PhysicsModel.cpp

#include "PhysicsModel.h"

PhysicsModel::PhysicsModel(Transform* newTransform, PhysicsBodyConfig bodyConfig) {
	_transform = newTransform;
	_bodyStats = bodyConfig;

	_velocity = Vector3();
	_acceleration = Vector3();

	_collider = nullptr;
}

PhysicsModel::~PhysicsModel() {
	_transform = nullptr; //don't destroy the transform, it's not ours.

	if (_collider) delete(_collider);
}

void PhysicsModel::Update(float deltaTime) {
	if (_bodyStats.kinematic) return;
	if (_bodyStats.mass == 0) return;

	//collect any other variables needed for now.
	Vector3 position = _transform->GetPosition();

	//gather net forces.
	if (_bodyStats.simulateGravity) _netForce += _bodyStats.gravity * _bodyStats.mass;
	_netForce += Vector3(QuickDrag1D(_velocity.x), QuickDrag1D(_velocity.y), QuickDrag1D(_velocity.z));

	//change acceleration based on it.
	//don't increment acceleration like we do velocity, as that's not how physics works.
	//theoretically this will set it to 0 if the net force is zero.
	_acceleration = _netForce * _bodyStats.massInverse;
	_netForce = Vector3(0, 0, 0); //make the result into a constant acceleration by resetting it here
	
	//apply the velocity and move the object
	_velocity += _acceleration * deltaTime;
	position += _velocity * deltaTime;

	_transform->SetPosition(position);

	//now update the collider so it can update any cache it has
	if (_collider) _collider->Update();
}

inline float PhysicsModel::QuickDrag1D(float velocity) {
	//utilize existing cache to quickly calculate the drag.
	//we aren't expecting the model to change between now and then
	//so unless the object gets setters called we can assume it is the same as last frame
	//across coefficients, volume and mass.

	//return a negative if incoming is positive, otherwise return a positive.
	if (velocity < 0.0f) return ((velocity * velocity) * 0.5f) * _bodyStats.dragMultiplier;
	return ((velocity * -velocity) * 0.5f) * _bodyStats.dragMultiplier;
}