#include <stdlib.h>
#include <math.h>
#include "SDL.h"
#include "time.h"

#define WINDOW_WIDTH  640
#define WINDOW_HEIGHT 480


#define STAR_COUNT 400

SDL_Surface *screen;

struct Star 
{
	float x, y, speed;
};

Star g_Stars[STAR_COUNT];

int g_midX, g_midY;
bool g_Hyperspeed;
 
inline bool PlotPixel(int x, int y, int color)
{
	if(x < 0 || y < 0 || x > WINDOW_WIDTH -1 || y > WINDOW_HEIGHT -1)
		return false;

	((unsigned int*)screen->pixels)[y * WINDOW_WIDTH + x] = color;

	return true;
}

inline void swap(int *x, int *y)
{
	int tmp = *x;
	*x = *y;
	*y = tmp;
}

// Bresenham's line algorithm as explained by wikipedia
void DrawLine(int color, int x0, int y0, int x1, int y1)
{
	bool steep = abs(y1 - y0) > abs(x1 - x0);

	if(steep)
	{
		swap(&x0, &y0);
		swap(&x1, &y1);
	}

	if(x0 > x1)
	{
		swap(&x0, &x1);
		swap(&y0, &y1);
	}

	signed int y = y0;
	int deltax = x1 - x0;
	int deltay = abs(y1 - y0);
	int error = -deltax / 2;   
	int ystep = (y0 < y1) ? 1 : -1;

	for(int x = x0; x <= x1; x++)
	{
		if(steep)
		{
			if(!PlotPixel(y, x, color))
				return;
		}
		else
		{
			if(!PlotPixel(x, y, color))
				return;
		}

		error = error + deltay;

		if(error > 0)
		{
			y = y + ystep;
			error = error - deltax;
		}
	}
}

void render()
{   
	// Lock surface if needed
	if (SDL_MUSTLOCK(screen)) 
		if (SDL_LockSurface(screen) < 0) 
			return;

	// Clear screen
	SDL_FillRect(screen, NULL, SDL_MapRGB(screen->format, 0, 0, 0));


	// Draw stars
	for(int i = 0; i < STAR_COUNT; i++)
	{
		int c = 0xff * g_Stars[i].speed;

		int x2 = g_Stars[i].x - (g_midX - g_Stars[i].x) * g_Stars[i].speed / (20.0f - (g_Hyperspeed * 15.0f));
		int y2 = g_Stars[i].y - (g_midY - g_Stars[i].y) * g_Stars[i].speed / (20.0f - (g_Hyperspeed * 15.0f));

		DrawLine((c << 16) + (c << 8) + c, g_Stars[i].x, g_Stars[i].y, x2, y2);
	}


	// Unlock if needed
	if (SDL_MUSTLOCK(screen)) 
		SDL_UnlockSurface(screen);

	// Tell SDL to update the whole screen
	SDL_UpdateRect(screen, 0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);  
}

void tick()
{
	// Ask SDL for the time in milliseconds
	static int lastTick = SDL_GetTicks() - 100;
	int iDelta = SDL_GetTicks() - lastTick;
	lastTick = SDL_GetTicks();

	if(iDelta > 33)
		iDelta = 33;


	// Move stars
	for(int i = 0; i < STAR_COUNT; i++)
	{
		g_Stars[i].x -= (g_midX - g_Stars[i].x) * iDelta * g_Stars[i].speed / (200.0f - (g_Hyperspeed * 150.0f));
		g_Stars[i].y -= (g_midY - g_Stars[i].y) * iDelta * g_Stars[i].speed / (200.0f - (g_Hyperspeed * 150.0f));

		// If the star is outside screen or in the exact middle, move them somewhere
		if((g_Stars[i].x < 0 || g_Stars[i].y < 0 || g_Stars[i].x > WINDOW_WIDTH -1 || g_Stars[i].y > WINDOW_HEIGHT -1) || (g_Stars[i].x == g_midX && g_Stars[i].y == g_midY))
		{
 			g_Stars[i].x = rand() % WINDOW_WIDTH;
			g_Stars[i].y = rand() % WINDOW_HEIGHT;
		}
	}
}


// Entry point
int main(int argc, char *argv[])
{
	srand((unsigned)time(NULL));

	g_midX = WINDOW_WIDTH / 2;
	g_midY = WINDOW_HEIGHT / 2;

	g_Hyperspeed = false;

	// Initialize SDL's subsystems - in this case, only video.
	if(SDL_Init(SDL_INIT_VIDEO) < 0)
	{
		fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
		exit(1);
	}

	// Register SDL_Quit to be called at exit; makes sure things are
	// cleaned up when we quit.
	atexit(SDL_Quit);

	// Attempt to create a 640x480 window with 32bit pixels.
	screen = SDL_SetVideoMode(WINDOW_WIDTH, WINDOW_HEIGHT, 32, SDL_SWSURFACE);

	// If we fail, return error.
	if(!screen)
	{
		fprintf(stderr, "Unable to set video: %s\n", SDL_GetError());
		exit(1);
	}


	// Initialize stars, place them randomly
	for(int i = 0; i < STAR_COUNT; i++)
	{
 		g_Stars[i].x = rand() % WINDOW_WIDTH;
		g_Stars[i].y = rand() % WINDOW_HEIGHT;
		g_Stars[i].speed = (rand() % 90) / 100.0f;
	}


	// Main loop: loop forever.
	while(true)
	{
		// Tick stuff;
		tick();

		// Render stuff
		render();

		// Poll for events, and handle the ones we care about.
		SDL_Event event;
		while (SDL_PollEvent(&event)) 
		{
			switch (event.type) 
			{
			case SDL_KEYDOWN:
				break;
			case SDL_KEYUP:
				// If escape is pressed, return (and thus, quit)
				if(event.key.keysym.sym == SDLK_ESCAPE)
					return 0;
				else if(event.key.keysym.sym == SDLK_SPACE)
					g_Hyperspeed = !g_Hyperspeed;
				break;
				
			case SDL_MOUSEMOTION:
				g_midX = event.motion.x;
				g_midY = event.motion.y;
				break;
				
			case SDL_QUIT:
				return 0;
			}
		}
	}
	return 0;
}