/* =====================================================================
   SGA — typographic system (loaded via book.toml additional-css, so it
   cascades AFTER mdBook's variables/general/chrome and wins). Mirrors the
   gentle-iut reading surface.

   SCALE NOTE: mdBook sets :root { font-size: 62.5% }, so 1rem = 10px.
   "20px body" is therefore 2rem; sizes below are spelled out accordingly.

   Strategy: a comfortable on-screen reading serif for the prose (STIX Two
   Text), paired with KaTeX's Computer-Modern math. STIX is the
   scientific-publishing companion to that math tradition, so the inline
   math sits naturally in the line without the thin Computer-Modern body
   that tired the eye over a long read.
   ===================================================================== */

/* ---------------------------------------------------------------------
   1. Prose font — self-hosted, so first paint lands in final metrics.

   Body is STIX Two Text (STIX v2.0.2, vendored under fonts/ → /fonts/):
   a Times-heritage scientific-publishing serif that reads comfortably on
   screen for long-form text and — unlike Computer Modern — actually covers
   the subscripts (₁), blackboard bold (ℚ ℝ ℤ ℂ) and primes the prose uses
   inline, so those no longer drop to a mismatched system fallback mid-word.

   STIX Two Text deliberately omits the heavy math operators and arrows
   (those live in STIX Two Math); CMU Serif is kept *below* it in the
   stack purely as the glyph fallback for the literal → ← ∗ √ … that
   appear in running prose, so they match the Computer-Modern math KaTeX
   renders. font-display:block (with the preloads in head.hbs) avoids a
   fallback→STIX reflow.
   --------------------------------------------------------------------- */
@font-face {
  font-family: "STIX Two Text";
  font-style: normal;
  font-weight: 400 500;
  font-display: block;
  src: url("../../fonts/stix-two-text-400-roman.woff2") format("woff2");
}
@font-face {
  font-family: "STIX Two Text";
  font-style: italic;
  font-weight: 400 500;
  font-display: block;
  src: url("../../fonts/stix-two-text-400-italic.woff2") format("woff2");
}
@font-face {
  font-family: "STIX Two Text";
  font-style: normal;
  font-weight: 600 700;
  font-display: block;
  src: url("../../fonts/stix-two-text-700-roman.woff2") format("woff2");
}
@font-face {
  font-family: "STIX Two Text";
  font-style: italic;
  font-weight: 600 700;
  font-display: block;
  src: url("../../fonts/stix-two-text-700-italic.woff2") format("woff2");
}

/* CMU Serif — kept ONLY as the symbol fallback for the arrows/operators
   STIX Two Text lacks; never preloaded, fetched lazily if such a glyph
   appears. Its Computer-Modern forms match the KaTeX math on the page. */
@font-face {
  font-family: "CMU Serif";
  font-style: normal;
  font-weight: 400 500;
  font-display: block;
  src: url("../../fonts/cmu-serif-500-roman.woff2") format("woff2");
}
@font-face {
  font-family: "CMU Serif";
  font-style: italic;
  font-weight: 400 500;
  font-display: block;
  src: url("../../fonts/cmu-serif-500-italic.woff2") format("woff2");
}
@font-face {
  font-family: "CMU Serif";
  font-style: normal;
  font-weight: 600 700;
  font-display: block;
  src: url("../../fonts/cmu-serif-700-roman.woff2") format("woff2");
}
@font-face {
  font-family: "CMU Serif";
  font-style: italic;
  font-weight: 600 700;
  font-display: block;
  src: url("../../fonts/cmu-serif-700-italic.woff2") format("woff2");
}

:root {
  --gi-prose:
    "STIX Two Text", "CMU Serif", Georgia, Cambria, "Times New Roman", serif;
  --gi-accent: var(--links);
}

/* ---------------------------------------------------------------------
   2. Warm, reduced-contrast palette. Pure #000/#fff haloes around the
   thin CM serifs and tires the eye over a long read; warm near-black on
   warm paper stays crisp without glare (~15:1, past WCAG AAA). Dark mode
   runs the same logic in reverse — never pure white on black.
   Only the visible reading-surface tokens are retuned; chrome niceties
   (scrollbar/icons) inherit mdBook's tuned values.
   --------------------------------------------------------------------- */
.light,
html:not(.js) {
  --bg: #fcfcfd; /* near-white, faintly cool — no warm cast */
  --fg: #1f2328; /* neutral near-black ink */
  --links: #2f5ca6; /* slate blue: the one calm accent */
  --sidebar-bg: #f4f5f7;
  --sidebar-fg: #3a4048;
  --sidebar-active: #2f5ca6;
  --inline-code-color: #34506f;
  --quote-bg: #f2f4f7;
  --quote-border: #e6e8eb;
  --search-mark-bg: #dbe6f4;
  --gi-ink-soft: #5c6370; /* tags, secondary text */
  --gi-rule: #e6e8eb; /* hairlines */
}

.coal {
  --bg: #15181c; /* cool near-black, never warm */
  --fg: #dde1e6; /* soft cool off-white, not pure white */
  --links: #84a9dd; /* lighter slate blue for dark bg */
  --sidebar-bg: #1b1f24;
  --sidebar-fg: #aab2bd;
  --sidebar-active: #84a9dd;
  --inline-code-color: #b8c6da;
  --quote-bg: #1c2127;
  --quote-border: #2a2f36;
  --search-mark-bg: #2a3a52;
  --gi-ink-soft: #9aa3af;
  --gi-rule: #2a2f36;
}

/* JS-disabled readers whose OS prefers dark get coal's values inline,
   not the .coal class — mirror the override so they get warm dark too. */
@media (prefers-color-scheme: dark) {
  html:not(.js) {
    --bg: #15181c;
    --fg: #dde1e6;
    --links: #84a9dd;
    --sidebar-bg: #1b1f24;
    --sidebar-fg: #aab2bd;
    --sidebar-active: #84a9dd;
    --inline-code-color: #b8c6da;
    --quote-bg: #1c2127;
    --quote-border: #2a2f36;
    --search-mark-bg: #2a3a52;
    --gi-ink-soft: #9aa3af;
    --gi-rule: #2a2f36;
  }
}

/* ---------------------------------------------------------------------
   3. The reading column. Only the content surface is restyled; the
   sidebar/menu chrome keep mdBook's sizing.
   --------------------------------------------------------------------- */
.content {
  font-family: var(--gi-prose);
  font-size: 2rem; /* 20px — deep reading of hard material */
  line-height: 1.65; /* looser than prose: tall inline math
                                         (∑ ∫ fractions) + a wide line both
                                         demand headroom */
  /* Ragged-right, NOT justified: CSS justification opens rivers and lacks
       TeX's hyphenation penalties, so left-aligned reads better here. */
  text-align: left;
  hyphens: none;
  text-rendering: optimizeLegibility;
  font-kerning: normal;
  font-variant-ligatures: common-ligatures contextual;
  /* Lining figures in prose: they sit at the same level as the digits
       in the Computer-Modern math, so a "rank 2" in text matches a "2" in
       an equation. (Old-style would descend and clash with the math.) */
  font-variant-numeric: lining-nums proportional-nums;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

/* ~72 characters: near the upper edge of the comfortable prose measure.
   An earlier 64ch read too sparse on wide screens — the eye returned too
   often — so this widens toward the optimum while staying under the point
   where the return sweep starts to miss lines (~75ch). At the 20px body set
   above this is ~720px, still inside mdBook's 750px --content-max-width,
   so no clamp is needed. ch is measured against that 20px body. */
.content main {
  max-width: 72ch;
}

.content p,
.content ol,
.content ul {
  line-height: 1.65; /* override mdBook's 1.45em */
}

/* Math, code, tables and tags want lining figures, not old-style. */
.content code,
.content .katex,
.content table {
  font-variant-numeric: lining-nums;
}

/* Match inline/display math size to the prose. katex.min.css sets
   `.katex { font-size: 1.21em }` — that 1.21 factor is calibrated for sans/Times
   bodies; against the Computer Modern KaTeX uses for math it overshoots and the
   math reads ~20% too big, making lines jump. Pulling it to ~1.0 lets the prose
   and math sit at one size. Higher specificity than katex.min.css's `.katex`,
   and loaded after it, so this wins. */
.content .katex {
  font-size: 1em;
}

/* ---------------------------------------------------------------------
   4. Headings — restrained scale (a textbook is not a landing page),
   bold, tight leading, balanced wrap. Sizes in rem (1rem = 10px).
   --------------------------------------------------------------------- */
.content h1,
.content h2,
.content h3,
.content h4 {
  font-family: var(--gi-prose);
  font-weight: 700;
  line-height: 1.15;
  letter-spacing: -0.006em;
  text-wrap: balance;
}
.content h1 {
  font-size: 3.4rem;
}
.content h2 {
  font-size: 2.8rem;
  margin-block-start: 2em;
}
.content h3 {
  font-size: 2.4rem;
  margin-block-start: 1.6em;
}
.content h4 {
  font-size: 2.05rem;
}

/* ---------------------------------------------------------------------
   5. Links — quiet by default, underline on hover (mdBook's content-link
   model already matches; just refine the hover underline).
   --------------------------------------------------------------------- */
.content a:hover {
  text-underline-offset: 0.14em;
  text-decoration-thickness: 0.06em;
}

/* ---------------------------------------------------------------------
   6. Display equations — vertical rhythm, and wide equations scroll
   WITHIN their own block rather than the whole page. KaTeX inherits
   currentColor, so math inverts cleanly with the prose in dark mode.
   --------------------------------------------------------------------- */
.content .katex-display {
  margin: 1.3em 0;
  overflow-x: auto;
  overflow-y: hidden;
  padding-block: 0.15em; /* room for descenders while scrolling */
}

/* mdbook-katex renders unparseable expressions in error-color; make sure
   they stay legible (and visible) rather than blending in. */
.content .katex-error {
  white-space: normal;
}

/* ---------------------------------------------------------------------
   7. Selection.
   --------------------------------------------------------------------- */
::selection {
  background: var(--search-mark-bg);
}

/* Volume/home breadcrumb injected at the top of each chapter by scripts/seo.py. */
.sga-breadcrumb {
  font-size: 0.85em;
  opacity: 0.75;
  margin-top: 0;
}