/* =========================================================
   Yixin (Sunny) Sun — personal site
   Aesthetic: cream background, DM Mono headings, gold accents
   Inspired by kristykim.github.io
   ========================================================= */

/* ---------- Tokens ---------- */
:root {
  --bg: #fdfaf3;             /* cream */
  --bg-alt: #f5f1e6;         /* slightly darker cream for alt section */
  --ink: #1a1a1a;            /* near black */
  --muted: #555;
  --accent: #f0b929;         /* warm gold (mirrors Bootstrap btn-warning) */
  --accent-hover: #d99e0d;
  --outline: #2f8f4a;        /* green for outline buttons */
  --outline-hover: #21683a;
  --link: #2f8f4a;
  --rule: rgba(0, 0, 0, 0.08);

  --font-display: "DM Mono", ui-monospace, "SF Mono", Menlo, monospace;
  --font-body: "Open Sans", system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;

  --max-width: 1100px;
  --gap: 2rem;
}

/* ---------- Reset-ish ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: var(--font-body);
  font-size: 17px;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
}
img { max-width: 100%; height: auto; display: block; }
a { color: var(--link); text-decoration: underline; text-decoration-thickness: 1px; text-underline-offset: 2px; }
a:hover { color: var(--outline-hover); }
.container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 1.5rem;
}

/* ---------- Nav ---------- */
.nav {
  position: sticky;
  top: 0;
  background: var(--bg);
  border-bottom: 1px solid var(--rule);
  z-index: 10;
}
.nav-inner {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1rem 1.5rem;
}
.nav-brand {
  display: inline-flex;
  align-items: center;
  gap: 0.6rem;
  font-family: var(--font-display);
  font-size: 1.05rem;
  font-weight: 500;
  color: var(--ink);
  text-decoration: none;
}
.nav-logo {
  height: 1.8em;     /* slightly taller than the "Yixin Sun" text */
  width: auto;
  display: block;
}
.nav-links {
  display: flex;
  gap: 1.75rem;
  list-style: none;
  margin: 0; padding: 0;
}
.nav-links a {
  font-family: var(--font-display);
  font-size: 0.95rem;
  color: var(--ink);
  text-decoration: none;
  transition: color 0.15s;
}
.nav-links a:hover { color: var(--accent-hover); }

/* ---------- Hero ---------- */
.hero {
  padding: 4rem 0 3rem;
}
.hero-inner {
  display: grid;
  grid-template-columns: 1.6fr 1fr;
  gap: 3rem;
  align-items: center;
}
.hero-title {
  font-family: var(--font-display);
  font-weight: 400;
  font-size: clamp(1.8rem, 3.2vw, 2.6rem);
  line-height: 1.25;
  margin: 0 0 1.5rem;
}
.hero-subtitle {
  font-size: 1.15rem;
  margin: 0 0 1.25rem;
  line-height: 1.6;
}
.hero-bio {
  font-size: 1.15rem;
  line-height: 1.6;
  margin: 0 0 1.75rem;
}
.hero-buttons {
  display: flex;
  gap: 0.75rem;
  flex-wrap: wrap;
}
.hero-image img {
  width: 100%;
  border-radius: 4px;
}

/* ---------- Buttons ---------- */
.btn {
  display: inline-block;
  padding: 0.65rem 1.4rem;
  font-family: var(--font-display);
  font-size: 0.95rem;
  font-weight: 500;
  text-decoration: none;
  border-radius: 2px;
  border: 2px solid transparent;
  transition: all 0.15s ease;
  cursor: pointer;
}
.btn-primary {
  background: var(--accent);
  color: var(--ink);
  border-color: var(--accent);
}
.btn-primary:hover {
  background: var(--accent-hover);
  border-color: var(--accent-hover);
  color: var(--ink);
}
.btn-outline {
  background: transparent;
  color: var(--outline);
  border-color: var(--outline);
}
.btn-outline:hover {
  background: var(--outline);
  color: #fff;
}
.btn-sm {
  padding: 0.45rem 1rem;
  font-size: 0.85rem;
}

/* ---------- Sections ---------- */
.section {
  padding: 4rem 0;
  border-top: 1px solid var(--rule);
}
.section-alt {
  background: var(--bg-alt);
}
.section-title {
  font-family: var(--font-display);
  font-weight: 400;
  font-size: clamp(1.6rem, 2.6vw, 2.2rem);
  margin: 0 0 3rem;
  color: var(--ink);
}

/* ---------- Paper block (Working papers, with image) ---------- */
.paper {
  display: grid;
  grid-template-columns: 1fr 2fr;
  gap: 2.5rem;
  margin-bottom: 4rem;
  align-items: start;
}
.paper:last-child { margin-bottom: 0; }
.paper-image img {
  width: 100%;
}
.paper-title {
  font-family: var(--font-display);
  font-size: 1.1rem;
  font-weight: 500;
  line-height: 1.4;
  margin: 0 0 0.5rem;
}
.paper-title a {
  color: var(--ink);
  text-decoration: none;
  border-bottom: 2px solid var(--accent);
  padding-bottom: 1px;
}
.paper-title a:hover {
  color: var(--accent-hover);
}
.paper-authors {
  font-size: 0.95rem;
  margin: 0 0 0.4rem;
  color: var(--muted);
}
.paper-authors a {
  color: var(--accent-hover);
}
.paper-status {
  font-size: 0.9rem;
  margin: 0 0 1rem;
  color: var(--muted);
}
.paper-abstract {
  font-size: 0.95rem;
  margin: 0 0 1.25rem;
  color: var(--ink);
}

/* Collapsible abstract */
.abstract {
  margin: 0 0 1.25rem;
}
.abstract > summary {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  cursor: pointer;
  font-family: var(--font-display);
  font-size: 0.85rem;
  font-weight: 500;
  color: var(--ink);
  padding: 0.35rem 0.85rem;
  border: 1.5px solid var(--ink);
  border-radius: 2px;
  list-style: none;
  user-select: none;
  transition: all 0.15s ease;
}
.abstract > summary::-webkit-details-marker { display: none; }
.abstract > summary::before {
  content: "+";
  font-weight: 500;
  display: inline-block;
  width: 0.75em;
  text-align: center;
}
.abstract[open] > summary::before { content: "−"; }
.abstract > summary:hover {
  background: var(--ink);
  color: var(--bg);
}
.abstract > p {
  font-size: 0.95rem;
  margin: 1rem 0 0;
  color: var(--ink);
  padding-left: 0.85rem;
  border-left: 2px solid var(--rule);
}
.paper-note {
  font-size: 0.9rem;
  color: var(--muted);
  margin: 0 0 1rem;
}
.paper-buttons {
  display: flex;
  gap: 0.6rem;
  flex-wrap: wrap;
}

/* ---------- WIP block (no image) ---------- */
.wip-paper {
  margin-bottom: 2.75rem;
  padding-bottom: 2.75rem;
  border-bottom: 1px solid var(--rule);
}
.wip-paper:last-child {
  border-bottom: none;
  margin-bottom: 0;
  padding-bottom: 0;
}

/* ---------- Footer ---------- */
.footer {
  border-top: 1px solid var(--rule);
  padding: 2rem 0;
  font-family: var(--font-display);
  font-size: 0.85rem;
  color: var(--muted);
  text-align: center;
}
.footer p { margin: 0; }

/* ---------- Mobile ---------- */
@media (max-width: 760px) {
  .hero {
    padding: 2.5rem 0 2rem;
  }
  .hero-inner {
    grid-template-columns: 1fr;
    gap: 2rem;
  }
  .hero-image {
    order: -1;
    max-width: 220px;
  }
  .nav-inner {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.75rem;
  }
  .nav-links {
    gap: 1.25rem;
    flex-wrap: wrap;
  }
  .section {
    padding: 2.5rem 0;
  }
  .section-title {
    margin-bottom: 2rem;
  }
  .paper {
    grid-template-columns: 1fr;
    gap: 1.5rem;
    margin-bottom: 3rem;
  }
}