implement barnes-hut particle repulsion using octree

include/octree.hpp

@@ -0,0 +1,53 @@
+#ifndef __OCTREE_HPP_
+#define __OCTREE_HPP_
+
+#include <raylib.h>
+#include <raymath.h>
+#include <vector>
+
+class OctreeNode {
+public:
+  Vector3 mass_center;
+  float mass_total;
+  Vector3 box_min; // area start
+  Vector3 box_max; // area end
+  int children[8];
+  int mass_id;
+  bool leaf;
+
+public:
+  OctreeNode()
+      : mass_center(Vector3Zero()), mass_total(0.0),
+        children(-1, -1, -1, -1, -1, -1, -1, -1), mass_id(-1), leaf(true) {}
+
+  ~OctreeNode() {}
+};
+
+class Octree {
+public:
+  std::vector<OctreeNode> nodes;
+
+public:
+  Octree() {}
+
+  Octree(const Octree &copy) = delete;
+  Octree &operator=(const Octree &copy) = delete;
+  Octree(Octree &&move) = delete;
+  Octree &operator=(Octree &&move) = delete;
+
+  ~Octree() {}
+
+public:
+  auto CreateNode(const Vector3 &box_min, const Vector3 &box_max) -> int;
+
+  auto GetOctant(int node_idx, const Vector3 &pos) -> int;
+
+  auto GetChildBounds(int node_idx, int octant) -> std::pair<Vector3, Vector3>;
+
+  auto Insert(int node_idx, int mass_id, const Vector3 &pos, float mass)
+      -> void;
+
+  auto CalculateForce(int node_idx, const Vector3 &pos) -> Vector3;
+};
+
+#endif