/* EC Problem Explorer — styles.
   System fonts only (no webfont/CDN dependency, consistent with the
   vendored-KaTeX, zero-CDN philosophy of the rest of the app) — except
   Atkinson Hyperlegible below, which is also vendored locally and used
   only when accessible mode is switched on. */

/* ---- Atkinson Hyperlegible (vendored; accessible-mode only) ----
   Same two-subset-per-style structure Google Fonts itself serves this
   family in; unicode-range lets the browser fetch only what it needs. */
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-Regular-latinext.woff2") format("woff2");
  unicode-range: U+0100-02BA, U+02BD-02C5, U+02C7-02CC, U+02CE-02D7, U+02DD-02FF, U+0304, U+0308, U+0329, U+1D00-1DBF, U+1E00-1E9F, U+1EF2-1EFF, U+2020, U+20A0-20AB, U+20AD-20C0, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-Regular-latin.woff2") format("woff2");
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: italic;
  font-weight: 400;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-Italic-latinext.woff2") format("woff2");
  unicode-range: U+0100-02BA, U+02BD-02C5, U+02C7-02CC, U+02CE-02D7, U+02DD-02FF, U+0304, U+0308, U+0329, U+1D00-1DBF, U+1E00-1E9F, U+1EF2-1EFF, U+2020, U+20A0-20AB, U+20AD-20C0, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: italic;
  font-weight: 400;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-Italic-latin.woff2") format("woff2");
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: normal;
  font-weight: 700;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-Bold-latinext.woff2") format("woff2");
  unicode-range: U+0100-02BA, U+02BD-02C5, U+02C7-02CC, U+02CE-02D7, U+02DD-02FF, U+0304, U+0308, U+0329, U+1D00-1DBF, U+1E00-1E9F, U+1EF2-1EFF, U+2020, U+20A0-20AB, U+20AD-20C0, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: normal;
  font-weight: 700;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-Bold-latin.woff2") format("woff2");
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: italic;
  font-weight: 700;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-BoldItalic-latinext.woff2") format("woff2");
  unicode-range: U+0100-02BA, U+02BD-02C5, U+02C7-02CC, U+02CE-02D7, U+02DD-02FF, U+0304, U+0308, U+0329, U+1D00-1DBF, U+1E00-1E9F, U+1EF2-1EFF, U+2020, U+20A0-20AB, U+20AD-20C0, U+2113, U+2C60-2C7F, U+A720-A7FF;
}
@font-face {
  font-family: "Atkinson Hyperlegible";
  font-style: italic;
  font-weight: 700;
  font-display: swap;
  src: url("../vendor/atkinson-hyperlegible/AtkinsonHyperlegible-BoldItalic-latin.woff2") format("woff2");
  unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+0304, U+0308, U+0329, U+2000-206F, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
}

:root {
  --bg: #f7f8fa;
  --surface: #ffffff;
  --surface-2: #eef0f4;
  --ink: #1a1d22;
  --ink-muted: #5b6470;
  --ink-faint: #889098;
  --line: #dde1e7;
  --accent: #2452a3;
  --accent-ink: #1b3d7a;
  --accent-contrast: #ffffff;
  --tone-hint: #0f766e;
  --tone-hint-bg: #e8f4f2;
  --tone-solution: #9a3412;
  --tone-solution-bg: #fbeee6;
  --shadow: 0 1px 2px rgba(20, 30, 50, 0.05), 0 6px 20px -10px rgba(20, 30, 50, 0.15);
  --radius: 10px;
  --font-body: -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  --font-heading: Georgia, "Times New Roman", Cambria, serif;
  --body-line-height: 1.6;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-a11y="on"]) {
    --bg: #14171c;
    --surface: #1b1f26;
    --surface-2: #232833;
    --ink: #e8eaed;
    --ink-muted: #9aa3ad;
    --ink-faint: #6b7480;
    --line: #2c323d;
    --accent: #7aa2e3;
    --accent-ink: #a9c3ee;
    --accent-contrast: #0e1116;
    --tone-hint: #4fbfa8;
    --tone-hint-bg: #172a27;
    --tone-solution: #e0965a;
    --tone-solution-bg: #2c2016;
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.3), 0 6px 20px -10px rgba(0, 0, 0, 0.5);
  }
}

/* ---- Accessible mode: color-vision-safe palette (Okabe–Ito) + Atkinson
   Hyperlegible type. The three semantic hues (accent/hint/solution) are
   drawn from Okabe–Ito specifically because that palette is documented to
   stay mutually distinguishable under protanopia, deuteranopia, and
   tritanopia — not just "probably fine" colors. Text contrast is pushed
   toward AAA (near-black-on-white / near-white-on-black) as a second,
   independent accessibility axis alongside hue-safety. */
:root[data-a11y="on"] {
  --bg: #ffffff;
  --surface: #ffffff;
  --surface-2: #ececec;
  --ink: #000000;
  --ink-muted: #3a3a3a;
  --ink-faint: #595959;
  --line: #767676;
  --accent: #0072b2;
  --accent-ink: #00507c;
  --accent-contrast: #ffffff;
  --tone-hint: #007a58;
  --tone-hint-bg: #e1f4ee;
  --tone-solution: #ad4c00;
  --tone-solution-bg: #fbe9dc;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.15), 0 6px 20px -10px rgba(0, 0, 0, 0.3);
  --font-body: "Atkinson Hyperlegible", -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  --font-heading: "Atkinson Hyperlegible", -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  --body-line-height: 1.7;
}

@media (prefers-color-scheme: dark) {
  :root[data-a11y="on"] {
    --bg: #000000;
    --surface: #0a0a0a;
    --surface-2: #1c1c1c;
    --ink: #ffffff;
    --ink-muted: #d9d9d9;
    --ink-faint: #b3b3b3;
    --line: #8f8f8f;
    --accent: #56b4e9;
    --accent-ink: #8ad0f5;
    --accent-contrast: #00131c;
    --tone-hint: #4ddba8;
    --tone-hint-bg: #072019;
    --tone-solution: #ff9d4d;
    --tone-solution-bg: #291505;
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.4), 0 6px 20px -10px rgba(0, 0, 0, 0.6);
  }
}

* { box-sizing: border-box; }

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

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: var(--font-body);
  font-size: 16px;
  line-height: var(--body-line-height);
}

h1, h2, h3 {
  font-family: var(--font-heading);
  font-weight: 700;
  line-height: 1.25;
  margin: 0 0 0.4em;
}
h1 { font-size: 1.9rem; }
h2 { font-size: 1.4rem; }

p { margin: 0 0 1em; }

a { color: var(--accent-ink); text-decoration-color: var(--line); text-underline-offset: 3px; }
a:hover { text-decoration-color: currentColor; }

:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 3px;
}

.skip-link {
  position: absolute;
  left: -999px;
  top: 0;
  background: var(--accent);
  color: var(--accent-contrast);
  padding: 10px 16px;
  z-index: 10;
  border-radius: 0 0 6px 0;
}
.skip-link:focus { left: 0; }

#app-root {
  max-width: 760px;
  margin: 0 auto;
  padding: 32px 20px 80px;
}
#app-root:focus { outline: none; }

/* ---------------------------------------------------------------- header */
.view-header { margin-bottom: 28px; }
.view-lede { color: var(--ink-muted); font-size: 1.05rem; max-width: 60ch; }
.filter-banner {
  display: inline-block;
  margin: 6px 0 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: var(--surface-2);
  color: var(--accent-ink);
  font-size: 0.82rem;
  font-weight: 600;
}
.empty-note { color: var(--ink-muted); font-style: italic; }

.breadcrumb {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  align-items: center;
  font-size: 0.85rem;
  color: var(--ink-faint);
  margin-bottom: 20px;
}
.breadcrumb-link { color: var(--ink-muted); }
.breadcrumb-current { color: var(--ink); font-weight: 600; }
.breadcrumb-sep { color: var(--ink-faint); }

/* ----------------------------------------------------------------- btns */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 11px 20px;
  border-radius: 999px;
  border: 1px solid var(--line);
  background: var(--surface);
  color: var(--ink);
  font-size: 0.95rem;
  font-weight: 600;
  cursor: pointer;
  text-decoration: none;
  min-height: 44px;
}
.btn:hover { border-color: var(--accent); color: var(--accent-ink); }

.btn-random {
  background: var(--accent);
  color: var(--accent-contrast);
  border-color: var(--accent);
}
.btn-random:hover { opacity: 0.92; color: var(--accent-contrast); }
.btn-random-hero { margin-top: 18px; font-size: 1.02rem; padding: 13px 24px; }

.btn-secondary { background: var(--surface); }

/* ------------------------------------------------------------- topic grid */
.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 16px;
}
.topic-card {
  display: flex;
  flex-direction: column;
  gap: 6px;
  padding: 20px;
  border-radius: var(--radius);
  border: 1px solid var(--line);
  background: var(--surface);
  box-shadow: var(--shadow);
  text-decoration: none;
  color: var(--ink);
}
.topic-card:hover { border-color: var(--accent); }
.topic-card-title { font-size: 1.15rem; margin: 0; }
.topic-card-desc { color: var(--ink-muted); font-size: 0.92rem; margin: 0; flex: 1; }
.topic-card-count {
  align-self: flex-start;
  margin-top: 6px;
  font-size: 0.78rem;
  font-weight: 600;
  letter-spacing: 0.03em;
  text-transform: uppercase;
  color: var(--accent-ink);
  background: var(--surface-2);
  padding: 4px 10px;
  border-radius: 999px;
}

/* --------------------------------------------------------------- sources */
.sources-section { margin-top: 44px; padding-top: 32px; border-top: 1px solid var(--line); }
.sources-heading { font-size: 1.3rem; margin: 0 0 6px; }
.source-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 16px;
  margin-top: 20px;
}
.source-card {
  display: flex;
  flex-direction: column;
  gap: 4px;
  padding: 18px;
  border-radius: var(--radius);
  border: 1px solid var(--line);
  background: var(--surface);
  text-decoration: none;
  color: var(--ink);
}
.source-card:hover { border-color: var(--accent); }
.source-card-title { font-size: 1.05rem; margin: 0; }
.source-card-subtitle { color: var(--accent-ink); font-size: 0.82rem; font-weight: 600; margin: 0; }
.source-card-desc { color: var(--ink-muted); font-size: 0.88rem; margin: 4px 0 8px; flex: 1; }

.source-topic-group { margin-bottom: 32px; }
.source-topic-heading { font-size: 1.1rem; margin: 0 0 14px; color: var(--ink-muted); }

/* ---------------------------------------------------------------- problem list */
.problem-list { list-style: none; margin: 0; padding: 0; display: grid; gap: 10px; }
.problem-list-link {
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 14px 16px;
  border-radius: var(--radius);
  border: 1px solid var(--line);
  background: var(--surface);
  text-decoration: none;
  color: var(--ink);
}
.problem-list-link:hover { border-color: var(--accent); }
.problem-list-index {
  font-family: Georgia, serif;
  font-weight: 700;
  color: var(--ink-faint);
  min-width: 1.4em;
}
.problem-list-title { font-weight: 600; }

/* ------------------------------------------------------------- statement */
.problem-statement {
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 24px;
  margin-bottom: 18px;
  font-size: 1.05rem;
  line-height: 1.75;
  /* Preserves the blank-line breaks authored in data.js between lettered
     parts (a), (b), ... without collapsing them to a single space. */
  white-space: pre-line;
}

.attribution {
  border-left: 3px solid var(--line);
  padding: 4px 0 4px 16px;
  margin-bottom: 30px;
  color: var(--ink-muted);
  font-size: 0.88rem;
}
.attribution-line { margin: 0 0 4px; }

/* ------------------------------------------------------------ reveal gate */
.reveal-section { display: grid; gap: 12px; margin-bottom: 32px; }

.reveal-trigger {
  width: 100%;
  text-align: left;
  padding: 13px 18px;
  border-radius: var(--radius);
  border: 1px solid var(--line);
  background: var(--surface);
  color: var(--ink);
  font-size: 0.98rem;
  font-weight: 600;
  cursor: pointer;
  min-height: 44px;
}
.reveal-hint .reveal-trigger:hover { border-color: var(--tone-hint); color: var(--tone-hint); }
.reveal-solution .reveal-trigger:hover { border-color: var(--tone-solution); color: var(--tone-solution); }

.reveal-panel { margin-top: 8px; }

.reveal-warning {
  padding: 14px 16px;
  border-radius: var(--radius);
  font-size: 0.92rem;
  margin: 0 0 12px;
}
.reveal-hint .reveal-warning { background: var(--tone-hint-bg); color: var(--tone-hint); }
.reveal-solution .reveal-warning { background: var(--tone-solution-bg); color: var(--tone-solution); }

.reveal-actions { display: flex; gap: 10px; flex-wrap: wrap; }
.reveal-confirm, .reveal-cancel {
  padding: 10px 18px;
  border-radius: 999px;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  min-height: 44px;
  border: 1px solid var(--line);
  background: var(--surface);
  color: var(--ink);
}
.reveal-hint .reveal-confirm { background: var(--tone-hint); border-color: var(--tone-hint); color: #fff; }
.reveal-solution .reveal-confirm { background: var(--tone-solution); border-color: var(--tone-solution); color: #fff; }

.reveal-content {
  padding: 16px 18px;
  border-radius: var(--radius);
  border: 1px dashed var(--line);
  background: var(--surface-2);
  line-height: 1.7;
  white-space: pre-line;
}
.reveal-content:focus { outline: none; }
.solution-unwritten { color: var(--ink-muted); font-style: italic; margin: 0; }

.problem-footer-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 24px;
}

.colophon {
  margin-top: 48px;
  padding-top: 20px;
  border-top: 1px solid var(--line);
  color: var(--ink-faint);
  font-size: 0.8rem;
}

/* KaTeX inherits surrounding color rather than forcing black, so it reads
   correctly in dark mode too. */
.katex { color: inherit; }

/* ------------------------------------------------------------- a11y bar */
.topbar {
  border-bottom: 1px solid var(--line);
  background: var(--surface);
}
.topbar-inner {
  max-width: 760px;
  margin: 0 auto;
  padding: 10px 20px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  flex-wrap: wrap;
}

.source-filter {
  display: inline-flex;
  align-items: center;
  gap: 8px;
}
.source-filter-label {
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--ink-muted);
}
.source-filter-select {
  font-family: var(--font-body);
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--ink);
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: 999px;
  padding: 6px 30px 6px 12px;
  min-height: 36px;
  cursor: pointer;
  /* a plain, theme-aware chevron so the control still reads as a dropdown
     without pulling in an image asset */
  appearance: none;
  -webkit-appearance: none;
  background-image: linear-gradient(45deg, transparent 50%, currentColor 50%), linear-gradient(135deg, currentColor 50%, transparent 50%);
  background-position: calc(100% - 16px) center, calc(100% - 11px) center;
  background-size: 5px 5px, 5px 5px;
  background-repeat: no-repeat;
  color: var(--ink-muted);
}
.source-filter-select:hover { border-color: var(--accent); }

.a11y-toggle {
  display: inline-flex;
  align-items: center;
  gap: 9px;
  border: 1px solid var(--line);
  background: var(--surface);
  color: var(--ink);
  border-radius: 999px;
  padding: 6px 14px 6px 6px;
  cursor: pointer;
  font-family: var(--font-body);
  font-size: 0.82rem;
  font-weight: 600;
  min-height: 36px;
}
.a11y-toggle:hover { border-color: var(--accent); }

.a11y-toggle-track {
  position: relative;
  width: 34px;
  height: 20px;
  border-radius: 999px;
  background: var(--line);
  flex-shrink: 0;
  transition: background-color 0.15s ease;
}
.a11y-toggle-thumb {
  position: absolute;
  top: 2px;
  left: 2px;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  background: var(--surface);
  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.3);
  transition: transform 0.15s ease;
}
.a11y-toggle[aria-pressed="true"] .a11y-toggle-track { background: var(--accent); }
.a11y-toggle[aria-pressed="true"] .a11y-toggle-thumb { transform: translateX(14px); }

.a11y-toggle-state {
  font-variant-numeric: tabular-nums;
  color: var(--ink-muted);
  min-width: 2.2em;
}
.a11y-toggle[aria-pressed="true"] .a11y-toggle-state { color: var(--accent-ink); }

/* --------------------------------------------------------------- mobile */
@media (max-width: 560px) {
  #app-root { padding: 20px 14px 60px; }
  h1 { font-size: 1.5rem; }
  .problem-statement { padding: 18px; font-size: 1rem; }
  .problem-footer-nav { flex-direction: column; align-items: stretch; }
  .problem-footer-nav .btn { justify-content: center; }
  .reveal-actions { flex-direction: column; }
  .reveal-confirm, .reveal-cancel { width: 100%; justify-content: center; }
  .topbar-inner { justify-content: center; padding: 8px 14px; }
  .a11y-toggle-text { display: none; }
  .source-filter-label { display: none; }
  .source-grid { grid-template-columns: 1fr; }
}

/* ---------------------------------------------------------------- print */
@media print {
  .topbar, .breadcrumb, .btn-random, .problem-footer-nav, .skip-link { display: none; }
  .reveal-trigger { display: none; }
  .reveal-panel:not(:has(.reveal-content)) { display: none; }
  body { background: #fff; color: #000; }
  .problem-statement, .attribution { box-shadow: none; border-color: #ccc; }
}