/* ===================================================
   INTEGRAL TRAINER — style.css
   Aesthetic: Academic dark — ink on slate
   =================================================== */

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

:root {
  --bg:        #0e0f11;
  --surface:   #16181c;
  --surface2:  #1e2127;
  --border:    #2a2d35;
  --border2:   #363a44;
  --text:      #e8e6e1;
  --text-dim:  #7a7f8e;
  --text-mute: #454952;
  --accent:    #c8a96e;       /* warm gold */
  --accent2:   #8bbcaa;       /* sage green */
  --correct:   #5bbd8a;
  --incorrect: #d9665e;
  --easy:      #5bbd8a;
  --medium:    #e8a44e;
  --hard:      #d9665e;

  --font-serif: 'Libre Baskerville', Georgia, serif;
  --font-mono:  'JetBrains Mono', 'Fira Code', monospace;

  --radius: 12px;
  --shadow: 0 8px 40px rgba(0,0,0,0.5);
}

/* ── Base ── */
html { font-size: 16px; }

body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--font-serif);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  position: relative;
  overflow-x: hidden;
}

/* ── Atmospheric background ── */
.page-bg {
  position: fixed;
  inset: 0;
  pointer-events: none;
  z-index: 0;
  background:
    radial-gradient(ellipse 80% 50% at 20% 10%, rgba(200, 169, 110, 0.06) 0%, transparent 60%),
    radial-gradient(ellipse 60% 40% at 80% 80%, rgba(139, 188, 170, 0.05) 0%, transparent 60%);
}

/* ── Header ── */
header {
  position: relative;
  z-index: 10;
  display: flex;
  align-items: center;
  gap: 1.25rem;
  padding: 1.5rem 2.5rem;
  border-bottom: 1px solid var(--border);
  background: rgba(14, 15, 17, 0.8);
  backdrop-filter: blur(12px);
}

.logo {
  font-size: 2.2rem;
  font-family: var(--font-serif);
  color: var(--accent);
  line-height: 1;
  opacity: 0.9;
  flex-shrink: 0;
}

.title-block h1 {
  font-size: 1.05rem;
  font-weight: 700;
  letter-spacing: 0.04em;
  color: var(--text);
}

.title-block .subtitle {
  font-size: 0.72rem;
  font-family: var(--font-mono);
  color: var(--text-dim);
  margin-top: 0.15rem;
  letter-spacing: 0.02em;
}

.header-stats {
  margin-left: auto;
  display: flex;
  gap: 2rem;
}

.stat {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.1rem;
}

.stat-val {
  font-family: var(--font-mono);
  font-size: 1.4rem;
  color: var(--accent);
  font-weight: 500;
  line-height: 1;
  transition: transform 0.2s, color 0.2s;
}

.stat-val.bump {
  transform: scale(1.3);
  color: var(--correct);
}

.stat-label {
  font-size: 0.65rem;
  color: var(--text-mute);
  font-family: var(--font-mono);
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

/* ── Main ── */
main {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 3rem 1.5rem;
  position: relative;
  z-index: 10;
}

/* ── Card ── */
.card {
  width: 100%;
  max-width: 640px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 2.5rem 2.8rem;
  position: relative;
  animation: card-in 0.4s ease both;
}

@keyframes card-in {
  from { opacity: 0; transform: translateY(16px); }
  to   { opacity: 1; transform: translateY(0); }
}

/* ── Difficulty badge ── */
.difficulty-badge {
  position: absolute;
  top: 1.2rem;
  right: 1.2rem;
  font-family: var(--font-mono);
  font-size: 0.65rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  padding: 0.25rem 0.65rem;
  border-radius: 100px;
  border: 1px solid currentColor;
  opacity: 0.75;
}

.difficulty-badge.easy   { color: var(--easy); }
.difficulty-badge.medium { color: var(--medium); }
.difficulty-badge.hard   { color: var(--hard); }

/* ── Problem number ── */
.problem-number {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  color: var(--text-mute);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: 1.5rem;
}

/* ── Integral display ── */
.integral-display {
  background: var(--surface2);
  border: 1px solid var(--border2);
  border-radius: 8px;
  padding: 2rem 1.5rem;
  text-align: center;
  margin-bottom: 2rem;
  min-height: 90px;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: opacity 0.25s;
}

.integral-display.changing {
  opacity: 0;
}

.katex { font-size: 1.8rem !important; }

/* ── Input group ── */
.input-group {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
  margin-bottom: 0.75rem;
}

label {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  color: var(--text-dim);
  letter-spacing: 0.06em;
}

.input-row {
  display: flex;
  gap: 0.6rem;
}

input[type="text"] {
  flex: 1;
  background: var(--surface2);
  border: 1px solid var(--border2);
  border-radius: 8px;
  color: var(--text);
  font-family: var(--font-mono);
  font-size: 1rem;
  padding: 0.8rem 1rem;
  outline: none;
  transition: border-color 0.2s, box-shadow 0.2s;
  caret-color: var(--accent);
}

input[type="text"]::placeholder {
  color: var(--text-mute);
  font-style: italic;
}

input[type="text"]:focus {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px rgba(200, 169, 110, 0.12);
}

input[type="text"].shake {
  animation: shake 0.35s ease;
}

@keyframes shake {
  0%,100% { transform: translateX(0); }
  20%      { transform: translateX(-6px); }
  40%      { transform: translateX(6px); }
  60%      { transform: translateX(-4px); }
  80%      { transform: translateX(4px); }
}

.input-hint {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  color: var(--text-mute);
}

/* ── Preview ── */
.preview-row {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  min-height: 2rem;
  margin-bottom: 1.25rem;
}

.preview-label {
  font-family: var(--font-mono);
  font-size: 0.65rem;
  color: var(--text-mute);
  flex-shrink: 0;
}

#answer-preview .katex {
  font-size: 1rem !important;
  color: var(--text-dim);
}

/* ── Buttons ── */
.btn {
  font-family: var(--font-mono);
  font-size: 0.82rem;
  letter-spacing: 0.04em;
  border: none;
  border-radius: 8px;
  cursor: pointer;
  padding: 0.8rem 1.4rem;
  transition: all 0.18s;
  white-space: nowrap;
}

.btn-primary {
  background: var(--accent);
  color: #0e0f11;
  font-weight: 500;
}

.btn-primary:hover {
  background: #dfc07e;
  transform: translateY(-1px);
  box-shadow: 0 4px 16px rgba(200, 169, 110, 0.3);
}

.btn-primary:active { transform: translateY(0); }

.btn-secondary {
  background: transparent;
  color: var(--text-dim);
  border: 1px solid var(--border2);
}

.btn-secondary:hover {
  border-color: var(--accent2);
  color: var(--accent2);
}

.btn-ghost {
  background: transparent;
  color: var(--text-mute);
  padding: 0.8rem 0.8rem;
  font-size: 0.75rem;
}

.btn-ghost:hover { color: var(--text-dim); }

/* ── Feedback ── */
.feedback {
  border-radius: 8px;
  font-family: var(--font-mono);
  font-size: 0.85rem;
  padding: 0;
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.3s ease, padding 0.3s ease, opacity 0.3s ease;
  opacity: 0;
  margin-bottom: 0;
}

.feedback.visible {
  max-height: 80px;
  padding: 0.8rem 1rem;
  opacity: 1;
  margin-bottom: 1.25rem;
}

.feedback.correct {
  background: rgba(91, 189, 138, 0.1);
  border: 1px solid rgba(91, 189, 138, 0.3);
  color: var(--correct);
}

.feedback.incorrect {
  background: rgba(217, 102, 94, 0.1);
  border: 1px solid rgba(217, 102, 94, 0.3);
  color: var(--incorrect);
}

.feedback.error {
  background: rgba(232, 164, 78, 0.08);
  border: 1px solid rgba(232, 164, 78, 0.25);
  color: var(--medium);
}

/* ── Card footer ── */
.card-footer {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

/* ── Hint ── */
.hint-block {
  background: var(--surface2);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.75rem 1rem;
  margin-bottom: 1.25rem;
  font-family: var(--font-mono);
  font-size: 0.78rem;
  color: var(--text-dim);
  animation: fadein 0.2s ease;
}

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

/* ── Progress track ── */
.progress-track {
  width: 100%;
  max-width: 640px;
  height: 3px;
  background: var(--surface2);
  border-radius: 100px;
  margin-top: 1rem;
  overflow: hidden;
}

.progress-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--accent2), var(--accent));
  border-radius: 100px;
  width: 0%;
  transition: width 0.5s cubic-bezier(0.4,0,0.2,1);
}

/* ── Footer ── */
footer {
  text-align: center;
  padding: 1.25rem;
  border-top: 1px solid var(--border);
  position: relative;
  z-index: 10;
}

footer p {
  font-family: var(--font-mono);
  font-size: 0.68rem;
  color: var(--text-mute);
  letter-spacing: 0.04em;
}

/* ── Responsive ── */
@media (max-width: 600px) {
  header { padding: 1rem 1.25rem; gap: 0.75rem; }
  .header-stats { gap: 1rem; }
  .stat-val { font-size: 1.1rem; }
  .card { padding: 1.75rem 1.4rem; }
  .katex { font-size: 1.4rem !important; }
  .input-row { flex-direction: column; }
  .btn-primary { width: 100%; }
}