/* ============ 计算机中的数学 · 练习本涂鸦风 ============ */
:root {
  --paper: #fdfcf7;
  --grid: #dcecf5;
  --ink: #23324a;
  --ink-soft: #6b7a94;
  --blue: #2563eb;
  --red: #e84c3d;
  --hl: #ffe9a3;
  --mint: #0ea87b;
  --pencil: #9aa5b5;
  --disp: "ZCOOL KuaiLe", "Noto Sans SC", sans-serif;
  --sans: "Noto Sans SC", "PingFang SC", sans-serif;
  --mono: "JetBrains Mono", Consolas, monospace;
}
* { margin: 0; padding: 0; box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  font-family: var(--sans);
  color: var(--ink);
  font-size: 17px; line-height: 1.95;
  background-color: var(--paper);
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 28px 28px;
}
a { color: var(--blue); text-decoration-thickness: 2px; text-underline-offset: 3px; }
a:hover { color: var(--red); }
code { font-family: var(--mono); background: #eef3f9; border: 1px solid #d8e2ee; border-radius: 6px; padding: 1px 7px; font-size: .87em; color: var(--blue); }

/* 顶栏：胶带贴纸 */
.topbar {
  padding: 18px 5vw 14px; display: flex; justify-content: space-between; align-items: baseline; flex-wrap: wrap; gap: 8px;
  border-bottom: 2px dashed var(--pencil); background: rgba(253,252,247,.92);
}
.brand {
  font-family: var(--disp); font-size: 24px; color: var(--ink); text-decoration: none;
  background: var(--hl); padding: 2px 14px; transform: rotate(-1.5deg); display: inline-block;
  box-shadow: 2px 3px 0 rgba(35,50,74,.15);
}
.brand small { font-size: 13px; color: var(--ink-soft); font-family: var(--sans); }
.topbar .tag { font-family: var(--mono); font-size: 12.5px; color: var(--ink-soft); }
nav.chapters {
  padding: 10px 5vw; display: flex; flex-wrap: wrap; gap: 6px 8px;
  border-bottom: 2px dashed var(--pencil);
  background: rgba(253,252,247,.92); position: sticky; top: 0; z-index: 50;
}
nav.chapters a {
  font-family: var(--disp); font-size: 14.5px; color: var(--ink); text-decoration: none;
  border: 2px solid var(--ink); border-radius: 999px; padding: 2px 14px;
  background: #fff; transition: all .15s;
}
nav.chapters a:hover, nav.chapters a.active { background: var(--ink); color: var(--hl); }

/* 页头 */
.hero { padding: 9vh 5vw 6vh; max-width: 980px; margin: 0 auto; }
.hero .sticker {
  display: inline-block; font-family: var(--disp); font-size: 15px; color: #fff;
  background: var(--red); padding: 4px 18px; border-radius: 999px; transform: rotate(-2deg);
  margin-bottom: 24px; box-shadow: 2px 3px 0 rgba(35,50,74,.2);
}
h1.display {
  font-family: var(--disp); font-weight: 400;
  font-size: clamp(36px, 6vw, 68px); line-height: 1.25;
}
h1.display .hl { background: linear-gradient(transparent 55%, var(--hl) 55%); padding: 0 4px; }
.standfirst { margin-top: 22px; max-width: 44em; color: var(--ink-soft); font-size: 18px; }
.standfirst b { color: var(--ink); }
.byline { margin-top: 18px; font-family: var(--mono); font-size: 13px; color: var(--ink-soft); }

/* 正文 */
main.body { max-width: 860px; margin: 0 auto; padding: 56px 5vw 30px; }
section.chapter { margin-bottom: 70px; }
.chap-no {
  display: inline-flex; align-items: center; justify-content: center;
  width: 44px; height: 44px; border-radius: 12px; background: var(--ink); color: var(--hl);
  font-family: var(--disp); font-size: 20px; margin-bottom: 10px;
  transform: rotate(-3deg);
}
h2 { font-family: var(--disp); font-weight: 400; font-size: clamp(25px, 3.2vw, 36px); margin-bottom: 18px; }
h3 { font-family: var(--disp); font-size: 20px; margin: 30px 0 10px; color: var(--blue); }
h3::before { content: "✎ "; }
p { margin-bottom: 16px; }
ul, ol { padding-left: 1.6em; margin-bottom: 18px; }
li { margin-bottom: 9px; }
li::marker { color: var(--red); font-weight: 700; }
strong { background: linear-gradient(transparent 58%, var(--hl) 58%); font-weight: 700; }

/* 类比贴纸框 */
.analogy {
  position: relative; background: #fffbea; border: 2px solid var(--ink); border-radius: 4px 18px 6px 16px;
  padding: 20px 22px 16px; margin: 28px 0; box-shadow: 4px 5px 0 rgba(35,50,74,.12);
}
.analogy::before {
  content: "📌 打个比方"; position: absolute; top: -16px; left: 14px;
  font-family: var(--disp); font-size: 14px; background: var(--mint); color: #fff;
  padding: 2px 14px; border-radius: 999px;
}
.analogy p { margin: 0; }

/* 公式便签 */
.formula {
  font-family: var(--mono); font-size: 16px; text-align: center;
  background: #fff; border: 2px dashed var(--blue); border-radius: 12px;
  padding: 16px 18px; margin: 22px 0; color: var(--ink);
}
.formula .ex { display: block; font-size: 13px; color: var(--ink-soft); margin-top: 8px; }

/* 表格 */
table { width: 100%; border-collapse: separate; border-spacing: 0; margin: 24px 0; font-size: 14.5px; background: #fff; border: 2px solid var(--ink); border-radius: 12px; overflow: hidden; }
th { background: var(--ink); color: var(--hl); text-align: left; padding: 10px 14px; font-family: var(--disp); font-weight: 400; }
td { border-top: 1px solid #e3e8f0; padding: 10px 14px; vertical-align: top; }
tr:nth-child(even) td { background: #f6f9fd; }

/* 进度式步骤 */
ol.steps { counter-reset: s; list-style: none; padding: 0; }
ol.steps li { counter-increment: s; position: relative; padding: 14px 16px 14px 58px; margin: 0; border-bottom: 2px dashed var(--grid); }
ol.steps li::before {
  content: counter(s); position: absolute; left: 4px; top: 12px;
  width: 34px; height: 34px; border-radius: 10px; background: var(--hl);
  border: 2px solid var(--ink); font-family: var(--disp); font-size: 16px;
  display: flex; align-items: center; justify-content: center;
}
ol.steps li b { font-family: var(--disp); font-weight: 400; font-size: 17px; display: block; }

/* FAQ */
.faq details { background: #fff; border: 2px solid #e3e8f0; border-radius: 12px; padding: 14px 18px; margin-bottom: 10px; }
.faq summary { font-family: var(--disp); font-size: 16.5px; cursor: pointer; }
.faq summary::marker { color: var(--red); }
.faq details p { margin: 10px 0 2px; color: var(--ink-soft); }

/* 下一课卡片 */
.next-cards { display: grid; grid-template-columns: repeat(auto-fit, minmax(220px, 1fr)); gap: 16px; margin-top: 18px; }
.next-cards a {
  display: block; text-decoration: none; color: var(--ink);
  background: #fff; border: 2px solid var(--ink); border-radius: 14px; padding: 18px;
  transition: transform .15s, box-shadow .15s;
}
.next-cards a:hover { transform: translate(-2px,-2px) rotate(-.5deg); box-shadow: 5px 6px 0 var(--hl); color: var(--ink); }
.next-cards .no { font-family: var(--mono); font-size: 12px; color: var(--red); }
.next-cards .ti { font-family: var(--disp); font-size: 18px; margin: 4px 0; }
.next-cards .de { font-size: 13px; color: var(--ink-soft); line-height: 1.6; }

/* 首页课程表 */
.lessons { max-width: 1060px; margin: 0 auto; padding: 4vh 5vw 6vh; display: grid; grid-template-columns: repeat(auto-fit, minmax(300px, 1fr)); gap: 20px; }
.lesson {
  display: block; text-decoration: none; color: var(--ink);
  background: #fff; border: 2px solid var(--ink); border-radius: 16px; padding: 24px 22px 20px;
  position: relative; transition: transform .15s, box-shadow .15s;
}
.lesson:nth-child(odd) { transform: rotate(-.6deg); }
.lesson:nth-child(even) { transform: rotate(.5deg); }
.lesson:hover { transform: rotate(0) translateY(-4px); box-shadow: 6px 7px 0 var(--hl); color: var(--ink); }
.lesson .no { font-family: var(--mono); font-size: 12px; color: var(--red); letter-spacing: 1px; }
.lesson h2 { font-family: var(--disp); font-size: 21px; margin: 6px 0 2px; }
.lesson .en { font-family: var(--mono); font-size: 12px; color: var(--mint); margin-bottom: 10px; }
.lesson p { font-size: 14px; color: var(--ink-soft); margin: 0; line-height: 1.75; }
.lesson .go { position: absolute; right: 16px; top: 18px; font-size: 18px; color: var(--pencil); }
.lesson:hover .go { color: var(--red); }

footer {
  margin-top: 60px; border-top: 2px dashed var(--pencil);
  padding: 24px 5vw 40px; font-size: 13px; color: var(--ink-soft);
  display: flex; justify-content: space-between; flex-wrap: wrap; gap: 10px; font-family: var(--mono);
}
footer nav a { margin-right: 14px; color: var(--ink-soft); }

@keyframes pop { from { opacity: 0; transform: translateY(14px) scale(.97); } to { opacity: 1; transform: none; } }
.lesson, section.chapter > *, .hero > * { animation: pop .5s ease both; }
@media (prefers-reduced-motion: reduce) { * { animation: none !important; } }
@media (max-width: 640px) { body { font-size: 16px; } }