:root {
  --bg: #0f0f10;
  --surface: #17171a;
  --surface2: #1e1e22;
  --border: #2a2a30;
  --accent: #7ee8a2;
  --accent2: #5bc8dc;
  --warn: #f4a261;
  --danger: #e76f51;
  --text: #e8e8ec;
  --muted: #6b6b7a;
  --beginner: #7ee8a2;
  --intermediate: #f4a261;
  --advanced: #e76f51;
}

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

body {
  background: var(--bg);
  color: var(--text);
  font-family: 'DM Mono', monospace;
  min-height: 100vh;
  display: grid;
  grid-template-rows: auto 1fr;
}

/* ─── HEADER ─────────────────────────────────────────── */

header {
  border-bottom: 1px solid var(--border);
  padding: 0 2rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 56px;
  background: var(--surface);
}

.logo {
  font-family: 'Fraunces', serif;
  font-size: 1.4rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--accent);
}

.logo span { color: var(--text); font-weight: 300; }

.progress-bar {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  font-size: 0.72rem;
  color: var(--muted);
}

.progress-track {
  width: 140px;
  height: 4px;
  background: var(--border);
  border-radius: 2px;
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--accent), var(--accent2));
  border-radius: 2px;
  transition: width 0.5s ease;
  width: 0%;
}

/* ─── MAIN LAYOUT ─────────────────────────────────────── */

.app {
  display: grid;
  grid-template-columns: 300px 1fr;
  overflow: hidden;
  height: calc(100vh - 56px);
}

/* ─── SIDEBAR ─────────────────────────────────────────── */

.sidebar {
  border-right: 1px solid var(--border);
  overflow-y: auto;
  padding: 1rem 0;
  background: var(--surface);
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}

.sidebar-section {
  margin-bottom: 0.25rem;
}

.sidebar-category {
  padding: 0.4rem 1.25rem;
  font-size: 0.65rem;
  text-transform: uppercase;
  letter-spacing: 0.12em;
  color: var(--muted);
  margin-top: 1rem;
}

.exercise-item {
  padding: 0.65rem 1.25rem;
  cursor: pointer;
  display: flex;
  align-items: center;
  gap: 0.6rem;
  border-left: 3px solid transparent;
  transition: all 0.15s;
  font-size: 0.82rem;
}

.exercise-item:hover { background: var(--surface2); }

.exercise-item.active {
  background: var(--surface2);
  border-left-color: var(--accent);
  color: var(--accent);
}

.exercise-item.done .ex-check { opacity: 1; }

.ex-check {
  width: 16px;
  height: 16px;
  border: 1.5px solid var(--border);
  border-radius: 50%;
  flex-shrink: 0;
  opacity: 0.4;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 9px;
  transition: all 0.2s;
}

.exercise-item.done .ex-check {
  background: var(--accent);
  border-color: var(--accent);
  color: var(--bg);
  opacity: 1;
}

.diff-dot {
  width: 6px;
  height: 6px;
  border-radius: 50%;
  flex-shrink: 0;
  margin-left: auto;
}

.diff-beginner    { background: var(--beginner); }
.diff-intermediate { background: var(--intermediate); }
.diff-advanced    { background: var(--advanced); }

/* ─── MAIN CONTENT ────────────────────────────────────── */

.main {
  display: grid;
  grid-template-rows: 1fr auto;
  overflow: hidden;
}

.workspace {
  display: grid;
  grid-template-columns: 1fr 1fr;
  overflow: hidden;
}

/* ─── TASK PANEL ──────────────────────────────────────── */

.task-panel {
  border-right: 1px solid var(--border);
  overflow-y: auto;
  padding: 2rem;
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}

.task-category-tag {
  font-size: 0.65rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--accent2);
  margin-bottom: 0.5rem;
}

.task-title {
  font-family: 'Fraunces', serif;
  font-size: 1.5rem;
  font-weight: 600;
  line-height: 1.3;
  margin-bottom: 1.2rem;
  color: var(--text);
}

.task-desc {
  font-size: 0.85rem;
  line-height: 1.7;
  color: #b0b0bf;
  margin-bottom: 1.5rem;
}

.task-desc code {
  background: var(--surface2);
  border: 1px solid var(--border);
  padding: 0.1em 0.4em;
  border-radius: 4px;
  font-size: 0.9em;
  color: var(--accent);
}

.hint-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  font-family: 'DM Mono', monospace;
  font-size: 0.75rem;
  padding: 0.4rem 0.75rem;
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.2s;
  margin-bottom: 1rem;
}

.hint-btn:hover { border-color: var(--warn); color: var(--warn); }

.hint-box {
  background: rgba(244,162,97,0.08);
  border: 1px solid rgba(244,162,97,0.25);
  border-radius: 8px;
  padding: 1rem;
  font-size: 0.8rem;
  line-height: 1.6;
  color: var(--warn);
  display: none;
  margin-bottom: 1rem;
}

.hint-box.visible { display: block; }

.hint-box code {
  background: rgba(244,162,97,0.15);
  padding: 0.1em 0.3em;
  border-radius: 3px;
}

.solution-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  font-family: 'DM Mono', monospace;
  font-size: 0.75rem;
  padding: 0.4rem 0.75rem;
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.2s;
  margin-left: 0.5rem;
}

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

.diff-badge {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  font-size: 0.68rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  padding: 0.25rem 0.6rem;
  border-radius: 20px;
  margin-bottom: 1.5rem;
  border: 1px solid;
}

.diff-badge.beginner     { color: var(--beginner);     border-color: rgba(126,232,162,0.3); background: rgba(126,232,162,0.06); }
.diff-badge.intermediate { color: var(--intermediate); border-color: rgba(244,162,97,0.3);  background: rgba(244,162,97,0.06); }
.diff-badge.advanced     { color: var(--advanced);     border-color: rgba(231,111,81,0.3);  background: rgba(231,111,81,0.06); }

/* ─── EDITOR PANEL ────────────────────────────────────── */

.editor-panel {
  display: flex;
  flex-direction: column;
  overflow: hidden;
}

.editor-toolbar {
  padding: 0.6rem 1rem;
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--surface2);
  flex-shrink: 0;
}

.editor-label {
  font-size: 0.68rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--muted);
  flex: 1;
}

.btn-run {
  background: var(--accent);
  color: var(--bg);
  border: none;
  font-family: 'DM Mono', monospace;
  font-size: 0.78rem;
  font-weight: 500;
  padding: 0.4rem 1rem;
  border-radius: 6px;
  cursor: pointer;
  display: flex;
  align-items: center;
  gap: 0.4rem;
  transition: all 0.15s;
  letter-spacing: 0.02em;
}

.btn-run:hover    { background: #9ef5b8; transform: translateY(-1px); }
.btn-run:active   { transform: translateY(0); }
.btn-run:disabled { opacity: 0.5; cursor: not-allowed; transform: none; }

.btn-reset {
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  font-family: 'DM Mono', monospace;
  font-size: 0.75rem;
  padding: 0.35rem 0.7rem;
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.15s;
}

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

.editor-wrap {
  flex: 1;
  overflow: hidden;
  position: relative;
}

/* ─── CODEMIRROR OVERRIDES ────────────────────────────── */

.CodeMirror {
  height: 100% !important;
  font-family: 'DM Mono', monospace !important;
  font-size: 0.85rem !important;
  background: var(--bg) !important;
  color: var(--text) !important;
  border: none !important;
}

.CodeMirror-scroll                { padding: 1rem 0; }
.CodeMirror-gutters               { background: var(--bg) !important; border-right: 1px solid var(--border) !important; }
.CodeMirror-linenumber            { color: var(--muted) !important; }
.CodeMirror-matchingbracket       { background: rgba(126,232,162,0.15) !important; color: var(--accent) !important; }
.CodeMirror-cursor                { border-left-color: var(--accent) !important; }
.CodeMirror-selected              { background: rgba(126,232,162,0.1) !important; }
.CodeMirror-activeline-background { background: rgba(255,255,255,0.02) !important; }

.cm-keyword  { color: #c792ea !important; }
.cm-def      { color: var(--accent2) !important; }
.cm-variable { color: var(--text) !important; }
.cm-string   { color: #c3e88d !important; }
.cm-number   { color: #f78c6c !important; }
.cm-comment  { color: var(--muted) !important; font-style: italic; }
.cm-operator { color: var(--accent) !important; }
.cm-atom     { color: var(--warn) !important; }
.cm-bracket  { color: #89ddff !important; }

/* ─── OUTPUT ──────────────────────────────────────────── */

.output-bar {
  border-top: 1px solid var(--border);
  background: var(--surface);
  flex-shrink: 0;
}

.output-tabs {
  display: flex;
  border-bottom: 1px solid var(--border);
}

.output-tab {
  padding: 0.5rem 1.2rem;
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--muted);
  cursor: pointer;
  border-bottom: 2px solid transparent;
  border-top: none;
  border-left: none;
  border-right: none;
  transition: all 0.15s;
  background: none;
  font-family: 'DM Mono', monospace;
}

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

.output-content {
  padding: 1rem;
  font-size: 0.82rem;
  height: 160px;
  overflow-y: auto;
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}

.output-placeholder {
  color: var(--muted);
  font-style: italic;
}

.output-text {
  white-space: pre-wrap;
  line-height: 1.6;
  color: #c8e6c9;
}

.output-error {
  white-space: pre-wrap;
  line-height: 1.6;
  color: var(--danger);
}

.result-banner {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  padding: 0.6rem 1rem;
  font-size: 0.8rem;
  border-radius: 6px;
  margin-bottom: 0.5rem;
  border: 1px solid;
  animation: slideIn 0.3s ease;
}

@keyframes slideIn {
  from { opacity: 0; transform: translateY(-4px); }
  to   { opacity: 1; transform: translateY(0); }
}

.result-banner.pass {
  background: rgba(126,232,162,0.08);
  border-color: rgba(126,232,162,0.3);
  color: var(--accent);
}

.result-banner.fail {
  background: rgba(231,111,81,0.08);
  border-color: rgba(231,111,81,0.3);
  color: var(--danger);
}

.exec-time {
  margin-left: auto;
  color: var(--muted);
  font-size: 0.7rem;
}

/* ─── WELCOME SCREEN ──────────────────────────────────── */

.welcome {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100%;
  text-align: center;
  flex-direction: column;
  gap: 1.5rem;
  padding: 2rem;
  color: var(--muted);
}

.welcome-icon {
  font-size: 3rem;
  opacity: 0.4;
}

.welcome h2 {
  font-family: 'Fraunces', serif;
  font-size: 1.2rem;
  color: var(--text);
  font-weight: 600;
}

.welcome p { font-size: 0.82rem; line-height: 1.6; max-width: 300px; }

/* ─── SPINNER ─────────────────────────────────────────── */

.spinner {
  display: inline-block;
  width: 12px;
  height: 12px;
  border: 2px solid var(--bg);
  border-top-color: transparent;
  border-radius: 50%;
  animation: spin 0.6s linear infinite;
}

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

/* ─── SCROLLBAR ───────────────────────────────────────── */

::-webkit-scrollbar       { width: 6px; }
::-webkit-scrollbar-track { background: transparent; }
::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }