:root {
  --bg: #f5f0e8;
  --ink: #1a1209;
  --card-bg: #fffdf8;
  --red: #c0392b;
  --green: #2c6e49;
  --border: #ddd4c0;
  --muted: #8a7560;
  --body-font: 'Source Sans 3', sans-serif;
  --heading-font: 'Playfair Display', Georgia, serif;
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  background: var(--bg);
  color: var(--ink);
  font-family: var(--body-font);
  font-size: 16px;
  line-height: 1.7;
}

.hidden {
  display: none !important;
}

/* navbar */
.navbar {
  display: flex;
  align-items: baseline;
  gap: 1rem;
  padding: 1.2rem 2.5rem;
  background: var(--ink);
}

.brand {
  font-family: var(--heading-font);
  font-size: 1.6rem;
  color: #f5f0e8;
}

.brand span {
  color: #e07b54;
}

.brand-sub {
  font-size: 0.85rem;
  color: #a89880;
  font-style: italic;
}

/* hero */
.hero-section {
  max-width: 780px;
  margin: 0 auto;
  padding: 5rem 2rem 3rem;
  text-align: center;
}

.hero-section h1 {
  font-family: var(--heading-font);
  font-size: clamp(2rem, 5vw, 3.2rem);
  line-height: 1.2;
  margin-bottom: 1rem;
}

.hero-section h1 em {
  color: var(--red);
  font-style: italic;
}

.hero-sub {
  font-size: 1.05rem;
  color: #5a4a35;
  margin-bottom: 2rem;
}

/* search */
.search-wrapper {
  display: flex;
  max-width: 640px;
  margin: 0 auto;
  border: 2px solid var(--ink);
  border-radius: 4px;
  overflow: hidden;
  box-shadow: 4px 4px 0 var(--ink);
}

#searchBox {
  flex: 1;
  padding: 0.85rem 1.2rem;
  font-size: 1rem;
  font-family: var(--body-font);
  border: none;
  outline: none;
  background: white;
}

#searchBtn {
  padding: 0.85rem 1.6rem;
  background: var(--red);
  color: white;
  font-family: var(--body-font);
  font-size: 1rem;
  font-weight: 600;
  border: none;
  cursor: pointer;
  transition: background 0.2s;
  white-space: nowrap;
}

#searchBtn:hover {
  background: #a93226;
}

.error-msg {
  color: var(--red);
  margin-top: 8px;
  font-size: 0.95rem;
}

/* filter bar */
.filter-bar {
  max-width: 900px;
  margin: 0 auto 1.5rem;
  padding: 1rem 2rem;
  display: flex;
  align-items: center;
  gap: 2rem;
  flex-wrap: wrap;
  border-bottom: 1px solid var(--border);
}

.filter-item {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.9rem;
}

.filter-item label {
  font-weight: 600;
  color: #5a4a35;
}

#yearFrom {
  width: 90px;
  padding: 5px 8px;
  border: 1px solid var(--border);
  border-radius: 3px;
  font-family: var(--body-font);
  font-size: 0.9rem;
  background: white;
}

#sortBy {
  padding: 5px 8px;
  border: 1px solid var(--border);
  border-radius: 3px;
  font-family: var(--body-font);
  font-size: 0.9rem;
  background: white;
  cursor: pointer;
}

.paper-count {
  margin-left: auto;
  font-size: 0.85rem;
  color: var(--muted);
  font-style: italic;
}

/* paper cards */
.paper-list {
  max-width: 900px;
  margin: 0 auto;
  padding: 0 2rem 4rem;
  display: grid;
  gap: 1.5rem;
}

.paper-card {
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1.5rem 1.8rem;
  box-shadow: 3px 3px 0 var(--border);
  transition:
    box-shadow 0.2s,
    transform 0.2s;
  animation: fadeUp 0.3s ease both;
}

.paper-card:hover {
  box-shadow: 5px 5px 0 var(--border);
  transform: translateY(-2px);
}

@keyframes fadeUp {
  from {
    opacity: 0;
    transform: translateY(12px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.paper-title {
  font-family: var(--heading-font);
  font-size: 1.15rem;
  margin-bottom: 0.5rem;
  line-height: 1.35;
}

.paper-title a {
  color: var(--ink);
  text-decoration: none;
  border-bottom: 2px solid var(--red);
}

.paper-title a:hover {
  color: var(--red);
}

.paper-meta {
  font-size: 0.85rem;
  color: var(--muted);
  margin-bottom: 0.8rem;
}

.paper-abstract {
  font-size: 0.95rem;
  color: #3a2e20;
  margin-bottom: 1rem;
  display: -webkit-box;
  -webkit-line-clamp: 3;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.card-footer {
  display: flex;
  align-items: center;
  gap: 1rem;
  flex-wrap: wrap;
}

.tag {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 20px;
  padding: 3px 10px;
  font-size: 12px;
  font-weight: 600;
  color: #5a4a35;
}

.explain-btn {
  margin-left: auto;
  padding: 6px 14px;
  background: var(--green);
  color: white;
  border: none;
  border-radius: 4px;
  font-family: var(--body-font);
  font-size: 0.88rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.2s;
}

.explain-btn:hover {
  background: #224f37;
}
.explain-btn:disabled {
  opacity: 0.6;
  cursor: wait;
}

/* loading spinner */
.loading-spinner {
  text-align: center;
  padding: 4rem 2rem;
}

.spinner-ring {
  width: 48px;
  height: 48px;
  border: 4px solid var(--border);
  border-top-color: var(--red);
  border-radius: 50%;
  margin: 0 auto 1rem;
  animation: spin 0.8s linear infinite;
}

@keyframes spin {
  to {
    transform: rotate(360deg);
  }
}

/* modal */
.modal-bg {
  position: fixed;
  inset: 0;
  background: rgba(26, 18, 9, 0.65);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 999;
  padding: 1.5rem;
  animation: fadeIn 0.2s ease;
}

@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

.modal-content {
  background: var(--card-bg);
  border: 2px solid var(--ink);
  border-radius: 8px;
  padding: 2.2rem 2.5rem;
  max-width: 680px;
  width: 100%;
  max-height: 80vh;
  overflow-y: auto;
  position: relative;
  box-shadow: 8px 8px 0 var(--ink);
  animation: popIn 0.25s ease;
}

@keyframes popIn {
  from {
    transform: scale(0.95);
    opacity: 0;
  }
  to {
    transform: scale(1);
    opacity: 1;
  }
}

.close-btn {
  position: absolute;
  top: 1rem;
  right: 1.2rem;
  background: none;
  border: none;
  font-size: 1.3rem;
  cursor: pointer;
  color: var(--muted);
}

.close-btn:hover {
  color: var(--red);
}

#modalPaperTitle {
  font-family: var(--heading-font);
  font-size: 1.35rem;
  margin-bottom: 0.5rem;
  padding-right: 2rem;
  line-height: 1.3;
}

.modal-tag {
  display: inline-block;
  background: var(--green);
  color: white;
  font-size: 0.75rem;
  font-weight: 600;
  padding: 3px 10px;
  border-radius: 20px;
  margin-bottom: 1.2rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.summary-text {
  font-size: 1rem;
  line-height: 1.8;
  color: #2a1f12;
  white-space: pre-wrap;
}

/* footer */
.site-footer {
  text-align: center;
  padding: 1.5rem;
  font-size: 0.85rem;
  color: var(--muted);
  border-top: 1px solid var(--border);
}

.site-footer a {
  color: var(--red);
  text-decoration: none;
}

/* mobile */
@media (max-width: 600px) {
  .hero-section {
    padding: 3rem 1.2rem 2rem;
  }
  .search-wrapper {
    flex-direction: column;
  }
  .filter-bar {
    gap: 1rem;
    padding: 1rem;
  }
  .paper-count {
    margin-left: 0;
  }
  .modal-content {
    padding: 1.5rem;
  }
}