/* ============================================================
   THINK IN BITS · BOOKLET ENGINE (our own interactive textbook)
   One reading page per section of the syllabus. Pairs with booklet.js.
   Tokens match deck.css so a booklet and its deck look like one product.
   ============================================================ */
:root {
  --ground: #0a0a0a; --ground-far: #1a1a2e;
  --surface: #1a1a1a; --surface-lift: #2a2a2a; --surface-deep: #0f0f0f;
  --accent: #00e5ff; --accent-deep: #0099cc;
  --accent-wash: rgba(0,229,255,0.10); --accent-line: rgba(0,229,255,0.30);
  --text: #ffffff; --text-2: #b0b0b0;
  --hairline: rgba(255,255,255,0.08);
  --ok: #4fe08a; --warn: #ff6b6b; --amber: #ffc46b;
  --shadow-card: 0 10px 30px rgba(0,0,0,0.5); --shadow-glow: 0 8px 20px rgba(0,229,255,0.3);
  --font: 'Poppins', ui-sans-serif, system-ui, -apple-system, sans-serif;
  --mono: 'JetBrains Mono', ui-monospace, SFMono-Regular, Menlo, monospace;
  --col: 820px;
  color-scheme: dark;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 84px; }
body {
  margin: 0; color: var(--text); font-family: var(--font); line-height: 1.65;
  background: var(--ground);
  background-image: linear-gradient(157deg, var(--ground) 0%, #12121f 52%, var(--ground-far) 100%);
  background-attachment: fixed; -webkit-font-smoothing: antialiased;
}
[hidden] { display: none !important; }
a { color: var(--accent); }
a:focus-visible, button:focus-visible { outline: 2px solid var(--accent); outline-offset: 3px; }

/* ---------- top bar: name, progress, contents ---------- */
.bk-bar {
  position: sticky; top: 0; z-index: 30; display: flex; align-items: center; gap: 14px;
  padding: 10px max(16px, calc((100vw - var(--col)) / 2 - 120px));
  background: rgba(10,10,14,0.88); border-bottom: 1px solid var(--hairline);
  backdrop-filter: blur(10px); -webkit-backdrop-filter: blur(10px);
}
.bk-bar .bk-back { font-size: 14px; color: var(--text-2); text-decoration: none; white-space: nowrap; }
.bk-bar .bk-back:hover { color: var(--accent); }
.bk-bar .bk-name { font-family: var(--mono); font-size: 13px; color: var(--text-2); letter-spacing: .06em; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; min-width: 0; flex: 1 1 auto; }
.bk-bar .bk-meter { flex: 0 1 220px; display: flex; flex-direction: column; gap: 4px; min-width: 120px; }
.bk-bar .bk-meter span { font-family: var(--mono); font-size: 11px; color: var(--text-2); white-space: nowrap; }
.bk-bar .bk-track { height: 6px; border-radius: 6px; background: var(--surface-lift); overflow: hidden; }
.bk-bar .bk-fill { height: 100%; width: 0; background: linear-gradient(90deg, var(--accent-deep), var(--accent)); transition: width .4s ease; }
.bk-btn, .bk-bar button {
  font-family: var(--font); font-size: 14px; font-weight: 500; cursor: pointer; color: var(--text);
  background: var(--surface-lift); border: 1px solid var(--hairline); border-radius: 50px; padding: 8px 14px; min-height: 40px;
}
.bk-btn:hover, .bk-bar button:hover { border-color: var(--accent-line); }
.bk-btn.go { color: var(--accent); background: var(--accent-wash); border-color: var(--accent-line); }

/* contents drawer */
.bk-toc { position: fixed; inset: 0; z-index: 40; display: none; background: rgba(5,5,8,0.82); }
.bk-toc.open { display: block; }
.bk-toc .panel {
  position: absolute; top: 0; right: 0; bottom: 0; width: min(380px, 92vw); overflow-y: auto;
  background: var(--surface); border-left: 1px solid var(--accent-line); padding: 22px 20px 30px;
}
.bk-toc h3 { margin: 0 0 12px; font-size: 18px; color: var(--accent); }
.bk-toc ol { list-style: none; margin: 0; padding: 0; display: flex; flex-direction: column; gap: 6px; }
.bk-toc a { display: flex; gap: 10px; align-items: baseline; padding: 9px 12px; border-radius: 12px; text-decoration: none; color: var(--text); border: 1px solid var(--hairline); font-size: 15px; line-height: 1.35; }
.bk-toc a:hover { border-color: var(--accent-line); }
.bk-toc a .n { font-family: var(--mono); color: var(--accent); min-width: 22px; }
.bk-toc a .tick { margin-left: auto; color: var(--ok); font-family: var(--mono); }
.bk-toc .row { display: flex; gap: 8px; margin-top: 18px; flex-wrap: wrap; }

/* ---------- reading column ---------- */
main.bk { max-width: var(--col); margin: 0 auto; padding: 40px 20px 120px; }
main.bk > * + * { margin-top: 22px; }
.eyebrow { font-family: var(--mono); font-size: 13px; text-transform: uppercase; letter-spacing: .16em; color: var(--accent); margin: 0; }
/* math inside a label keeps its case: x is not X */
.eyebrow .katex, .pair .h .katex { text-transform: none; letter-spacing: 0; }
h1 { font-size: clamp(38px, 7vw, 64px); line-height: 1.08; letter-spacing: -0.02em; margin: 6px 0 0; }
@supports ((-webkit-background-clip: text) or (background-clip: text)) {
  h1 { background-image: linear-gradient(100deg, var(--text) 0%, var(--accent) 62%, var(--accent-deep) 100%);
       -webkit-background-clip: text; background-clip: text; color: transparent; padding: 0.18em 0; margin: calc(6px - 0.18em) 0 -0.18em; }
}
h2 { font-size: clamp(26px, 4.2vw, 36px); line-height: 1.18; letter-spacing: -0.015em; color: var(--accent); margin: 0; }
h3 { font-size: 21px; line-height: 1.3; margin: 0; color: var(--text); }
p, li { font-size: 18px; margin: 0; }
p + p { margin-top: 12px; }
ul, ol { padding-left: 1.2em; margin: 0; }
li + li { margin-top: 8px; }
.muted, .small { color: var(--text-2); }
.small { font-size: 15.5px; }
.lead { font-size: 20px; color: var(--text); }
b, strong { font-weight: 600; }
.bad { color: var(--warn); }
.mono { font-family: var(--mono); }

/* cover */
.cover { padding: 26px 26px 24px; border-radius: 24px; border: 1px solid var(--accent-line);
  background: radial-gradient(120% 140% at 100% 0%, rgba(0,229,255,0.12), rgba(0,0,0,0) 60%), var(--surface); box-shadow: var(--shadow-card); }
.cover .can { display: grid; grid-template-columns: 1fr 1fr; gap: 14px 22px; margin-top: 18px; }
.cover .can > div { border-top: 1px solid var(--hairline); padding-top: 12px; }
.cover .can h3 { font-size: 15px; font-family: var(--mono); color: var(--accent); text-transform: uppercase; letter-spacing: .1em; font-weight: 500; margin-bottom: 6px; }
.cover .can li, .cover .can p { font-size: 16px; }
.cover .start { margin-top: 20px; display: flex; gap: 10px; flex-wrap: wrap; align-items: center; }

/* sections */
section.bsec { margin-top: 64px; scroll-margin-top: 84px; }
section.bsec > * + * { margin-top: 18px; }
.sec-head { display: flex; gap: 14px; align-items: flex-start; }
.sec-head .num { flex: 0 0 auto; width: 46px; height: 46px; border-radius: 14px; display: grid; place-items: center;
  font-family: var(--mono); font-weight: 700; font-size: 18px; color: var(--accent); background: var(--accent-wash); border: 1px solid var(--accent-line); margin-top: 2px; }
.sec-head .sub { font-size: 16px; color: var(--text-2); margin-top: 6px; }

/* boxes */
.card { background: var(--surface); border: 1px solid var(--hairline); border-radius: 20px; padding: 20px 22px; box-shadow: var(--shadow-card); }
.card > * + * { margin-top: 12px; }
.card .eyebrow { font-size: 12px; }
.idea { border-left: 4px solid var(--accent); background: linear-gradient(90deg, var(--accent-wash), rgba(0,0,0,0) 70%), var(--surface); border-radius: 0 18px 18px 0; padding: 16px 20px; }
.idea > * + * { margin-top: 10px; }
.idea .eyebrow { font-size: 12px; }
.why { border: 1px dashed rgba(255,196,107,0.45); border-radius: 18px; padding: 14px 18px; background: rgba(255,196,107,0.05); }
.why .eyebrow { color: var(--amber); font-size: 12px; }
.why > * + * { margin-top: 8px; }
.local { border: 1px solid rgba(79,224,138,0.35); background: rgba(79,224,138,0.06); border-radius: 18px; padding: 16px 20px; }
.local .eyebrow { color: var(--ok); font-size: 12px; }
.local > * + * { margin-top: 10px; }
.two { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }
.two > .card, .two > div { min-width: 0; }

/* math */
.m { font-size: 19px; padding: 10px 14px; border-radius: 12px; background: var(--surface-deep); border: 1px solid var(--hairline); overflow-x: auto; overflow-y: hidden; }
.m .lbl, .lbl { display: block; font-family: var(--mono); font-size: 13px; color: var(--text-2); margin-bottom: 4px; }
.m.result { border-color: var(--accent-line); background: var(--accent-wash); color: var(--accent); font-weight: 600; }
.m.result .katex { color: var(--accent); }
.katex { font-size: 1.08em; }
.m .katex-display { margin: 0.2em 0; }

/* side-by-side derivation: general on the left, numbers on the right */
.pair { display: grid; grid-template-columns: 1fr 1fr; gap: 10px 16px; align-items: start; }
.pair .h { font-family: var(--mono); font-size: 12px; text-transform: uppercase; letter-spacing: .12em; color: var(--accent); }
.pair .lbl { grid-column: 1 / -1; margin: 8px 0 -4px; }
.pair .m { font-size: 17px; }

/* step-by-step reveal */
.steps > .step { display: none; }
.steps > .step.on { display: block; animation: rise .3s ease-out both; }
.steps > .step.pair.on { display: grid; }
.steps > * + * { margin-top: 10px; }
.steps .more { margin-top: 12px; }
@keyframes rise { from { opacity: 0; transform: translateY(6px); } to { opacity: 1; transform: none; } }

/* predict first, then see */
.predict { border: 1px solid var(--accent-line); border-radius: 20px; padding: 18px 20px; background: var(--surface); }
.predict > * + * { margin-top: 12px; }
.predict .eyebrow::before { content: "Predict · "; }
.predict .opts { display: flex; flex-wrap: wrap; gap: 8px; }
.predict .opts button {
  font-family: var(--font); font-size: 16px; cursor: pointer; color: var(--text); background: var(--surface-lift);
  border: 1px solid var(--hairline); border-radius: 14px; padding: 9px 14px; min-height: 44px; text-align: left;
}
.predict .opts button:hover:not(:disabled) { border-color: var(--accent-line); }
.predict .opts button.right { border-color: var(--ok); background: rgba(79,224,138,0.10); color: var(--ok); }
.predict .opts button.wrong { border-color: var(--warn); background: rgba(255,107,107,0.10); color: var(--warn); }
.predict .opts button:disabled { cursor: default; }
.predict .say { font-size: 16px; }
.predict .say.right { color: var(--ok); }
.predict .say.wrong { color: var(--warn); }
.predict .why-it { display: none; }
.predict.done .why-it { display: block; animation: rise .3s ease-out both; }

/* examiner-error ghosts: a wrong line that visibly misses */
.ghost { border: 1px solid rgba(255,107,107,0.4); background: rgba(255,107,107,0.06); border-radius: 18px; padding: 14px 18px; }
.ghost > * + * { margin-top: 8px; }
.ghost .eyebrow { color: var(--warn); font-size: 12px; }
.ghost .wrongline { text-decoration: line-through; text-decoration-color: var(--warn); text-decoration-thickness: 2px; opacity: 0.9; }
.ghost .fix { color: var(--ok); }
.ghosts { display: grid; grid-template-columns: repeat(3, 1fr); gap: 12px; }
.ghosts .ghost p, .ghosts .ghost .m { font-size: 16px; }

/* a real Cambridge question */
.exam { border: 1px solid var(--hairline); border-radius: 20px; background: #121218; padding: 20px 22px; }
.exam > * + * { margin-top: 12px; }
.exam .src { font-family: var(--mono); font-size: 12.5px; color: var(--accent); letter-spacing: .08em; text-transform: uppercase; }
.exam .qtext { font-family: Georgia, 'Times New Roman', serif; font-size: 18px; line-height: 1.6; }
.exam .mk { float: right; font-family: var(--mono); font-size: 14px; color: var(--text-2); }

/* marks lighting up line by line */
.marks > .mline { display: grid; grid-template-columns: 62px 1fr; gap: 6px 14px; align-items: start; padding: 12px 14px; border-radius: 14px;
  border: 1px solid var(--hairline); background: var(--surface-deep); opacity: 0.28; filter: grayscale(1); transition: opacity .35s ease, border-color .35s ease, filter .35s ease; }
.marks > .mline + .mline { margin-top: 8px; }
.marks > .mline.lit { opacity: 1; filter: none; border-color: var(--accent-line); }
.marks .code { font-family: var(--mono); font-weight: 700; font-size: 17px; color: var(--accent); background: var(--accent-wash); border: 1px solid var(--accent-line); border-radius: 10px; text-align: center; padding: 4px 0; }
.marks .mline.lit .code { box-shadow: var(--shadow-glow); }
.marks .earn { grid-column: 2; font-size: 15px; color: var(--text-2); }
.marks .tot { margin-top: 12px; display: flex; align-items: center; gap: 12px; flex-wrap: wrap; font-family: var(--mono); color: var(--text-2); }
.marks .tot b { color: var(--accent); }

/* check yourself */
.try { border: 1px solid var(--hairline); border-radius: 20px; background: var(--surface); padding: 18px 20px; }
.try > * + * { margin-top: 12px; }
.try .qh { display: flex; gap: 10px; align-items: baseline; }
.try .qh .qn { font-family: var(--mono); color: var(--accent); font-weight: 700; }
.try .qh .tag { margin-left: auto; font-family: var(--mono); font-size: 12px; color: var(--text-2); white-space: nowrap; }
.try .sol { display: none; border-top: 1px solid var(--hairline); padding-top: 12px; }
.try .sol > * + * { margin-top: 10px; }
.try.open .sol { display: block; animation: rise .3s ease-out both; }
.try .acts { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; }
.try .self { display: none; gap: 8px; align-items: center; flex-wrap: wrap; }
.try.open .self { display: flex; }
.try .self span { font-size: 15px; color: var(--text-2); }
.try .self button[aria-pressed="true"].got { color: var(--ok); border-color: var(--ok); background: rgba(79,224,138,0.10); }
.try .self button[aria-pressed="true"].not { color: var(--amber); border-color: var(--amber); background: rgba(255,196,107,0.10); }
.try.got { border-color: rgba(79,224,138,0.45); }
.try.not { border-color: rgba(255,196,107,0.45); }
.tally { font-family: var(--mono); font-size: 15px; color: var(--text-2); }
.tally b { color: var(--accent); }

/* figures */
svg.cg { width: 100%; max-width: 420px; height: auto; display: block; margin: 0 auto; background: var(--surface-deep); border-radius: 14px; cursor: zoom-in; }
figure { margin: 0; }
figcaption, .key { font-size: 14.5px; color: var(--text-2); margin-top: 8px; text-align: center; }
.key .sw { display: inline-block; width: 26px; margin-right: 6px; height: 0; border-top: 3px solid; vertical-align: middle; }
.key .sw.circ { border-color: #ffffff; } .key .sw.dash { border-top-style: dashed; border-color: #b0b0b0; }
.key .sw.ans { border-color: #00e5ff; } .key .sw.bad { border-color: #ff6b6b; } .key .sw.leg { border-top-style: dashed; border-color: #ffc46b; }
#cgZoom { position: fixed; inset: 0; z-index: 60; background: rgba(5,5,8,0.94); display: none; flex-direction: column; align-items: center; justify-content: center; gap: 2vh; cursor: zoom-out; padding: 3vh 3vw; }
#cgZoom.on { display: flex; }
#cgZoom svg { width: auto; height: min(84vh, 92vw); max-width: 92vw; }
#cgZoom p { color: #b0b0b0; font-family: var(--mono); font-size: 14px; margin: 0; }

/* labs (same look as in the decks) */
.lab { display: flex; gap: 20px; align-items: center; flex-wrap: wrap; background: var(--surface); border: 1px solid var(--accent-line);
  border-radius: 20px; padding: 18px; box-shadow: var(--shadow-card); }
.lab .fig, .lab canvas { display: block; width: min(400px, 100%); height: auto; flex: 0 0 auto; background: var(--surface-deep); border-radius: 14px; margin: 0 auto; }
.lab .panel { display: flex; flex-direction: column; gap: 9px; flex: 1 1 260px; min-width: 240px; }
.lab label { font-family: var(--mono); font-size: 14px; color: var(--text-2); letter-spacing: .03em; }
.lab input[type=range] { width: 100%; accent-color: var(--accent); cursor: pointer; }
.lab .read { font-family: var(--mono); font-size: 15.5px; line-height: 1.75; font-variant-numeric: tabular-nums; }
.lab .read .lw { color: var(--text-2); }
.lab .read b { color: var(--accent); }
.lab .verdict { font-family: var(--mono); font-size: 15px; font-weight: 600; border-radius: 12px; padding: 9px 12px; border: 1px solid var(--hairline); line-height: 1.45; }
.lab .verdict.ok { color: var(--ok); border-color: rgba(79,224,138,0.45); background: rgba(79,224,138,0.08); }
.lab .verdict.in { color: var(--accent); border-color: var(--accent-line); background: var(--accent-wash); }
.lab .verdict.no { color: var(--warn); border-color: rgba(255,107,107,0.45); background: rgba(255,107,107,0.08); }
.picks { display: flex; gap: 6px; flex-wrap: wrap; }
.picks button { font-family: var(--mono); font-size: 14px; cursor: pointer; color: var(--text); background: var(--surface-lift); border: 1px solid var(--hairline); border-radius: 50px; padding: 7px 12px; min-height: 40px; }
.picks button:hover { border-color: var(--accent-line); }
.picks button[aria-pressed="true"] { background: var(--accent-wash); border-color: var(--accent); color: var(--accent); }
.lab .toggle { align-self: flex-start; font-family: var(--mono); font-size: 14px; cursor: pointer; color: var(--accent); background: transparent; border: 1px solid var(--accent-line); border-radius: 50px; padding: 7px 12px; min-height: 40px; }
.lab .toggle[aria-pressed="true"] { background: var(--accent-wash); }
.lab .pr-hint { font-size: 13.5px; color: var(--text-2); }
.labhead { display: flex; align-items: baseline; gap: 10px; flex-wrap: wrap; }
.labhead .eyebrow { margin: 0; }

.toolchip { display: inline-flex; align-items: center; gap: 8px; font-family: var(--mono); font-size: 14px; color: var(--accent); text-decoration: none;
  border: 1px solid var(--accent-line); background: var(--accent-wash); border-radius: 50px; padding: 8px 14px; }
.toolchip:hover { box-shadow: var(--shadow-glow); }
.chips { display: flex; flex-wrap: wrap; gap: 8px; }

/* end of booklet */
.next { border-radius: 22px; border: 1px solid var(--accent-line); padding: 22px; background: radial-gradient(120% 140% at 0% 100%, rgba(0,229,255,0.10), rgba(0,0,0,0) 60%), var(--surface); }
.next > * + * { margin-top: 12px; }
footer.bk-foot { max-width: var(--col); margin: 0 auto; padding: 0 20px 60px; color: var(--text-2); font-size: 14px; }

/* ---------- phone ---------- */
@media (max-width: 720px) {
  main.bk { padding: 26px 16px 100px; }
  p, li { font-size: 17px; }
  .lead { font-size: 18px; }
  .two, .pair, .ghosts, .cover .can { grid-template-columns: 1fr; }
  .pair .h.right { margin-top: 6px; }
  .m { font-size: 17px; }
  .bk-bar { gap: 8px; padding: 8px 12px; }
  .bk-bar .bk-name { display: none; }
  .bk-bar .bk-meter { flex: 1 1 auto; }
  .marks > .mline { grid-template-columns: 52px 1fr; padding: 10px; }
  .exam .mk { float: none; display: block; }
}
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  .steps > .step.on, .predict.done .why-it, .try.open .sol { animation: none; }
  .marks > .mline, .bk-bar .bk-fill { transition: none; }
}

/* ---------- print: every step, answer and mark shown ---------- */
@media print {
  body { background: #fff; color: #111; }
  .bk-bar, .bk-toc, .acts, .self, .steps .more, .predict .opts, .marks .tot button, .lab input, .picks, .lab .toggle, .pr-hint, #cgZoom { display: none !important; }
  .steps > .step, .try .sol, .predict .why-it { display: block !important; animation: none !important; }
  .steps > .step.pair { display: grid !important; }
  .marks > .mline { opacity: 1 !important; filter: none !important; }
  h1 { background: none !important; color: #111 !important; -webkit-text-fill-color: #111; }
  h2, .eyebrow, .sec-head .num { color: #00607a !important; }
  .card, .try, .predict, .exam, .idea, .why, .ghost, .local, .lab, .cover, .next, .m { background: #fff !important; color: #111 !important; border-color: #bbb !important; box-shadow: none !important; break-inside: avoid; }
  p, li, .muted, .small, .lbl, figcaption { color: #222 !important; }
  .katex { color: #111; }
  section.bsec { break-before: auto; }
}