/* IterMath — shared styles
   Spacing: 4pt scale. Radius: 12px everywhere. One font family (system UI). */

:root {
  --s1: 4px;  --s2: 8px;  --s3: 12px; --s4: 16px;
  --s5: 24px; --s6: 32px; --s7: 48px; --s8: 64px;

  --radius: 12px;
  --control: 48px;

  --bg: #fafafa;
  --surface: #ffffff;
  --text: #18181b;
  --muted: #52525b;
  --border: #e4e4e7;
  --border-strong: #a1a1aa;
  --brand: #89d957;
  --brand-ink: #356b14;
  --brand-soft: #f1f9e8;
  --danger-ink: #b42318;
  --danger-soft: #fef3f2;
  --danger-border: #fecdca;
  --focus: 0 0 0 3px rgba(137, 217, 87, .55);

  --font: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --ease: cubic-bezier(.2, 0, 0, 1);
}

*, *::before, *::after { box-sizing: border-box; }
* { margin: 0; }

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

body {
  min-height: 100vh;
  min-height: 100dvh;
  display: flex;
  flex-direction: column;
  background: var(--bg);
  color: var(--text);
  font: 400 16px/24px var(--font);
  -webkit-font-smoothing: antialiased;
}

img, svg { display: block; max-width: 100%; }
a { color: inherit; }
button, input { font: inherit; color: inherit; }

:focus-visible { outline: none; box-shadow: var(--focus); }

.visually-hidden {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}

/* ---------- Layout ---------- */

.container {
  width: 100%;
  max-width: 960px;
  margin: 0 auto;
  padding: 0 var(--s4);
}
.container--narrow { max-width: 560px; }

main { flex: 1; }

@media (min-width: 640px) {
  .container { padding: 0 var(--s5); }
}

/* ---------- Type ramp ----------
   12/16 caption · 14/20 small · 16/24 body · 20/28 title · 24/32 h1 · 32/40 display (40/48 ≥640) */

.h1      { font-size: 24px; line-height: 32px; font-weight: 600; letter-spacing: -.01em; }
.h2      { font-size: 12px; line-height: 16px; font-weight: 600; letter-spacing: .06em; text-transform: uppercase; color: var(--muted); }
.lead    { font-size: 16px; line-height: 24px; color: var(--muted); }
.small   { font-size: 14px; line-height: 20px; }
.muted   { color: var(--muted); }

/* ---------- Components ---------- */

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
}

.input {
  height: var(--control);
  width: 100%;
  padding: 0 var(--s4);
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  transition: border-color .15s var(--ease), box-shadow .15s var(--ease);
  -webkit-appearance: none;
  appearance: none;
}
.input::placeholder { color: #71717a; }
.input:hover { border-color: var(--border-strong); }
.input:focus { outline: none; border-color: var(--brand-ink); box-shadow: var(--focus); }
.input[aria-invalid="true"] { border-color: var(--danger-ink); }
.input::-webkit-search-cancel-button { -webkit-appearance: none; }
.input::-webkit-outer-spin-button,
.input::-webkit-inner-spin-button { -webkit-appearance: none; margin: 0; }
.input[type="number"] { -moz-appearance: textfield; }

.btn {
  position: relative;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--s2);
  height: var(--control);
  padding: 0 var(--s5);
  border: 1px solid var(--text);
  border-radius: var(--radius);
  background: var(--text);
  color: #fff;
  font-weight: 600;
  white-space: nowrap;
  cursor: pointer;
  transition: background-color .15s var(--ease), border-color .15s var(--ease);
  -webkit-tap-highlight-color: transparent;
  touch-action: manipulation;
}
.btn:hover { background: #3f3f46; border-color: #3f3f46; }
.btn:active { background: #52525b; }
.btn:disabled { cursor: default; }

/* Loading state: label hides, spinner shows, width stays put */
.btn[aria-busy="true"] { color: transparent; }
.btn[aria-busy="true"]::after {
  content: "";
  position: absolute;
  width: 16px; height: 16px;
  border: 2px solid rgba(255, 255, 255, .35);
  border-top-color: #fff;
  border-radius: 50%;
  animation: spin .6s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }

.tile {
  flex: none;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 40px; height: 40px;
  border-radius: var(--radius);
  background: var(--brand-soft);
  color: var(--brand-ink);
  font-size: 20px; line-height: 28px; font-weight: 600;
}
.tile--sm { font-size: 14px; line-height: 20px; }

/* ---------- Home ---------- */

.home-header { padding-top: var(--s7); padding-bottom: var(--s6); }
.home-header .logo { width: 220px; height: auto; margin: 0 0 var(--s5) calc(var(--s3) * -1); }
.home-header .lead { max-width: 480px; }

.search { position: relative; max-width: 400px; margin-bottom: var(--s6); }
.search svg {
  position: absolute; left: var(--s4); top: 50%;
  width: 16px; height: 16px; margin-top: -8px;
  color: var(--muted); pointer-events: none;
}
.search .input { padding-left: 40px; }

.group + .group { margin-top: var(--s6); }
.group[hidden] + .group { margin-top: 0; }
.group .h2 { margin-bottom: var(--s3); }

.module-list {
  list-style: none;
  padding: 0;
  display: grid;
  gap: var(--s3);
}

.module {
  display: flex;
  align-items: flex-start;
  gap: var(--s4);
  height: 100%;
  padding: var(--s4);
  text-decoration: none;
  transition: border-color .15s var(--ease);
  -webkit-tap-highlight-color: transparent;
}
.module:hover { border-color: var(--border-strong); }
.module:active { background: var(--bg); }
.module-title { font-weight: 600; }
.module-desc { display: block; margin-top: var(--s1); }
.module-grade {
  display: block;
  margin-top: var(--s2);
  color: var(--muted);
  font-size: 12px; line-height: 16px; font-weight: 600;
  letter-spacing: .02em;
  font-variant-numeric: tabular-nums;
}

.filters {
  display: flex;
  flex-wrap: wrap;
  gap: var(--s2);
  margin: calc(var(--s6) * -1 + var(--s4)) 0 var(--s6);
}
.chip {
  height: 36px;
  padding: 0 var(--s4);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  background: var(--surface);
  color: var(--muted);
  font-size: 14px; line-height: 20px; font-weight: 500;
  cursor: pointer;
  transition: border-color .15s var(--ease), background-color .15s var(--ease), color .15s var(--ease);
  -webkit-tap-highlight-color: transparent;
}
.chip:hover { border-color: var(--border-strong); color: var(--text); }
.chip[aria-pressed="true"] { background: var(--text); border-color: var(--text); color: #fff; }

.count { margin: 0 0 var(--s5); }

.empty { padding: 0; }

@media (min-width: 640px) {
  .home-header { padding-top: var(--s8); padding-bottom: var(--s7); }
  .home-header .logo { width: 260px; }
  .module-list { grid-template-columns: repeat(2, 1fr); gap: var(--s4); }
}
@media (min-width: 900px) {
  .module-list { grid-template-columns: repeat(3, 1fr); }
}

.site-footer {
  margin-top: var(--s8);
  padding: var(--s5) 0;
  border-top: 1px solid var(--border);
}
.site-footer .container {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  gap: var(--s2) var(--s5);
}
.site-footer a { text-decoration: none; }
.site-footer a:hover { color: var(--text); text-decoration: underline; text-underline-offset: 3px; }
.site-footer nav { display: flex; gap: var(--s5); }

/* ---------- Practice ---------- */

.topbar { padding-top: var(--s3); padding-bottom: var(--s3); }
.back {
  display: inline-flex;
  align-items: center;
  gap: var(--s2);
  min-height: 44px;
  padding-right: var(--s2);
  border-radius: var(--radius);
  text-decoration: none;
}
.back svg { width: 16px; height: 16px; color: var(--muted); transition: transform .15s var(--ease); }
.back:hover svg { transform: translateX(-2px); }
.back img { width: 104px; height: auto; }

.practice-header { padding: var(--s4) 0 var(--s5); }
.practice-header .lead { margin-top: var(--s1); }

.panel { padding: var(--s5) var(--s4); }

.question {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 96px;
  margin-bottom: var(--s5);
  font-size: 32px; line-height: 40px; font-weight: 600;
  font-variant-numeric: tabular-nums;
  text-align: center;
}
.question--long { font-size: 24px; line-height: 32px; }
.question--xl { font-size: 20px; line-height: 32px; }
.question-prompt {
  display: block;
  max-width: 440px;
  margin-bottom: var(--s3);
  color: var(--muted);
  font-size: 16px; line-height: 24px; font-weight: 400;
  text-wrap: pretty;
}
.question-main { max-width: 100%; text-wrap: balance; }
.question i { font-family: "Times New Roman", Times, serif; font-weight: 400; }
.question.is-entering { animation: enter .16s var(--ease); }

.answer-row { display: flex; gap: var(--s2); }
.answer-row .input {
  flex: 1;
  min-width: 0;
  font-size: 20px;
  line-height: 28px;
  font-variant-numeric: tabular-nums;
}
.answer-row .btn { min-width: 112px; }

.field-hint { margin-top: var(--s2); }
.field-hint[hidden] { display: none; }

.field-error {
  margin-top: var(--s2);
  color: var(--danger-ink);
  font-size: 14px; line-height: 20px;
}

.feedback:not(:empty) { margin-top: var(--s4); }
.feedback > div { animation: enter .16s var(--ease); }

.result {
  padding: var(--s4);
  border: 1px solid;
  border-radius: var(--radius);
}
.result--correct { background: var(--brand-soft); border-color: #cfe9b6; color: var(--brand-ink); font-weight: 600; }
.result--wrong { background: var(--danger-soft); border-color: var(--danger-border); }
.result-title { color: var(--danger-ink); font-weight: 600; }
.result-sub { margin-top: var(--s1); }
.solution {
  margin-top: var(--s4);
  padding: var(--s4);
  background: var(--surface);
  border: 1px solid var(--danger-border);
  border-radius: var(--radius);
  overflow-x: auto;
}
.solution-note { margin-top: var(--s3); color: var(--muted); font-size: 14px; line-height: 20px; }

@media (min-width: 640px) {
  .practice-header { padding: var(--s6) 0 var(--s5); }
  .panel { padding: var(--s6); }
  .question { min-height: 112px; font-size: 40px; line-height: 48px; }
  .question--long { font-size: 32px; line-height: 40px; }
  .question--xl { font-size: 24px; line-height: 36px; }
}

@keyframes enter {
  from { opacity: 0; transform: translateY(4px); }
  to   { opacity: 1; transform: none; }
}

/* ---------- Math ---------- */

.math { font-variant-numeric: tabular-nums; white-space: nowrap; }
.math i { font-family: "Times New Roman", Times, serif; font-size: 1.1em; }

.frac {
  display: inline-flex;
  flex-direction: column;
  align-items: center;
  vertical-align: middle;
  margin: 0 var(--s1);
  line-height: 1.25;
}
.frac > span { padding: 0 var(--s1); }
.frac > span:first-child { border-bottom: 1px solid currentColor; }

.column {
  display: inline-grid;
  grid-template-columns: auto auto;
  column-gap: var(--s3);
  font-size: 20px; line-height: 28px;
  font-variant-numeric: tabular-nums;
  text-align: right;
}
.column .rule { grid-column: 1 / -1; height: 0; margin: var(--s1) 0; border-top: 2px solid var(--text); }
.column strong { grid-column: 2; }

.steps {
  display: grid;
  grid-template-columns: auto auto;
  justify-content: start;
  align-items: center;
  gap: var(--s3) var(--s2);
  font-size: 16px; line-height: 24px;
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
}
.steps > :nth-child(odd) { text-align: right; }

.solution--center { text-align: center; }

sup, sub { font-size: .62em; line-height: 0; }
sup { vertical-align: .75em; }
sub { vertical-align: -.25em; }

.sqrt { white-space: nowrap; }
.sqrt > span {
  display: inline-block;
  padding: 0 .1em;
  margin-left: .02em;
  border-top: .08em solid currentColor;
  line-height: 1.15;
}
.root-index { margin-right: -.45em; vertical-align: .9em; }

.matrix {
  display: inline-grid;
  gap: 0 .7em;
  vertical-align: middle;
  margin: 0 .15em;
  padding: .1em .45em;
  border-left: .1em solid currentColor;
  border-right: .1em solid currentColor;
  border-radius: .3em;
  line-height: 1.35;
  text-align: center;
}
.matrix--det { border-radius: 0; }

.bigop, .lim {
  display: inline-flex;
  align-items: center;
  vertical-align: middle;
  margin-right: .15em;
}
.bigop-sign { font-size: 1.6em; line-height: 1; font-weight: 400; }
.bigop-limits {
  display: inline-flex;
  flex-direction: column;
  justify-content: space-between;
  align-self: stretch;
  margin-left: .2em;
  margin-right: .15em;
  padding: .1em 0;
  font-size: .5em;
  line-height: 1.25;
}
.lim { flex-direction: column; line-height: 1.1; }
.lim > span:last-child { font-size: .55em; line-height: 1.2; font-weight: 400; }

.figure {
  display: block;
  width: 240px;
  height: auto;
  margin: 0 auto var(--s2);
  font-size: 16px;
  font-weight: 500;
  fill: currentColor;
}
.figure text { font-family: var(--font); }

.system {
  display: inline-flex;
  flex-direction: column;
  gap: .1em;
  vertical-align: middle;
  padding-left: .5em;
  border-left: .1em solid currentColor;
  text-align: left;
}

.eyebrow { margin-bottom: var(--s1); }

.prob-list {
  display: inline-flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: .25em .6em;
}

.solution .steps { white-space: normal; }
.solution .steps > span { white-space: nowrap; }

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation-duration: .01ms !important; transition-duration: .01ms !important; }
  .btn[aria-busy="true"]::after { animation-duration: .6s !important; }
}