/* Import Comfortaa font */
@import url('https://fonts.googleapis.com/css2?family=Comfortaa:wght@400;500;600;700&display=swap');

/* Base Root Variables */
:root {
  --primary: #2563eb;
  --primary-hover: #1d4ed8;
  --bg-gradient: linear-gradient(135deg, #dbeafe 0%, #e0f2fe 50%, #bae6fd 100%);
  --card-bg: rgba(255, 255, 255, 0.85);
  --card-border: rgba(255, 255, 255, 0.7);
  --text-main: #0f172a;
  --text-muted: #64748b;
  --pill-outline: rgba(37, 99, 235, 0.25);
  --input-bg: rgba(255, 255, 255, 0.9);
  --table-border: rgba(226, 232, 240, 0.8);
}

* { 
  box-sizing: border-box; 
}

body {
  font-family: 'Comfortaa', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, cursive, sans-serif;
  min-height: 100vh;
  margin: 0;
  padding: 2.5rem 1rem;
  color: var(--text-main);
  background: var(--bg-gradient);
  background-attachment: fixed;
}

/* Page Structure */
.container {
  max-width: 1020px;
  margin: 0 auto;
  background: var(--card-bg);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border: 1px solid var(--card-border);
  border-radius: 24px;
  padding: 2rem 2.5rem;
  box-shadow: 0 20px 40px rgba(15, 23, 42, 0.08);
  animation: slideUpFade 0.4s cubic-bezier(0.16, 1, 0.3, 1) forwards;
}

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

/* Header */
.header-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 1.5rem;
}

h1 {
  font-size: 1.8rem;
  font-weight: 700;
  margin: 0;
  color: var(--text-main);
  letter-spacing: -0.02em;
}

/* Controls Area */
.controls { 
  display: flex; 
  flex-direction: column; 
  gap: 1rem; 
  margin-bottom: 1.75rem; 
}

.search-row { 
  display: flex; 
  gap: 12px; 
  flex-wrap: wrap; 
  position: relative; 
  z-index: 10; 
}

.search-input {
  font-family: inherit;
  padding: 10px 18px;
  border-radius: 20px;
  border: 1px solid var(--pill-outline);
  background: var(--input-bg);
  font-size: 0.9rem;
  color: var(--text-main);
  outline: none;
  width: 100%;
  max-width: 280px;
  transition: border-color 0.2s ease, box-shadow 0.2s ease, transform 0.2s ease;
}

.search-input:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.15);
  transform: translateY(-1px);
}

/* Custom Dropdown Menus */
.custom-select-container {
  position: relative;
  user-select: none;
  min-width: 155px;
}

.custom-select-trigger {
  font-family: inherit;
  padding: 10px 18px;
  border-radius: 20px;
  border: 1px solid var(--pill-outline);
  background: var(--input-bg);
  font-size: 0.9rem;
  color: var(--text-main);
  cursor: pointer;
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 10px;
  transition: all 0.2s cubic-bezier(0.16, 1, 0.3, 1);
}

.custom-select-trigger:hover {
  border-color: var(--primary);
  transform: translateY(-1px);
}

.custom-select-container.open .custom-select-trigger {
  border-color: var(--primary);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.15);
  border-bottom-left-radius: 12px;
  border-bottom-right-radius: 12px;
}

.chevron-arrow {
  width: 10px;
  height: 6px;
  border-left: 2px solid var(--text-muted);
  border-bottom: 2px solid var(--text-muted);
  transform: rotate(-45deg);
  transition: transform 0.25s cubic-bezier(0.16, 1, 0.3, 1);
  margin-top: -2px;
}

.custom-select-container.open .chevron-arrow {
  transform: rotate(135deg);
  margin-top: 3px;
}

.custom-options {
  position: absolute;
  top: calc(100% + 6px);
  left: 0;
  right: 0;
  background: var(--card-bg);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border: 1px solid var(--card-border);
  border-radius: 16px;
  box-shadow: 0 12px 28px rgba(15, 23, 42, 0.12);
  padding: 6px;
  display: flex;
  flex-direction: column;
  gap: 2px;
  opacity: 0;
  visibility: hidden;
  transform: translateY(-8px) scale(0.98);
  transition: all 0.2s cubic-bezier(0.16, 1, 0.3, 1);
  z-index: 100;
}

.custom-select-container.open .custom-options {
  opacity: 1;
  visibility: visible;
  transform: translateY(0) scale(1);
}

.custom-option {
  padding: 8px 14px;
  font-size: 0.85rem;
  border-radius: 10px;
  cursor: pointer;
  color: var(--text-main);
  transition: background-color 0.15s ease, color 0.15s ease;
}

.custom-option:hover {
  background: rgba(37, 99, 235, 0.08);
  color: var(--primary);
}

.custom-option.selected {
  background: var(--primary);
  color: #fff;
  font-weight: 600;
}

/* Filters Control Bar Base */
.filters { 
  display: flex; 
  gap: 8px; 
  flex-wrap: wrap; 
}

.filter-btn {
  font-family: 'Segoe UI Emoji', 'Apple Color Emoji', 'Noto Color Emoji', 'Comfortaa', sans-serif;
  padding: 6px 16px;
  border-radius: 20px;
  border: 1.5px solid #cbd5e1;
  background: #ffffff;
  color: #0f172a;
  cursor: pointer;
  font-size: 0.82rem;
  font-weight: 700;
  white-space: nowrap;
  transition: all 0.2s cubic-bezier(0.16, 1, 0.3, 1);
}

.filter-btn:hover {
  transform: translateY(-1px);
  box-shadow: 0 4px 10px rgba(0,0,0,0.1);
}

/* Tag Color Schemes */
.filter-btn[data-tag="All"] { border-color: #0f172a; }
.filter-btn[data-tag="All"].active { background: #0f172a; color: #ffffff; }

.tag-badge[data-tag="Classical Mechanics"] { background-color: #2563eb !important; }
.filter-btn[data-tag="Classical Mechanics"] { border-color: #2563eb; color: #1e40af; }
.filter-btn[data-tag="Classical Mechanics"].active { background: #2563eb; color: #ffffff; }

.tag-badge[data-tag="Quantum Mechanics"] { background-color: #0284c7 !important; }
.filter-btn[data-tag="Quantum Mechanics"] { border-color: #0284c7; color: #075985; }
.filter-btn[data-tag="Quantum Mechanics"].active { background: #0284c7; color: #ffffff; }

.tag-badge:not([data-tag]) { background-color: #475569 !important; }

/* Difficulty Filter Button Color Schemes */
.filter-btn[data-diff="All"] { border-color: #64748b; color: #475569; }
.filter-btn[data-diff="All"].active { background: #64748b; color: #ffffff; }

.filter-btn[data-diff="Easy"] { border-color: #10b981; color: #047857; }
.filter-btn[data-diff="Easy"].active { background: #10b981; color: #ffffff; }

.filter-btn[data-diff="Medium"] { border-color: #f59e0b; color: #b45309; }
.filter-btn[data-diff="Medium"].active { background: #f59e0b; color: #ffffff; }

.filter-btn[data-diff="Hard"] { border-color: #dc2626; color: #b91c1c; }
.filter-btn[data-diff="Hard"].active { background: #dc2626; color: #ffffff; }

/* Table Styling */
.table-container { 
  overflow-x: auto; 
  -webkit-overflow-scrolling: touch;
}

.problem-table { 
  width: 100%; 
  border-collapse: separate; 
  border-spacing: 0; 
}

.problem-table th {
  padding: 14px 16px;
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-muted);
  border-bottom: 2px solid var(--table-border);
  white-space: nowrap;
}

.problem-table th:nth-child(1) { text-align: left; }
.problem-table th:nth-child(2),
.problem-table th:nth-child(3),
.problem-table th:nth-child(4) { text-align: center; }

.problem-table tbody tr.problem-row {
  transition: background-color 0.2s ease;
  height: 68px;
}

.problem-table tbody tr.problem-row:hover {
  background-color: rgba(255, 255, 255, 0.5);
}

.problem-table td {
  padding: 12px 16px;
  border-bottom: 1px solid var(--table-border);
  vertical-align: middle;
  height: 68px;
  box-sizing: border-box;
}

.problem-table td:nth-child(1) { text-align: left; }
.problem-table td:nth-child(2),
.problem-table td:nth-child(3),
.problem-table td:nth-child(4) { text-align: center; }

.problem-name {
  font-weight: 600;
  color: var(--primary);
  cursor: pointer;
  text-decoration: none;
  display: inline-block;
  transition: color 0.2s ease, transform 0.2s ease;
}

.problem-name:hover { 
  text-decoration: underline; 
  color: var(--primary-hover);
  transform: translateX(2px);
}

.tags-cell-container {
  display: flex;
  flex-wrap: nowrap;
  gap: 6px;
  align-items: center;
  justify-content: center;
}

.tag-badge {
  font-family: 'Segoe UI Emoji', 'Apple Color Emoji', 'Noto Color Emoji', 'Comfortaa', sans-serif;
  color: #ffffff !important;
  font-size: 0.78rem;
  font-weight: 700;
  padding: 6px 14px;
  border-radius: 20px;
  white-space: nowrap;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: max-content;
  cursor: pointer;
  border: none !important;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.12);
  transition: all 0.2s cubic-bezier(0.16, 1, 0.3, 1);
}

.tag-badge:hover {
  filter: brightness(1.15);
  transform: translateY(-2px);
  box-shadow: 0 6px 14px rgba(0, 0, 0, 0.18);
}

/* Difficulty Badges */
.difficulty-badge {
  font-family: 'Segoe UI Emoji', 'Apple Color Emoji', 'Noto Color Emoji', 'Comfortaa', sans-serif;
  font-size: 0.78rem;
  font-weight: 700;
  padding: 6px 14px;
  border-radius: 20px;
  white-space: nowrap;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: max-content;
  cursor: pointer;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.08);
  transition: all 0.2s cubic-bezier(0.16, 1, 0.3, 1);
}

.difficulty-badge:hover {
  filter: brightness(1.15);
  transform: translateY(-2px);
  box-shadow: 0 6px 14px rgba(0, 0, 0, 0.18);
}

.difficulty-badge[data-level="Easy"] {
  background-color: #10b981 !important;
  color: #ffffff !important;
}

.difficulty-badge[data-level="Medium"] {
  background-color: #f59e0b !important;
  color: #ffffff !important;
}

.difficulty-badge[data-level="Hard"] {
  background-color: #dc2626 !important;
  color: #ffffff !important;
}

/* Status Buttons Group Alignment */
.status-btns { 
  display: flex; 
  gap: 6px; 
  align-items: center; 
  justify-content: center;
}

.action-btn {
  font-family: inherit;
  border: 1px solid var(--pill-outline);
  background: var(--input-bg);
  color: var(--text-main);
  border-radius: 20px;
  padding: 0 14px;
  cursor: pointer;
  font-size: 0.78rem;
  font-weight: 600;
  height: 34px;
  min-width: 90px;
  line-height: 32px;
  white-space: nowrap;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  box-sizing: border-box;
  transition: background-color 0.2s ease, border-color 0.2s ease, color 0.2s ease, box-shadow 0.2s ease;
}

.action-btn:hover { 
  border-color: var(--primary); 
}

.action-btn.solved.active {
  background: #10b981;
  color: #fff;
  border: 1px solid #059669;
  box-shadow: 0 2px 6px rgba(16, 185, 129, 0.25);
}

.action-btn.review.active {
  background: #f59e0b;
  color: #fff;
  border: 1px solid #d97706;
  box-shadow: 0 2px 6px rgba(245, 158, 11, 0.25);
}

/* Accordion Expansion */
.details-row { 
  display: table-row; 
}

.details-cell {
  padding: 0 16px !important;
  border-bottom: 1px solid transparent;
  height: auto !important;
}

.details-wrapper {
  display: grid;
  grid-template-rows: 0fr;
  transition: grid-template-rows 0.35s cubic-bezier(0.16, 1, 0.3, 1);
}

.details-row.active .details-wrapper {
  grid-template-rows: 1fr;
}

.details-inner {
  overflow: hidden;
}

.details-content {
  padding: 1.5rem;
  background: var(--input-bg);
  border-radius: 16px;
  margin: 12px 0 18px 0;
  border: 1px solid var(--table-border);
  box-shadow: 0 8px 20px rgba(0,0,0,0.03);
  animation: fadeIn 0.25s ease;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(-4px); }
  to { opacity: 1; transform: translateY(0); }
}

.pdf-frame { 
  width: 100%; 
  height: 500px; 
  border: 1px solid var(--table-border); 
  border-radius: 12px; 
  margin-top: 0.75rem; 
}

.solution-btn {
  font-family: inherit;
  background: var(--primary);
  color: #fff;
  border: 1px solid var(--primary);
  padding: 10px 20px;
  border-radius: 20px;
  cursor: pointer;
  font-size: 0.82rem;
  font-weight: 600;
  margin-top: 1.25rem;
  transition: all 0.2s cubic-bezier(0.16, 1, 0.3, 1);
}

.solution-btn:hover { 
  background: var(--primary-hover); 
  transform: translateY(-1px);
}

.solution-wrapper {
  display: grid;
  grid-template-rows: 0fr;
  transition: grid-template-rows 0.35s cubic-bezier(0.16, 1, 0.3, 1);
}

.solution-box.active .solution-wrapper {
  grid-template-rows: 1fr;
}

.solution-inner {
  overflow: hidden;
}

.solution-padding {
  padding-top: 1.25rem;
}

/* Empty State */
.empty-state {
  text-align: center;
  padding: 3rem 1rem;
  animation: fadeIn 0.3s ease;
}

.empty-icon {
  font-size: 2.5rem;
  margin-bottom: 0.5rem;
}

.empty-text {
  color: var(--text-muted);
  font-size: 0.95rem;
  margin-bottom: 1rem;
}

.reset-btn {
  font-family: inherit;
  background: var(--primary);
  color: #fff;
  border: none;
  padding: 8px 18px;
  border-radius: 20px;
  cursor: pointer;
  font-size: 0.82rem;
  font-weight: 600;
  transition: all 0.2s ease;
}

.reset-btn:hover { 
  background: var(--primary-hover); 
  transform: translateY(-1px);
}