/* ============================================================
   Engineering Tools — Design System & Global Styles
   ============================================================ */

/* ---------- Google Fonts ---------- */
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&display=swap');

/* ---------- CSS Custom Properties ---------- */
:root {
  /* Colors */
  --color-bg:          #f5f7fa;
  --color-surface:     #ffffff;
  --color-surface-alt: #f0f2f5;
  --color-border:      #e2e8f0;
  --color-border-focus:#2563eb;
  --color-text:        #1e293b;
  --color-text-sec:    #64748b;
  --color-text-muted:  #94a3b8;
  --color-accent:      #2563eb;
  --color-accent-hover:#1d4ed8;
  --color-accent-light:#dbeafe;
  --color-accent-bg:   #eff6ff;
  --color-success:     #16a34a;
  --color-success-bg:  #f0fdf4;
  --color-warning:     #d97706;
  --color-warning-bg:  #fffbeb;
  --color-error:       #dc2626;
  --color-error-bg:    #fef2f2;

  /* Typography */
  --font-family:  'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  --font-mono:    'SF Mono', 'Fira Code', 'Cascadia Code', Consolas, monospace;

  /* Sizing */
  --radius-sm:    6px;
  --radius-md:    10px;
  --radius-lg:    14px;
  --radius-xl:    20px;
  --shadow-sm:    0 1px 2px rgba(0,0,0,.05);
  --shadow-md:    0 4px 12px rgba(0,0,0,.08);
  --shadow-lg:    0 8px 30px rgba(0,0,0,.1);
  --shadow-card:  0 1px 3px rgba(0,0,0,.06), 0 1px 2px rgba(0,0,0,.04);
  --shadow-card-hover: 0 10px 25px rgba(0,0,0,.1);
  --transition:   .2s ease;
  --max-width:    1200px;
  --max-width-tool: 900px;
}

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

html {
  font-size: 16px;
  scroll-behavior: smooth;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

body {
  font-family: var(--font-family);
  color: var(--color-text);
  background: var(--color-bg);
  line-height: 1.6;
  min-height: 100vh;
}

img, svg { display: block; max-width: 100%; }
a { color: var(--color-accent); text-decoration: none; }
a:hover { color: var(--color-accent-hover); }

/* ---------- Scrollbar ---------- */
::-webkit-scrollbar { width: 8px; height: 8px; }
::-webkit-scrollbar-track { background: var(--color-bg); }
::-webkit-scrollbar-thumb { background: var(--color-border); border-radius: 4px; }
::-webkit-scrollbar-thumb:hover { background: var(--color-text-muted); }

/* ============================================================
   Header / Navigation
   ============================================================ */
.site-header {
  position: sticky;
  top: 0;
  z-index: 1000;
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  backdrop-filter: blur(8px);
  background: rgba(255,255,255,.92);
}

.header-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 24px;
  height: 60px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.header-logo {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  font-size: 1.125rem;
  color: var(--color-text);
  text-decoration: none;
}

.header-logo svg {
  width: 28px;
  height: 28px;
  color: var(--color-accent);
}

.header-logo:hover { color: var(--color-accent); }

/* Nav links */
.header-nav {
  display: flex;
  align-items: center;
  gap: 6px;
}

.header-nav a {
  padding: 6px 12px;
  border-radius: var(--radius-sm);
  font-size: .875rem;
  font-weight: 500;
  color: var(--color-text-sec);
  transition: all var(--transition);
}

.header-nav a:hover {
  background: var(--color-accent-light);
  color: var(--color-accent);
}

.header-nav a.active {
  background: var(--color-accent);
  color: #fff;
}

/* Dropdown */
.nav-dropdown {
  position: relative;
}

.nav-dropdown-btn {
  display: flex;
  align-items: center;
  gap: 5px;
  padding: 6px 14px;
  border-radius: var(--radius-sm);
  font-size: .875rem;
  font-weight: 500;
  color: var(--color-text-sec);
  background: none;
  border: 1px solid var(--color-border);
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-family);
}

.nav-dropdown-btn:hover {
  background: var(--color-accent-light);
  color: var(--color-accent);
  border-color: var(--color-accent);
}

.nav-dropdown-btn svg {
  width: 14px;
  height: 14px;
  transition: transform var(--transition);
}

.nav-dropdown.open .nav-dropdown-btn svg {
  transform: rotate(180deg);
}

.nav-dropdown-menu {
  position: absolute;
  right: 0;
  top: calc(100% + 6px);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-lg);
  min-width: 240px;
  padding: 6px;
  opacity: 0;
  visibility: hidden;
  transform: translateY(-8px);
  transition: all .15s ease;
  z-index: 100;
}

.nav-dropdown.open .nav-dropdown-menu {
  opacity: 1;
  visibility: visible;
  transform: translateY(0);
}

.nav-dropdown-menu a {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 12px;
  border-radius: var(--radius-sm);
  font-size: .85rem;
  color: var(--color-text);
  transition: all var(--transition);
}

.nav-dropdown-menu a:hover {
  background: var(--color-accent-light);
  color: var(--color-accent);
}

.nav-dropdown-menu a svg {
  width: 16px;
  height: 16px;
  flex-shrink: 0;
  color: var(--color-text-muted);
}

.nav-dropdown-menu a:hover svg {
  color: var(--color-accent);
}

/* Mobile menu button */
.mobile-menu-btn {
  display: none;
  background: none;
  border: none;
  padding: 6px;
  cursor: pointer;
  color: var(--color-text);
}

.mobile-menu-btn svg {
  width: 24px;
  height: 24px;
}

/* ============================================================
   Home Page — Hero
   ============================================================ */
.hero {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 60px 24px 20px;
  text-align: center;
}

.hero h1 {
  font-size: 2.5rem;
  font-weight: 700;
  letter-spacing: -.02em;
  line-height: 1.2;
  color: var(--color-text);
}

.hero h1 span {
  color: var(--color-accent);
}

.hero p {
  margin-top: 12px;
  font-size: 1.125rem;
  color: var(--color-text-sec);
  max-width: 560px;
  margin-left: auto;
  margin-right: auto;
}

/* Category labels */
.category-label {
  max-width: var(--max-width);
  margin: 36px auto 14px;
  padding: 0 24px;
  font-size: .8rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: .08em;
  color: var(--color-text-muted);
}

/* ============================================================
   Home Page — Card Grid
   ============================================================ */
.card-grid {
  max-width: var(--max-width);
  margin: 0 auto 60px;
  padding: 0 24px;
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 18px;
}

.tool-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 24px;
  display: flex;
  flex-direction: column;
  gap: 12px;
  text-decoration: none;
  color: inherit;
  transition: all var(--transition);
  box-shadow: var(--shadow-card);
}

.tool-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-card-hover);
  border-color: var(--color-accent);
}

.tool-card-icon {
  width: 44px;
  height: 44px;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--color-accent-bg);
  color: var(--color-accent);
}

.tool-card-icon svg {
  width: 22px;
  height: 22px;
}

.tool-card h3 {
  font-size: 1rem;
  font-weight: 600;
  color: var(--color-text);
}

.tool-card p {
  font-size: .85rem;
  color: var(--color-text-sec);
  line-height: 1.5;
  flex: 1;
}

.tool-card-arrow {
  align-self: flex-end;
  color: var(--color-text-muted);
  transition: all var(--transition);
}

.tool-card:hover .tool-card-arrow {
  color: var(--color-accent);
  transform: translateX(3px);
}

/* ============================================================
   Tool Page — Layout
   ============================================================ */
.tool-page {
  max-width: var(--max-width-tool);
  margin: 0 auto;
  padding: 36px 24px 60px;
}

.tool-page-header {
  margin-bottom: 32px;
}

.tool-page-header .breadcrumb {
  font-size: .8rem;
  color: var(--color-text-muted);
  margin-bottom: 8px;
  display: flex;
  align-items: center;
  gap: 6px;
}

.tool-page-header .breadcrumb a {
  color: var(--color-text-sec);
}

.tool-page-header .breadcrumb a:hover {
  color: var(--color-accent);
}

.tool-page-header h1 {
  font-size: 1.75rem;
  font-weight: 700;
  letter-spacing: -.01em;
}

.tool-page-header p {
  margin-top: 6px;
  color: var(--color-text-sec);
  font-size: .95rem;
}

/* ============================================================
   Form Controls
   ============================================================ */
.form-group {
  margin-bottom: 18px;
}

.form-group label {
  display: block;
  font-size: .825rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: 6px;
}

.form-group label span {
  font-weight: 400;
  color: var(--color-text-muted);
}

.form-input,
.form-select,
.form-textarea {
  width: 100%;
  padding: 10px 14px;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  font-family: var(--font-family);
  font-size: .9rem;
  color: var(--color-text);
  background: var(--color-surface);
  transition: border-color var(--transition), box-shadow var(--transition);
  outline: none;
}

.form-input:focus,
.form-select:focus,
.form-textarea:focus {
  border-color: var(--color-accent);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, .12);
}

.form-input::placeholder,
.form-textarea::placeholder {
  color: var(--color-text-muted);
}

.form-input[type="number"] {
  font-family: var(--font-mono);
}

.form-select {
  cursor: pointer;
  appearance: none;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' fill='none' stroke='%2364748b' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpath d='m2 4 4 4 4-4'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 12px center;
  padding-right: 36px;
}

.form-textarea {
  resize: vertical;
  min-height: 80px;
}

/* Inline form row */
.form-row {
  display: flex;
  gap: 14px;
  flex-wrap: wrap;
}

.form-row > .form-group {
  flex: 1;
  min-width: 140px;
}

/* ============================================================
   Buttons
   ============================================================ */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 10px 20px;
  border: none;
  border-radius: var(--radius-sm);
  font-family: var(--font-family);
  font-size: .875rem;
  font-weight: 600;
  cursor: pointer;
  transition: all var(--transition);
  white-space: nowrap;
}

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

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

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

.btn-secondary:hover {
  background: var(--color-surface-alt);
  border-color: var(--color-text-muted);
}

.btn-sm {
  padding: 6px 14px;
  font-size: .8rem;
}

.btn-icon {
  padding: 8px;
  border-radius: var(--radius-sm);
}

.btn svg {
  width: 16px;
  height: 16px;
}

/* ============================================================
   Result Panels
   ============================================================ */
.result-panel {
  background: var(--color-accent-bg);
  border: 1px solid var(--color-accent-light);
  border-radius: var(--radius-md);
  padding: 20px 24px;
  margin-top: 24px;
}

.result-panel h3 {
  font-size: .8rem;
  text-transform: uppercase;
  letter-spacing: .06em;
  color: var(--color-accent);
  margin-bottom: 12px;
  font-weight: 600;
}

.result-row {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  padding: 8px 0;
  border-bottom: 1px solid rgba(37, 99, 235, .1);
}

.result-row:last-child {
  border-bottom: none;
}

.result-label {
  font-size: .875rem;
  color: var(--color-text-sec);
}

.result-value {
  font-family: var(--font-mono);
  font-size: 1rem;
  font-weight: 600;
  color: var(--color-text);
}

.result-value.large {
  font-size: 1.5rem;
}

/* ============================================================
   Info / Formula Panels
   ============================================================ */
.info-panel {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 20px 24px;
  margin-top: 24px;
}

.info-panel h3 {
  font-size: .9rem;
  font-weight: 600;
  margin-bottom: 10px;
}

.formula {
  font-family: var(--font-mono);
  font-size: .9rem;
  background: var(--color-surface-alt);
  padding: 12px 16px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--color-border);
  margin: 8px 0;
  overflow-x: auto;
}

/* ============================================================
   Tabs
   ============================================================ */
.tabs {
  display: flex;
  gap: 4px;
  border-bottom: 2px solid var(--color-border);
  margin-bottom: 24px;
}

.tab-btn {
  padding: 10px 18px;
  font-family: var(--font-family);
  font-size: .875rem;
  font-weight: 500;
  border: none;
  background: none;
  color: var(--color-text-sec);
  cursor: pointer;
  border-bottom: 2px solid transparent;
  margin-bottom: -2px;
  transition: all var(--transition);
}

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

.tab-btn.active {
  color: var(--color-accent);
  border-bottom-color: var(--color-accent);
  font-weight: 600;
}

.tab-content {
  display: none;
}

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

/* ============================================================
   Toggle / Switch
   ============================================================ */
.toggle-group {
  display: inline-flex;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  overflow: hidden;
}

.toggle-btn {
  padding: 8px 16px;
  font-family: var(--font-family);
  font-size: .825rem;
  font-weight: 500;
  border: none;
  background: var(--color-surface);
  color: var(--color-text-sec);
  cursor: pointer;
  transition: all var(--transition);
  border-right: 1px solid var(--color-border);
}

.toggle-btn:last-child {
  border-right: none;
}

.toggle-btn.active {
  background: var(--color-accent);
  color: #fff;
}

.toggle-btn:hover:not(.active) {
  background: var(--color-accent-light);
  color: var(--color-accent);
}

/* ============================================================
   SVG Diagram Container
   ============================================================ */
.diagram-container {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 24px;
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 200px;
  margin-bottom: 24px;
}

.diagram-container svg {
  max-width: 100%;
}

/* ============================================================
   Bit Display (for binary/bitwise tools)
   ============================================================ */
.bit-row {
  display: flex;
  gap: 2px;
  flex-wrap: wrap;
  margin: 12px 0;
}

.bit-cell {
  width: 36px;
  height: 36px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: var(--font-mono);
  font-size: .85rem;
  font-weight: 600;
  border-radius: 4px;
  cursor: pointer;
  transition: all var(--transition);
  user-select: none;
}

.bit-cell.zero {
  background: var(--color-surface-alt);
  color: var(--color-text-muted);
  border: 1px solid var(--color-border);
}

.bit-cell.one {
  background: var(--color-accent);
  color: #fff;
  border: 1px solid var(--color-accent);
}

.bit-label {
  font-size: .65rem;
  color: var(--color-text-muted);
  text-align: center;
  margin-top: 2px;
}

/* ============================================================
   Table
   ============================================================ */
.data-table {
  width: 100%;
  border-collapse: collapse;
  font-size: .85rem;
}

.data-table th {
  text-align: left;
  padding: 10px 14px;
  font-weight: 600;
  color: var(--color-text-sec);
  border-bottom: 2px solid var(--color-border);
  font-size: .8rem;
  text-transform: uppercase;
  letter-spacing: .04em;
}

.data-table td {
  padding: 10px 14px;
  border-bottom: 1px solid var(--color-border);
}

.data-table tr:hover td {
  background: var(--color-accent-bg);
}

/* ============================================================
   Footer
   ============================================================ */
.site-footer {
  border-top: 1px solid var(--color-border);
  padding: 24px;
  text-align: center;
  font-size: .8rem;
  color: var(--color-text-muted);
  background: var(--color-surface);
}

/* ============================================================
   Utility Classes
   ============================================================ */
.text-mono  { font-family: var(--font-mono); }
.text-muted { color: var(--color-text-muted); }
.text-accent{ color: var(--color-accent); }
.text-sm    { font-size: .85rem; }
.mt-0       { margin-top: 0; }
.mt-1       { margin-top: 8px; }
.mt-2       { margin-top: 16px; }
.mt-3       { margin-top: 24px; }
.mt-4       { margin-top: 32px; }
.mb-1       { margin-bottom: 8px; }
.mb-2       { margin-bottom: 16px; }
.mb-3       { margin-bottom: 24px; }
.gap-sm     { gap: 8px; }
.flex       { display: flex; }
.flex-wrap  { flex-wrap: wrap; }
.items-center { align-items: center; }
.justify-between { justify-content: space-between; }
.hidden     { display: none !important; }

/* ============================================================
   Responsive
   ============================================================ */
@media (max-width: 768px) {
  .hero h1 { font-size: 1.75rem; }
  .hero p { font-size: 1rem; }
  .card-grid { grid-template-columns: 1fr; }
  .header-nav { display: none; }
  .mobile-menu-btn { display: block; }
  .form-row { flex-direction: column; }
  .tool-page { padding: 24px 16px 48px; }
  .result-row { flex-direction: column; gap: 4px; }
}

@media (max-width: 480px) {
  .header-inner { padding: 0 16px; }
  .hero { padding: 36px 16px 16px; }
  .card-grid { padding: 0 16px; }
  .category-label { padding: 0 16px; }
}