queryCircle method

List<int> queryCircle(double centerX, double centerY, double radius)

Queries particles within a circular area. Returns list of particle indices within the circle.

Mathematical Operations:

1. First filters nodes using rectangle-circle intersection.
2. Then checks exact distance using: sqrt((x2-x1)² + (y2-y1)²) <= radius.

Implementation

List<int> queryCircle(double centerX, double centerY, double radius) {
  return _root
      .queryCircle(centerX, centerY, radius)
      .map((p) => p.index)
      .toList();
}