/* ==========================================================================
   Algorithm Lab — core styles
   ========================================================================== */

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  color-scheme: dark;
}

body {
  margin: 0;
  font-family: var(--font-sans);
  font-size: var(--fs-base);
  background: var(--bg-app);
  color: var(--text-primary);
  -webkit-font-smoothing: antialiased;
  /* Full-height column so the footer sits at the bottom of the screen on
     short pages (landing, contact) instead of right under the content. */
  display: flex;
  flex-direction: column;
  min-height: 100vh;
  min-height: 100dvh;
  transition: background var(--dur-med) var(--ease-standard), color var(--dur-med) var(--ease-standard);
}

h1, h2, h3, h4, p {
  margin: 0;
}

button {
  font-family: inherit;
}

a {
  color: inherit;
}

/* Focus states — visible everywhere, keyboard-first */
:focus-visible {
  outline: 2px solid var(--accent-primary);
  outline-offset: 2px;
  border-radius: var(--radius-sm);
}

.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

/* ---------------------------------------------------------------------- */
/* App shell                                                              */
/* ---------------------------------------------------------------------- */

.app-header {
  position: sticky;
  top: 0;
  z-index: 20;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-4);
  padding: var(--space-3) var(--space-5);
  background: var(--bg-surface);
  border-bottom: var(--border-width) solid var(--border-subtle);
}

.app-header__brand {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-weight: 700;
  font-size: var(--fs-md);
  letter-spacing: -0.01em;
}

.app-header__brand .brand-mark {
  display: inline-flex;
  width: 28px;
  height: 28px;
  border-radius: var(--radius-sm);
  background: linear-gradient(135deg, var(--color-current), var(--color-special));
}

.app-header__tagline {
  font-weight: 400;
  font-size: var(--fs-xs);
  color: var(--text-muted);
  letter-spacing: normal;
  padding-left: var(--space-3);
  border-left: var(--border-width) solid var(--border-subtle);
  margin-left: var(--space-1);
}

.app-header__nav {
  display: flex;
  gap: var(--space-1);
  background: var(--bg-inset);
  padding: var(--space-1);
  border-radius: var(--radius-md);
  border: var(--border-width) solid var(--border-subtle);
}

.app-header__nav button {
  border: none;
  background: transparent;
  color: var(--text-secondary);
  padding: var(--space-2) var(--space-4);
  border-radius: var(--radius-sm);
  font-size: var(--fs-sm);
  font-weight: 600;
  cursor: pointer;
  transition: background var(--dur-fast) var(--ease-standard), color var(--dur-fast) var(--ease-standard);
}

.app-header__nav button:hover {
  color: var(--text-primary);
}

.app-header__nav button.is-active {
  background: var(--bg-elevated);
  color: var(--text-primary);
  box-shadow: var(--shadow-card);
}

.app-header__right {
  display: flex;
  align-items: center;
  gap: var(--space-4);
}

.progress-pill {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-size: var(--fs-sm);
  color: var(--text-secondary);
  white-space: nowrap;
}

.progress-pill strong {
  color: var(--text-primary);
}

.progress-pill__track {
  width: 96px;
  height: 6px;
  border-radius: 999px;
  background: var(--bg-inset);
  overflow: hidden;
  border: var(--border-width) solid var(--border-subtle);
}

.progress-pill__fill {
  height: 100%;
  background: linear-gradient(90deg, var(--color-current), var(--color-visited));
  transition: width var(--dur-med) var(--ease-standard);
}

.theme-toggle {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: var(--radius-md);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-elevated);
  color: var(--text-primary);
  cursor: pointer;
  font-size: var(--fs-base);
}

.theme-toggle:hover {
  background: var(--bg-elevated-hover);
}

.code-lang-switch {
  display: inline-flex;
  margin-left: auto;
  margin-right: var(--space-2);
  padding: 2px;
  border-radius: var(--radius-sm);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-inset);
}

.code-lang-switch__btn {
  white-space: nowrap;
  padding: 1px var(--space-2);
  border: none;
  border-radius: 4px;
  background: none;
  color: var(--text-muted);
  cursor: pointer;
  font-size: var(--fs-xs);
  font-weight: 600;
}

.code-lang-switch__btn:hover {
  color: var(--text-primary);
}

.code-lang-switch__btn[aria-pressed="true"] {
  background: var(--accent-primary);
  color: var(--on-accent);
}

.code-copy-btn {
  white-space: nowrap;
  display: inline-flex;
  align-items: center;
  gap: var(--space-1);
  padding: 2px var(--space-2);
  border-radius: var(--radius-sm);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-elevated);
  color: var(--text-secondary);
  cursor: pointer;
  font-size: var(--fs-xs);
  font-weight: 600;
}

.code-copy-btn:hover {
  background: var(--bg-elevated-hover);
  color: var(--text-primary);
}

.app-main {
  flex: 1 0 auto;
  width: 100%;
  max-width: 1180px;
  margin: 0 auto;
  padding: var(--space-6) var(--space-5) var(--space-7);
}

.app-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: var(--space-2) var(--space-5);
  /* Line the footer text up with the 1180px .app-main column on wide screens. */
  padding: var(--space-4) max(var(--space-5), calc((100% - 1180px) / 2 + var(--space-5)));
  padding-bottom: calc(var(--space-4) + env(safe-area-inset-bottom, 0px));
  border-top: var(--border-width) solid var(--border-subtle);
  color: var(--text-muted);
  font-size: var(--fs-xs);
  line-height: 1.6;
}

.app-footer__tagline {
  min-width: 0;
}

.app-footer__meta {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  white-space: nowrap;
}

/* Short disclaimer at the end of every article (articleNotice.js). */
.article-notice {
  margin: var(--space-6) 0 0;
  padding-top: var(--space-3);
  border-top: var(--border-width) solid var(--border-subtle);
  color: var(--text-muted);
  font-size: var(--fs-xs);
  line-height: 1.6;
}

/* Topic pages are narrower than algorithm labs; line up with the article. */
.article-updated {
  color: var(--text-muted);
  font-size: var(--fs-xs);
  white-space: nowrap;
}

.topic-page + .article-notice {
  max-width: 920px;
}

.article-notice a {
  color: inherit;
  text-decoration: underline;
}

.article-notice a:hover {
  color: var(--text-primary);
}

.app-footer__dot {
  color: var(--border-subtle);
}

.app-footer__link {
  border: none;
  background: none;
  padding: 0;
  color: var(--text-muted);
  font-size: inherit;
  text-decoration: underline;
  cursor: pointer;
}

.app-footer__link:hover {
  color: var(--text-primary);
}

.policy-content {
  padding: var(--space-5);
  color: var(--text-secondary);
  line-height: 1.7;
}

.policy-content h2 {
  color: var(--text-primary);
  font-size: var(--fs-lg);
  margin: var(--space-6) 0 var(--space-3);
}

.policy-content h2:first-child {
  margin-top: 0;
}

.policy-content h3 {
  color: var(--text-primary);
  font-size: var(--fs-md);
  margin: var(--space-4) 0 var(--space-2);
}

.policy-content p {
  margin: 0 0 var(--space-3);
}

.policy-summary {
  padding: var(--space-3) var(--space-4);
  border-left: 3px solid var(--color-attention);
  border-radius: var(--radius-sm);
  background: var(--color-attention-bg);
  color: var(--text-primary);
}

.policy-content ul {
  margin: 0 0 var(--space-3);
  padding-left: var(--space-5);
}

.policy-content li {
  margin-bottom: var(--space-1);
}

.policy-credit {
  padding: var(--space-2) var(--space-4);
  border-left: 3px solid var(--border-subtle);
  color: var(--text-primary);
}

.policy-content code {
  font-family: var(--font-mono);
  font-size: 0.9em;
  background: var(--bg-inset);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-sm);
  padding: 1px 5px;
}

/* ---------------------------------------------------------------------- */
/* Buttons                                                                */
/* ---------------------------------------------------------------------- */

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  padding: var(--space-2) var(--space-4);
  border-radius: var(--radius-md);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-elevated);
  color: var(--text-primary);
  font-size: var(--fs-sm);
  font-weight: 600;
  cursor: pointer;
  transition: background var(--dur-fast) var(--ease-standard), border-color var(--dur-fast) var(--ease-standard), transform var(--dur-fast) var(--ease-standard);
}

.btn:hover:not(:disabled) {
  background: var(--bg-elevated-hover);
  border-color: var(--border-strong);
}

.btn:active:not(:disabled) {
  transform: translateY(1px);
}

.btn:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

.btn-primary {
  background: var(--accent-primary);
  border-color: var(--accent-primary);
  color: var(--on-accent);
}

.btn-primary:hover:not(:disabled) {
  background: var(--accent-primary-strong);
  border-color: var(--accent-primary-strong);
}

.btn-icon {
  width: 40px;
  padding: var(--space-2);
}

/* ---------------------------------------------------------------------- */
/* Section headings                                                       */
/* ---------------------------------------------------------------------- */

.page-title {
  font-size: var(--fs-2xl);
  font-weight: 800;
  letter-spacing: -0.02em;
  margin-bottom: var(--space-2);
}

.page-subtitle {
  color: var(--text-secondary);
  font-size: var(--fs-md);
  margin-bottom: var(--space-6);
}

.section-heading {
  font-size: var(--fs-lg);
  font-weight: 700;
  margin-bottom: var(--space-4);
}

/* ---------------------------------------------------------------------- */
/* Roadmap                                                                 */
/* ---------------------------------------------------------------------- */

.roadmap {
  display: flex;
  flex-direction: column;
  gap: 0;
}

.roadmap__connector {
  display: flex;
  justify-content: flex-start;
  padding-left: 27px;
  color: var(--border-strong);
  font-size: var(--fs-base);
  line-height: 1;
  height: var(--space-4);
}

.phase-card {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
  box-shadow: var(--shadow-card);
  cursor: pointer;
  transition: border-color var(--dur-fast) var(--ease-standard), background var(--dur-fast) var(--ease-standard);
}

.phase-card:hover {
  border-color: var(--border-strong);
}

.phase-card[aria-expanded="true"] {
  border-color: var(--accent-primary);
}

.phase-card__status {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  font-size: var(--fs-md);
  flex-shrink: 0;
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-inset);
}

.phase-card__status.status-completed {
  color: var(--color-visited);
  border-color: var(--color-visited);
  background: var(--color-visited-bg);
}

.phase-card__status.status-in-progress {
  color: var(--color-current);
  border-color: var(--color-current);
  background: var(--color-current-bg);
}

.phase-card__status.status-locked {
  color: var(--text-muted);
}

.phase-card__body {
  flex: 1;
  min-width: 0;
}

.phase-card__eyebrow {
  font-size: var(--fs-xs);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-muted);
  font-weight: 700;
}

.phase-card__title {
  font-size: var(--fs-md);
  font-weight: 700;
}

.phase-card__meta {
  font-size: var(--fs-sm);
  color: var(--text-secondary);
}

.phase-card__chevron {
  color: var(--text-muted);
  transition: transform var(--dur-fast) var(--ease-standard);
  flex-shrink: 0;
}

.phase-card[aria-expanded="true"] .phase-card__chevron {
  transform: rotate(90deg);
}

.phase-panel {
  overflow: hidden;
  max-height: 0;
  transition: max-height var(--dur-med) var(--ease-standard);
}

.phase-panel.is-open {
  max-height: 2000px;
}

.phase-panel__grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: var(--space-4);
  padding: var(--space-4) var(--space-2) var(--space-6);
}

/* ---------------------------------------------------------------------- */
/* Algorithm card                                                         */
/* ---------------------------------------------------------------------- */

.algo-card {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
  padding: var(--space-4);
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
  box-shadow: var(--shadow-card);
}

.algo-card.is-disabled {
  opacity: 0.55;
}

.algo-card__title {
  font-size: var(--fs-base);
  font-weight: 700;
}

.algo-card__desc {
  font-size: var(--fs-sm);
  color: var(--text-secondary);
  flex: 1;
}

.algo-card__difficulty {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-size: var(--fs-xs);
  color: var(--text-muted);
}

.difficulty-dots {
  display: inline-flex;
  gap: 3px;
}

.difficulty-dots span {
  width: 7px;
  height: 7px;
  border-radius: 50%;
  background: var(--color-unvisited);
}

.difficulty-dots span.is-filled {
  background: var(--color-attention);
}

.algo-card__features {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-1) var(--space-3);
  font-size: var(--fs-xs);
  color: var(--text-secondary);
}

.algo-card__features li {
  list-style: none;
}

.algo-card__features {
  padding: 0;
  margin: 0;
}

.algo-card__footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-2);
}

.badge {
  display: inline-flex;
  align-items: center;
  padding: 2px var(--space-2);
  border-radius: 999px;
  font-size: var(--fs-xs);
  font-weight: 700;
  background: var(--bg-inset);
  color: var(--text-muted);
  border: var(--border-width) solid var(--border-subtle);
}

.badge-soon {
  color: var(--color-attention);
  border-color: var(--color-attention);
  background: var(--color-attention-bg);
}

/* ---------------------------------------------------------------------- */
/* Learning page layout                                                   */
/* ---------------------------------------------------------------------- */

.learn-page {
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}

.learn-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-3);
}

.learn-header__title {
  display: flex;
  align-items: center;
  gap: var(--space-3);
}

.learn-header__title h1 {
  font-size: var(--fs-lg);
  font-weight: 700;
}

.back-link {
  display: inline-flex;
  align-items: center;
  gap: var(--space-1);
  background: none;
  border: none;
  color: var(--text-secondary);
  cursor: pointer;
  font-size: var(--fs-sm);
  font-weight: 600;
  padding: var(--space-2);
  border-radius: var(--radius-sm);
}

.back-link:hover {
  color: var(--text-primary);
}

.step-counter {
  font-size: var(--fs-sm);
  color: var(--text-secondary);
  font-variant-numeric: tabular-nums;
}

.viz-panel {
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
  padding: var(--space-4);
  min-height: 320px;
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.viz-panel__toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: var(--space-2);
}

.viz-panel__toolbar-group {
  display: flex;
  gap: var(--space-2);
  flex-wrap: wrap;
}

.graph-canvas-wrap {
  flex: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 260px;
}

.graph-canvas-wrap svg {
  width: 100%;
  height: 100%;
  max-height: 420px;
}

.graph-node circle,
.graph-node rect {
  fill: var(--bg-inset);
  stroke: var(--color-unvisited);
  stroke-width: 2px;
  transition: fill var(--dur-med) var(--ease-standard), stroke var(--dur-med) var(--ease-standard);
}

.graph-node text {
  fill: var(--text-primary);
  font-family: var(--font-mono);
  font-size: 13px;
  font-weight: 700;
  text-anchor: middle;
  dominant-baseline: central;
  pointer-events: none;
}

.graph-node {
  cursor: pointer;
}

.graph-node.state-unvisited circle,
.graph-node.state-unvisited rect {
  fill: var(--bg-inset);
  stroke: var(--color-unvisited);
}

.graph-node.state-queued circle,
.graph-node.state-queued rect {
  fill: var(--color-special-bg);
  stroke: var(--color-special);
}

.graph-node.state-current circle,
.graph-node.state-current rect {
  fill: var(--color-current-bg);
  stroke: var(--color-current);
  stroke-width: 3px;
}

.graph-node.state-visited circle,
.graph-node.state-visited rect {
  fill: var(--color-visited-bg);
  stroke: var(--color-visited);
}

.graph-node.state-error circle,
.graph-node.state-error rect {
  fill: var(--color-error-bg);
  stroke: var(--color-error);
  stroke-width: 3px;
}

.graph-node.is-start circle,
.graph-node.is-start rect {
  stroke-dasharray: 4 2;
}

.graph-edge {
  stroke: var(--border-strong);
  color: var(--border-strong);
  stroke-width: 2px;
  transition: stroke var(--dur-med) var(--ease-standard), stroke-width var(--dur-med) var(--ease-standard), color var(--dur-med) var(--ease-standard);
}

.graph-edge.is-active {
  stroke: var(--color-current);
  color: var(--color-current);
  stroke-width: 3px;
}

.graph-edge-weight {
  fill: var(--text-primary);
  font-family: var(--font-mono);
  font-size: 11px;
  font-weight: 700;
  text-anchor: middle;
  dominant-baseline: middle;
  paint-order: stroke;
  stroke: var(--bg-surface);
  stroke-width: 4px;
  stroke-linejoin: round;
}

/* Linked list */
.linked-list-arrow {
  stroke: var(--border-strong);
  stroke-width: 2px;
  color: var(--border-strong);
  transition: stroke var(--dur-med) var(--ease-standard);
}

.linked-list-arrow--back {
  stroke: var(--color-special);
  color: var(--color-special);
  stroke-width: 2.5px;
}

.linked-list-null {
  fill: var(--text-muted);
  font-family: var(--font-mono);
  font-size: 12px;
  font-weight: 700;
}

.ll-pointer-label {
  fill: var(--color-special);
  font-family: var(--font-mono);
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.graph-legend {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-3);
  font-size: var(--fs-xs);
  color: var(--text-secondary);
}

.graph-legend__item {
  display: inline-flex;
  align-items: center;
  gap: 6px;
}

.graph-legend__swatch {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  border: 2px solid currentColor;
  display: inline-block;
}

.viz-hint {
  font-size: var(--fs-xs);
  color: var(--text-muted);
  text-align: center;
}

.viz-goal {
  margin: 0;
  padding: var(--space-2) var(--space-4);
  font-size: var(--fs-sm);
  font-weight: 600;
  color: var(--color-attention);
  text-align: center;
}

.viz-goal:empty {
  display: none;
}

/* Code + Debugger row */
.work-grid {
  display: grid;
  grid-template-columns: 1.4fr 1fr;
  gap: var(--space-4);
}

.panel {
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
  overflow: hidden;
  display: flex;
  flex-direction: column;
}

.panel__header {
  padding: var(--space-3) var(--space-4);
  border-bottom: var(--border-width) solid var(--border-subtle);
  font-size: var(--fs-sm);
  font-weight: 700;
  color: var(--text-secondary);
  display: flex;
  align-items: center;
  justify-content: space-between;
}

/* Code panel */
.code-panel {
  background: var(--code-bg);
  font-family: var(--font-mono);
  font-size: var(--fs-sm);
  overflow-x: auto;
  max-height: 360px;
  overflow-y: auto;
}

.code-line {
  display: flex;
  padding: 2px var(--space-4);
  border-left: 3px solid transparent;
  white-space: pre;
}

.code-line__num {
  width: 28px;
  flex-shrink: 0;
  color: var(--text-muted);
  user-select: none;
  text-align: right;
  margin-right: var(--space-3);
}

.code-line.is-active {
  background: var(--code-active-line);
  border-left-color: var(--code-active-border);
}

.code-line.is-active .code-line__num {
  color: var(--code-active-border);
  font-weight: 700;
}

/* Debugger panel */
.debugger-panel {
  padding: var(--space-4);
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
  overflow-y: auto;
  max-height: 360px;
}

.debug-var {
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.debug-var__label {
  font-size: var(--fs-xs);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  font-weight: 700;
}

.debug-var__label--has-tip {
  cursor: help;
  border-bottom: 1px dotted var(--text-muted);
  width: fit-content;
}

.debug-var__hint {
  font-size: var(--fs-xs);
  color: var(--text-secondary);
  line-height: 1.5;
  font-weight: 400;
  text-transform: none;
  letter-spacing: normal;
}

.debug-var__value {
  font-family: var(--font-mono);
  font-size: var(--fs-sm);
  background: var(--bg-inset);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-sm);
  padding: var(--space-2) var(--space-3);
  word-break: break-word;
  transition: background var(--dur-fast) var(--ease-standard);
}

.debug-var.just-changed .debug-var__value {
  background: var(--color-current-bg);
  border-color: var(--color-current);
}

.debug-var__old {
  color: var(--text-muted);
  text-decoration: line-through;
  margin-right: var(--space-2);
}

/* Explanation panel */
.explanation-panel {
  padding: var(--space-4) var(--space-5);
}

.explanation-panel__step {
  font-size: var(--fs-xs);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent-primary);
  font-weight: 700;
  margin-bottom: var(--space-2);
}

.explanation-panel__text {
  font-size: var(--fs-base);
  line-height: 1.6;
  color: var(--text-primary);
}

/* Controls */
.controls-panel {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-5);
}

.controls-panel__buttons {
  display: flex;
  gap: var(--space-2);
  flex-wrap: wrap;
}

.speed-control {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-size: var(--fs-xs);
  color: var(--text-secondary);
}

.speed-control input[type="range"] {
  accent-color: var(--accent-primary);
}

/* Complexity card */
.complexity-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-4);
}

.complexity-card {
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
  padding: var(--space-5);
}

.complexity-card__label {
  font-size: var(--fs-xs);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  font-weight: 700;
  margin-bottom: var(--space-2);
}

.complexity-card__value {
  font-family: var(--font-mono);
  font-size: var(--fs-xl);
  font-weight: 700;
  color: var(--color-special);
  margin-bottom: var(--space-2);
}

.complexity-card__desc {
  color: var(--text-secondary);
  font-size: var(--fs-sm);
  line-height: 1.5;
}

/* Quiz */
.quiz-card {
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
  padding: var(--space-5);
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}

.quiz-question {
  padding-bottom: var(--space-4);
  border-bottom: var(--border-width) solid var(--border-subtle);
}

.quiz-question:last-of-type {
  border-bottom: none;
  padding-bottom: 0;
}

.quiz-question__text {
  font-weight: 700;
  margin-bottom: var(--space-3);
}

.quiz-options {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.quiz-option {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  border: var(--border-width) solid var(--border-subtle);
  cursor: pointer;
  font-size: var(--fs-sm);
}

.quiz-option:hover {
  border-color: var(--border-strong);
}

.quiz-option input {
  accent-color: var(--accent-primary);
}

.quiz-question.is-correct .quiz-option.is-answer {
  border-color: var(--color-visited);
  background: var(--color-visited-bg);
}

.quiz-question.is-incorrect .quiz-option.is-selected {
  border-color: var(--color-error);
  background: var(--color-error-bg);
}

.quiz-question.is-incorrect .quiz-option.is-answer {
  border-color: var(--color-visited);
  background: var(--color-visited-bg);
}

.quiz-feedback {
  font-size: var(--fs-sm);
  color: var(--text-secondary);
  margin-top: var(--space-2);
  display: none;
}

.quiz-question.is-answered .quiz-feedback {
  display: block;
}

.quiz-score {
  font-size: var(--fs-sm);
  color: var(--text-secondary);
}

.quiz-score strong {
  color: var(--text-primary);
}

/* Tooltip */
[data-tooltip] {
  position: relative;
}

[data-tooltip]:hover::after,
[data-tooltip]:focus-visible::after {
  content: attr(data-tooltip);
  position: absolute;
  bottom: calc(100% + 6px);
  left: 50%;
  transform: translateX(-50%);
  background: var(--bg-elevated);
  border: var(--border-width) solid var(--border-strong);
  color: var(--text-primary);
  font-size: var(--fs-xs);
  padding: var(--space-1) var(--space-2);
  border-radius: var(--radius-sm);
  white-space: nowrap;
  z-index: 30;
  box-shadow: var(--shadow-elevated);
}

/* Empty / loading states */
.empty-state {
  text-align: center;
  padding: var(--space-7) var(--space-4);
  color: var(--text-muted);
}

/* Common Uses table */
.uses-table {
  width: 100%;
  border-collapse: collapse;
  font-size: var(--fs-sm);
}

.uses-table th,
.uses-table td {
  text-align: left;
  padding: var(--space-2) var(--space-5);
  border-bottom: var(--border-width) solid var(--border-subtle);
}

.uses-table th {
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-size: var(--fs-xs);
  font-weight: 700;
}

.uses-table td:first-child {
  font-weight: 600;
  color: var(--text-primary);
  white-space: nowrap;
}

.uses-table td:last-child {
  color: var(--text-secondary);
}

.uses-table tr:last-child td {
  border-bottom: none;
}

/* Worked Example */
.worked-example {
  padding: var(--space-4) var(--space-5);
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}

.worked-example__graph {
  font-family: var(--font-mono);
  white-space: pre;
  background: var(--code-bg);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: var(--space-4);
  overflow-x: auto;
  font-size: var(--fs-sm);
  color: var(--text-primary);
}

.worked-example__question {
  font-weight: 700;
  color: var(--text-primary);
}

.worked-example__levels {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.worked-example__level {
  display: flex;
  gap: var(--space-3);
  align-items: baseline;
  font-size: var(--fs-sm);
}

.worked-example__level-label {
  font-family: var(--font-mono);
  font-weight: 700;
  color: var(--color-special);
  flex-shrink: 0;
  width: 70px;
}

/* Concept Guide's step labels are full phrases ("Visit A", "C's neighbor
   is A"), not short "Level N" tags — give them a much wider column so
   they don't wrap awkwardly, and let them wrap onto 2 lines instead of
   forcing baseline-alignment with a single-line value. */
.concept-guide .worked-example__level {
  align-items: flex-start;
}

.concept-guide .worked-example__level-label {
  width: 220px;
}

.worked-example__answer {
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-visited-bg);
  border: var(--border-width) solid var(--color-visited);
  color: var(--text-primary);
  font-size: var(--fs-sm);
}

.worked-example__practice {
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  background: var(--color-attention-bg);
  border: var(--border-width) solid var(--color-attention);
  color: var(--text-primary);
  font-size: var(--fs-sm);
  line-height: 1.6;
}

.worked-example__practice strong {
  color: var(--color-attention);
}

.worked-example__item {
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}

.worked-example__item + .worked-example__item {
  padding-top: var(--space-4);
  border-top: var(--border-width) dashed var(--border-subtle);
}

.worked-example__item-heading {
  font-family: var(--font-mono);
  font-weight: 700;
  font-size: var(--fs-sm);
  color: var(--accent-primary-strong);
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.cheat-sheet {
  padding: var(--space-4) var(--space-5);
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
}

.cheat-sheet__section {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.cheat-sheet__heading {
  font-weight: 700;
  color: var(--color-special);
  font-size: var(--fs-sm);
}

.cheat-sheet__lines {
  margin: 0;
  font-family: var(--font-mono);
  font-size: var(--fs-sm);
  white-space: pre-wrap;
  background: var(--code-bg);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: var(--space-3) var(--space-4);
  color: var(--text-primary);
  line-height: 1.6;
}

.concept-guide {
  padding: var(--space-4) var(--space-5);
  display: flex;
  flex-direction: column;
  gap: var(--space-6, var(--space-5));
}

.concept-guide__section {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.concept-guide__section + .concept-guide__section {
  padding-top: var(--space-5);
  border-top: var(--border-width) dashed var(--border-subtle);
}

.concept-guide__heading {
  font-size: var(--fs-md, 1.05rem);
  font-weight: 800;
  color: var(--text-primary);
}

.concept-guide__text {
  margin: 0;
  color: var(--text-secondary);
  line-height: 1.7;
  white-space: pre-line;
}

.concept-guide__text--question {
  color: var(--text-primary);
  font-weight: 700;
}

.concept-guide__text--note {
  font-style: italic;
  font-size: var(--fs-sm);
}

.concept-guide__block {
  margin: 0;
  font-family: var(--font-mono);
  font-size: var(--fs-sm);
  white-space: pre;
  overflow-x: auto;
  background: var(--code-bg);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: var(--space-4);
  color: var(--text-primary);
}

.concept-guide__qa {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-2) 0;
}

.concept-guide__qa-question {
  color: var(--text-primary);
  font-style: italic;
}

.concept-guide__qa-code {
  font-family: var(--font-mono);
  background: var(--code-bg);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-sm, 4px);
  padding: 2px 8px;
  color: var(--color-special);
}

.concept-guide__qa-result {
  font-weight: 700;
  color: var(--color-visited);
}

.concept-guide__problem {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
  padding: var(--space-4);
  border: var(--border-width) solid var(--border-subtle);
  border-radius: var(--radius-md);
}

.concept-guide__problem-title {
  font-weight: 700;
  color: var(--color-current);
  font-family: var(--font-mono);
  font-size: var(--fs-sm);
}

/* Array visualization */
.array-viz {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  align-items: flex-start;
  gap: var(--space-2);
  padding: var(--space-5) var(--space-3);
}

.array-cell {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: var(--space-1);
}

.array-cell__value {
  width: 48px;
  height: 48px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: var(--font-mono);
  font-weight: 700;
  font-size: var(--fs-md);
  border-radius: var(--radius-md);
  border: 2px solid var(--color-unvisited);
  background: var(--bg-inset);
  color: var(--text-primary);
  transition: background var(--dur-med) var(--ease-standard), border-color var(--dur-med) var(--ease-standard), transform var(--dur-fast) var(--ease-standard);
}

.array-cell__index {
  font-size: var(--fs-xs);
  color: var(--text-muted);
  font-family: var(--font-mono);
}

.array-cell__pointer {
  font-size: var(--fs-xs);
  font-weight: 700;
  color: var(--color-special);
  min-height: 1.2em;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.array-cell.state-current .array-cell__value,
.array-cell.state-mid .array-cell__value {
  border-color: var(--color-current);
  background: var(--color-current-bg);
  transform: translateY(-4px);
}

.array-cell.state-left .array-cell__value,
.array-cell.state-right .array-cell__value {
  border-color: var(--color-special);
  background: var(--color-special-bg);
}

.array-cell.state-compare .array-cell__value {
  border-color: var(--color-attention);
  background: var(--color-attention-bg);
}

.array-cell.state-swap .array-cell__value {
  border-color: var(--color-error);
  background: var(--color-error-bg);
}

.array-cell.state-found .array-cell__value {
  border-color: var(--color-visited);
  background: var(--color-visited-bg);
  transform: translateY(-4px);
}

.array-cell.state-eliminated .array-cell__value {
  border-color: var(--border-subtle);
  background: transparent;
  color: var(--text-muted);
  opacity: 0.4;
}

/* 2D DP grid visualization */
.grid-viz {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-5) var(--space-3);
  overflow-x: auto;
}

.grid-viz__row {
  display: flex;
  gap: var(--space-2);
}

.grid-viz__cell .array-cell__value {
  width: 40px;
  height: 40px;
  font-size: var(--fs-sm);
}

/* Stack visualization */
.stack-viz {
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: var(--space-4);
}

.stack-cell {
  display: flex;
  align-items: center;
  gap: var(--space-3);
}

.stack-cell__value {
  width: 72px;
  height: 42px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: var(--font-mono);
  font-weight: 700;
  font-size: var(--fs-md);
  border: 2px solid var(--color-unvisited);
  border-top-width: 0;
  background: var(--bg-inset);
  color: var(--text-primary);
  transition: background var(--dur-med) var(--ease-standard), border-color var(--dur-med) var(--ease-standard);
}

.stack-cell:first-child .stack-cell__value {
  border-top-width: 2px;
  border-top-left-radius: var(--radius-md);
  border-top-right-radius: var(--radius-md);
}

.stack-cell:last-child .stack-cell__value {
  border-bottom-left-radius: var(--radius-md);
  border-bottom-right-radius: var(--radius-md);
}

.stack-cell__value.state-current {
  border-color: var(--color-current);
  background: var(--color-current-bg);
}

.stack-cell__top-label {
  font-size: var(--fs-xs);
  font-weight: 700;
  color: var(--color-special);
  text-transform: uppercase;
  letter-spacing: 0.04em;
  white-space: nowrap;
}

.stack-viz__empty {
  color: var(--text-muted);
  font-style: italic;
  padding: var(--space-4);
}

/* ---------------------------------------------------------------------- */
/* Contact form                                                           */
/* ---------------------------------------------------------------------- */

.contact-page {
  max-width: 720px;
}

.contact-page .back-link {
  margin-left: calc(-1 * var(--space-2));
  margin-bottom: var(--space-2);
}

.contact-form {
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
  padding: var(--space-5);
  border-radius: var(--radius-lg);
  border: var(--border-width) solid var(--border-subtle);
  background: var(--bg-surface);
}

.contact-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-4);
}

.contact-field {
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
  min-width: 0;
}

.contact-field label {
  font-size: var(--fs-sm);
  font-weight: 700;
}

.contact-required {
  color: var(--color-error);
}

.contact-field input,
.contact-field select,
.contact-field textarea {
  width: 100%;
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  border: var(--border-width) solid var(--border-strong);
  background: var(--bg-inset);
  color: var(--text-primary);
  font: inherit;
  font-size: var(--fs-sm);
}

.contact-field textarea {
  resize: vertical;
  min-height: 140px;
  line-height: 1.6;
}

.contact-field input:focus,
.contact-field select:focus,
.contact-field textarea:focus {
  outline: 2px solid var(--accent-primary);
  outline-offset: 1px;
}

.contact-field [aria-invalid="true"] {
  border-color: var(--color-error);
}

.contact-hint {
  font-size: var(--fs-xs);
  color: var(--text-muted);
}

.contact-count {
  align-self: flex-end;
}

.contact-honey {
  position: absolute;
  left: -9999px;
  width: 1px;
  height: 1px;
  opacity: 0;
}

.contact-actions {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: var(--space-3);
}

.contact-status {
  font-size: var(--fs-sm);
  font-weight: 600;
}

.contact-status.is-success {
  color: var(--color-visited);
}

.contact-status.is-error {
  color: var(--color-error);
}

@media (max-width: 640px) {
  .contact-row {
    grid-template-columns: 1fr;
  }

  .contact-form {
    padding: var(--space-4);
  }
}