cabana: use a segment tree to solve range queries in O(log n) time complexity (#27498)

tools/cabana/chartswidget.cc

@@ -562,6 +562,9 @@ void ChartView::updateSeries(const Signal *sig, const std::vector<Event *> *even
       if (events->size()) {
         s.last_value_mono_time = events->back()->mono_time;
       }
+      if (!can->liveStreaming()) {
+        s.segment_tree.build(s.vals);
+      }
       s.series->replace(series_type == SeriesType::StepLine ? s.step_vals : s.vals);
     }
   }
@@ -586,9 +589,15 @@ void ChartView::updateAxisY() {
 
     auto first = std::lower_bound(s.vals.begin(), s.vals.end(), axis_x->min(), [](auto &p, double x) { return p.x() < x; });
     auto last = std::lower_bound(first, s.vals.end(), axis_x->max(), [](auto &p, double x) { return p.x() < x; });
-    for (auto it = first; it != last; ++it) {
-      if (it->y() < min) min = it->y();
-      if (it->y() > max) max = it->y();
+    if (can->liveStreaming()) {
+      for (auto it = first; it != last; ++it) {
+        if (it->y() < min) min = it->y();
+        if (it->y() > max) max = it->y();
+      }
+    } else {
+      auto [min_y, max_y] = s.segment_tree.minmax(std::distance(s.vals.begin(), first), std::distance(s.vals.begin(), last));
+      min = std::min(min, min_y);
+      max = std::max(max, max_y);
     }
   }
   if (min == std::numeric_limits<double>::max()) min = 0;
@@ -781,7 +790,7 @@ void ChartView::drawForeground(QPainter *painter, const QRectF &rect) {
   painter->setPen(Qt::NoPen);
   qreal track_line_x = -1;
   for (auto &s : sigs) {
-    if (!s.track_pt.isNull() && s.series->isVisible()) {  
+    if (!s.track_pt.isNull() && s.series->isVisible()) {
       painter->setBrush(s.series->color().darker(125));
       painter->drawEllipse(s.track_pt, 5.5, 5.5);
       track_line_x = std::max(track_line_x, s.track_pt.x());

tools/cabana/chartswidget.h

@@ -46,6 +46,7 @@ public:
     QVector<QPointF> step_vals;
     uint64_t last_value_mono_time = 0;
     QPointF track_pt{};
+    SegmentTree segment_tree;
   };
 
 signals:

tools/cabana/util.cc

@@ -67,6 +67,40 @@ void ChangeTracker::clear() {
   colors.clear();
 }
 
+
+// SegmentTree
+
+void SegmentTree::build(const QVector<QPointF> &arr) {
+  size = arr.size();
+  tree.resize(4 * size);  // size of the tree is 4 times the size of the array
+  if (size > 0) {
+    build_tree(arr, 1, 0, size - 1);
+  }
+}
+
+void SegmentTree::build_tree(const QVector<QPointF> &arr, int n, int left, int right) {
+  if (left == right) {
+    const double y = arr[left].y();
+    tree[n] = {y, y};
+  } else {
+    const int mid = (left + right) >> 1;
+    build_tree(arr, 2 * n, left, mid);
+    build_tree(arr, 2 * n + 1, mid + 1, right);
+    tree[n] = {std::min(tree[2 * n].first, tree[2 * n + 1].first), std::max(tree[2 * n].second, tree[2 * n + 1].second)};
+  }
+}
+
+std::pair<double, double> SegmentTree::get_minmax(int n, int left, int right, int range_left, int range_right) const {
+  if (range_left > right || range_right < left)
+    return {std::numeric_limits<double>::max(), std::numeric_limits<double>::lowest()};
+  if (range_left <= left && range_right >= right)
+    return tree[n];
+  int mid = (left + right) >> 1;
+  auto l = get_minmax(2 * n, left, mid, range_left, range_right);
+  auto r = get_minmax(2 * n + 1, mid + 1, right, range_left, range_right);
+  return {std::min(l.first, r.first), std::max(l.second, r.second)};
+}
+
 // MessageBytesDelegate
 
 MessageBytesDelegate::MessageBytesDelegate(QObject *parent) : QStyledItemDelegate(parent) {

tools/cabana/util.h

@@ -35,6 +35,19 @@ enum {
   BytesRole = Qt::UserRole + 2
 };
 
+class SegmentTree {
+public:
+  SegmentTree() = default;
+  void build(const QVector<QPointF> &arr);
+  inline std::pair<double, double> minmax(int left, int right) const { return get_minmax(1, 0, size - 1, left, right); }
+
+private:
+  std::pair<double, double> get_minmax(int n, int left, int right, int range_left, int range_right) const;
+  void build_tree(const QVector<QPointF> &arr, int n, int left, int right);
+  std::vector<std::pair<double ,double>> tree;
+  int size = 0;
+};
+
 class MessageBytesDelegate : public QStyledItemDelegate {
   Q_OBJECT
 public: