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

#define WINDOW_WIDTH  640
#define WINDOW_HEIGHT 480

#define METABALL_COUNT 3

SDL_Surface *screen;

struct Metaball
{
	int x, y;
	float size;
	int dx, dy;
};

Metaball g_Metaballs[METABALL_COUNT];

inline int clamp(int value, int min, int max)
{
	return (value < min) ? min : ((value > max) ? max : value);
}

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 metaballs
	int x, y, i, c;
	int xDiff, yDiff, iPow;
	float fMetaballValue;
	for(x = 0; x < WINDOW_WIDTH; x++)
		for(y = 0; y < WINDOW_HEIGHT; y++)
		{
			fMetaballValue = 0.0f;

			for(i = 0; i < METABALL_COUNT; i++)
			{
				xDiff = g_Metaballs[i].x - x;
				yDiff = g_Metaballs[i].y - y;
				iPow = xDiff * xDiff + yDiff * yDiff;
				fMetaballValue += g_Metaballs[i].size / ((float)iPow * (float)iPow);
			}

			c = clamp(fMetaballValue * 255.0f * 20.0f, 0, 256);
			((unsigned int*)screen->pixels)[y * WINDOW_WIDTH + x] = (0 << 16) + (c << 8) + 0;
		}


	// 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 the metaballs
	for(int i = 0; i < METABALL_COUNT; i++)
	{
		g_Metaballs[i].x += g_Metaballs[i].dx * iDelta / 20.0f;
		g_Metaballs[i].y += g_Metaballs[i].dy * iDelta / 20.0f;

		if(g_Metaballs[i].x < 0 || g_Metaballs[i].x > WINDOW_WIDTH -1)
			g_Metaballs[i].dx = -g_Metaballs[i].dx;
		if(g_Metaballs[i].y < 0  || g_Metaballs[i].y > WINDOW_HEIGHT -1)
			g_Metaballs[i].dy = -g_Metaballs[i].dy;
	}
}


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

	// 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);
	}


	// Init metaballs
	for(int i = 0; i < METABALL_COUNT; i++)
	{
		g_Metaballs[i].x = rand() % WINDOW_WIDTH;
		g_Metaballs[i].y = rand() % WINDOW_HEIGHT;
		g_Metaballs[i].size = 2500000;
		g_Metaballs[i].dx = rand() % 10 - 5;
		g_Metaballs[i].dy = rand() % 10 - 5;
	}


	// 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;
				break;
				
			case SDL_QUIT:
				return 0;
			}
		}
	}
	return 0;
}