ViewDesignEngine/include/vde/spatial/bvh.h

#pragma once
#include <optional>
#include "vde/spatial/spatial_index.h"
#include "vde/core/aabb.h"
#include "vde/core/triangle.h"

namespace vde::spatial {
using core::Point3D;
using core::Vector3D;
using core::Ray3Dd;
using core::Triangle3D;

enum class BVHSplitStrategy { Middle, Equal, SAH };

struct BVHBuildOptions {
    BVHSplitStrategy strategy = BVHSplitStrategy::SAH;
    int leaf_size = 4;
    int max_depth = 64;
};

class BVH : public SpatialIndex<Triangle3D> {
public:
    explicit BVH(const BVHBuildOptions& opts = {}) : opts_(opts) {}

    void build(const std::vector<Triangle3D>& tris) override;
    void insert(const Triangle3D& tri) override;
    bool remove(const Triangle3D& tri) override;
    std::vector<Triangle3D> query_range(const AABB3D& range) const override;
    std::vector<Triangle3D> query_knn(const Point3D& point, size_t k) const override;
    std::vector<Triangle3D> query_ray(const Ray3Dd& ray) const override;
    void clear() override;
    size_t size() const override { return primitives_.size(); }

    /// Find closest hit along ray
    struct HitResult { double t; Triangle3D tri; Point3D point; };
    std::optional<HitResult> query_ray_nearest(const Ray3Dd& ray) const;

private:
    struct Node {
        AABB3D bounds;
        int left = -1, right = -1;
        int first_prim = 0, prim_count = 0;
        bool leaf() const { return left < 0; }
    };
    std::vector<Node> nodes_;
    std::vector<Triangle3D> primitives_;
    BVHBuildOptions opts_;

    void build_recursive(int node, int first, int count, int depth);
    double sah_cost(int n_left, int n_right, const AABB3D& left, const AABB3D& right) const;
};

}  // namespace vde::spatial