/* ===== CSS Custom Properties ===== */
:root {
   --bg-primary: #0f1117;
   --bg-secondary: #1a1d27;
   --bg-tertiary: #252836;
   --bg-board: #1e2130;
   --bg-cell: #2a2d3e;
   --bg-cell-hover: #333650;
   --bg-selected: #3b50a9;
   --bg-highlight: #2a3560;
   --bg-matching: #253055;
   --bg-error: #4a1a2e;
   --bg-prefilled: #1e2235;
   --text-primary: #e8eaf0;
   --text-secondary: #9ca3b8;
   --text-muted: #6b7280;
   --text-selected: #ffffff;
   --text-error: #f87171;
   --text-prefilled: #94a3b8;
   --accent: #6366f1;
   --accent-hover: #818cf8;
   --accent-glow: rgba(99, 102, 241, 0.3);
   --border: #2e3148;
   --border-thick: #4a4e6a;
   --success: #34d399;
   --warning: #fbbf24;
   --danger: #f87171;
   --shadow: 0 4px 24px rgba(0, 0, 0, 0.4);
   --radius: 8px;
   --radius-lg: 12px;
   --transition: 0.15s ease;
}

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

body {
   font-family:
      'Segoe UI',
      system-ui,
      -apple-system,
      sans-serif;
   background: var(--bg-primary);
   color: var(--text-primary);
   min-height: 100vh;
   display: flex;
   justify-content: center;
   align-items: flex-start;
   padding: 20px;
   line-height: 1.5;
}

/* ===== App Container ===== */
.app {
   width: 100%;
   max-width: 960px;
   display: flex;
   flex-direction: column;
   gap: 20px;
}

/* ===== Header ===== */
.app-header {
   display: flex;
   justify-content: space-between;
   align-items: center;
   flex-wrap: wrap;
   gap: 16px;
   padding: 0 8px;
}

.app-title {
   font-size: 2rem;
   font-weight: 700;
   background: linear-gradient(135deg, var(--accent), var(--accent-hover));
   -webkit-background-clip: text;
   -webkit-text-fill-color: transparent;
   background-clip: text;
   letter-spacing: -0.5px;
}

.difficulty-selector {
   display: flex;
   gap: 4px;
   background: var(--bg-secondary);
   border-radius: var(--radius);
   padding: 4px;
   border: 1px solid var(--border);
}

.diff-btn {
   padding: 8px 20px;
   border: none;
   background: transparent;
   color: var(--text-secondary);
   font-size: 0.875rem;
   font-weight: 500;
   border-radius: 6px;
   cursor: pointer;
   transition: all var(--transition);
}

.diff-btn:hover {
   color: var(--text-primary);
   background: var(--bg-tertiary);
}

.diff-btn.active {
   background: var(--accent);
   color: var(--text-selected);
   box-shadow: 0 2px 8px var(--accent-glow);
}

/* ===== Game Container ===== */
.game-container {
   display: flex;
   gap: 24px;
   align-items: flex-start;
   justify-content: center;
}

.board-panel {
   display: flex;
   flex-direction: column;
   gap: 16px;
   flex-shrink: 0;
}

.controls-panel {
   display: flex;
   flex-direction: column;
   gap: 20px;
   align-items: center;
   flex-shrink: 0;
}

/* ===== Stats Bar ===== */
.stats-bar {
   display: flex;
   gap: 24px;
   justify-content: center;
   padding: 12px 20px;
   background: var(--bg-secondary);
   border-radius: var(--radius);
   border: 1px solid var(--border);
}

.timer-group {
   display: flex;
   align-items: center;
   gap: 6px;
}

.pause-btn {
   display: inline-flex;
   align-items: center;
   justify-content: center;
   width: 24px;
   height: 24px;
   padding: 0;
   border: none;
   background: transparent;
   color: var(--text-muted);
   border-radius: 4px;
   cursor: pointer;
   transition: all var(--transition);
}

.pause-btn:hover {
   color: var(--text-primary);
   background: var(--bg-tertiary);
}

.pause-btn.paused svg rect {
   fill: var(--accent);
   stroke: var(--accent);
}

.stat {
   display: flex;
   flex-direction: column;
   align-items: center;
   gap: 2px;
}

.stat-label {
   font-size: 0.7rem;
   text-transform: uppercase;
   letter-spacing: 1px;
   color: var(--text-muted);
   font-weight: 600;
}

.stat-value {
   font-size: 1.1rem;
   font-weight: 700;
   color: var(--text-primary);
   font-variant-numeric: tabular-nums;
}

.stat-value.error {
   color: var(--danger);
}

.stat-value.success {
   color: var(--success);
}

/* ===== Board ===== */
.board-wrapper {
   background: var(--bg-secondary);
   border-radius: var(--radius-lg);
   padding: 12px;
   border: 1px solid var(--border);
   box-shadow: var(--shadow);
   position: relative;
   margin: 0 auto;
}

.board-pause-overlay {
   position: absolute;
   inset: 12px;
   z-index: 10;
   display: none;
   align-items: center;
   justify-content: center;
   pointer-events: none;
   background: rgba(15, 17, 23, 0.85);
   backdrop-filter: blur(4px);
   border-radius: var(--radius);
}

.board-pause-overlay.visible {
   display: flex;
}

.board-pause-overlay .overlay-card {
   background: var(--bg-secondary);
   border: 1px solid var(--border);
   box-shadow: var(--shadow);
}

.sudoku-board {
   display: grid;
   grid-template-columns: repeat(9, 1fr);
   gap: 1px;
   background: var(--border);
   border-radius: var(--radius);
   overflow: hidden;
   width: min(480px, calc(100vw - 60px));
   height: min(480px, calc(100vw - 60px));
}

.cell {
   position: relative;
   display: flex;
   justify-content: center;
   align-items: center;
   background: var(--bg-cell);
   cursor: pointer;
   font-size: clamp(1.2rem, 3vw, 1.8rem);
   font-weight: 600;
   color: var(--text-primary);
   transition: background var(--transition);
   user-select: none;
   -webkit-user-select: none;
   aspect-ratio: 1;
}

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

.cell.prefilled {
   color: var(--text-prefilled);
   font-weight: 700;
   background: var(--bg-prefilled);
   cursor: default;
}

.cell.selected {
   background: var(--bg-selected) !important;
   box-shadow: inset 0 0 0 2px var(--accent-hover);
   z-index: 1;
}

.cell.highlighted {
   background: var(--bg-highlight);
}

.cell.matching {
   background: var(--bg-matching);
   color: var(--accent-hover);
}

.cell.error {
   background: var(--bg-error) !important;
   color: var(--text-error) !important;
   animation: shake 0.3s ease;
}

.cell.error .cell-pencils {
   color: var(--text-error) !important;
}

.cell.win-cell {
   animation: popIn 0.3s ease;
}

/* 3x3 sub-grid borders */
.cell.border-right {
   border-right: 2px solid var(--border-thick);
}

.cell.border-bottom {
   border-bottom: 2px solid var(--border-thick);
}

/* Pencil marks */
.cell-pencils {
   position: absolute;
   inset: 0;
   display: grid;
   grid-template-columns: repeat(3, 1fr);
   grid-template-rows: repeat(3, 1fr);
   padding: 2px;
   pointer-events: none;
}

.cell-pencils .pencil-num {
   display: flex;
   justify-content: center;
   align-items: center;
   font-size: clamp(0.45rem, 1.2vw, 0.65rem);
   font-weight: 500;
   color: var(--text-secondary);
   line-height: 1;
}

/* ===== Controls ===== */
.controls {
   display: flex;
   gap: 8px;
   justify-content: center;
   flex-wrap: wrap;
}

.control-btn {
   display: flex;
   align-items: center;
   gap: 6px;
   padding: 10px 16px;
   border: 1px solid var(--border);
   background: var(--bg-secondary);
   color: var(--text-secondary);
   font-size: 0.8rem;
   font-weight: 500;
   border-radius: var(--radius);
   cursor: pointer;
   transition: all var(--transition);
}

.control-btn:hover {
   background: var(--bg-tertiary);
   color: var(--text-primary);
   border-color: var(--accent);
}

.control-btn.active {
   background: var(--accent);
   color: var(--text-selected);
   border-color: var(--accent);
   box-shadow: 0 2px 8px var(--accent-glow);
}

.control-btn svg {
   flex-shrink: 0;
}

/* ===== Numpad ===== */
.numpad {
   display: grid;
   grid-template-columns: repeat(3, 1fr);
   gap: 6px;
   width: min(200px, calc(100vw - 60px));
}

.num-btn {
   aspect-ratio: 1;
   display: flex;
   justify-content: center;
   align-items: center;
   font-size: 1.4rem;
   font-weight: 700;
   color: var(--text-primary);
   background: var(--bg-secondary);
   border: 1px solid var(--border);
   border-radius: var(--radius);
   cursor: pointer;
   transition: all var(--transition);
}

.num-btn:hover {
   background: var(--bg-tertiary);
   border-color: var(--accent);
   color: var(--accent-hover);
}

.num-btn:active {
   transform: scale(0.95);
}

.num-btn.completed {
   opacity: 0.25;
   pointer-events: none;
}

/* ===== Scores Panel ===== */
.scores-panel {
   width: min(200px, calc(100vw - 60px));
   background: var(--bg-secondary);
   border-radius: var(--radius);
   padding: 16px;
   border: 1px solid var(--border);
}

.scores-title {
   font-size: 0.8rem;
   text-transform: uppercase;
   letter-spacing: 1px;
   color: var(--text-muted);
   font-weight: 600;
   margin-bottom: 12px;
   text-align: center;
}

.scores-grid {
   display: flex;
   flex-direction: column;
   gap: 8px;
}

.score-item {
   display: flex;
   justify-content: space-between;
   align-items: center;
   padding: 6px 10px;
   background: var(--bg-tertiary);
   border-radius: 6px;
}

.score-label {
   font-size: 0.8rem;
   color: var(--text-secondary);
   font-weight: 500;
}

.score-value {
   font-size: 0.9rem;
   font-weight: 700;
   color: var(--accent-hover);
   font-variant-numeric: tabular-nums;
}

/* ===== Overlays ===== */
.overlay {
   position: fixed;
   inset: 0;
   background: rgba(0, 0, 0, 0.7);
   backdrop-filter: blur(4px);
   display: none;
   justify-content: center;
   align-items: center;
   z-index: 100;
}

.overlay.visible {
   display: flex;
}

.overlay-card {
   background: var(--bg-secondary);
   border: 1px solid var(--border);
   border-radius: var(--radius-lg);
   padding: 40px 48px;
   text-align: center;
   box-shadow: var(--shadow);
   animation: slideUp 0.3s ease;
}

.overlay-title {
   font-size: 1.8rem;
   font-weight: 700;
   margin-bottom: 8px;
   background: linear-gradient(135deg, var(--accent), var(--accent-hover));
   -webkit-background-clip: text;
   -webkit-text-fill-color: transparent;
   background-clip: text;
}

.overlay-message {
   color: var(--text-secondary);
   margin-bottom: 24px;
   font-size: 1rem;
}

.overlay-btn {
   padding: 12px 32px;
   background: var(--accent);
   color: var(--text-selected);
   border: none;
   border-radius: var(--radius);
   font-size: 1rem;
   font-weight: 600;
   cursor: pointer;
   transition: all var(--transition);
}

.overlay-btn:hover {
   background: var(--accent-hover);
   box-shadow: 0 4px 16px var(--accent-glow);
}

/* ===== Animations ===== */
@keyframes shake {
   0%,
   100% {
      transform: translateX(0);
   }
   20% {
      transform: translateX(-4px);
   }
   40% {
      transform: translateX(4px);
   }
   60% {
      transform: translateX(-3px);
   }
   80% {
      transform: translateX(3px);
   }
}

@keyframes popIn {
   0% {
      transform: scale(0.8);
      opacity: 0.5;
   }
   50% {
      transform: scale(1.05);
   }
   100% {
      transform: scale(1);
      opacity: 1;
   }
}

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

/* ===== Responsive ===== */
@media (max-width: 768px) {
   .game-container {
      flex-direction: column;
      align-items: center;
   }

   .board-panel {
      width: 100%;
      max-width: 480px;
   }

   .controls-panel {
      width: 100%;
      max-width: 480px;
      flex-direction: column;
      align-items: center;
      justify-content: center;
   }

   .numpad {
      width: 100%;
      display: flex;
      flex-direction: row;
      flex-wrap: nowrap;
      justify-content: center;
      gap: 3px;
   }

   .numpad .num-btn {
      flex: 1 1 0;
      height: 36px;
      font-size: 0.95rem;
      aspect-ratio: unset;
   }

   .scores-panel {
      width: 100%;
      margin-top: 8px;
   }

   .sudoku-board {
      width: min(400px, calc(100vw - 24px));
      height: min(400px, calc(100vw - 24px));
   }

   .board-wrapper {
      padding: 8px;
   }

   .app-title {
      font-size: 1.6rem;
   }
}

@media (max-width: 480px) {
   body {
      padding: 10px;
   }

   .app-header {
      flex-direction: column;
      align-items: center;
      gap: 12px;
   }

   .stats-bar {
      gap: 16px;
      padding: 10px 14px;
   }

   .stat-value {
      font-size: 0.95rem;
   }

   .control-btn {
      padding: 8px 12px;
      font-size: 0.75rem;
   }

   .control-btn span {
      display: none;
   }

   .control-btn svg {
      margin: 0;
   }

   .overlay-card {
      padding: 32px 24px;
      margin: 16px;
   }
}

@media (orientation: landscape) and (max-height: 500px) {
   .game-container {
      flex-direction: row;
   }

   .sudoku-board {
      width: min(420px, 70vh);
      height: min(420px, 70vh);
   }

   .app {
      max-width: 1100px;
   }
}

/* ===== Focus styles for accessibility ===== */
.cell:focus-visible {
   outline: 2px solid var(--accent-hover);
   outline-offset: -2px;
   z-index: 2;
}

.num-btn:focus-visible,
.control-btn:focus-visible,
.diff-btn:focus-visible,
.overlay-btn:focus-visible {
   outline: 2px solid var(--accent-hover);
   outline-offset: 2px;
}

/* ===== Scrollbar ===== */
::-webkit-scrollbar {
   width: 8px;
}

::-webkit-scrollbar-track {
   background: var(--bg-primary);
}

::-webkit-scrollbar-thumb {
   background: var(--bg-tertiary);
   border-radius: 4px;
}

::-webkit-scrollbar-thumb:hover {
   background: var(--border);
}