text: #ifndef ENTITY_H
  #define ENTITY_H
  
  #include <glm/glm.hpp> //glm::mat4
  #include <list> //std::list
  #include <array> //std::array
  #include <memory> //std::unique_ptr
  
  class Transform
  {
  protected:
  	//Local space information
  	glm::vec3 m_pos = { 0.0f, 0.0f, 0.0f };
  	glm::vec3 m_eulerRot = { 0.0f, 0.0f, 0.0f }; //In degrees
  	glm::vec3 m_scale = { 1.0f, 1.0f, 1.0f };
  
  	//Global space informaiton concatenate in matrix
  	glm::mat4 m_modelMatrix = glm::mat4(1.0f);
  
  	//Dirty flag
  	bool m_isDirty = true;
  
  protected:
  	glm::mat4 getLocalModelMatrix()
  	{
  		const glm::mat4 transformX = glm::rotate(glm::mat4(1.0f), glm::radians(m_eulerRot.x), glm::vec3(1.0f, 0.0f, 0.0f));
  		const glm::mat4 transformY = glm::rotate(glm::mat4(1.0f), glm::radians(m_eulerRot.y), glm::vec3(0.0f, 1.0f, 0.0f));
  		const glm::mat4 transformZ = glm::rotate(glm::mat4(1.0f), glm::radians(m_eulerRot.z), glm::vec3(0.0f, 0.0f, 1.0f));
  
  		// Y * X * Z
  		const glm::mat4 roationMatrix = transformY * transformX * transformZ;
  
  		// translation * rotation * scale (also know as TRS matrix)
  		return glm::translate(glm::mat4(1.0f), m_pos) * roationMatrix * glm::scale(glm::mat4(1.0f), m_scale);
  	}
  public:
  
  	void computeModelMatrix()
  	{
  		m_modelMatrix = getLocalModelMatrix();
  	}
  
  	void computeModelMatrix(const glm::mat4& parentGlobalModelMatrix)
  	{
  		m_modelMatrix = parentGlobalModelMatrix * getLocalModelMatrix();
  	}
  
  	void setLocalPosition(const glm::vec3& newPosition)
  	{
  		m_pos = newPosition;
  		m_isDirty = true;
  	}
  
  	void setLocalRotation(const glm::vec3& newRotation)
  	{
  		m_eulerRot = newRotation;
  		m_isDirty = true;
  	}
  
  	void setLocalScale(const glm::vec3& newScale)
  	{
  		m_scale = newScale;
  		m_isDirty = true;
  	}
  
  	const glm::vec3& getGlobalPosition() const
  	{
  		return m_modelMatrix[3];
  	}
  
  	const glm::vec3& getLocalPosition() const
  	{
  		return m_pos;
  	}
  
  	const glm::vec3& getLocalRotation() const
  	{
  		return m_eulerRot;
  	}
  
  	const glm::vec3& getLocalScale() const
  	{
  		return m_scale;
  	}
  
  	const glm::mat4& getModelMatrix() const
  	{
  		return m_modelMatrix;
  	}
  
  	glm::vec3 getRight() const
  	{
  		return m_modelMatrix[0];
  	}
  
  
  	glm::vec3 getUp() const
  	{
  		return m_modelMatrix[1];
  	}
  
  	glm::vec3 getBackward() const
  	{
  		return m_modelMatrix[2];
  	}
  
  	glm::vec3 getForward() const
  	{
  		return -m_modelMatrix[2];
  	}
  
  	glm::vec3 getGlobalScale() const
  	{
  		return { glm::length(getRight()), glm::length(getUp()), glm::length(getBackward()) };
  	}
  
  	bool isDirty() const
  	{
  		return m_isDirty;
  	}
  };
  
  struct Plan
  {
  	glm::vec3 normal = { 0.f, 1.f, 0.f }; // unit vector
  	float     distance = 0.f;        // Distance with origin
  
  	Plan() = default;
  
  	Plan(const glm::vec3& p1, const glm::vec3& norm)
  		: normal(glm::normalize(norm)),
  		distance(glm::dot(normal, p1))
  	{}
  
  	float getSignedDistanceToPlan(const glm::vec3& point) const
  	{
  		return glm::dot(normal, point) - distance;
  	}
  };
  
  struct Frustum
  {
  	Plan topFace;
  	Plan bottomFace;
  
  	Plan rightFace;
  	Plan leftFace;
  
  	Plan farFace;
  	Plan nearFace;
  };
  
  struct BoundingVolume
  {
  	virtual bool isOnFrustum(const Frustum& camFrustum, const Transform& transform) const = 0;
  
  	virtual bool isOnOrForwardPlan(const Plan& plan) const = 0;
  
  	bool isOnFrustum(const Frustum& camFrustum) const
  	{
  		return (isOnOrForwardPlan(camFrustum.leftFace) &&
  			isOnOrForwardPlan(camFrustum.rightFace) &&
  			isOnOrForwardPlan(camFrustum.topFace) &&
  			isOnOrForwardPlan(camFrustum.bottomFace) &&
  			isOnOrForwardPlan(camFrustum.nearFace) &&
  			isOnOrForwardPlan(camFrustum.farFace));
  	};
  };
  
  struct Sphere : public BoundingVolume
  {
  	glm::vec3 center{ 0.f, 0.f, 0.f };
  	float radius{ 0.f };
  
  	Sphere(const glm::vec3& inCenter, float inRadius)
  		: BoundingVolume{}, center{ inCenter }, radius{ inRadius }
  	{}
  
  	bool isOnOrForwardPlan(const Plan& plan) const final
  	{
  		return plan.getSignedDistanceToPlan(center) > -radius;
  	}
  
  	bool isOnFrustum(const Frustum& camFrustum, const Transform& transform) const final
  	{
  		//Get global scale thanks to our transform
  		const glm::vec3 globalScale = transform.getGlobalScale();
  
  		//Get our global center with process it with the global model matrix of our transform
  		const glm::vec3 globalCenter{ transform.getModelMatrix() * glm::vec4(center, 1.f) };
  
  		//To wrap correctly our shape, we need the maximum scale scalar.
  		const float maxScale = std::max(std::max(globalScale.x, globalScale.y), globalScale.z);
  
  		//Max scale is assuming for the diameter. So, we need the half to apply it to our radius
  		Sphere globalSphere(globalCenter, radius * (maxScale * 0.5f));
  
  		//Check Firstly the result that have the most chance to faillure to avoid to call all functions.
  		return (globalSphere.isOnOrForwardPlan(camFrustum.leftFace) &&
  			globalSphere.isOnOrForwardPlan(camFrustum.rightFace) &&
  			globalSphere.isOnOrForwardPlan(camFrustum.farFace) &&
  			globalSphere.isOnOrForwardPlan(camFrustum.nearFace) &&
  			globalSphere.isOnOrForwardPlan(camFrustum.topFace) &&
  			globalSphere.isOnOrForwardPlan(camFrustum.bottomFace));
  	};
  };
  
  struct SquareAABB : public BoundingVolume
  {
  	glm::vec3 center{ 0.f, 0.f, 0.f };
  	float extent{ 0.f };
  
  	SquareAABB(const glm::vec3& inCenter, float inExtent)
  		: BoundingVolume{}, center{ inCenter }, extent{ inExtent }
  	{}
  
  	bool isOnOrForwardPlan(const Plan& plan) const final
  	{
  		// Compute the projection interval radius of b onto L(t) = b.c + t * p.n
  		const float r = extent * (std::abs(plan.normal.x) + std::abs(plan.normal.y) + std::abs(plan.normal.z));
  		return -r <= plan.getSignedDistanceToPlan(center);
  	}
  
  	bool isOnFrustum(const Frustum& camFrustum, const Transform& transform) const final
  	{
  		//Get global scale thanks to our transform
  		const glm::vec3 globalCenter{ transform.getModelMatrix() * glm::vec4(center, 1.f) };
  
  		// Scaled orientation
  		const glm::vec3 right = transform.getRight() * extent;
  		const glm::vec3 up = transform.getUp() * extent;
  		const glm::vec3 forward = transform.getForward() * extent;
  
  		const float newIi = std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, forward));
  
  		const float newIj = std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, forward));
  
  		const float newIk = std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, forward));
  
  		const SquareAABB globalAABB(globalCenter, std::max(std::max(newIi, newIj), newIk));
  
  		return (globalAABB.isOnOrForwardPlan(camFrustum.leftFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.rightFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.topFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.bottomFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.nearFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.farFace));
  	};
  };
  
  struct AABB : public BoundingVolume
  {
  	glm::vec3 center{ 0.f, 0.f, 0.f };
  	glm::vec3 extents{ 0.f, 0.f, 0.f };
  
  	AABB(const glm::vec3& min, const glm::vec3& max)
  		: BoundingVolume{}, center{ (max + min) * 0.5f }, extents{ max.x - center.x, max.y - center.y, max.z - center.z }
  	{}
  
  	AABB(const glm::vec3& inCenter, float iI, float iJ, float iK)
  		: BoundingVolume{}, center{ inCenter }, extents{ iI, iJ, iK }
  	{}
  
  	std::array<glm::vec3, 8> getVertice() const
  	{
  		std::array<glm::vec3, 8> vertice;
  		vertice[0] = { center.x - extents.x, center.y - extents.y, center.z - extents.z };
  		vertice[1] = { center.x + extents.x, center.y - extents.y, center.z - extents.z };
  		vertice[2] = { center.x - extents.x, center.y + extents.y, center.z - extents.z };
  		vertice[3] = { center.x + extents.x, center.y + extents.y, center.z - extents.z };
  		vertice[4] = { center.x - extents.x, center.y - extents.y, center.z + extents.z };
  		vertice[5] = { center.x + extents.x, center.y - extents.y, center.z + extents.z };
  		vertice[6] = { center.x - extents.x, center.y + extents.y, center.z + extents.z };
  		vertice[7] = { center.x + extents.x, center.y + extents.y, center.z + extents.z };
  		return vertice;
  	}
  
  	//see https://gdbooks.gitbooks.io/3dcollisions/content/Chapter2/static_aabb_plan.html
  	bool isOnOrForwardPlan(const Plan& plan) const final
  	{
  		// Compute the projection interval radius of b onto L(t) = b.c + t * p.n
  		const float r = extents.x * std::abs(plan.normal.x) + extents.y * std::abs(plan.normal.y) +
  			extents.z * std::abs(plan.normal.z);
  
  		return -r <= plan.getSignedDistanceToPlan(center);
  	}
  
  	bool isOnFrustum(const Frustum& camFrustum, const Transform& transform) const final
  	{
  		//Get global scale thanks to our transform
  		const glm::vec3 globalCenter{ transform.getModelMatrix() * glm::vec4(center, 1.f) };
  
  		// Scaled orientation
  		const glm::vec3 right = transform.getRight() * extents.x;
  		const glm::vec3 up = transform.getUp() * extents.y;
  		const glm::vec3 forward = transform.getForward() * extents.z;
  
  		const float newIi = std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, forward));
  
  		const float newIj = std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, forward));
  
  		const float newIk = std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, forward));
  
  		const AABB globalAABB(globalCenter, newIi, newIj, newIk);
  
  		return (globalAABB.isOnOrForwardPlan(camFrustum.leftFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.rightFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.topFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.bottomFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.nearFace) &&
  			globalAABB.isOnOrForwardPlan(camFrustum.farFace));
  	};
  };
  
  Frustum createFrustumFromCamera(const Camera& cam, float aspect, float fovY, float zNear, float zFar)
  {
  	Frustum     frustum;
  	const float halfVSide = zFar * tanf(fovY * .5f);
  	const float halfHSide = halfVSide * aspect;
  	const glm::vec3 frontMultFar = zFar * cam.Front;
  
  	frustum.nearFace = { cam.Position + zNear * cam.Front, cam.Front };
  	frustum.farFace = { cam.Position + frontMultFar, -cam.Front };
  	frustum.rightFace = { cam.Position, glm::cross(cam.Up, frontMultFar + cam.Right * halfHSide) };
  	frustum.leftFace = { cam.Position, glm::cross(frontMultFar - cam.Right * halfHSide, cam.Up) };
  	frustum.topFace = { cam.Position, glm::cross(cam.Right, frontMultFar - cam.Up * halfVSide) };
  	frustum.bottomFace = { cam.Position, glm::cross(frontMultFar + cam.Up * halfVSide, cam.Right) };
  
  	return frustum;
  }
  
  AABB generateAABB(const Model& model)
  {
  	glm::vec3 minAABB = glm::vec3(std::numeric_limits<float>::max());
  	glm::vec3 maxAABB = glm::vec3(std::numeric_limits<float>::min());
  	for (auto&& mesh : model.meshes)
  	{
  		for (auto&& vertex : mesh.vertices)
  		{
  			minAABB.x = std::min(minAABB.x, vertex.Position.x);
  			minAABB.y = std::min(minAABB.y, vertex.Position.y);
  			minAABB.z = std::min(minAABB.z, vertex.Position.z);
  
  			maxAABB.x = std::max(maxAABB.x, vertex.Position.x);
  			maxAABB.y = std::max(maxAABB.y, vertex.Position.y);
  			maxAABB.z = std::max(maxAABB.z, vertex.Position.z);
  		}
  	}
  	return AABB(minAABB, maxAABB);
  }
  
  Sphere generateSphereBV(const Model& model)
  {
  	glm::vec3 minAABB = glm::vec3(std::numeric_limits<float>::max());
  	glm::vec3 maxAABB = glm::vec3(std::numeric_limits<float>::min());
  	for (auto&& mesh : model.meshes)
  	{
  		for (auto&& vertex : mesh.vertices)
  		{
  			minAABB.x = std::min(minAABB.x, vertex.Position.x);
  			minAABB.y = std::min(minAABB.y, vertex.Position.y);
  			minAABB.z = std::min(minAABB.z, vertex.Position.z);
  
  			maxAABB.x = std::max(maxAABB.x, vertex.Position.x);
  			maxAABB.y = std::max(maxAABB.y, vertex.Position.y);
  			maxAABB.z = std::max(maxAABB.z, vertex.Position.z);
  		}
  	}
  
  	return Sphere((maxAABB + minAABB) * 0.5f, glm::length(minAABB - maxAABB));
  }
  
  class Entity
  {
  public:
  	//Scene graph
  	std::list<std::unique_ptr<Entity>> children;
  	Entity* parent = nullptr;
  
  	//Space information
  	Transform transform;
  
  	Model* pModel = nullptr;
  	std::unique_ptr<AABB> boundingVolume;
  
  
  	// constructor, expects a filepath to a 3D model.
  	Entity(Model& model) : pModel{ &model }
  	{
  		boundingVolume = std::make_unique<AABB>(generateAABB(model));
  		//boundingVolume = std::make_unique<Sphere>(generateSphereBV(model));
  	}
  
  	AABB getGlobalAABB()
  	{
  		//Get global scale thanks to our transform
  		const glm::vec3 globalCenter{ transform.getModelMatrix() * glm::vec4(boundingVolume->center, 1.f) };
  
  		// Scaled orientation
  		const glm::vec3 right = transform.getRight() * boundingVolume->extents.x;
  		const glm::vec3 up = transform.getUp() * boundingVolume->extents.y;
  		const glm::vec3 forward = transform.getForward() * boundingVolume->extents.z;
  
  		const float newIi = std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 1.f, 0.f, 0.f }, forward));
  
  		const float newIj = std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 1.f, 0.f }, forward));
  
  		const float newIk = std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, right)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, up)) +
  			std::abs(glm::dot(glm::vec3{ 0.f, 0.f, 1.f }, forward));
  
  		return AABB(globalCenter, newIi, newIj, newIk);
  	}
  
  	//Add child. Argument input is argument of any constructor that you create. By default you can use the default constructor and don't put argument input.
  	template<typename... TArgs>
  	void addChild(TArgs&... args)
  	{
  		children.emplace_back(std::make_unique<Entity>(args...));
  		children.back()->parent = this;
  	}
  
  	//Update transform if it was changed
  	void updateSelfAndChild()
  	{
  		if (!transform.isDirty())
  			return;
  
  		forceUpdateSelfAndChild();
  	}
  
  	//Force update of transform even if local space don't change
  	void forceUpdateSelfAndChild()
  	{
  		if (parent)
  			transform.computeModelMatrix(parent->transform.getModelMatrix());
  		else
  			transform.computeModelMatrix();
  
  		for (auto&& child : children)
  		{
  			child->forceUpdateSelfAndChild();
  		}
  	}
  
  
  	void drawSelfAndChild(const Frustum& frustum, Shader& ourShader, unsigned int& display, unsigned int& total)
  	{
  		if (boundingVolume->isOnFrustum(frustum, transform))
  		{
  			ourShader.setMat4("model", transform.getModelMatrix());
  			pModel->Draw(ourShader);
  			display++;
  		}
  		total++;
  
  		for (auto&& child : children)
  		{
  			child->drawSelfAndChild(frustum, ourShader, display, total);
  		}
  	}
  };
  #endif
  