: 25px auto;
    display: block;
    border-radius: 12px;
    box-shadow: 0 6px 18px rgba(0, 0, 0, 0.15);
}
.home-image {
    margin: 5px 0;
    display: flex;
    justify-content: center;
}

.home-image img {
    width: 100%;
    max-width: 200px;
    border-radius: 12px;
    box-shadow: 0 12px 30px rgba(0, 0, 0, 0.15);
}
.description {
    margin-bottom: 6px;
}
/* ===== GAME FRAME ===== */
.game-frame {
    background: #ffffff;
    border-radius: 18px;
    padding: 26px;
    box-shadow: 0 14px 35px rgba(0,0,0,0.15);
    border: 2px solid #3f51b5;
}
#game-page .container {
    display: flex;
    justify-content: center;
}
/* ===== HOME FRAME – GRADIENT ===== */
.home-frame {
    background: linear-gradient(
        135deg,
        #3f51b5,
        #cb79a2
    );
    color: #ffffff;
    border-radius: 18px;
    padding: 32px;
    box-shadow: 0 14px 35px rgba(63,81,181,0.45);
}
.home-frame p,
.home-frame h1 {
    color: #ffffff;
}
/* --- Score and Hint Styles --- */
.score-display {
    font-size: 1.5em;
    font-weight: 600;
    color: var(--primary-color);
    background: #f0f4ff;
    padding: 8px 15px;
    border-radius: 8px;
    border: 2px solid var(--primary-color);
}

.hint-section {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: 100%;
    margin: 15px 0;
    padding: 10px;
    background: #f9f9f9;
    border-radius: 8px;
    border: 1px solid var(--border-color);
}

.hint-display {
    font-size: 1.2em;
    font-weight: 600;
    color: var(--secondary-color);
}

.btn-hint {
    background-color: #9c27b0;
    color: white;
}

.btn-hint:hover {
    background-color: #7b1fa2;
    transform: translateY(-2px);
}

.btn-hint:disabled {
    background-color: #ccc;
    cursor: not-allowed;
    transform: none;
}

/* Add to game-controls for better layout */
.game-controls {
    display: flex;
    justify-content: space-between;
    align-items: center;
    width: 100%;
    margin-bottom: 15px;
    flex-wrap: wrap;
    gap: 10px;
}

/* Mobile responsive for game controls */
@media (max-width: 768px) {
    .game-controls {
        flex-direction: column;
        align-items: stretch;
    }
    
    .score-display, .hint-section {
        width: 100%;
    }
    
    .hint-section {
        flex-direction: column;
        gap: 10px;
        text-align: center;
    }
}