/* =============================================
   DESIGN TOKENS
   ============================================= */
:root {
  --bg-primary: #0D1117;
  --bg-secondary: #161B22;
  --bg-tertiary: #21262D;
  --text-primary: #F0F6FC;
  --text-secondary: #8B949E;
  --text-muted: #484F58;
  --accent: #3B82F6;
  --border: #30363D;
  --btn-bg: #21262D;
  --btn-active: #30363D;
  --overlay-bg: rgba(0, 0, 0, 0.85);
  --board-bg: #0A0E14;
}

@media (prefers-color-scheme: light) {
  :root {
    --bg-primary: #FFFFFF;
    --bg-secondary: #F6F8FA;
    --bg-tertiary: #E9ECF0;
    --text-primary: #1B1E28;
    --text-secondary: #57606A;
    --text-muted: #8C959F;
    --accent: #3182F6;
    --border: #D0D7DE;
    --btn-bg: #E9ECF0;
    --btn-active: #D0D7DE;
    --overlay-bg: rgba(255, 255, 255, 0.9);
    --board-bg: #F0F2F5;
  }
}

/* =============================================
   RESET & BASE
   ============================================= */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  -webkit-tap-highlight-color: transparent;
}

html {
  height: 100%;
  font-size: 16px;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Pretendard', 'Segoe UI', Roboto, sans-serif;
  background: var(--bg-primary);
  color: var(--text-primary);
  min-height: 100dvh;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  position: relative;
}

/* =============================================
   SCREENS
   ============================================= */
.screen {
  display: none;
  position: fixed;
  inset: 0;
  width: 100%;
  height: 100dvh;
  align-items: center;
  justify-content: center;
  background: var(--bg-primary);
  overflow-y: auto;
}

.screen.active {
  display: flex;
}

/* =============================================
   START SCREEN
   ============================================= */
.screen-content {
  width: 100%;
  max-width: 480px;
  padding: 48px 24px 40px;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 28px;
  min-height: 100dvh;
  justify-content: center;
}

.title-gradient {
  font-size: clamp(2.8rem, 10vw, 3.8rem);
  font-weight: 800;
  letter-spacing: 0.15em;
  background: linear-gradient(135deg, var(--accent) 0%, #60A5FA 40%, #A78BFA 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  text-align: center;
  line-height: 1.1;
}

.subtitle {
  font-size: 0.9rem;
  color: var(--text-secondary);
  letter-spacing: 0.05em;
  margin-top: -16px;
}

.section-label {
  font-size: 0.75rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  text-align: center;
  margin-bottom: 12px;
}

.difficulty-select {
  width: 100%;
}

.diff-buttons {
  display: flex;
  gap: 8px;
  width: 100%;
}

.diff-btn {
  flex: 1;
  padding: 12px 8px;
  background: var(--btn-bg);
  border: 1.5px solid var(--border);
  border-radius: 10px;
  color: var(--text-secondary);
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.15s ease;
  font-family: inherit;
}

.diff-btn:hover {
  border-color: var(--accent);
  color: var(--text-primary);
}

.diff-btn.selected {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
  box-shadow: 0 0 16px rgba(59, 130, 246, 0.35);
}

/* Best Times */
.best-times {
  width: 100%;
  background: var(--bg-secondary);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 16px;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.best-times-title {
  font-size: 0.72rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: 4px;
}

.best-time-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 0.88rem;
}

.best-time-row .label {
  color: var(--text-secondary);
}

.best-time-row .value {
  color: var(--text-primary);
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

.best-time-row .value.new-record {
  color: var(--accent);
}

/* Primary / Secondary buttons */
.btn-primary {
  width: 100%;
  padding: 15px;
  background: var(--accent);
  border: none;
  border-radius: 12px;
  color: #fff;
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  transition: opacity 0.15s, transform 0.1s;
  font-family: inherit;
  letter-spacing: 0.03em;
}

.btn-primary:hover {
  opacity: 0.9;
}

.btn-primary:active {
  transform: scale(0.98);
}

.btn-secondary {
  flex: 1;
  padding: 13px;
  background: var(--btn-bg);
  border: 1.5px solid var(--border);
  border-radius: 12px;
  color: var(--text-primary);
  font-size: 0.95rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.15s;
  font-family: inherit;
}

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

/* =============================================
   GAME SCREEN
   ============================================= */
#screen-game {
  padding: 0;
}

.game-wrapper {
  width: 100%;
  max-width: 480px;
  height: 100dvh;
  display: flex;
  flex-direction: column;
  padding: 12px 12px 16px;
  gap: 12px;
}

/* Header */
.game-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 4px 0;
  flex-shrink: 0;
}

.header-center {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 4px;
}

.diff-badge {
  font-size: 0.7rem;
  font-weight: 600;
  color: var(--accent);
  background: rgba(59, 130, 246, 0.12);
  padding: 2px 10px;
  border-radius: 20px;
  letter-spacing: 0.06em;
  border: 1px solid rgba(59, 130, 246, 0.25);
}

.timer-display {
  font-size: 1.5rem;
  font-weight: 700;
  font-variant-numeric: tabular-nums;
  letter-spacing: 0.05em;
  color: var(--text-primary);
  line-height: 1;
}

.icon-btn {
  width: 40px;
  height: 40px;
  background: var(--btn-bg);
  border: 1.5px solid var(--border);
  border-radius: 10px;
  color: var(--text-secondary);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all 0.15s;
}

.icon-btn:hover {
  color: var(--text-primary);
  border-color: var(--text-secondary);
}

/* =============================================
   SUDOKU BOARD
   ============================================= */
.board-container {
  flex: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 0;
}

.sudoku-board {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  grid-template-rows: repeat(9, 1fr);
  background: var(--board-bg);
  border: 2.5px solid var(--text-secondary);
  border-radius: 8px;
  overflow: hidden;
  aspect-ratio: 1;
  width: min(100%, calc(100dvh - 240px));
  max-width: 100%;
  box-shadow: 0 4px 24px rgba(0, 0, 0, 0.3);
}

/* Cell base */
.cell {
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  position: relative;
  background: var(--board-bg);
  border-right: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
  transition: background 0.1s;
  user-select: none;
  -webkit-user-select: none;
}

.cell:nth-child(9n) {
  border-right: none;
}

/* Remove bottom border for last row */
.cell:nth-child(n+73) {
  border-bottom: none;
}

/* Thick borders for 3x3 boxes — right side */
.cell:nth-child(3n):not(:nth-child(9n)) {
  border-right: 2px solid var(--text-secondary);
}

/* Thick borders for 3x3 boxes — bottom side (rows 3 and 6) */
.cell:nth-child(n+19):nth-child(-n+27),
.cell:nth-child(n+46):nth-child(-n+54) {
  border-bottom: 2px solid var(--text-secondary);
}

/* Cell number */
.cell-value {
  font-size: clamp(13px, 4vw, 22px);
  font-weight: 600;
  line-height: 1;
  pointer-events: none;
  color: var(--text-primary);
  z-index: 1;
}

.cell.given .cell-value {
  color: var(--text-primary);
  font-weight: 700;
}

.cell.user-filled .cell-value {
  color: var(--accent);
}

/* Notes grid inside cell */
.cell-notes {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: repeat(3, 1fr);
  width: 100%;
  height: 100%;
  padding: 1px;
  pointer-events: none;
  z-index: 1;
}

.note-digit {
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: clamp(6px, 1.5vw, 9px);
  color: var(--text-secondary);
  font-weight: 500;
  line-height: 1;
}

.note-digit.visible {
  color: var(--accent);
  opacity: 0.85;
}

/* Highlight states */
.cell.highlight-region {
  background: var(--bg-secondary);
}

.cell.highlight-same {
  background: rgba(59, 130, 246, 0.15);
}

.cell.selected {
  background: rgba(59, 130, 246, 0.28) !important;
}

.cell.error .cell-value {
  color: #F87171 !important;
}

.cell.error {
  background: rgba(248, 113, 113, 0.1);
}

/* Complete animation */
@keyframes cellComplete {
  0%   { transform: scale(1); }
  40%  { transform: scale(1.15); }
  100% { transform: scale(1); }
}

.cell.complete-anim {
  animation: cellComplete 0.35s ease forwards;
}

/* =============================================
   CONTROLS
   ============================================= */
.controls {
  display: flex;
  flex-direction: column;
  gap: 10px;
  flex-shrink: 0;
}

.action-row {
  display: flex;
  gap: 8px;
}

.action-btn {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 5px;
  padding: 10px 6px;
  background: var(--btn-bg);
  border: 1.5px solid var(--border);
  border-radius: 10px;
  color: var(--text-secondary);
  font-size: 0.7rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.15s;
  font-family: inherit;
  letter-spacing: 0.04em;
}

.action-btn:hover {
  color: var(--text-primary);
  border-color: var(--text-secondary);
}

.action-btn.active {
  background: var(--btn-active);
  border-color: var(--accent);
  color: var(--accent);
}

/* Number Pad */
.numpad {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  gap: 5px;
}

.num-btn {
  aspect-ratio: 1;
  background: var(--btn-bg);
  border: 1.5px solid var(--border);
  border-radius: 8px;
  color: var(--text-primary);
  font-size: clamp(16px, 4vw, 22px);
  font-weight: 700;
  cursor: pointer;
  transition: all 0.12s;
  font-family: inherit;
  display: flex;
  align-items: center;
  justify-content: center;
}

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

.num-btn:active {
  transform: scale(0.93);
}

.num-btn.complete {
  opacity: 0.35;
  cursor: default;
  pointer-events: none;
}

/* =============================================
   OVERLAYS
   ============================================= */
.overlay {
  position: fixed;
  inset: 0;
  z-index: 100;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 24px;
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  background: var(--overlay-bg);
  animation: overlayIn 0.25s ease;
}

.overlay.hidden {
  display: none;
}

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

.overlay-card {
  width: 100%;
  max-width: 360px;
  background: var(--bg-secondary);
  border: 1px solid var(--border);
  border-radius: 20px;
  padding: 32px 28px;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 16px;
  box-shadow: 0 20px 60px rgba(0, 0, 0, 0.4);
  animation: cardIn 0.3s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.overlay-card.small {
  max-width: 300px;
  padding: 28px 24px;
}

@keyframes cardIn {
  from { transform: scale(0.85) translateY(20px); opacity: 0; }
  to   { transform: scale(1) translateY(0); opacity: 1; }
}

.victory-icon {
  font-size: 3.5rem;
  line-height: 1;
}

.overlay-title {
  font-size: 1.7rem;
  font-weight: 800;
  color: var(--text-primary);
}

.overlay-sub {
  font-size: 0.9rem;
  color: var(--text-secondary);
  text-align: center;
}

.result-stats {
  width: 100%;
  display: flex;
  flex-direction: column;
  gap: 8px;
  background: var(--bg-tertiary);
  border-radius: 12px;
  padding: 16px;
}

.stat-item {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.stat-label {
  font-size: 0.82rem;
  color: var(--text-secondary);
}

.stat-value {
  font-size: 0.95rem;
  font-weight: 700;
  color: var(--text-primary);
  font-variant-numeric: tabular-nums;
}

.stat-value.new-record {
  color: var(--accent);
}

.overlay-buttons {
  display: flex;
  gap: 10px;
  width: 100%;
  margin-top: 4px;
}

/* =============================================
   RESPONSIVE — very small screens
   ============================================= */
@media (max-height: 650px) {
  .game-wrapper {
    gap: 8px;
    padding: 8px 10px 12px;
  }

  .timer-display {
    font-size: 1.25rem;
  }

  .action-btn {
    padding: 8px 4px;
    font-size: 0.65rem;
  }

  .numpad {
    gap: 3px;
  }
}

@media (max-width: 360px) {
  .game-wrapper {
    padding: 8px 8px 12px;
  }
}