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

:root {
  --primary: #2563eb;
  --primary-hover: #1d4ed8;
  --primary-light: #3b82f6;
  --primary-50: #eff6ff;
  --primary-100: #dbeafe;
  --sidebar-bg: #0f1a33;
  --sidebar-hover: #1a2d4e;
  --sidebar-text: #94a3b8;
  --sidebar-active: #fff;
  --bg: #f0f5ff;
  --card-bg: #fff;
  --text: #1e293b;
  --text-secondary: #64748b;
  --text-light: #94a3b8;
  --border: #e2e8f0;
  --success: #16a34a;
  --success-bg: #f0fdf4;
  --success-border: #bbf7d0;
  --success-text: #166534;
  --error: #dc2626;
  --error-bg: #fef2f2;
  --error-border: #fecaca;
  --error-text: #991b1b;
  --shadow: 0 1px 3px rgba(0,0,0,0.08), 0 1px 2px rgba(0,0,0,0.06);
  --shadow-md: 0 4px 6px rgba(0,0,0,0.07), 0 2px 4px rgba(0,0,0,0.06);
  --radius: 8px;
  --radius-sm: 6px;
  --sidebar-width: 240px;
}

html, body {
  height: 100%;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Noto Sans SC", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
}

#app {
  display: flex;
  height: 100vh;
}

/* ── Sidebar ── */

#sidebar {
  width: var(--sidebar-width);
  background: var(--sidebar-bg);
  color: var(--sidebar-text);
  display: flex;
  flex-direction: column;
  flex-shrink: 0;
  overflow-y: auto;
}

.sidebar-header {
  padding: 20px;
  display: flex;
  align-items: center;
  gap: 10px;
  border-bottom: 1px solid rgba(255,255,255,0.08);
}

.sidebar-header h1 {
  font-size: 18px;
  font-weight: 600;
  color: #fff;
  letter-spacing: 1px;
}

.sidebar-logo {
  color: var(--primary-light);
  flex-shrink: 0;
}

#nav {
  padding: 12px 0;
  flex: 1;
}

.nav-item {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px 20px;
  color: var(--sidebar-text);
  text-decoration: none;
  font-size: 13px;
  transition: all .2s;
  cursor: pointer;
  border-left: 3px solid transparent;
}

.nav-item:hover {
  background: var(--sidebar-hover);
  color: #fff;
}

.nav-item.active {
  background: var(--sidebar-hover);
  color: var(--sidebar-active);
  border-left-color: var(--primary-light);
}

.nav-icon {
  width: 18px;
  height: 18px;
  flex-shrink: 0;
  opacity: .7;
}

.nav-item.active .nav-icon {
  opacity: 1;
  color: var(--primary-light);
}

/* ── Chapter headers ── */

.chapter-header {
  display: flex;
  align-items: center;
  gap: 6px;
  padding: 8px 16px;
  cursor: pointer;
  user-select: none;
  font-size: 14px;
  font-weight: 700;
  color: rgba(255,255,255,0.55);
  letter-spacing: 0.5px;
  transition: color .2s;
  margin-top: 4px;
}

.chapter-number {
  color: var(--primary-light);
  opacity: .7;
  font-size: 12px;
  flex-shrink: 0;
}

.chapter-header:hover {
  color: rgba(255,255,255,0.7);
}

.chapter-header.empty {
  cursor: default;
  opacity: .35;
}

.chapter-header.empty:hover {
  color: inherit;
}

.chapter-arrow {
  width: 12px;
  font-size: 10px;
  flex-shrink: 0;
}

.chapter-algos {
  overflow: hidden;
}

/* ── Main ── */

#main {
  flex: 1;
  display: flex;
  flex-direction: column;
  overflow-y: auto;
  min-width: 0;
}

#header {
  padding: 24px 32px;
  background: var(--card-bg);
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

#algo-title {
  font-size: 22px;
  font-weight: 600;
}

#algo-desc {
  font-size: 14px;
  color: var(--text-secondary);
  margin-top: 4px;
}

#content {
  flex: 1;
  padding: 24px 32px;
}

/* ── Welcome (empty state) ── */

.empty-state {
  text-align: center;
  padding: 80px 20px;
  color: var(--text-light);
}

.empty-state svg {
  margin-bottom: 16px;
  opacity: .35;
}

.empty-state h3 {
  font-size: 18px;
  color: var(--text-secondary);
  margin-bottom: 8px;
}

.empty-state p {
  font-size: 14px;
}

/* ── Cards ── */

.algo-card {
  background: var(--card-bg);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  margin-bottom: 20px;
}

.algo-card-header {
  padding: 16px 20px;
  border-bottom: 1px solid var(--border);
  font-size: 15px;
  font-weight: 600;
}

.algo-card-body {
  padding: 20px;
}

/* ── Form ── */

.form-group {
  margin-bottom: 16px;
}

.form-label {
  display: block;
  font-size: 13px;
  font-weight: 500;
  color: var(--text-secondary);
  margin-bottom: 6px;
}

.form-input {
  width: 100%;
  padding: 8px 12px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-size: 14px;
  font-family: 'Courier New', monospace;
  transition: border-color .2s, box-shadow .2s;
  outline: none;
  background: var(--card-bg);
  color: var(--text);
}

.form-input:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 3px var(--primary-50);
}

.form-input.error {
  border-color: var(--error);
  box-shadow: 0 0 0 3px #fef2f2;
}

.form-hint {
  font-size: 12px;
  color: var(--text-light);
  margin-top: 4px;
}

.form-hint code {
  background: var(--primary-50);
  padding: 1px 5px;
  border-radius: 3px;
  font-size: 12px;
  color: var(--primary);
}

.form-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
}

.form-error {
  font-size: 13px;
  color: var(--error);
  margin-top: 8px;
  display: none;
}

.form-error.visible {
  display: block;
}

/* ── Buttons ── */

.btn {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 10px 24px;
  border: none;
  border-radius: var(--radius-sm);
  font-size: 14px;
  font-weight: 500;
  cursor: pointer;
  transition: all .2s;
  font-family: inherit;
}

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

.btn-primary:hover {
  background: var(--primary-hover);
  box-shadow: var(--shadow-md);
}

.btn-primary:disabled {
  opacity: .6;
  cursor: not-allowed;
  box-shadow: none;
}

/* ── Table ── */

.table-wrapper {
  overflow-x: auto;
  margin-top: 16px;
}

.result-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 14px;
}

.result-table th {
  background: var(--primary);
  color: #fff;
  padding: 10px 14px;
  text-align: center;
  font-weight: 500;
  white-space: nowrap;
}

.result-table td {
  padding: 8px 14px;
  text-align: center;
  border-bottom: 1px solid var(--border);
  font-family: 'Courier New', monospace;
  white-space: nowrap;
}

.result-table tbody tr {
  animation: rowAppear .5s ease-out;
}

.result-table tbody tr:hover {
  background: var(--primary-50);
}

@keyframes rowAppear {
  from { opacity: 0; background: var(--primary-100); }
  to { opacity: 1; background: transparent; }
}

/* ── Result Summary ── */

.result-summary {
  margin-top: 20px;
  padding: 16px 20px;
  border-radius: var(--radius-sm);
  font-size: 15px;
  line-height: 1.8;
}

.result-summary.success {
  background: var(--success-bg);
  border: 1px solid var(--success-border);
  color: var(--success-text);
}

.result-summary.error {
  background: var(--error-bg);
  border: 1px solid var(--error-border);
  color: var(--error-text);
}

.result-summary strong {
  font-weight: 600;
}

/* ── Calculating indicator ── */

.calc-status {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 13px;
  color: var(--text-secondary);
  margin-top: 12px;
}

.spinner {
  width: 16px;
  height: 16px;
  border: 2px solid var(--border);
  border-top-color: var(--primary);
  border-radius: 50%;
  animation: spin .6s linear infinite;
}

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

/* ── Scrollbar ── */

#sidebar::-webkit-scrollbar {
  width: 4px;
}

#sidebar::-webkit-scrollbar-track {
  background: transparent;
}

#sidebar::-webkit-scrollbar-thumb {
  background: rgba(255,255,255,0.15);
  border-radius: 2px;
}

#main::-webkit-scrollbar {
  width: 6px;
}

#main::-webkit-scrollbar-track {
  background: transparent;
}

#main::-webkit-scrollbar-thumb {
  background: var(--border);
  border-radius: 3px;
}

/* ── Matrix Input ── */

.matrix-display {
  display: inline-flex;
  align-items: center;
  gap: 2px;
}

.matrix-display .bracket {
  font-size: 2em;
  font-weight: 100;
  color: var(--text-secondary);
  padding: 0 2px;
  user-select: none;
  font-family: serif;
  opacity: .7;
  line-height: 1;
}

.matrix-table {
  border-collapse: collapse;
}

.matrix-table td {
  padding: 3px;
}

.matrix-table.input-mode td {
  padding: 2px;
}

.matrix-table .sep {
  font-size: 1.3em;
  color: var(--text-secondary);
  padding: 0 4px;
  vertical-align: middle;
  user-select: none;
  font-family: serif;
  opacity: .6;
}

.matrix-input {
  width: 64px;
  padding: 4px 6px;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-size: 14px;
  font-family: 'Courier New', monospace;
  text-align: center;
  outline: none;
  background: var(--card-bg);
  color: var(--text);
  transition: border-color .2s;
}

.matrix-input:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 2px var(--primary-50);
}

/* ── Step display ── */

.step-header {
  font-size: 16px;
  font-weight: 600;
  color: var(--primary);
  margin: 16px 0 12px;
  padding: 8px 0;
  border-bottom: 2px solid var(--primary-100);
}

.step-item {
  padding: 12px 16px;
  margin-bottom: 12px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: #fafcff;
}

.step-desc {
  font-size: 14px;
  margin-bottom: 10px;
  color: var(--text);
  line-height: 1.7;
}

.step-desc strong {
  color: var(--primary);
}

.step-item hr {
  border: none;
  border-top: 1px solid var(--border);
  margin: 16px 0;
}

.step-item.backsub {
  background: var(--primary-50);
  border-color: var(--primary-100);
  padding: 8px 16px;
  font-size: 14px;
}

.step-item.backsub .step-marker {
  color: var(--primary);
  margin-right: 6px;
}

.step-item.det {
  background: #f8faff;
  border-color: var(--primary-100);
  font-size: 15px;
  line-height: 1.8;
  font-family: 'Courier New', monospace;
}

/* ── Matrix step table ── */

.step-item .matrix-table {
  margin: 6px auto;
}

.step-item .matrix-table td {
  padding: 3px 10px;
  font-family: 'Courier New', monospace;
  font-size: 14px;
  text-align: center;
}

.step-item .matrix-table td.hl {
  background: #fff3cd;
}

.step-item .matrix-table .sep {
  font-size: 1.1em;
  color: var(--text-secondary);
  font-family: serif;
}

/* ── Fraction display ── */

.frac-stack {
  display: inline-flex;
  flex-direction: column;
  align-items: center;
  vertical-align: middle;
  line-height: 1.1;
  font-size: 13px;
}

.frac-num {
  padding: 0 4px;
}

.frac-bar {
  display: block;
  width: 100%;
  min-width: 1em;
  height: 1px;
  background: var(--text);
  margin: 1px 0;
}

.frac-den {
  padding: 0 4px;
}

/* ── Solution ── */

.result-block {
  margin-bottom: 8px;
}

.result-block strong {
  font-size: 15px;
  font-weight: 600;
  color: var(--text);
  display: block;
  margin-bottom: 8px;
}

.solution-vector {
  display: inline-block;
  padding: 8px 20px;
  background: var(--primary-50);
  border: 1px solid var(--primary-100);
  border-radius: var(--radius-sm);
  font-family: 'Courier New', monospace;
  font-size: 18px;
  font-weight: 600;
  color: var(--primary);
  letter-spacing: 1px;
}

.det-display {
  padding: 12px 16px;
  background: #f8faff;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-family: 'Courier New', monospace;
  font-size: 15px;
  line-height: 1.8;
}

.det-display sup {
  font-size: 12px;
}

/* ── Responsive ── */

@media (max-width: 768px) {
  #sidebar {
    width: 56px;
  }
  .sidebar-header h1,
  .nav-item span {
    display: none;
  }
  .sidebar-header {
    justify-content: center;
    padding: 16px;
  }
  .nav-item {
    justify-content: center;
    padding: 12px;
  }
  .form-row {
    grid-template-columns: 1fr;
  }
  #header,
  #content {
    padding: 16px;
  }
}