/* ==========================================================================
   Automata A→Z — stylesheet
   ========================================================================== */

:root {
  --bg: #f8f7f3;
  --surface: #ffffff;
  --surface-2: #f1efe9;
  --ink: #1c1b19;
  --ink-2: #4a4843;
  --muted: #77746c;
  --line: #e2dfd6;
  --accent: #4b45d6;
  --accent-ink: #ffffff;
  --accent-soft: #e6e5fb;
  --good: #1f7a4d;
  --good-soft: #e1f3e8;
  --bad: #b3261e;
  --bad-soft: #fbe6e4;
  --warn: #8a5a00;
  --warn-soft: #fbf0d9;
  --idea: #6b3fa0;
  --idea-soft: #f1e8fb;
  --def: #0b5f8a;
  --def-soft: #e2f1f9;
  --shadow: 0 1px 2px rgba(20, 18, 10, .06), 0 4px 18px rgba(20, 18, 10, .05);
  --radius: 12px;
  --font-body: "Source Serif 4", Georgia, "Times New Roman", serif;
  --font-ui: "Inter", system-ui, -apple-system, "Segoe UI", sans-serif;
  --font-mono: "JetBrains Mono", ui-monospace, "Cascadia Code", Consolas, monospace;
  --sidebar-w: 300px;
  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #111216;
    --surface: #191b21;
    --surface-2: #20232b;
    --ink: #ecebe6;
    --ink-2: #c3c1ba;
    --muted: #8f8d86;
    --line: #2d3039;
    --accent: #9b98ff;
    --accent-ink: #12131a;
    --accent-soft: #2a2950;
    --good: #6fd29c;
    --good-soft: #17301f;
    --bad: #ff8a80;
    --bad-soft: #3a1a18;
    --warn: #f0c060;
    --warn-soft: #33290f;
    --idea: #cfa8ff;
    --idea-soft: #2a1f3a;
    --def: #7cc7f0;
    --def-soft: #13283a;
    --shadow: 0 1px 2px rgba(0, 0, 0, .4), 0 4px 18px rgba(0, 0, 0, .25);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #111216;
  --surface: #191b21;
  --surface-2: #20232b;
  --ink: #ecebe6;
  --ink-2: #c3c1ba;
  --muted: #8f8d86;
  --line: #2d3039;
  --accent: #9b98ff;
  --accent-ink: #12131a;
  --accent-soft: #2a2950;
  --good: #6fd29c;
  --good-soft: #17301f;
  --bad: #ff8a80;
  --bad-soft: #3a1a18;
  --warn: #f0c060;
  --warn-soft: #33290f;
  --idea: #cfa8ff;
  --idea-soft: #2a1f3a;
  --def: #7cc7f0;
  --def-soft: #13283a;
  --shadow: 0 1px 2px rgba(0, 0, 0, .4), 0 4px 18px rgba(0, 0, 0, .25);
  color-scheme: dark;
}

*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 76px; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: var(--font-body);
  font-size: 18.5px;
  line-height: 1.68;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}
a { color: var(--accent); text-underline-offset: 3px; text-decoration-thickness: 1px; }
a:hover { text-decoration-thickness: 2px; }

/* ---------- Top bar ---------- */
.topbar {
  position: sticky; top: 0; z-index: 40;
  height: 58px;
  display: flex; align-items: center; gap: 12px;
  padding: 0 16px;
  background: color-mix(in srgb, var(--bg) 88%, transparent);
  backdrop-filter: saturate(1.4) blur(10px);
  border-bottom: 1px solid var(--line);
  font-family: var(--font-ui);
}
.brand { display: flex; align-items: center; gap: 10px; color: var(--ink); text-decoration: none; font-weight: 700; letter-spacing: -.01em; font-size: 16px; }
.brand-mark { width: 30px; height: 30px; flex: none; }
.brand small { font-weight: 500; color: var(--muted); font-size: 13px; }
.topbar .spacer { flex: 1; }
.icon-btn {
  appearance: none; border: 1px solid var(--line); background: var(--surface);
  color: var(--ink-2); width: 38px; height: 38px; border-radius: 10px;
  display: inline-grid; place-items: center; cursor: pointer;
}
.icon-btn:hover { color: var(--ink); border-color: var(--muted); }
.icon-btn svg { width: 18px; height: 18px; }
#menu-btn { display: none; }
.read-progress { position: absolute; left: 0; bottom: -1px; height: 2px; background: var(--accent); width: 0; transition: width .1s linear; }

/* ---------- Layout ---------- */
.layout { display: grid; grid-template-columns: var(--sidebar-w) minmax(0, 1fr); }
.sidebar {
  position: sticky; top: 58px; height: calc(100vh - 58px); overflow-y: auto;
  border-right: 1px solid var(--line);
  padding: 20px 14px 40px;
  font-family: var(--font-ui);
  font-size: 14px;
  background: var(--bg);
}
.progress-card {
  border: 1px solid var(--line); background: var(--surface); border-radius: 10px;
  padding: 10px 12px; margin: 0 4px 18px;
}
.progress-card .row { display: flex; justify-content: space-between; color: var(--ink-2); font-size: 12.5px; font-weight: 600; }
.progress-bar { height: 6px; border-radius: 99px; background: var(--surface-2); margin-top: 8px; overflow: hidden; }
.progress-bar span { display: block; height: 100%; background: var(--good); border-radius: inherit; transition: width .3s; }
.nav-part { margin: 18px 8px 6px; font-size: 11px; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; color: var(--muted); }
.nav-list { list-style: none; margin: 0; padding: 0; }
.nav-list a {
  display: flex; align-items: flex-start; gap: 10px;
  padding: 7px 8px; border-radius: 8px; color: var(--ink-2); text-decoration: none; line-height: 1.35;
}
.nav-list a:hover { background: var(--surface-2); color: var(--ink); }
.nav-list a.current { background: var(--accent-soft); color: var(--ink); font-weight: 600; }
.nav-num {
  flex: none; width: 22px; height: 22px; border-radius: 50%;
  display: grid; place-items: center; font-size: 11px; font-weight: 700;
  border: 1.5px solid var(--line); color: var(--muted); background: var(--surface);
}
.nav-list a.current .nav-num { border-color: var(--accent); color: var(--accent); }
.nav-list a.done .nav-num { background: var(--good); border-color: var(--good); color: var(--surface); }
.nav-toc { list-style: none; margin: 2px 0 6px 40px; padding: 0 0 0 10px; border-left: 1.5px solid var(--line); }
.nav-toc a { display: block; padding: 3px 6px; font-size: 13px; color: var(--muted); }
.nav-toc a:hover { background: none; color: var(--accent); }

.content { min-width: 0; padding: 40px 48px 80px; }
.prose { max-width: 780px; margin: 0 auto; }

@media (max-width: 960px) {
  .layout { grid-template-columns: minmax(0, 1fr); }
  #menu-btn { display: inline-grid; }
  .sidebar {
    position: fixed; z-index: 50; top: 58px; left: 0; bottom: 0; height: auto;
    width: min(320px, 86vw); transform: translateX(-102%); transition: transform .22s ease;
    box-shadow: var(--shadow);
  }
  body.nav-open .sidebar { transform: none; }
  body.nav-open .scrim { position: fixed; inset: 58px 0 0 0; background: rgba(0,0,0,.35); z-index: 45; }
  .content { padding: 28px 16px 64px; }
  body { font-size: 17.5px; }
}

/* ---------- Typography ---------- */
.prose h1, .prose h2, .prose h3, .prose h4 { font-family: var(--font-ui); letter-spacing: -.015em; line-height: 1.25; color: var(--ink); }
.prose h1 { font-size: clamp(30px, 4.4vw, 42px); margin: 6px 0 12px; font-weight: 800; letter-spacing: -.025em; }
.prose h2 { font-size: 26px; margin: 2.4em 0 .6em; padding-top: .6em; border-top: 1px solid var(--line); font-weight: 750; }
.prose h3 { font-size: 20px; margin: 1.8em 0 .5em; font-weight: 700; }
.prose h4 { font-size: 16.5px; margin: 1.4em 0 .4em; font-weight: 700; }
.prose p, .prose ul, .prose ol { margin: 0 0 1em; }
.prose li { margin: .25em 0; }
.prose li > ul, .prose li > ol { margin: .25em 0; }
.kicker { font-family: var(--font-ui); font-size: 13px; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; color: var(--accent); }
.lede { font-size: 1.15em; color: var(--ink-2); line-height: 1.6; margin-bottom: 1.4em; }
.chapter-meta { display: flex; flex-wrap: wrap; gap: 8px; margin: 0 0 28px; font-family: var(--font-ui); font-size: 13px; }
.chip { display: inline-flex; align-items: center; gap: 6px; padding: 4px 10px; border-radius: 99px; background: var(--surface); border: 1px solid var(--line); color: var(--ink-2); }
code, kbd, .mono { font-family: var(--font-mono); font-size: .86em; }
:not(pre) > code { background: var(--surface-2); border: 1px solid var(--line); padding: .08em .35em; border-radius: 5px; }
pre {
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 14px 16px; overflow-x: auto; line-height: 1.55; font-size: 15.5px; margin: 0 0 1.2em;
}
pre code { font-size: .92em; }
strong { font-weight: 650; }
hr { border: 0; border-top: 1px solid var(--line); margin: 2em 0; }
.katex { font-size: 1.06em; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: 4px 0; }

/* ---------- Tables ---------- */
.table-wrap { overflow-x: auto; margin: 0 0 1.3em; border: 1px solid var(--line); border-radius: var(--radius); background: var(--surface); }
table { border-collapse: collapse; width: 100%; font-size: .9em; }
th, td { padding: 9px 12px; text-align: left; border-bottom: 1px solid var(--line); vertical-align: top; }
th { font-family: var(--font-ui); font-size: 13px; font-weight: 700; color: var(--ink-2); background: var(--surface-2); }
tr:last-child td { border-bottom: 0; }
td.c, th.c { text-align: center; }
.yes { color: var(--good); font-weight: 700; }
.no { color: var(--bad); font-weight: 700; }

/* ---------- Callouts ---------- */
.box {
  position: relative; border-radius: var(--radius); padding: 16px 18px 6px; margin: 1.4em 0;
  border: 1px solid var(--line); background: var(--surface);
}
.box > .box-title {
  display: flex; align-items: center; gap: 8px;
  font-family: var(--font-ui); font-size: 13px; font-weight: 750; letter-spacing: .04em; text-transform: uppercase;
  margin-bottom: 6px;
}
.box-title::before { content: ""; width: 8px; height: 8px; border-radius: 2px; background: currentColor; }
.box.intuition { background: var(--idea-soft); border-color: color-mix(in srgb, var(--idea) 30%, transparent); }
.box.intuition .box-title { color: var(--idea); }
.box.definition { background: var(--def-soft); border-color: color-mix(in srgb, var(--def) 30%, transparent); }
.box.definition .box-title { color: var(--def); }
.box.theorem { background: var(--accent-soft); border-color: color-mix(in srgb, var(--accent) 35%, transparent); }
.box.theorem .box-title { color: var(--accent); }
.box.pitfall { background: var(--warn-soft); border-color: color-mix(in srgb, var(--warn) 30%, transparent); }
.box.pitfall .box-title { color: var(--warn); }
.box.example { background: var(--surface); }
.box.example .box-title { color: var(--ink-2); }
.box.takeaways { background: var(--good-soft); border-color: color-mix(in srgb, var(--good) 30%, transparent); }
.box.takeaways .box-title { color: var(--good); }
.proof { border-left: 3px solid var(--line); padding: 2px 0 2px 16px; margin: 0 0 1.2em; color: var(--ink-2); }
.proof::before { content: "Proof. "; font-style: italic; font-weight: 650; color: var(--ink); }
.proof.sketch::before { content: "Proof sketch. "; }
.proof::after { content: " ∎"; }

/* ---------- Exercises ---------- */
.exercises { counter-reset: ex; }
details.ex {
  counter-increment: ex;
  border: 1px solid var(--line); border-radius: var(--radius); background: var(--surface);
  margin: 0 0 12px; overflow: hidden;
}
details.ex > summary {
  list-style: none; cursor: pointer; padding: 14px 16px 14px 52px; position: relative;
}
details.ex > summary::-webkit-details-marker { display: none; }
details.ex > summary::before {
  content: counter(ex); position: absolute; left: 14px; top: 13px;
  width: 26px; height: 26px; border-radius: 50%; display: grid; place-items: center;
  font: 700 12px var(--font-ui); background: var(--surface-2); color: var(--ink-2);
}
details.ex > summary::after {
  content: "Show solution"; display: block; margin-top: 6px;
  font: 600 12.5px var(--font-ui); color: var(--accent);
}
details.ex[open] > summary::after { content: "Hide solution"; }
details.ex .sol { padding: 4px 18px 6px 52px; border-top: 1px dashed var(--line); background: var(--surface-2); }
details.ex .sol > :first-child { margin-top: 12px; }
.lvl { font: 700 11px var(--font-ui); letter-spacing: .05em; text-transform: uppercase; padding: 2px 7px; border-radius: 99px; margin-left: 6px; vertical-align: 2px; }
.lvl.easy { background: var(--good-soft); color: var(--good); }
.lvl.med { background: var(--warn-soft); color: var(--warn); }
.lvl.hard { background: var(--bad-soft); color: var(--bad); }

/* ---------- Figures / automata ---------- */
figure { margin: 1.6em 0; }
figure.fa, figure.tree, figure.diagram {
  border: 1px solid var(--line); border-radius: var(--radius); background: var(--surface);
  padding: 12px 12px 10px; box-shadow: var(--shadow);
}
figcaption { font-family: var(--font-ui); font-size: 13.5px; color: var(--muted); margin-top: 8px; line-height: 1.5; padding: 0 4px; }
figcaption b { color: var(--ink-2); }
.fa-canvas svg, .tree-canvas svg, figure.diagram svg { display: block; width: 100%; height: auto; margin: 0 auto; }
.fa-canvas svg { max-height: 460px; }

.fa-edge { fill: none; stroke: var(--ink-2); stroke-width: 1.6; transition: stroke .2s, stroke-width .2s; }
.fa-edge.hot { stroke: var(--accent); stroke-width: 2.8; }
.fa-head { fill: var(--ink-2); }
.fa-head.hot { fill: var(--accent); }
.fa-start { stroke: var(--ink-2); stroke-width: 1.6; fill: none; }
.fa-state circle { fill: var(--surface); stroke: var(--ink); stroke-width: 1.8; transition: fill .2s, stroke .2s; }
.fa-state.on circle { fill: var(--accent-soft); stroke: var(--accent); stroke-width: 2.6; }
.fa-state.dead circle { stroke: var(--muted); stroke-dasharray: 4 3; }
.fa-state text { font: 600 14px var(--font-ui); fill: var(--ink); text-anchor: middle; dominant-baseline: central; }
.fa-label { font: 500 13px var(--font-mono); fill: var(--ink); text-anchor: middle; dominant-baseline: central; paint-order: stroke; stroke: var(--surface); stroke-width: 5px; stroke-linejoin: round; }
.fa-label.hot { fill: var(--accent); font-weight: 700; }

.sim {
  display: flex; flex-wrap: wrap; align-items: center; gap: 8px;
  margin-top: 10px; padding-top: 10px; border-top: 1px dashed var(--line);
  font-family: var(--font-ui); font-size: 14px;
}
.sim label { color: var(--muted); font-weight: 600; font-size: 12.5px; }
.sim input[type="text"] {
  font: 500 15px var(--font-mono); padding: 7px 10px; width: 170px; max-width: 100%;
  border: 1px solid var(--line); border-radius: 8px; background: var(--bg); color: var(--ink);
}
.sim input[type="text"]:focus { outline: 2px solid var(--accent); outline-offset: 1px; }
.btn {
  appearance: none; font: 600 13.5px var(--font-ui); padding: 7px 12px; border-radius: 8px; cursor: pointer;
  border: 1px solid var(--line); background: var(--surface-2); color: var(--ink);
}
.btn:hover { border-color: var(--muted); }
.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.btn:disabled { opacity: .45; cursor: default; }
.tape { display: flex; flex-wrap: wrap; gap: 3px; width: 100%; margin-top: 4px; min-height: 30px; }
.tape span {
  min-width: 26px; height: 30px; padding: 0 4px; display: grid; place-items: center;
  font: 600 14px var(--font-mono); border: 1px solid var(--line); border-radius: 6px; background: var(--bg); color: var(--ink);
}
.tape span.done { color: var(--muted); background: var(--surface-2); }
.tape span.next { border-color: var(--accent); box-shadow: 0 0 0 2px var(--accent-soft); }
.tape span.head { border-color: var(--accent); background: var(--accent-soft); box-shadow: 0 0 0 2px var(--accent); }
.tape.tm { flex-wrap: nowrap; overflow: hidden; justify-content: center; }
.status { width: 100%; font-size: 13.5px; color: var(--ink-2); min-height: 1.4em; }
.status .yes, .status .no { font-size: 14px; }
.status code { font-size: 12.5px; }
.trace { width: 100%; max-height: 180px; overflow: auto; font: 12.5px/1.6 var(--font-mono); color: var(--muted); background: var(--bg); border: 1px solid var(--line); border-radius: 8px; padding: 6px 10px; }
.trace:empty { display: none; }

/* Parse trees */
.tree-node circle { fill: var(--accent-soft); stroke: var(--accent); stroke-width: 1.4; }
.tree-node.leaf circle { fill: var(--surface); stroke: var(--line); }
.tree-node text { font: 600 14px var(--font-ui); fill: var(--ink); text-anchor: middle; dominant-baseline: central; }
.tree-node.leaf text { font: 600 14px var(--font-mono); fill: var(--good); }
.tree-edge { stroke: var(--muted); stroke-width: 1.3; }

/* Hand-drawn diagrams */
.d-fill-1 { fill: var(--accent-soft); } .d-fill-2 { fill: var(--def-soft); } .d-fill-3 { fill: var(--idea-soft); }
.d-fill-4 { fill: var(--good-soft); } .d-fill-5 { fill: var(--warn-soft); } .d-fill-6 { fill: var(--bad-soft); }
.d-stroke { stroke: var(--ink-2); stroke-width: 1.4; fill: none; }
.d-line { stroke: var(--line); }
.d-text { font: 600 14px var(--font-ui); fill: var(--ink); }
.d-small { font: 500 12px var(--font-ui); fill: var(--ink-2); }
.d-mono { font: 500 13px var(--font-mono); fill: var(--ink); }
.d-accent { fill: var(--accent); }

/* Widgets */
.widget {
  border: 1px solid var(--line); border-radius: var(--radius); background: var(--surface);
  padding: 16px; margin: 1.6em 0; box-shadow: var(--shadow); font-family: var(--font-ui); font-size: 14.5px;
}
.widget h4 { margin: 0 0 4px; font-size: 15px; }
.widget p { margin: 0 0 10px; color: var(--ink-2); }
.widget .row { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; margin: 8px 0; }
.widget input[type="text"], .widget input[type="number"] {
  font: 500 15px var(--font-mono); padding: 7px 10px; border: 1px solid var(--line); border-radius: 8px;
  background: var(--bg); color: var(--ink); min-width: 0;
}
.regex-results { display: grid; grid-template-columns: repeat(auto-fill, minmax(130px, 1fr)); gap: 6px; margin-top: 8px; }
.regex-results div { padding: 6px 10px; border-radius: 8px; font: 500 13.5px var(--font-mono); border: 1px solid var(--line); display: flex; justify-content: space-between; gap: 8px; }
.regex-results .m { background: var(--good-soft); border-color: transparent; }
.regex-results .n { background: var(--bad-soft); border-color: transparent; }
.split { font: 600 18px var(--font-mono); letter-spacing: .04em; word-break: break-all; }
.split .x { background: var(--def-soft); color: var(--def); border-radius: 4px; padding: 1px 2px; }
.split .y { background: var(--accent); color: var(--accent-ink); border-radius: 4px; padding: 1px 2px; }
.split .z { background: var(--idea-soft); color: var(--idea); border-radius: 4px; padding: 1px 2px; }
.log { font-size: 13.5px; color: var(--ink-2); }
.pill { display: inline-block; padding: 2px 8px; border-radius: 99px; font-weight: 700; font-size: 12px; }
.pill.yes { background: var(--good-soft); }
.pill.no { background: var(--bad-soft); }

/* PCP dominos */
.dominos { display: flex; flex-wrap: wrap; gap: 10px; margin: 1em 0 1.4em; }
.domino { display: grid; grid-template-rows: 1fr 1fr; border: 1.5px solid var(--ink-2); border-radius: 8px; min-width: 54px; background: var(--surface); font: 600 15px var(--font-mono); text-align: center; overflow: hidden; }
.domino span { padding: 4px 10px; }
.domino span:first-child { border-bottom: 1.5px solid var(--ink-2); }
.domino small { display: block; font: 600 10px var(--font-ui); color: var(--muted); }

/* Cards (home) */
.hero { padding: 18px 0 8px; }
.hero h1 { font-size: clamp(34px, 6vw, 56px); line-height: 1.05; }
.hero .lede { max-width: 640px; }
.cta { display: flex; flex-wrap: wrap; gap: 10px; margin: 18px 0 8px; font-family: var(--font-ui); }
.cta a { text-decoration: none; }
.cta .btn { padding: 10px 16px; font-size: 14.5px; display: inline-block; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 12px; margin: 1em 0 1.6em; }
.card {
  display: block; text-decoration: none; color: var(--ink);
  border: 1px solid var(--line); border-radius: var(--radius); background: var(--surface); padding: 14px 16px;
  font-family: var(--font-ui); transition: border-color .15s, transform .15s;
}
.card:hover { border-color: var(--accent); transform: translateY(-1px); }
.card .n { font-size: 12px; font-weight: 700; color: var(--accent); letter-spacing: .05em; }
.card h3 { margin: 2px 0 4px !important; font-size: 16.5px !important; }
.card p { margin: 0; font-size: 13.5px; color: var(--muted); line-height: 1.45; }
.card.done { border-color: color-mix(in srgb, var(--good) 50%, var(--line)); }
.card.done .n::after { content: "  ✓ understood"; color: var(--good); }

/* Chapter footer */
.chapter-end { margin-top: 56px; font-family: var(--font-ui); }
.mark-done {
  display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap;
  border: 1px solid var(--line); border-radius: var(--radius); padding: 14px 16px; background: var(--surface); margin-bottom: 16px;
}
.mark-done p { margin: 0; font-size: 14.5px; color: var(--ink-2); }
.mark-done.is-done { border-color: var(--good); background: var(--good-soft); }
.pager { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
.pager a {
  display: block; padding: 14px 16px; border: 1px solid var(--line); border-radius: var(--radius);
  text-decoration: none; color: var(--ink); background: var(--surface);
}
.pager a:hover { border-color: var(--accent); }
.pager small { display: block; font-size: 12px; color: var(--muted); font-weight: 600; text-transform: uppercase; letter-spacing: .05em; }
.pager span { font-weight: 650; font-size: 15px; }
.pager .next { text-align: right; grid-column: 2; }
.site-foot { margin-top: 40px; font: 13px var(--font-ui); color: var(--muted); text-align: center; }

@media (max-width: 560px) {
  .pager { grid-template-columns: 1fr; }
  .pager .next { grid-column: 1; }
  .box { padding: 14px 14px 4px; }
  details.ex > summary { padding-left: 46px; }
  details.ex .sol { padding: 4px 14px 6px 16px; }
  .prose h2 { font-size: 23px; }
}

@media print {
  .topbar, .sidebar, .chapter-end, .sim { display: none !important; }
  .layout { display: block; }
  details.ex .sol { display: block; }
}