/* ============================================================
   System Design Fundamentals — shared lesson stylesheet
   Palette + type matched to the Alex Xu source decks, but laid
   out as a calm, print-friendly reading document (Tufte-ish):
   one narrow column, generous leading, restrained color.
   Linked by every lesson and reference doc. Reuse, don't inline.
   ============================================================ */

:root {
  --navy:  #0B1B3B;
  --deep:  #065A82;
  --teal:  #1C7293;
  --ice:   #CADCFC;
  --gold:  #F6AE2D;
  --coral: #E5564E;
  --green: #15803D;
  --bg:    #F5F9FC;
  --paper: #FFFFFF;
  --ink:   #0F172A;
  --mute:  #5A6B82;
  --line:  #D9E2EC;
  --rule:  #CBD5E1;
}

* { box-sizing: border-box; }

html { -webkit-text-size-adjust: 100%; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  font-size: 18px;
  line-height: 1.62;
  -webkit-font-smoothing: antialiased;
}

/* ---- Reading column ---------------------------------------- */
.lesson, .doc {
  max-width: 760px;
  margin: 0 auto;
  padding: 64px 28px 120px;
}

/* ---- Masthead ---------------------------------------------- */
.eyebrow {
  font-size: 12.5px;
  letter-spacing: 3px;
  text-transform: uppercase;
  color: var(--teal);
  font-weight: 700;
  margin: 0 0 14px;
}
h1.title {
  font-family: Georgia, 'Times New Roman', serif;
  font-size: 42px;
  line-height: 1.12;
  color: var(--navy);
  font-weight: 700;
  margin: 0 0 18px;
  letter-spacing: -0.01em;
}
.lede {
  font-size: 20px;
  line-height: 1.55;
  color: var(--mute);
  margin: 0 0 22px;
}
.meta {
  display: flex;
  flex-wrap: wrap;
  gap: 10px 18px;
  align-items: center;
  font-size: 13px;
  color: var(--mute);
  border-top: 1px solid var(--line);
  border-bottom: 1px solid var(--line);
  padding: 14px 0;
  margin: 0 0 40px;
}
.meta b { color: var(--navy); font-weight: 700; }

/* ---- Headings + prose -------------------------------------- */
h2 {
  font-family: Georgia, serif;
  font-size: 27px;
  color: var(--navy);
  margin: 52px 0 14px;
  line-height: 1.2;
}
h3 {
  font-size: 18px;
  color: var(--deep);
  margin: 32px 0 8px;
  letter-spacing: -0.005em;
}
p { margin: 0 0 18px; }
a { color: var(--deep); text-decoration-color: var(--ice); text-underline-offset: 2px; }
a:hover { text-decoration-color: var(--teal); }
strong { color: var(--navy); }
ul, ol { margin: 0 0 18px; padding-left: 24px; }
li { margin: 6px 0; }
code {
  font-family: 'SF Mono', 'JetBrains Mono', 'Fira Code', ui-monospace, Menlo, monospace;
  font-size: 0.86em;
  background: #EEF3F8;
  border: 1px solid var(--line);
  border-radius: 5px;
  padding: 1px 6px;
  color: var(--navy);
}
kbd {
  font-family: ui-monospace, Menlo, monospace;
  font-size: 0.8em;
  background: var(--navy);
  color: #fff;
  border-radius: 5px;
  padding: 2px 7px;
}

/* ---- Tags / pills ------------------------------------------ */
.tag {
  display: inline-block;
  font-size: 11.5px;
  font-weight: 700;
  letter-spacing: 0.6px;
  text-transform: uppercase;
  padding: 2px 9px;
  border-radius: 999px;
  vertical-align: middle;
}
.tag.break { background: #FDECEA; color: #B23029; border: 1px solid #F4C7C2; }
.tag.fix   { background: #E7F4EC; color: #176436; border: 1px solid #BDE2CB; }
.tag.term  { background: #EAF1FB; color: #1E3A8A; border: 1px solid #C9DAF4; }

/* ---- Callouts ---------------------------------------------- */
.callout {
  border-left: 4px solid var(--teal);
  background: var(--paper);
  border-radius: 0 10px 10px 0;
  padding: 16px 20px;
  margin: 24px 0;
  box-shadow: 0 1px 3px rgba(11,27,59,.05);
}
.callout p:last-child { margin-bottom: 0; }
.callout .lbl {
  font-size: 12px; font-weight: 700; letter-spacing: 1.5px;
  text-transform: uppercase; color: var(--teal); display: block; margin-bottom: 6px;
}
.callout.key  { border-color: var(--gold); }
.callout.key  .lbl { color: #B45309; }
.callout.warn { border-color: var(--coral); }
.callout.warn .lbl { color: var(--coral); }

/* ---- The "ladder" rung component (reusable) ---------------- */
.ladder { margin: 28px 0; border: 1px solid var(--line); border-radius: 14px; overflow: hidden; background: var(--paper); }
.rung {
  display: grid;
  grid-template-columns: 34px 1fr;
  gap: 14px;
  padding: 18px 20px;
  border-top: 1px solid var(--line);
}
.rung:first-child { border-top: none; }
.rung .n {
  grid-row: 1 / span 3;
  width: 34px; height: 34px; border-radius: 50%;
  background: var(--deep); color: #fff;
  display: flex; align-items: center; justify-content: center;
  font-weight: 700; font-size: 15px;
}
.rung .have { font-weight: 700; color: var(--navy); }
.rung .pair { font-size: 15.5px; color: var(--ink); margin-top: 3px; }
.rung .pair em { color: var(--mute); font-style: normal; }

/* ---- Generic card grid ------------------------------------- */
.cards { display: grid; gap: 16px; margin: 24px 0; }
@media (min-width: 620px) { .cards.two { grid-template-columns: 1fr 1fr; } }
.card {
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 12px;
  padding: 18px 20px;
}
.card h4 { margin: 0 0 6px; color: var(--navy); font-size: 16px; }
.card p { margin: 0; font-size: 15px; color: var(--ink); }

/* ---- Source / citation line -------------------------------- */
.src {
  font-size: 13.5px;
  color: var(--mute);
  border-left: 3px solid var(--line);
  padding: 2px 0 2px 12px;
  margin: 8px 0 22px;
}
.cite { font-size: 0.72em; vertical-align: super; line-height: 0; }

/* ============================================================
   Interactive widgets (driven by assets/quiz.js)
   ============================================================ */

/* Active-recall flashcards */
.sd-recall { display: grid; gap: 14px; margin: 22px 0; }
.sd-card {
  border: 1px solid var(--line);
  border-radius: 12px;
  background: var(--paper);
  padding: 18px 20px;
  cursor: pointer;
  transition: border-color .15s, box-shadow .15s;
}
.sd-card:hover { border-color: var(--teal); box-shadow: 0 2px 10px rgba(11,27,59,.06); }
.sd-card .prompt { font-weight: 600; color: var(--navy); }
.sd-card .hint { font-size: 13px; color: var(--mute); margin-top: 8px; }
.sd-card .answer {
  margin-top: 14px; padding-top: 14px; border-top: 1px dashed var(--rule);
  font-size: 15.5px; color: var(--ink); display: none;
}
.sd-card.open .answer { display: block; }
.sd-card.open .hint { display: none; }

/* Multiple-choice quiz */
.sd-quiz { margin: 22px 0; }
.sd-q {
  border: 1px solid var(--line);
  border-radius: 12px;
  background: var(--paper);
  padding: 20px 22px;
  margin-bottom: 16px;
}
.sd-q .stem { font-weight: 600; color: var(--navy); margin-bottom: 14px; }
.sd-q .stem .qn { color: var(--teal); font-weight: 700; margin-right: 8px; }
.sd-opt {
  display: block; width: 100%; text-align: left;
  font: inherit; color: var(--ink);
  background: var(--bg);
  border: 1px solid var(--line);
  border-radius: 9px;
  padding: 11px 15px;
  margin: 8px 0;
  cursor: pointer;
  transition: background .12s, border-color .12s;
}
.sd-opt:hover:not(:disabled) { border-color: var(--teal); background: #fff; }
.sd-opt:disabled { cursor: default; }
.sd-opt.correct { background: #E7F4EC; border-color: var(--green); color: #14532D; font-weight: 600; }
.sd-opt.wrong   { background: #FDECEA; border-color: var(--coral); color: #7F1D1D; }
.sd-explain {
  display: none;
  margin-top: 12px; padding: 12px 15px;
  background: #F1F6FB; border: 1px solid var(--line); border-radius: 9px;
  font-size: 15px; color: var(--ink);
}
.sd-explain.show { display: block; }
.sd-explain b { color: var(--navy); }
.sd-score {
  font-size: 15px; font-weight: 600; color: var(--navy);
  background: var(--ice); border-radius: 999px;
  padding: 8px 18px; display: inline-block; margin-top: 6px;
}

/* ---- Teacher reminder + footer ----------------------------- */
.ask-teacher {
  margin: 48px 0 0;
  background: linear-gradient(135deg, #0B1B3B 0%, #112a52 100%);
  color: var(--ice);
  border-radius: 14px;
  padding: 24px 26px;
}
.ask-teacher h3 { color: var(--gold); margin: 0 0 8px; }
.ask-teacher p { margin: 0; color: var(--ice); font-size: 15.5px; }

.lesson-foot {
  margin-top: 40px; padding-top: 20px;
  border-top: 1px solid var(--line);
  display: flex; flex-wrap: wrap; gap: 12px; justify-content: space-between;
  font-size: 13.5px; color: var(--mute);
}
.lesson-foot a { font-weight: 600; }

/* ---- Glossary (reference doc) ------------------------------ */
.glossary { margin: 24px 0; }
.gloss-term {
  border-top: 1px solid var(--line);
  padding: 18px 0;
}
.gloss-term:first-child { border-top: none; }
.gloss-term dt {
  font-family: Georgia, serif; font-size: 19px; color: var(--navy); font-weight: 700;
}
.gloss-term .also { font-size: 13px; color: var(--mute); font-style: italic; margin-left: 8px; }
.gloss-term dd { margin: 6px 0 0; font-size: 16px; }
.gloss-term .when { display: block; margin-top: 6px; font-size: 14.5px; color: var(--teal); }
.gloss-term .when b { color: var(--deep); }

/* ============================================================
   Diagrams — reusable SVG component (shared by every chapter)
   Define the marker once per page in a hidden <svg>, then every
   figure references url(#arrow). Node + connector classes give
   all chapter diagrams one consistent visual vocabulary.
   ============================================================ */
figure.diagram {
  margin: 30px 0;
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 22px 22px 12px;
  box-shadow: 0 1px 3px rgba(11,27,59,.05);
  break-inside: avoid;
}
figure.diagram svg { width: 100%; height: auto; display: block; }
figure.diagram figcaption {
  margin-top: 14px;
  padding-top: 12px;
  border-top: 1px solid var(--line);
  font-size: 13.5px;
  line-height: 1.5;
  color: var(--mute);
  text-align: center;
}
figure.diagram figcaption b { color: var(--navy); }

/* Two diagrams side by side on wide screens (e.g. SQL vs NoSQL) */
.figure-pair { display: grid; gap: 18px; margin: 30px 0; }
@media (min-width: 640px) { .figure-pair { grid-template-columns: 1fr 1fr; } }
.figure-pair figure.diagram { margin: 0; }

/* SVG node + connector vocabulary */
.node-box   { fill: var(--paper); stroke: var(--deep);  stroke-width: 2; }
.node-db    { fill: #E0F2FE; stroke: var(--deep);  stroke-width: 2; }
.node-cache { fill: #FEF3C7; stroke: #B45309; stroke-width: 2; }
.node-cdn   { fill: #FAE8FF; stroke: #86198F; stroke-width: 2; }
.node-lb    { fill: #DBEAFE; stroke: #1E40AF; stroke-width: 2; }
.node-mq    { fill: #DCFCE7; stroke: var(--green); stroke-width: 2; }
.node-user  { fill: #F1F5F9; stroke: #334155; stroke-width: 2; }
.node-warn  { fill: #FEE2E2; stroke: #B91C1C; stroke-width: 2; }
.node-soft  { fill: var(--bg);  stroke: var(--rule); stroke-width: 1.5; }
.arrow      { stroke: #475569; stroke-width: 1.7; fill: none; marker-end: url(#arrow); }
.arrow-dash { stroke: #475569; stroke-width: 1.7; fill: none; marker-end: url(#arrow); stroke-dasharray: 5,4; }
.svg-title  { font: 700 15px Inter, sans-serif; fill: var(--navy); }
.svg-label  { font: 600 13px Inter, sans-serif; fill: var(--ink); }
.svg-sub    { font: 500 11px Inter, sans-serif; fill: var(--mute); }
.svg-em     { font: italic 500 11px Inter, sans-serif; fill: var(--mute); }
.svg-step   { font: 700 11px Inter, sans-serif; fill: var(--teal); }

/* ---- "Jump to slide N" chip + companion-deck button -------- */
.slidelink {
  display: inline-flex; align-items: center; gap: 5px;
  font-size: 11px; font-weight: 700; letter-spacing: .4px;
  text-transform: uppercase;
  color: var(--deep); text-decoration: none;
  background: var(--ice); border: 1px solid #C9DAF4;
  border-radius: 999px; padding: 3px 11px;
  vertical-align: middle; margin-left: 12px; white-space: nowrap;
}
.slidelink:hover { background: #DCE8FB; text-decoration: none; }
h2 .slidelink { font-size: 11px; position: relative; top: -3px; }

.deck-cta {
  display: inline-flex; align-items: center; gap: 9px;
  font-size: 14.5px; font-weight: 700;
  color: #fff; text-decoration: none;
  background: linear-gradient(135deg, #0B1B3B 0%, #112a52 100%);
  border-radius: 11px; padding: 11px 18px; margin: 4px 0 8px;
}
.deck-cta:hover { background: #16386a; text-decoration: none; }
.deck-cta .play {
  width: 22px; height: 22px; border-radius: 50%;
  background: var(--gold); color: var(--navy);
  display: inline-flex; align-items: center; justify-content: center;
  font-size: 11px;
}

/* ============================================================
   Data components — reusable across number-heavy chapters:
   tables, formula blocks, worked-example steps, stat chips.
   ============================================================ */

/* ---- Data table -------------------------------------------- */
table.tbl {
  width: 100%; border-collapse: collapse; margin: 26px 0;
  background: var(--paper); border: 1px solid var(--line);
  border-radius: 12px; overflow: hidden;
  font-size: 15px;
  box-shadow: 0 1px 3px rgba(11,27,59,.05);
}
table.tbl th {
  background: var(--navy); color: #fff; text-align: left;
  padding: 10px 14px; font-size: 11.5px; letter-spacing: 1.2px;
  text-transform: uppercase; font-weight: 700;
}
table.tbl td { padding: 9px 14px; border-top: 1px solid var(--line); vertical-align: middle; }
table.tbl tbody tr:nth-child(even) td { background: #F4F8FC; }
table.tbl td.num {
  font-family: 'SF Mono', ui-monospace, Menlo, monospace;
  font-weight: 600; color: var(--deep); white-space: nowrap;
}
table.tbl td.mut {
  font-family: 'SF Mono', ui-monospace, Menlo, monospace;
  color: var(--mute); font-size: 13px; white-space: nowrap;
}
.bar {
  display: inline-block; height: 8px; border-radius: 4px;
  background: linear-gradient(90deg, var(--teal), var(--coral));
  vertical-align: middle;
}

/* ---- Formula / math block ---------------------------------- */
.formula {
  background: var(--navy); color: #fff;
  border-radius: 12px; padding: 16px 20px; margin: 24px 0;
  font-family: 'SF Mono', 'JetBrains Mono', ui-monospace, Menlo, monospace;
  font-size: 15.5px; line-height: 1.75; overflow-x: auto;
}
.formula .v { color: var(--gold); }            /* variable */
.formula .o { color: #8FD3E8; }                /* operator */
.formula .r { color: #86EFAC; }                /* result   */

/* ---- Worked-example steps ---------------------------------- */
.steps { margin: 24px 0; }
.step {
  background: var(--paper); border: 1px solid var(--line);
  border-left: 4px solid var(--teal);
  border-radius: 0 10px 10px 0; padding: 11px 18px; margin: 10px 0;
  box-shadow: 0 1px 3px rgba(11,27,59,.05);
}
.step .lbl {
  font-size: 11px; letter-spacing: 1.5px; text-transform: uppercase;
  color: var(--teal); font-weight: 700; margin-bottom: 4px;
}
.step .calc {
  font-family: 'SF Mono', ui-monospace, Menlo, monospace;
  font-size: 14.5px; color: var(--ink); line-height: 1.55;
}
.step .calc b { color: var(--coral); font-weight: 700; }
.step.concl { border-left-color: var(--gold); background: #FFFCF5; }
.step.concl .lbl { color: #B45309; }

/* ---- Stat chips (powers of two, key constants) ------------- */
.chips {
  display: grid; gap: 12px; margin: 24px 0;
  grid-template-columns: repeat(auto-fit, minmax(118px, 1fr));
}
.chip {
  background: var(--paper); border: 1px solid var(--line);
  border-radius: 10px; padding: 12px 14px;
}
.chip .exp {
  font-family: 'SF Mono', ui-monospace, Menlo, monospace;
  font-size: 13px; color: var(--teal); font-weight: 700;
}
.chip .big { font-family: Georgia, serif; font-size: 22px; color: var(--navy); margin: 3px 0 1px; }
.chip .cap { font-size: 11px; color: var(--mute); text-transform: uppercase; letter-spacing: .6px; }

/* ---- Time-budget / proportion bars ------------------------- */
.timeline { margin: 24px 0; }
.tl-row {
  display: grid; grid-template-columns: 210px 1fr 60px;
  align-items: center; gap: 14px; margin: 10px 0;
}
.tl-row .tl-lbl { font-size: 14px; color: var(--ink); font-weight: 600; }
.tl-row .tl-lbl small { display: block; font-weight: 400; color: var(--mute); font-size: 12px; margin-top: 2px; }
.tl-track { background: #EAF1FB; height: 20px; border-radius: 5px; overflow: hidden; }
.tl-track > span {
  display: block; height: 100%; border-radius: 5px;
  background: linear-gradient(90deg, var(--deep), var(--teal));
}
.tl-row .tl-track.alt > span { background: linear-gradient(90deg, var(--gold), #E8B85B); }
.tl-row .tl-track.mut > span { background: #9CA3AF; }
.tl-min { font-size: 13px; font-weight: 700; color: var(--mute); text-align: right; font-variant-numeric: tabular-nums; }
@media (max-width: 560px) {
  .tl-row { grid-template-columns: 1fr 56px; }
  .tl-row .tl-track { grid-column: 1 / -1; order: 3; }
}

/* ============================================================
   Read-aloud player — reusable text-to-speech (Web Speech API),
   driven by assets/readaloud.js. Speaks every .ra-text block and
   highlights the spoken sentence. Used by the Hinglish lessons,
   but language-agnostic.
   ============================================================ */
.readaloud {
  position: sticky; top: 8px; z-index: 30;
  display: flex; flex-wrap: wrap; align-items: center; gap: 8px 12px;
  background: rgba(255,255,255,.94);
  -webkit-backdrop-filter: blur(8px); backdrop-filter: blur(8px);
  border: 1px solid var(--line); border-radius: 12px;
  padding: 10px 14px; margin: 0 0 30px;
  box-shadow: 0 4px 16px rgba(11,27,59,.08);
}
.readaloud .ra-title {
  font-size: 12px; font-weight: 700; letter-spacing: .5px;
  text-transform: uppercase; color: var(--teal);
}
.ra-btn {
  font: 600 14px Inter, sans-serif;
  display: inline-flex; align-items: center; gap: 6px;
  border: 1px solid var(--line); background: var(--paper); color: var(--navy);
  border-radius: 9px; padding: 7px 13px; cursor: pointer;
  transition: background .12s, border-color .12s;
}
.ra-btn:hover { border-color: var(--teal); }
.ra-btn.primary { background: var(--deep); border-color: var(--deep); color: #fff; }
.ra-btn.primary:hover { background: #054a6b; }
.ra-btn:disabled { opacity: .45; cursor: default; }
.ra-field { display: inline-flex; align-items: center; gap: 6px; font-size: 13px; color: var(--mute); }
.ra-field select, .ra-field input { font: inherit; }
.ra-field select {
  max-width: 200px; border: 1px solid var(--line); border-radius: 8px;
  padding: 6px 8px; background: var(--paper); color: var(--ink);
}
.ra-hint { font-size: 12.5px; color: var(--mute); flex-basis: 100%; margin: 2px 0 0; }

/* Block + sentence highlighting while speaking */
.ra-block { scroll-margin-top: 90px; border-radius: 10px; }
.ra-block.ra-speaking {
  background: linear-gradient(90deg, rgba(28,114,147,.07), transparent 65%);
  box-shadow: -16px 0 0 rgba(28,114,147,.07);
}
.ra-listen {
  display: inline-flex; align-items: center; gap: 5px;
  font: 700 11px Inter, sans-serif; letter-spacing: .4px; text-transform: uppercase;
  color: var(--teal); background: #EAF4F7; border: 1px solid #C5E0E8;
  border-radius: 999px; padding: 3px 11px; margin-left: 10px; cursor: pointer;
  vertical-align: middle; white-space: nowrap;
}
.ra-listen:hover { background: #DCEDF2; }
.ra-listen.playing { background: var(--teal); color: #fff; border-color: var(--teal); }
h2 .ra-listen { position: relative; top: -3px; }
.ra-sent { border-radius: 4px; transition: background .15s, color .15s; }
.ra-sent.ra-on { background: var(--gold); color: var(--navy); box-shadow: 0 0 0 3px var(--gold); }

/* ---- Print ------------------------------------------------- */
@media print {
  body { background: #fff; font-size: 12pt; }
  .lesson, .doc { max-width: none; padding: 0; }
  .ask-teacher { background: none; color: var(--ink); border: 1px solid var(--line); }
  .ask-teacher h3, .ask-teacher p { color: var(--ink); }
  .sd-card .answer { display: block; }
  .sd-explain { display: block; }
  .sd-opt { background: #fff; }
  a { color: var(--ink); text-decoration: none; }
  .slidelink, .deck-cta, .readaloud, .ra-listen { display: none; }
  .ra-sent.ra-on { background: none; box-shadow: none; }
  figure.diagram { box-shadow: none; break-inside: avoid; }
}