// SDL Experiment 07, Barra Ó Catháin.
// ===================================
#include <SDL2/SDL.h>
#include <SDL2/SDL_timer.h>
#include <math.h>
#include <stdbool.h>
#include <stdint.h>

typedef struct xyVector
{
	double xComponent;
	double yComponent;
} xyVector;

// Calculate the vector from point A to point B:
static inline void xyVectorBetweenPoints(long ax, long ay, long bx, long by, xyVector * vector)
{
	vector->xComponent = bx - ax;
	vector->yComponent = by - ay;
}

// Normalize a vector, returning the magnitude:
static inline double normalizeXYVector(xyVector * vector)
{
	double magnitude = sqrt(pow(vector->xComponent, 2) + pow(vector->yComponent, 2));
	vector->xComponent /= magnitude;
	vector->yComponent /= magnitude;
	return magnitude;
}

// Add vector B to vector A:
static inline void addXYVector(xyVector * vectorA, xyVector * vectorB)
{
	vectorA->xComponent += vectorB->xComponent;
	vectorA->yComponent += vectorB->yComponent; 
}

// Add vector B to vector A, scaled for units per frame:
static inline void addXYVectorDeltaScaled(xyVector * vectorA, xyVector * vectorB, double deltaTime)
{
	vectorA->xComponent += vectorB->xComponent * (0.001 * deltaTime) * 60;
	vectorA->yComponent += vectorB->yComponent * (0.001 * deltaTime) * 60; 
}

// Multiply a vector by a scalar constant:
static inline void multiplyXYVector(xyVector * vector, double scalar)
{	
	vector->xComponent *= scalar;
	vector->yComponent *= scalar;
}
void DrawCircle(SDL_Renderer * renderer, int32_t centreX, int32_t centreY, int32_t radius)
{
   const int32_t diameter = (radius * 2);

   int32_t x = (radius - 1);
   int32_t y = 0;
   int32_t tx = 1;
   int32_t ty = 1;
   int32_t error = (tx - diameter);

   while (x >= y)
   {
	 //  Each of the following renders an octant of the circle
      SDL_RenderDrawPoint(renderer, centreX + x, centreY - y);
      SDL_RenderDrawPoint(renderer, centreX + x, centreY + y);
      SDL_RenderDrawPoint(renderer, centreX - x, centreY - y);
      SDL_RenderDrawPoint(renderer, centreX - x, centreY + y);
      SDL_RenderDrawPoint(renderer, centreX + y, centreY - x);
      SDL_RenderDrawPoint(renderer, centreX + y, centreY + x);
      SDL_RenderDrawPoint(renderer, centreX - y, centreY - x);
      SDL_RenderDrawPoint(renderer, centreX - y, centreY + x);

      if (error <= 0)
      {
         ++y;
         error += ty;
         ty += 2;
      }

      if (error > 0)
      {
         --x;
         tx += 2;
         error += (tx - diameter);
      }
   }
}

int main(int argc, char ** argv)
{
	SDL_Event event;
	bool quit = false;
	int width = 0, height = 0;
	uint32_t rendererFlags = SDL_RENDERER_ACCELERATED;
	double deltaTime = 0, gravityMagnitude = 0, gravityAcceleration = 0;	
	uint64_t thisFrameTime = SDL_GetPerformanceCounter(), lastFrameTime = 0;
	long positionX = 0, positionY = 0;
	xyVector positionVector = {100, 100}, velocityVector = {20, 0}, gravityVector = {0, 0};
	
	// Initialize the SDL library, video, sound, and input:
	if (SDL_Init(SDL_INIT_EVERYTHING) != 0)
	{
		printf("SDL Initialization Error: %s\n", SDL_GetError());
	}
	
	// Create an SDL window and rendering context in that window:
	SDL_Window * window = SDL_CreateWindow("SDL_TEST", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 640, 0);
	SDL_Renderer * renderer = SDL_CreateRenderer(window, -1, rendererFlags);
	
	// Enable resizing the window:
	SDL_SetWindowResizable(window, SDL_TRUE);

	while (!quit)
	{
		lastFrameTime = thisFrameTime;
		thisFrameTime = SDL_GetPerformanceCounter();
		deltaTime = (double)(((thisFrameTime - lastFrameTime) * 1000) / (double)SDL_GetPerformanceFrequency());
		
		// Check if the user wants to quit:
		while (SDL_PollEvent(&event))
        {
            switch (event.type)
            {
                case SDL_QUIT:
				{
					quit = true;
					break;
				}
				default:
				{
					break;
				}
            }
        }

        // Store the window's current width and height:
		SDL_GetWindowSize(window, &width, &height);

		// Calculate the vector between the star and ship:
		xyVectorBetweenPoints(positionVector.xComponent, positionVector.yComponent, width/2, height/2, &gravityVector);

		// Make it into a unit vector:
		gravityMagnitude = normalizeXYVector(&gravityVector);

		// Calculate the gravity between the star and ship:
		gravityAcceleration = (gravityMagnitude > 15) ?
			2 * (2500 / pow(gravityMagnitude, 2)):
			0.02 * (2500 / pow(gravityMagnitude, 2));
		
		// Scale the vector:
		multiplyXYVector(&gravityVector, gravityAcceleration);
		
		// Wrap the position if the ship goes off-screen:
		if(positionVector.xComponent > width + 15)
		{
			positionVector.xComponent = 0;
			velocityVector.xComponent *= 0.6;
		}
		if(positionVector.yComponent > height + 15)
		{
			positionVector.yComponent = 0;
			velocityVector.yComponent *= 0.6;
		}
		if(positionVector.xComponent < -15)
		{
			positionVector.xComponent = width;
			velocityVector.xComponent *= 0.6;
		}
		if(positionVector.yComponent < -15)
		{
			positionVector.yComponent = height;
			velocityVector.yComponent *= 0.6;
		}

		// Calculate the new current velocity:
		addXYVectorDeltaScaled(&velocityVector, &gravityVector, deltaTime);

		// Calculate the new position:
		addXYVectorDeltaScaled(&positionVector, &velocityVector, deltaTime);
		
		positionX = (long)positionVector.xComponent;
		positionY = (long)positionVector.yComponent;

		// Set the colour to black:
		SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);

		// Clear the screen, filling it with black:
		SDL_RenderClear(renderer);

		// Set the colour to green:
		SDL_SetRenderDrawColor(renderer, 0, 255, 0, 255);

		// Draw a circle "ship" around the current position:
		DrawCircle(renderer, positionX, positionY, 15);
		
		// Set the colour to yellow:
		SDL_SetRenderDrawColor(renderer, 255, 255, 0, 255);

		// Draw a circle in the center as the star:
		DrawCircle(renderer, width/2, height/2, 30);

		// Draw a line representing the velocity:
		SDL_RenderDrawLine(renderer, positionX, positionY,
						   (long)(positionVector.xComponent + velocityVector.xComponent * 3),
						   (long)(positionVector.yComponent + velocityVector.yComponent * 3));
		
		// Present the rendered graphics:
		SDL_RenderPresent(renderer);
	}
	return 0;
}
// ===========================================================================================
// Local Variables:
// compile-command: "gcc `sdl2-config --libs --cflags` SDL2-Experiment-07.c  -lm"
// End: