/* ============================================================
   Euler's Method Calculator — Design System
   Futuristic mathematical laboratory aesthetic
   ============================================================ */

:root {
  /* Core palette — deep laboratory dark */
  --bg-deep: #060a12;
  --bg-base: #0b1120;
  --bg-elevated: #111827;
  --bg-panel: #0f172a;
  --bg-input: #1e293b;

  --border-subtle: rgba(148, 163, 184, 0.12);
  --border-medium: rgba(148, 163, 184, 0.22);
  --border-strong: rgba(34, 211, 238, 0.35);

  --text-primary: #f1f5f9;
  --text-secondary: #94a3b8;
  --text-muted: #64748b;
  --text-accent: #67e8f9;

  --accent: #22d3ee;
  --accent-dim: #0891b2;
  --accent-glow: rgba(34, 211, 238, 0.25);

  --success: #34d399;
  --warning: #fbbf24;
  --danger: #f87171;
  --purple: #a78bfa;

  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --radius-xl: 22px;

  --shadow-panel: 0 4px 24px rgba(0, 0, 0, 0.4);
  --shadow-glow: 0 0 40px rgba(34, 211, 238, 0.08);

  --font-sans: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', 'SF Mono', Consolas, monospace;

  --header-h: 56px;
  --transition: 180ms cubic-bezier(0.4, 0, 0.2, 1);
}

/* Light theme overrides */
[data-theme="light"] {
  --bg-deep: #f8fafc;
  --bg-base: #f1f5f9;
  --bg-elevated: #ffffff;
  --bg-panel: #ffffff;
  --bg-input: #f1f5f9;

  --border-subtle: rgba(15, 23, 42, 0.08);
  --border-medium: rgba(15, 23, 42, 0.14);
  --border-strong: rgba(8, 145, 178, 0.4);

  --text-primary: #0f172a;
  --text-secondary: #475569;
  --text-muted: #64748b;
  --text-accent: #0e7490;

  --accent: #0891b2;
  --accent-dim: #0e7490;
  --accent-glow: rgba(8, 145, 178, 0.15);

  --shadow-panel: 0 4px 20px rgba(15, 23, 42, 0.08);
  --shadow-glow: 0 0 30px rgba(8, 145, 178, 0.06);
}

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

html {
  font-size: 16px;
  scroll-behavior: smooth;
  -webkit-text-size-adjust: 100%;
}

body {
  font-family: var(--font-sans);
  background: var(--bg-deep);
  color: var(--text-primary);
  line-height: 1.55;
  min-height: 100vh;
  overflow-x: hidden;
  -webkit-font-smoothing: antialiased;
}

/* Typography */
h1, h2, h3, h4 {
  font-weight: 650;
  letter-spacing: -0.02em;
  line-height: 1.25;
  color: var(--text-primary);
}

h1 { font-size: clamp(1.65rem, 4vw, 2.15rem); }
h2 { font-size: clamp(1.35rem, 3vw, 1.65rem); margin-bottom: 0.75rem; }
h3 { font-size: 1.15rem; margin-bottom: 0.5rem; }

p { margin-bottom: 1rem; color: var(--text-secondary); }
a { color: var(--accent); text-decoration: none; transition: color var(--transition); }
a:hover { color: var(--text-accent); }

code, .mono {
  font-family: var(--font-mono);
  font-size: 0.9em;
}

/* Layout */
.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  height: var(--header-h);
  background: rgba(11, 17, 32, 0.85);
  backdrop-filter: blur(14px);
  border-bottom: 1px solid var(--border-subtle);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 1.25rem;
}

[data-theme="light"] .site-header {
  background: rgba(248, 250, 252, 0.9);
}

.logo {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  font-weight: 700;
  font-size: 1.05rem;
  color: var(--text-primary);
  letter-spacing: -0.03em;
}

.logo-mark {
  width: 28px;
  height: 28px;
  border-radius: 7px;
  background: linear-gradient(135deg, #22d3ee, #0891b2);
  display: grid;
  place-items: center;
  font-size: 14px;
  color: #0b1120;
  font-weight: 800;
}

.header-actions {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.theme-toggle, .icon-btn {
  background: var(--bg-input);
  border: 1px solid var(--border-medium);
  color: var(--text-secondary);
  width: 36px;
  height: 36px;
  border-radius: var(--radius-md);
  display: grid;
  place-items: center;
  cursor: pointer;
  transition: all var(--transition);
}

.theme-toggle:hover, .icon-btn:hover {
  border-color: var(--accent);
  color: var(--accent);
}

/* Main calculator area */
.calculator-hero {
  max-width: 1280px;
  margin: 0 auto;
  padding: 1.5rem 1.25rem 2rem;
}

.hero-intro {
  text-align: center;
  margin-bottom: 1.75rem;
}

.hero-intro h1 {
  margin-bottom: 0.4rem;
}

.hero-intro .subtitle {
  color: var(--text-secondary);
  font-size: 1.05rem;
  max-width: 540px;
  margin: 0 auto;
}

/* Calculator shell */
.calc-shell {
  display: grid;
  grid-template-columns: 1fr;
  gap: 1.25rem;
}

@media (min-width: 960px) {
  .calc-shell {
    grid-template-columns: 380px 1fr;
    align-items: start;
  }
}

.panel {
  background: var(--bg-panel);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-panel);
  overflow: hidden;
}

.panel-header {
  padding: 0.9rem 1.15rem;
  border-bottom: 1px solid var(--border-subtle);
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: rgba(255,255,255,0.02);
}

.panel-header h2, .panel-header h3 {
  margin: 0;
  font-weight: 600;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  font-size: 0.78rem;
  color: var(--text-secondary);
}

.panel-body {
  padding: 1.15rem;
}

/* Inputs */
.form-group {
  margin-bottom: 1rem;
}

.form-group label {
  display: block;
  font-size: 0.8rem;
  font-weight: 500;
  color: var(--text-secondary);
  margin-bottom: 0.35rem;
  letter-spacing: 0.02em;
}

.input-with-prefix {
  display: flex;
  align-items: stretch;
  background: var(--bg-input);
  border: 1px solid var(--border-medium);
  border-radius: var(--radius-md);
  overflow: hidden;
  transition: border-color var(--transition), box-shadow var(--transition);
}

.input-with-prefix:focus-within {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-glow);
}

.input-prefix {
  padding: 0.65rem 0.75rem;
  background: rgba(0,0,0,0.2);
  color: var(--text-muted);
  font-family: var(--font-mono);
  font-size: 0.9rem;
  display: flex;
  align-items: center;
  border-right: 1px solid var(--border-subtle);
  white-space: nowrap;
}

.input-with-prefix input,
.form-control {
  flex: 1;
  background: transparent;
  border: none;
  color: var(--text-primary);
  font-family: var(--font-mono);
  font-size: 0.95rem;
  padding: 0.65rem 0.85rem;
  outline: none;
  width: 100%;
}

.form-control {
  background: var(--bg-input);
  border: 1px solid var(--border-medium);
  border-radius: var(--radius-md);
  transition: border-color var(--transition), box-shadow var(--transition);
}

.form-control:focus {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-glow);
}

.form-row {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.75rem;
}

/* Function buttons */
.fn-buttons {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
  margin-top: 0.5rem;
}

.fn-btn {
  background: var(--bg-input);
  border: 1px solid var(--border-subtle);
  color: var(--text-secondary);
  font-family: var(--font-mono);
  font-size: 0.75rem;
  padding: 0.3rem 0.5rem;
  border-radius: 5px;
  cursor: pointer;
  transition: all var(--transition);
}

.fn-btn:hover {
  border-color: var(--accent);
  color: var(--accent);
  background: rgba(34, 211, 238, 0.08);
}

/* Primary button */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 0.45rem;
  font-weight: 600;
  font-size: 0.9rem;
  padding: 0.7rem 1.2rem;
  border-radius: var(--radius-md);
  border: none;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-sans);
}

.btn-primary {
  background: linear-gradient(135deg, #22d3ee, #0891b2);
  color: #0b1120;
  width: 100%;
  box-shadow: 0 4px 14px rgba(34, 211, 238, 0.3);
}

.btn-primary:hover {
  transform: translateY(-1px);
  box-shadow: 0 6px 20px rgba(34, 211, 238, 0.4);
}

.btn-primary:active {
  transform: translateY(0);
}

.btn-primary:disabled {
  opacity: 0.55;
  cursor: not-allowed;
  transform: none;
}

.btn-ghost {
  background: transparent;
  border: 1px solid var(--border-medium);
  color: var(--text-secondary);
}

.btn-ghost:hover {
  border-color: var(--accent);
  color: var(--accent);
}

.btn-sm {
  padding: 0.4rem 0.75rem;
  font-size: 0.8rem;
}

/* Advanced options */
.advanced-toggle {
  display: flex;
  align-items: center;
  justify-content: space-between;
  width: 100%;
  background: transparent;
  border: none;
  color: var(--text-secondary);
  font-size: 0.85rem;
  padding: 0.5rem 0;
  cursor: pointer;
  margin-top: 0.5rem;
}

.advanced-panel {
  display: none;
  padding-top: 0.75rem;
  border-top: 1px solid var(--border-subtle);
  margin-top: 0.5rem;
}

.advanced-panel.open {
  display: block;
}

/* Results area */
.results-area {
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

.result-summary {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
  gap: 0.75rem;
}

.stat-card {
  background: var(--bg-elevated);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: 0.9rem 1rem;
}

.stat-label {
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  margin-bottom: 0.25rem;
}

.stat-value {
  font-family: var(--font-mono);
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--accent);
}

.stat-value.small {
  font-size: 1rem;
}

/* Graph container */
.graph-container {
  position: relative;
  width: 100%;
  height: 340px;
  background: #060a12;
  border-radius: var(--radius-md);
  overflow: hidden;
  border: 1px solid var(--border-subtle);
}

[data-theme="light"] .graph-container {
  background: #f8fafc;
}

.graph-container canvas {
  width: 100%;
  height: 100%;
  display: block;
  cursor: grab;
}

.graph-container canvas:active {
  cursor: grabbing;
}

.graph-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-top: 0.75rem;
  align-items: center;
}

/* Table */
.table-wrapper {
  overflow-x: auto;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-subtle);
  max-height: 380px;
  overflow-y: auto;
}

table.results-table {
  width: 100%;
  border-collapse: collapse;
  font-family: var(--font-mono);
  font-size: 0.82rem;
}

table.results-table th {
  position: sticky;
  top: 0;
  background: var(--bg-elevated);
  color: var(--text-muted);
  font-weight: 500;
  text-align: right;
  padding: 0.55rem 0.75rem;
  border-bottom: 1px solid var(--border-medium);
  white-space: nowrap;
}

table.results-table td {
  text-align: right;
  padding: 0.45rem 0.75rem;
  border-bottom: 1px solid var(--border-subtle);
  color: var(--text-secondary);
}

table.results-table tr:hover td {
  background: rgba(34, 211, 238, 0.04);
  color: var(--text-primary);
}

table.results-table tr.highlight td {
  background: rgba(251, 191, 36, 0.1);
  color: var(--warning);
}

/* Steps explanation */
.steps-list {
  display: flex;
  flex-direction: column;
  gap: 0.85rem;
  max-height: 420px;
  overflow-y: auto;
}

.step-card {
  background: var(--bg-elevated);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: 0.85rem 1rem;
  font-size: 0.9rem;
}

.step-card .step-num {
  font-weight: 600;
  color: var(--accent);
  margin-bottom: 0.35rem;
  font-size: 0.8rem;
  letter-spacing: 0.04em;
}

.step-card .math-line {
  font-family: var(--font-mono);
  color: var(--text-secondary);
  margin: 0.2rem 0;
  line-height: 1.5;
}

.step-card .math-result {
  color: var(--text-primary);
  font-weight: 500;
}

/* Tabs */
.tabs {
  display: flex;
  gap: 0.25rem;
  border-bottom: 1px solid var(--border-subtle);
  margin-bottom: 1rem;
  overflow-x: auto;
}

.tab-btn {
  background: transparent;
  border: none;
  color: var(--text-muted);
  padding: 0.6rem 1rem;
  font-size: 0.85rem;
  font-weight: 500;
  cursor: pointer;
  border-bottom: 2px solid transparent;
  white-space: nowrap;
  transition: all var(--transition);
}

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

.tab-btn.active {
  color: var(--accent);
  border-bottom-color: var(--accent);
}

.tab-panel {
  display: none;
}

.tab-panel.active {
  display: block;
}

/* Presets */
.presets-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 0.5rem;
}

.preset-btn {
  text-align: left;
  background: var(--bg-input);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: 0.7rem 0.9rem;
  cursor: pointer;
  transition: all var(--transition);
  color: var(--text-secondary);
}

.preset-btn:hover {
  border-color: var(--accent);
  background: rgba(34, 211, 238, 0.06);
}

.preset-btn .preset-name {
  font-weight: 600;
  color: var(--text-primary);
  font-size: 0.88rem;
  margin-bottom: 0.15rem;
}

.preset-btn .preset-eq {
  font-family: var(--font-mono);
  font-size: 0.78rem;
  color: var(--text-muted);
}

/* Alerts / messages */
.alert {
  padding: 0.85rem 1rem;
  border-radius: var(--radius-md);
  font-size: 0.9rem;
  margin-bottom: 1rem;
  border: 1px solid;
}

.alert-error {
  background: rgba(248, 113, 113, 0.1);
  border-color: rgba(248, 113, 113, 0.3);
  color: #fca5a5;
}

.alert-warning {
  background: rgba(251, 191, 36, 0.1);
  border-color: rgba(251, 191, 36, 0.3);
  color: #fcd34d;
}

.alert-info {
  background: rgba(34, 211, 238, 0.08);
  border-color: rgba(34, 211, 238, 0.25);
  color: var(--text-accent);
}

.alert-success {
  background: rgba(52, 211, 153, 0.1);
  border-color: rgba(52, 211, 153, 0.3);
  color: #6ee7b7;
}

/* Content sections below calculator */
.content-section {
  max-width: 800px;
  margin: 0 auto;
  padding: 2.5rem 1.25rem;
}

.content-section h2 {
  margin-top: 2.5rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border-subtle);
}

.content-section h2:first-child {
  margin-top: 0;
  padding-top: 0;
  border-top: none;
}

.content-section ul, .content-section ol {
  margin: 1rem 0 1.25rem 1.4rem;
  color: var(--text-secondary);
}

.content-section li {
  margin-bottom: 0.45rem;
}

.formula-box {
  background: var(--bg-panel);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-md);
  padding: 1.1rem 1.25rem;
  margin: 1.25rem 0;
  font-family: var(--font-mono);
  font-size: 1.05rem;
  text-align: center;
  color: var(--text-accent);
  overflow-x: auto;
}

.faq-item {
  margin-bottom: 1.5rem;
}

.faq-item h3 {
  font-size: 1.05rem;
  margin-bottom: 0.4rem;
}

/* Footer */
.site-footer {
  border-top: 1px solid var(--border-subtle);
  padding: 2.5rem 1.25rem;
  margin-top: 3rem;
  background: var(--bg-base);
}

.footer-inner {
  max-width: 1000px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 2rem;
}

.footer-col h4 {
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  margin-bottom: 0.85rem;
}

.footer-col a {
  display: block;
  color: var(--text-secondary);
  font-size: 0.9rem;
  margin-bottom: 0.4rem;
}

.footer-bottom {
  max-width: 1000px;
  margin: 2rem auto 0;
  padding-top: 1.25rem;
  border-top: 1px solid var(--border-subtle);
  font-size: 0.82rem;
  color: var(--text-muted);
  text-align: center;
}

/* Utility */
.hidden { display: none !important; }
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  border: 0;
}

.flex-gap {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
  align-items: center;
}

/* Animation controls */
.anim-controls {
  display: flex;
  gap: 0.4rem;
  align-items: center;
}

/* Responsive refinements */
@media (max-width: 640px) {
  .form-row {
    grid-template-columns: 1fr;
  }
  .graph-container {
    height: 260px;
  }
  .result-summary {
    grid-template-columns: 1fr 1fr;
  }
  .panel-body {
    padding: 1rem;
  }
  .tabs {
    gap: 0;
  }
  .tab-btn {
    padding: 0.5rem 0.65rem;
    font-size: 0.8rem;
  }
  .graph-controls {
    gap: 0.35rem;
  }
  .btn-sm {
    padding: 0.45rem 0.65rem;
    min-height: 36px;
  }
}

/* Reduced motion */
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}

/* Focus visible */
:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

button:focus:not(:focus-visible) {
  outline: none;
}