/* ===================== RESET & BIẾN MÀU ===================== */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --bg: #0b1020;
  --bg-soft: #121a30;
  --bg-card: #141d33;
  --bg-card-2: #18233f;
  --line: #24304f;
  --text: #e6ecff;
  --muted: #93a0c4;
  --accent: #5eead4;     /* teal */
  --accent-2: #818cf8;   /* indigo */
  --accent-3: #f472b6;   /* pink */
  --green: #4ade80;
  --yellow: #facc15;
  --red: #f87171;
  --code-bg: #0d1426;
  --radius: 16px;
  --shadow: 0 18px 50px rgba(0,0,0,.45);
  --font: 'Segoe UI', system-ui, -apple-system, sans-serif;
  --mono: 'Fira Code', 'Cascadia Code', 'Consolas', monospace;
}

html { scroll-behavior: smooth; scroll-padding-top: 24px; }
body {
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  overflow-x: hidden;
}

/* nền canvas */
#bg-canvas { position: fixed; inset: 0; z-index: 0; opacity: .5; }

/* gradient nền */
body::before {
  content: ""; position: fixed; inset: 0; z-index: -1;
  background:
    radial-gradient(900px 500px at 80% -10%, rgba(129,140,248,.18), transparent 60%),
    radial-gradient(800px 500px at 0% 20%, rgba(94,234,212,.12), transparent 55%),
    radial-gradient(700px 600px at 90% 90%, rgba(244,114,182,.12), transparent 60%);
}

/* thanh tiến độ */
#progress-bar {
  position: fixed; top: 0; left: 0; height: 4px; width: 0%;
  background: linear-gradient(90deg, var(--accent), var(--accent-2), var(--accent-3));
  z-index: 200; transition: width .1s linear;
  box-shadow: 0 0 12px rgba(94,234,212,.6);
}

/* ===================== SIDEBAR ===================== */
.sidebar {
  position: fixed; top: 0; left: 0; bottom: 0; width: 268px; z-index: 100;
  background: rgba(10,15,28,.85);
  backdrop-filter: blur(14px);
  border-right: 1px solid var(--line);
  padding: 22px 16px; display: flex; flex-direction: column;
  transition: transform .3s ease;
}
.brand { display: flex; align-items: center; gap: 12px; margin-bottom: 26px; padding: 0 6px; }
.brand-logo {
  width: 44px; height: 44px; display: grid; place-items: center;
  font-family: var(--mono); font-weight: 700; font-size: 18px;
  border-radius: 12px; color: #04241f;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  box-shadow: 0 8px 22px rgba(94,234,212,.35);
}
.brand-text { display: flex; flex-direction: column; line-height: 1.2; }
.brand-text strong { font-size: 16px; }
.brand-text span { font-size: 11.5px; color: var(--muted); }

.nav { flex: 1; overflow-y: auto; }
.nav-label {
  font-size: 11px; text-transform: uppercase; letter-spacing: .12em;
  color: var(--accent); margin: 18px 8px 8px; font-weight: 700;
}
.nav-link {
  display: block; padding: 8px 12px; margin: 2px 0; border-radius: 10px;
  color: var(--muted); text-decoration: none; font-size: 14px;
  border-left: 2px solid transparent; transition: all .18s;
}
.nav-link:hover { color: var(--text); background: var(--bg-card); transform: translateX(3px); }
.nav-link.active {
  color: var(--text); background: var(--bg-card-2);
  border-left-color: var(--accent);
}
.sidebar-footer {
  font-size: 12.5px; color: var(--muted); padding: 12px 8px 0;
  border-top: 1px solid var(--line);
}
.sidebar-footer span { color: var(--accent-3); }

#menu-toggle {
  position: fixed; top: 14px; left: 14px; z-index: 150; display: none;
  width: 44px; height: 44px; border-radius: 12px; border: 1px solid var(--line);
  background: var(--bg-card); color: var(--text); font-size: 20px; cursor: pointer;
}

/* ===================== CONTENT ===================== */
.content {
  margin-left: 268px; position: relative; z-index: 1;
  max-width: 880px; padding: 40px 32px 80px;
}

/* HERO */
.hero { padding: 36px 0 20px; }
.hero-badges { display: flex; gap: 10px; flex-wrap: wrap; margin-bottom: 22px; }
.badge {
  font-size: 12px; padding: 6px 13px; border-radius: 999px;
  background: var(--bg-card); border: 1px solid var(--line); color: var(--muted);
}
.badge-green { color: var(--green); border-color: rgba(74,222,128,.4); }
.hero-title { font-size: clamp(36px, 6vw, 58px); line-height: 1.08; font-weight: 800; letter-spacing: -1px; }
.grad {
  background: linear-gradient(120deg, var(--accent), var(--accent-2) 60%, var(--accent-3));
  -webkit-background-clip: text; background-clip: text; -webkit-text-fill-color: transparent;
}
.type-wrap { font-size: clamp(22px, 4vw, 34px); color: var(--muted); font-weight: 600; }
#typed { color: var(--accent); font-family: var(--mono); }
.cursor { color: var(--accent); animation: blink 1s steps(2) infinite; font-weight: 300; }
@keyframes blink { 50% { opacity: 0; } }
.hero-sub { color: var(--muted); font-size: 17px; margin: 18px 0 26px; max-width: 620px; }

.hero-term { margin: 24px 0 28px; }
.cta {
  display: inline-block; padding: 14px 28px; border-radius: 12px; font-weight: 700;
  color: #04241f; text-decoration: none;
  background: linear-gradient(120deg, var(--accent), var(--accent-2));
  box-shadow: 0 12px 30px rgba(94,234,212,.3); transition: transform .2s, box-shadow .2s;
}
.cta:hover { transform: translateY(-3px); box-shadow: 0 16px 38px rgba(94,234,212,.45); }

/* TERMINAL */
.terminal {
  border-radius: 14px; overflow: hidden; border: 1px solid var(--line);
  background: var(--code-bg); box-shadow: var(--shadow);
}
.term-bar {
  display: flex; align-items: center; gap: 8px; padding: 11px 14px;
  background: #0a1122; border-bottom: 1px solid var(--line);
}
.dot { width: 12px; height: 12px; border-radius: 50%; }
.dot.red { background: #ff5f56; } .dot.yellow { background: #ffbd2e; } .dot.green { background: #27c93f; }
.term-title { margin-left: 10px; font-size: 12.5px; color: var(--muted); font-family: var(--mono); }
.term-body { padding: 18px; font-family: var(--mono); font-size: 14px; min-height: 150px; }

/* DIVIDER buổi học */
.lesson-divider { margin: 64px 0 28px; text-align: center; }
.lesson-divider span {
  display: inline-block; font-family: var(--mono); font-size: 13px; font-weight: 700;
  letter-spacing: .25em; color: var(--accent); padding: 5px 16px; border-radius: 999px;
  background: var(--bg-card); border: 1px solid var(--line);
}
.lesson-divider h2 {
  font-size: clamp(26px, 4vw, 36px); margin-top: 14px; font-weight: 800; letter-spacing: -.5px;
}

/* CARD */
.card {
  background: linear-gradient(180deg, var(--bg-card), var(--bg-card-2));
  border: 1px solid var(--line); border-radius: var(--radius);
  padding: 28px 28px 30px; margin: 22px 0; box-shadow: var(--shadow);
}
.card h3 { font-size: 23px; display: flex; align-items: center; gap: 12px; margin-bottom: 14px; }
.card h4.sub { margin: 22px 0 10px; font-size: 16px; color: var(--accent); }
.card p { color: #c8d2f0; margin: 10px 0; }
.card b { color: var(--text); }
.num {
  width: 34px; height: 34px; flex: none; display: grid; place-items: center;
  border-radius: 10px; font-size: 16px; font-weight: 800; color: #04241f;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
}
.muted { color: var(--muted); font-size: 14px; }

code {
  font-family: var(--mono); font-size: .9em; padding: 2px 7px; border-radius: 6px;
  background: rgba(129,140,248,.14); color: #c7d2fe; border: 1px solid rgba(129,140,248,.2);
}
kbd {
  font-family: var(--mono); font-size: 12.5px; padding: 3px 9px; border-radius: 7px;
  background: #0e1730; border: 1px solid var(--line); border-bottom-width: 3px; color: var(--text);
}

/* FLOW (mã nguồn -> compiler -> chương trình) */
.flow { display: flex; align-items: stretch; gap: 8px; margin: 22px 0; flex-wrap: wrap; justify-content: center; }
.flow-step {
  flex: 1; min-width: 130px; text-align: center; padding: 18px 12px; border-radius: 14px;
  background: var(--code-bg); border: 1px solid var(--line);
  transition: transform .25s, border-color .25s;
}
.flow-step:hover { transform: translateY(-5px); border-color: var(--accent); }
.flow-step b { display: block; margin: 6px 0 2px; }
.flow-step small { color: var(--muted); font-size: 12.5px; }
.flow-icon { font-size: 30px; }
.flow-arrow { display: grid; place-items: center; color: var(--accent); font-size: 24px; font-weight: 700; }

/* CALLOUT */
.callout {
  border-radius: 12px; padding: 14px 18px; margin: 18px 0; font-size: 14.5px;
  border-left: 4px solid var(--accent); background: rgba(94,234,212,.08);
}
.callout-info { border-color: var(--accent-2); background: rgba(129,140,248,.1); }
.callout-warn { border-color: var(--yellow); background: rgba(250,204,21,.1); }
.callout-tip  { border-color: var(--green); background: rgba(74,222,128,.1); }
.callout b { color: var(--text); }

/* UI MOCK Dev-C++ */
.ui-mock { border: 1px solid var(--line); border-radius: 12px; overflow: hidden; margin: 18px 0; }
.ui-toolbar { display: flex; gap: 6px; flex-wrap: wrap; padding: 10px; background: #0a1122; border-bottom: 1px solid var(--line); }
.ui-tool { font-size: 12px; padding: 6px 11px; border-radius: 8px; background: var(--bg-card); border: 1px solid var(--line); color: var(--muted); }
.ui-tool.ui-hot { color: var(--accent); border-color: rgba(94,234,212,.4); }
.ui-editor { padding: 30px 18px; background: var(--code-bg); color: var(--muted); font-family: var(--mono); font-size: 13px; }
.ui-log { padding: 14px 18px; background: #0a1122; border-top: 1px solid var(--line); color: var(--muted); font-size: 12.5px; font-family: var(--mono); }

/* PHÍM TẮT GRID */
.keys { display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: 12px; margin: 16px 0; }
.key-item {
  display: flex; align-items: center; gap: 12px; padding: 14px 16px; border-radius: 12px;
  background: var(--code-bg); border: 1px solid var(--line); transition: transform .2s, border-color .2s;
}
.key-item:hover { transform: translateY(-3px); border-color: var(--accent-2); }
.key-item span { font-size: 13.5px; color: var(--muted); }
.key-item.key-hot { border-color: var(--accent); background: rgba(94,234,212,.07); }

/* CODE BLOCK */
.code-block { border: 1px solid var(--line); border-radius: 14px; overflow: hidden; margin: 18px 0; background: var(--code-bg); }
.code-head { display: flex; justify-content: space-between; align-items: center; padding: 10px 14px; background: #0a1122; border-bottom: 1px solid var(--line); }
.code-file { font-family: var(--mono); font-size: 13px; color: var(--muted); }
.copy-btn {
  font-size: 12px; padding: 6px 13px; border-radius: 8px; cursor: pointer;
  background: var(--bg-card); color: var(--muted); border: 1px solid var(--line); transition: all .18s;
}
.copy-btn:hover { color: var(--accent); border-color: var(--accent); }
.copy-btn.done { color: var(--green); border-color: var(--green); }
pre.code { padding: 18px 20px; font-family: var(--mono); font-size: 13.6px; line-height: 1.7; overflow-x: auto; }
pre.code.mini { font-size: 12.8px; padding: 14px 16px; }

/* syntax highlight tokens */
.tok-key { color: #c792ea; }      /* keyword */
.tok-pre { color: #82aaff; }      /* preprocessor */
.tok-str { color: #c3e88d; }      /* string */
.tok-fn  { color: #82aaff; }      /* function */
.tok-num { color: #f78c6c; }      /* number */
.tok-com { color: #5c6a89; font-style: italic; } /* comment */
.tok-pun { color: #89ddff; }      /* punctuation */

/* RUN */
.code-run { border-top: 1px solid var(--line); padding: 14px; background: #0a1122; }
.run-btn {
  font-size: 13.5px; font-weight: 700; padding: 10px 20px; border-radius: 10px; cursor: pointer;
  color: #04241f; border: none; background: linear-gradient(120deg, var(--green), var(--accent));
  transition: transform .2s, box-shadow .2s;
}
.run-btn:hover { transform: translateY(-2px); box-shadow: 0 10px 24px rgba(74,222,128,.3); }
.console { margin-top: 14px; border-radius: 10px; overflow: hidden; border: 1px solid var(--line); background: #000; }
.console-bar { padding: 8px 12px; font-size: 12px; color: var(--muted); background: #0a0f1c; font-family: var(--mono); }
.console-body { padding: 16px; font-family: var(--mono); font-size: 13.5px; color: #d7ffe0; white-space: pre-wrap; min-height: 40px; }
.console-body .ln { opacity: 0; animation: typeIn .25s forwards; }
@keyframes typeIn { to { opacity: 1; } }
.console-body .sys { color: var(--muted); }

/* EXPLAIN list */
.explain { list-style: none; margin: 8px 0; }
.explain li { padding: 9px 14px; margin: 6px 0; border-radius: 10px; background: var(--code-bg); border: 1px solid var(--line); font-size: 14px; }
.explain li.hl { border-color: var(--yellow); background: rgba(250,204,21,.08); }

/* COMPARE đúng/sai */
.compare { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin: 16px 0; }
.compare-col { border-radius: 12px; overflow: hidden; border: 1px solid var(--line); background: var(--code-bg); }
.compare-col.bad { border-color: rgba(248,113,113,.5); }
.compare-col.good { border-color: rgba(74,222,128,.5); }
.compare-tag { padding: 8px 12px; font-size: 12.5px; font-weight: 700; }
.bad .compare-tag { background: rgba(248,113,113,.12); color: var(--red); }
.good .compare-tag { background: rgba(74,222,128,.12); color: var(--green); }
.err-log { padding: 12px 14px; font-family: var(--mono); font-size: 12px; color: var(--red); background: #1a0e12; border-top: 1px solid var(--line); }
.err-log span { color: var(--muted); }
.ok-log { padding: 12px 14px; font-family: var(--mono); font-size: 12px; color: var(--green); background: #0e1a12; border-top: 1px solid var(--line); }

/* HOMEWORK — phần riêng cho mỗi buổi */
.homework-section {
  margin: 26px 0; padding: 30px 28px 32px; border-radius: var(--radius);
  background:
    radial-gradient(600px 200px at 100% 0%, rgba(244,114,182,.1), transparent 60%),
    linear-gradient(180deg, #161033, #1a1340);
  border: 1px solid rgba(244,114,182,.25); box-shadow: var(--shadow);
}
.hw-head { margin-bottom: 20px; }
.hw-badge {
  display: inline-block; font-size: 12.5px; font-weight: 700; letter-spacing: .04em;
  color: #04241f; padding: 6px 14px; border-radius: 999px;
  background: linear-gradient(120deg, var(--accent-3), var(--accent-2));
  box-shadow: 0 8px 20px rgba(244,114,182,.3);
}
.hw-head h3 { font-size: 22px; margin: 12px 0 4px; }
.hw-head .muted { margin: 0; }

.hw-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(230px, 1fr)); gap: 16px; }
.hw-card {
  position: relative; padding: 18px 18px 20px; border-radius: 14px;
  background: var(--code-bg); border: 1px solid var(--line);
  transition: transform .25s, border-color .25s, box-shadow .25s;
}
.hw-card:hover { transform: translateY(-6px); border-color: var(--accent-3); box-shadow: 0 16px 34px rgba(244,114,182,.18); }
.hw-card-top { display: flex; align-items: center; justify-content: space-between; margin-bottom: 10px; }
.hw-no { font-family: var(--mono); font-size: 13px; font-weight: 700; color: var(--accent); }
.hw-tag { font-size: 11px; font-weight: 700; padding: 3px 10px; border-radius: 999px; }
.hw-tag.easy { color: var(--green); background: rgba(74,222,128,.14); }
.hw-tag.mid  { color: var(--yellow); background: rgba(250,204,21,.14); }
.hw-title { font-size: 16px; font-weight: 700; color: var(--text); margin: 0 0 6px; }
.hw-desc { font-size: 14px; color: #c8d2f0; margin: 0 0 12px; }
.hw-hint {
  font-size: 12.8px; color: var(--muted); margin: 0; padding: 10px 12px;
  border-radius: 10px; background: rgba(129,140,248,.08); border-left: 3px solid var(--accent-2);
}

/* nav link bài tập */
.nav-hw { color: var(--accent-3); font-weight: 600; }
.nav-hw:hover { color: #fff; }

/* BOXES biến */
.boxes { display: flex; gap: 18px; flex-wrap: wrap; margin: 22px 0; justify-content: center; }
.box-var { width: 150px; text-align: center; transition: transform .25s; }
.box-var:hover { transform: translateY(-6px) rotate(-1deg); }
.box-label { font-family: var(--mono); font-size: 13px; color: var(--accent); margin-bottom: 6px; }
.box-body {
  height: 78px; display: grid; place-items: center; font-family: var(--mono); font-size: 22px; font-weight: 700;
  border-radius: 12px; background: linear-gradient(160deg, var(--bg-card-2), var(--code-bg));
  border: 2px solid var(--accent-2); box-shadow: inset 0 -8px 20px rgba(0,0,0,.4);
}
.box-type { margin-top: 8px; font-family: var(--mono); font-size: 12px; color: var(--muted); }

/* TYPE GRID */
.type-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(180px, 1fr)); gap: 14px; margin: 18px 0; }
.type-card { padding: 22px 18px; text-align: center; border-radius: 14px; background: var(--code-bg); border: 1px solid var(--line); transition: transform .25s, border-color .25s; }
.type-card:hover { transform: translateY(-6px); border-color: var(--accent); }
.type-icon { font-size: 34px; }
.type-card b { display: block; font-family: var(--mono); font-size: 20px; color: var(--accent); margin: 8px 0 2px; }
.type-card p { font-size: 13.5px; color: var(--muted); margin: 0; }
.type-ex { display: inline-block; margin-top: 10px; font-family: var(--mono); font-size: 12.5px; color: #c3e88d; padding: 4px 10px; border-radius: 8px; background: rgba(195,232,141,.1); }

/* FMT TABLE */
.fmt-table { width: 100%; border-collapse: collapse; margin: 16px 0; font-size: 14px; border-radius: 12px; overflow: hidden; }
.fmt-table th, .fmt-table td { padding: 12px 14px; text-align: left; border-bottom: 1px solid var(--line); }
.fmt-table thead { background: #0a1122; }
.fmt-table th { color: var(--accent); font-size: 13px; text-transform: uppercase; letter-spacing: .05em; }
.fmt-table tbody tr { background: var(--code-bg); transition: background .15s; }
.fmt-table tbody tr:hover { background: var(--bg-card-2); }

/* TASK LIST */
.task-list { margin: 14px 0; }
.task { display: flex; gap: 12px; align-items: center; padding: 13px 16px; margin: 8px 0; border-radius: 12px; background: var(--code-bg); border: 1px solid var(--line); }
.task-no { font-size: 18px; color: var(--accent); font-weight: 700; }

/* QUIZ */
.quiz { padding: 18px 20px; margin: 14px 0; border-radius: 14px; background: var(--code-bg); border: 1px solid var(--line); }
.quiz-q { font-weight: 600; margin-bottom: 12px; color: var(--text); }
.quiz-opt {
  display: block; width: 100%; text-align: left; padding: 11px 16px; margin: 7px 0; cursor: pointer;
  font-size: 14px; border-radius: 10px; background: var(--bg-card); color: var(--text);
  border: 1px solid var(--line); transition: all .18s;
}
.quiz-opt:hover { border-color: var(--accent-2); transform: translateX(4px); }
.quiz-opt.correct { background: rgba(74,222,128,.15); border-color: var(--green); color: var(--green); }
.quiz-opt.wrong { background: rgba(248,113,113,.13); border-color: var(--red); color: var(--red); }

/* FOOTER */
.footer { margin-top: 60px; padding-top: 28px; border-top: 1px solid var(--line); text-align: center; color: var(--muted); }
.footer-author { margin-top: 6px; font-size: 14px; }
.footer-author b { color: var(--accent-3); }

/* REVEAL animation */
.reveal { opacity: 0; transform: translateY(30px); transition: opacity .6s ease, transform .6s ease; }
.reveal.shown { opacity: 1; transform: none; }

/* ===================== RESPONSIVE ===================== */
@media (max-width: 980px) {
  .sidebar { transform: translateX(-100%); }
  .sidebar.open { transform: none; }
  #menu-toggle { display: block; }
  .content { margin-left: 0; padding: 70px 18px 60px; }
  .compare { grid-template-columns: 1fr; }
  .flow-arrow { transform: rotate(90deg); }
}
@media (max-width: 520px) {
  .card { padding: 20px 16px; }
  .boxes { gap: 12px; }
  .box-var { width: 120px; }
}