Picking and seeing through a tree of boxes per object (made again only when it changes)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/bvh.h

@@ -0,0 +1,51 @@
+#pragma once
+// A tree of boxes round a mesh's triangles (a bounding volume hierarchy): what a ray hits first,
+// or whether anything's in its way, without trying every triangle.
+#include "mesh.h"
+
+#include <glm/glm.hpp>
+
+#include <array>
+#include <cstdint>
+#include <optional>
+#include <vector>
+
+namespace fry {
+
+class TriangleTree {
+public:
+    TriangleTree() = default;
+    // The mesh's triangles, its points moved by `matrix` (into the world).
+    TriangleTree(const Mesh& m, const glm::mat4& matrix);
+
+    struct Hit {
+        float t = 0;
+        int face = -1;           // (the mesh's face it's part of)
+        std::array<int, 3> tri{}; // (its points)
+    };
+    // The nearest hit along the ray (dir needn't be unit length: t is in its lengths), before maxT.
+    std::optional<Hit> nearest(const glm::vec3& origin, const glm::vec3& dir, float maxT = 1e30f) const;
+    // Anything in the way before maxT, but triangles with point `skip` (the point being looked at).
+    bool blocked(const glm::vec3& origin, const glm::vec3& dir, float maxT, int skip = -1) const;
+    bool empty() const { return tris_.empty(); }
+    const std::vector<glm::vec3>& points() const { return points_; } // (in the world)
+
+private:
+    struct Tri {
+        std::array<int, 3> v;
+        int face;
+    };
+    struct Node {
+        glm::vec3 lo, hi;
+        int first, count; // (a leaf: its triangles; else count 0 and `first` is the right child, the left one next)
+    };
+    std::vector<glm::vec3> points_;
+    std::vector<Tri> tris_;
+    std::vector<Node> nodes_;
+    int build(int first, int count);
+};
+
+// Where a ray meets a triangle (Möller–Trumbore), in the ray's lengths.
+std::optional<float> rayTriangle(const glm::vec3& origin, const glm::vec3& dir, const glm::vec3& a, const glm::vec3& b, const glm::vec3& c);
+
+} // namespace fry