SHADE_Y3/SHADE_Engine/src/FRC/SHFramerateController.cpp

/*********************************************************************
 * \file   SHFramerateController.cpp
 * \author Ryan Wang Nian Jing
 * \brief  Definition for functions of the framerate controller
 *				 Handles changing of scenes and manages loop (timestep, etc.)
 *
 * \copyright Copyright (c) 2022 DigiPen Institute of Technology. Reproduction
	 or disclosure of this file or its contents without the prior written
	 consent of DigiPen Institute of Technology is prohibited.
 *********************************************************************/

#include <chrono>
#include <cassert>
#include <SHpch.h>
#include "SHFramerateController.h"
#include "../Tools/SHLogger.h"

namespace SHADE
{
	//Init statics
	double SHFramerateController::fixedTimestep = 0.01;
	SHScene* SHFramerateController::previousScene = nullptr;
	SHScene* SHFramerateController::currentScene = nullptr;
	SHScene* SHFramerateController::nextScene = nullptr;
	bool SHFramerateController::toRestart = false;
	bool SHFramerateController::toQuit = false;

	//Scene manager loop
	void SHFramerateController::Run(SHScene* firstScene)
	{
		if (firstScene == nullptr)
		{
			SHLOG_ERROR("Do not pass a nullptr as the firstScene");
			return;
		}

		//Set quit and restart flags to false
		toQuit = false;
		toRestart = false;

		//Set the first scene to run
		previousScene = firstScene;
		currentScene = firstScene;
		nextScene = firstScene;

		while (!toQuit)
		{
			if (toRestart)
			{
				//Restart current scene
				currentScene = previousScene;
				nextScene = previousScene;
				toRestart = false;
			}
			else
			{
				//Move to a new scene
				currentScene->Load();
			}

			//Call init function of current scene
			currentScene->Init();

			//Have an initial value
			//This frame time will fluctuate
			//SHOULD be larger than the fixed timestep
			//TODO this might need to be changed
			double variableLastFrameTime = fixedTimestep;

			//Time accumulator for meshing between fixed and variable timesteps
			double accumulator = 0.0;

			//Start state loop
			while (currentScene == nextScene && !toQuit && !toRestart)
			{
				//Use of new STL timing functions
				//https://en.cppreference.com/w/cpp/chrono
				std::chrono::duration<double> deltaTime;

				auto startTime = std::chrono::high_resolution_clock::now();

				//Whittle down the accumulator by continuously simulating
				for (; accumulator > fixedTimestep; accumulator -= fixedTimestep)
				{
					MSG msg;
					if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
					{
						TranslateMessage(&msg);
						DispatchMessage(&msg);
					}

					//TODO change to double
					currentScene->Update((float)fixedTimestep);
				}

				//Interpolation
				//Manage the alpha value well
				//https://randomascii.wordpress.com/2012/02/13/dont-store-that-in-a-float/
				//Key points:
				//1) Any time you add or subtract floats of widely varying
				//   magnitudes, you need to watch for loss of precision
				//2) Sometimes using double instead of float is the correct
				//   solution, but often a more stable algorithm is more important
				//3) calcT() should probably use double (to give sufficient
				//   precision after many hours of gameplay)

				//TODO awaiting approval to use this
				//double alpha = accumulator / fixedTimestep;

				//assert alpha does not go out of range

				currentScene->Render();

				auto endTime = std::chrono::high_resolution_clock::now();
				deltaTime = endTime - startTime;
				variableLastFrameTime = deltaTime.count();

				//Increase accumulator
				accumulator += variableLastFrameTime;
			}

			//Free once out of scene loop
			currentScene->Free();

			//Check if not to restart state
			//If so, unload
			if (!toRestart) currentScene->Unload();

			//Shift forward scenes
			previousScene = currentScene;
			currentScene = nextScene;
		}
	}
}