.fenwick-card-visual {
  display: grid;
  grid-template-columns: repeat(8, 1fr);
  align-items: end;
  gap: 8px;
  padding: 28px 24px;
  background:
    radial-gradient(circle at 25% 15%, rgba(215, 239, 114, .55), transparent 30%),
    linear-gradient(135deg, #eef3df, #fffdf8);
}

.fenwick-card-visual span {
  height: var(--h);
  min-height: 34px;
  border-radius: 12px 12px 6px 6px;
  background: linear-gradient(180deg, var(--green), #6aa771);
  color: white;
  display: grid;
  place-items: end center;
  padding-bottom: 8px;
  font: 800 12px ui-monospace, monospace;
  box-shadow: 0 10px 22px rgba(30, 89, 61, .16);
}

.lesson-main {
  max-width: 1180px;
  margin: 0 auto;
  padding: 46px 0 90px;
}

.lesson-hero {
  display: grid;
  grid-template-columns: minmax(0, 1fr) 300px;
  gap: 18px;
  align-items: stretch;
  margin-bottom: 22px;
}

.lesson-hero > div:first-child,
.lesson-summary-card {
  border: 1px solid var(--line);
  border-radius: 18px;
  background:
    linear-gradient(135deg, rgba(255, 253, 248, .98), rgba(246, 248, 240, .96));
  box-shadow: var(--shadow);
}

.lesson-hero > div:first-child {
  padding: 30px 34px;
}

.lesson-hero h1 {
  max-width: 760px;
  margin: 14px 0 14px;
  color: var(--ink);
  font: 800 46px/1.18 "Microsoft YaHei", "PingFang SC", "Noto Sans CJK SC", sans-serif;
  letter-spacing: 0;
}

.lesson-hero h1 em {
  color: var(--green);
  font-style: normal;
  font-weight: 800;
}

.lesson-hero p {
  max-width: 720px;
  color: var(--muted);
  font-size: 16px;
  line-height: 1.85;
  margin: 0;
}

.lesson-hero code,
.lesson-content code {
  background: #edf0e6;
  border: 1px solid var(--line);
  border-radius: 6px;
  padding: 1px 5px;
  color: var(--green);
  font-family: ui-monospace, "Cascadia Code", Consolas, monospace;
}

.lesson-summary-card {
  padding: 24px;
  display: grid;
  align-content: center;
  gap: 12px;
}

.lesson-summary-card b {
  color: var(--green);
  font-size: 13px;
  letter-spacing: 1px;
}

.lesson-summary-card code {
  display: block;
  font: 800 18px/1.45 ui-monospace, "Cascadia Code", Consolas, monospace;
  color: var(--ink);
  background: white;
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 13px 14px;
}

.lesson-summary-card span {
  color: var(--muted);
  line-height: 1.7;
}

.lesson-layout {
  display: block;
}

.lesson-content {
  display: grid;
  gap: 18px;
}

.lesson-section {
  position: relative;
  border: 1px solid var(--line);
  border-radius: 18px;
  background: var(--card);
  box-shadow: var(--shadow);
  padding: 28px;
}

.lesson-step {
  display: inline-grid;
  place-items: center;
  min-width: 40px;
  height: 26px;
  border-radius: 999px;
  background: var(--lime);
  color: var(--green);
  font: 900 11px ui-monospace, monospace;
  margin-bottom: 12px;
}

.lesson-section h2 {
  margin: 0 0 12px;
  font-size: 26px;
  letter-spacing: 0;
}

.lesson-section p {
  color: var(--muted);
  line-height: 1.8;
  margin: 10px 0;
}

.lesson-callout {
  margin-top: 16px;
  border-left: 4px solid var(--green);
  background: #f2f3e9;
  border-radius: 10px;
  padding: 13px 15px;
  color: var(--green);
  font-size: 13px;
  font-weight: 800;
}

.code-block {
  margin: 18px 0 0;
  border-radius: 14px;
  background: #18251e;
  color: #dfe9df;
  overflow: auto;
  padding: 18px;
  font: 13px/1.65 ui-monospace, "Cascadia Code", Consolas, monospace;
}

.code-block code {
  background: transparent;
  border: 0;
  padding: 0;
  color: inherit;
}

.code-block span {
  box-sizing: border-box;
  display: inline-block;
  width: 100%;
  padding: 0 8px;
  border-radius: 6px;
}

.code-block span.active {
  background: rgba(215, 239, 114, .18);
  color: #f2ffd0;
}

.visual-demo {
  padding: 28px;
}

.visual-heading {
  display: flex;
  align-items: flex-start;
  gap: 14px;
  margin-bottom: 18px;
}

.visual-heading .lesson-step {
  flex: 0 0 auto;
  margin: 4px 0 0;
}

.visual-heading h2 {
  margin-bottom: 8px;
}

.visual-heading p {
  margin: 0;
}

.visual-control-bar {
  display: grid;
  grid-template-columns: 150px 150px minmax(0, 1fr);
  gap: 12px;
  align-items: end;
  margin-bottom: 16px;
}

.visual-control-bar.single-control {
  grid-template-columns: 180px minmax(420px, 520px);
}

.visual-control-bar.diff-control {
  grid-template-columns: repeat(4, minmax(92px, 120px)) minmax(420px, 1fr);
}

.visual-control-bar label {
  display: grid;
  gap: 7px;
  color: var(--muted);
  font-size: 12px;
}

.visual-control-bar input {
  border: 1px solid var(--line);
  background: #faf9f4;
  border-radius: 10px;
  padding: 10px;
  outline: none;
}

.bit-actions {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 10px;
}

.bit-actions.one-action {
  grid-template-columns: 1fr;
}

.bit-actions.two-actions {
  grid-template-columns: 1fr 1fr;
}

.bit-actions.three-actions {
  grid-template-columns: repeat(3, minmax(0, 1fr));
}

.bit-actions.four-actions {
  grid-template-columns: repeat(4, minmax(0, 1fr));
}

.bit-actions .ghost-btn {
  width: 100%;
  text-align: center;
  background: rgba(255,255,255,.55);
  white-space: nowrap;
}

.bit-actions .danger {
  color: var(--red);
}

.visual-grid {
  display: grid;
  grid-template-columns: minmax(0, 1.08fr) minmax(360px, .92fr);
  gap: 16px;
  align-items: start;
}

.bit-message,
.bit-formula {
  border: 1px solid var(--line);
  border-radius: 12px;
  background: linear-gradient(135deg, #fffdf8, #f2f3e9);
  color: var(--green);
  padding: 12px 14px;
  font-size: 13px;
  line-height: 1.65;
}

.trace-panel {
  display: grid;
  gap: 12px;
}

.trace-panel .code-block {
  margin: 0;
}

.compact-code {
  max-width: 430px;
}

.range-diagram {
  display: grid;
  gap: 16px;
}

.add-diagram {
  display: grid;
  gap: 16px;
}

.query-diagram {
  display: grid;
  gap: 16px;
}

.range-summary-grid {
  display: grid;
  grid-template-columns: 96px repeat(8, minmax(62px, 1fr));
  gap: 6px;
  align-items: stretch;
}

.range-label {
  display: grid;
  align-items: center;
  color: var(--ink);
  font-size: 13px;
  font-weight: 800;
  line-height: 1.35;
}

.range-cell {
  border: 1px solid var(--line);
  border-radius: 9px;
  background: #f0f0ee;
  color: var(--ink);
  min-height: 42px;
  padding: 8px 6px;
  box-shadow: 0 2px 5px rgba(12, 24, 18, .12);
  cursor: pointer;
  font: 800 14px/1.2 ui-monospace, "Cascadia Code", Consolas, monospace;
  transition: .18s;
}

.range-cell.interval-cell {
  background: var(--tone);
  font-size: 13px;
}

.range-cell.active,
.range-line-row.active .range-cell {
  border-color: var(--green);
  box-shadow: 0 0 0 2px rgba(30, 89, 61, .12), 0 8px 18px rgba(30, 89, 61, .12);
  transform: translateY(-1px);
}

.range-map {
  display: grid;
  gap: 10px;
  padding: 8px 0 4px;
}

.add-map {
  display: grid;
  gap: 10px;
  padding: 8px 0 4px;
}

.query-map {
  display: grid;
  gap: 10px;
  padding: 8px 0 4px;
}

.range-axis,
.range-line-row {
  display: grid;
  grid-template-columns: 76px minmax(0, 1fr);
  gap: 10px;
  align-items: center;
}

.range-axis-track,
.range-track {
  display: grid;
  grid-template-columns: repeat(8, minmax(0, 1fr));
  align-items: center;
  column-gap: 0;
  min-height: 24px;
}

.range-axis-track span {
  color: var(--ink);
  font: 900 13px ui-monospace, "Cascadia Code", Consolas, monospace;
  text-align: center;
}

.range-row-label {
  color: var(--ink);
  font: 800 13px ui-monospace, "Cascadia Code", Consolas, monospace;
}

.range-track {
  position: relative;
  min-height: 34px;
}

.range-segment {
  position: relative;
  height: 3px;
  border: 0;
  border-radius: 999px;
  background: var(--line-color);
  box-shadow: 0 2px 5px rgba(12, 24, 18, .18);
  cursor: pointer;
}

.range-segment::before,
.range-segment::after {
  content: "";
  position: absolute;
  top: 50%;
  width: 10px;
  height: 10px;
  border-radius: 999px;
  background: var(--line-color);
  transform: translate(-50%, -50%);
  box-shadow: 0 1px 4px rgba(12, 24, 18, .22);
}

.range-segment::before {
  left: 0;
}

.range-segment::after {
  left: 100%;
}

.range-segment .range-start,
.range-segment .range-end {
  position: absolute;
  top: -22px;
  color: var(--ink);
  font: 800 12px ui-monospace, "Cascadia Code", Consolas, monospace;
  white-space: nowrap;
}

.range-segment .range-start {
  left: 0;
  transform: translateX(-50%);
}

.range-segment .range-end {
  right: 0;
  transform: translateX(50%);
}

.range-line-row.active .range-segment {
  height: 5px;
  box-shadow: 0 0 0 4px rgba(30, 89, 61, .08), 0 8px 18px rgba(30, 89, 61, .16);
}

.add-line-row.candidate .range-segment {
  opacity: .78;
}

.add-line-row.current .range-segment {
  height: 5px;
  box-shadow: 0 0 0 4px rgba(30, 89, 61, .09), 0 8px 18px rgba(30, 89, 61, .16);
}

.add-line-row.done .range-segment {
  height: 5px;
}

.add-line-row.done .range-row-label {
  color: var(--green);
}

.add-line-row.dimmed {
  opacity: .42;
}

.query-line-row.candidate .range-segment {
  opacity: .78;
}

.query-line-row.current .range-segment {
  height: 5px;
  box-shadow: 0 0 0 4px rgba(30, 89, 61, .09), 0 8px 18px rgba(30, 89, 61, .16);
}

.query-line-row.done .range-segment {
  height: 5px;
}

.query-line-row.done .range-row-label {
  color: var(--green);
}

.query-line-row.dimmed {
  opacity: .42;
}

.add-status {
  position: absolute;
  right: -2px;
  top: -34px;
  border: 1px solid rgba(30, 89, 61, .18);
  border-radius: 999px;
  background: #fffdf8;
  color: var(--green);
  padding: 3px 8px;
  font: 800 11px "Microsoft YaHei", "PingFang SC", sans-serif;
  white-space: nowrap;
}

.query-status {
  position: absolute;
  right: -2px;
  top: -34px;
  border: 1px solid rgba(30, 89, 61, .18);
  border-radius: 999px;
  background: #fffdf8;
  color: var(--green);
  padding: 3px 8px;
  font: 800 11px "Microsoft YaHei", "PingFang SC", sans-serif;
  white-space: nowrap;
}

.add-point-marker {
  align-self: stretch;
  justify-self: center;
  width: 2px;
  min-height: 34px;
  border-radius: 999px;
  background: rgba(207, 58, 46, .52);
  position: relative;
  z-index: 0;
}

.add-point-marker::before {
  content: "x";
  position: absolute;
  left: 50%;
  top: -18px;
  transform: translateX(-50%);
  color: var(--red);
  font: 900 11px ui-monospace, "Cascadia Code", Consolas, monospace;
}

.query-prefix-marker {
  align-self: center;
  height: 18px;
  border: 1px dashed rgba(30, 89, 61, .35);
  border-radius: 999px;
  background: rgba(215, 239, 114, .2);
  color: var(--green);
  display: grid;
  place-items: center;
  font: 800 11px "Microsoft YaHei", "PingFang SC", sans-serif;
  white-space: nowrap;
}

.range-explain-grid {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(0, 1fr);
  gap: 12px;
}

.bit-board {
  margin: 0;
  border: 1px solid var(--line);
  border-radius: 16px;
  background:
    radial-gradient(circle at 20px 20px, rgba(30, 89, 61, .08) 2px, transparent 2px) 0 0/22px 22px,
    #faf9f4;
  padding: 16px;
  overflow: hidden;
}

.bit-board h3 {
  margin: 0 0 8px;
  color: var(--muted);
  font-size: 12px;
}

.bit-row {
  display: grid;
  grid-template-columns: repeat(8, 1fr);
  gap: 6px;
  margin-bottom: 16px;
}

.bit-cell {
  border: 1px solid var(--line);
  border-radius: 10px;
  background: white;
  min-height: 54px;
  display: grid;
  place-items: center;
  padding: 6px 4px;
  cursor: pointer;
  transition: .18s;
}

.bit-cell b {
  color: var(--green);
  font: 900 11px ui-monospace, monospace;
}

.bit-cell span {
  color: var(--ink);
  font: 800 16px ui-monospace, monospace;
}

.bit-cell.cover {
  background: #eff8d4;
  border-color: #b9d765;
}

.bit-cell.visit {
  background: var(--green);
  border-color: var(--green);
  color: white;
  transform: translateY(-3px);
  box-shadow: 0 10px 20px rgba(30, 89, 61, .18);
}

.bit-cell.visit b,
.bit-cell.visit span {
  color: white;
}

.tree-row .bit-cell {
  min-height: 64px;
}

.diff-row .bit-cell {
  background: #f7fbea;
  min-height: 58px;
}

.diff-row .bit-cell.visit {
  background: #8ab657;
  border-color: #8ab657;
}

.bit-range-line {
  min-height: 24px;
  color: var(--muted);
  font-size: 12px;
  line-height: 1.5;
}

@media (max-width: 1050px) {
  .lesson-main {
    margin-left: 20px;
    margin-right: 20px;
  }

  .lesson-hero,
  .visual-control-bar,
  .visual-control-bar.single-control,
  .visual-control-bar.diff-control,
  .visual-grid,
  .range-explain-grid {
    grid-template-columns: 1fr;
  }

  .range-summary-grid {
    grid-template-columns: 84px repeat(8, minmax(56px, 1fr));
    overflow-x: auto;
    padding-bottom: 4px;
  }
}

@media (max-width: 620px) {
  .lesson-hero > div:first-child,
  .lesson-summary-card,
  .lesson-section,
  .visual-demo {
    padding: 20px;
  }

  .lesson-hero h1 {
    font-size: 34px;
  }

  .bit-actions {
    grid-template-columns: 1fr 1fr;
  }

  .bit-actions.four-actions {
    grid-template-columns: 1fr 1fr;
  }

  .range-summary-grid {
    grid-template-columns: 78px repeat(8, 64px);
  }

  .range-axis,
  .range-line-row {
    grid-template-columns: 64px minmax(480px, 1fr);
  }

  .range-map,
  .add-map,
  .query-map {
    overflow-x: auto;
    padding-top: 12px;
  }

  .bit-row {
    gap: 4px;
  }

  .bit-cell {
    min-height: 48px;
    border-radius: 8px;
  }

  .bit-cell b {
    font-size: 10px;
  }

  .bit-cell span {
    font-size: 14px;
  }
}