#include "random.h"
#include "planet.h"

static void putpixel(struct SDL_Surface *screen, const int x, const int y,
		     const unsigned char r, const unsigned char g,
		     const unsigned char b)
{
	int offset = y * screen->pitch + x * 4;
	unsigned char *buf = screen->pixels;

	buf[offset++] = b;
	buf[offset++] = g;
	buf[offset]   = r;
}

void init_planet(const SDL_Surface *screen, struct planet *p)
{
	p->speed.x = 0;
	p->speed.y = 0;
	p->pos.x = get_random() % screen->w;
	p->pos.y = get_random() % screen->h;
	p->mass = get_random() % 1000 + 100;
	p->r = get_random() % 256;
	p->g = get_random() % 256;
	p->b = get_random() % 256;
}

void draw_planet(SDL_Surface *screen, struct planet *p)
{
	if (p->pos.x < screen->w &&
	    p->pos.y < screen->h &&
	    p->pos.x >= 0 &&
	    p->pos.y >= 0)
		putpixel(screen, p->pos.x, p->pos.y, p->r, p->g, p->b);
}

void gravitize_planets(struct planet *a, struct planet *b, const double time)
{
	struct vector distance, sum;
	double dist, f, acc;

	vector_sub(&a->pos, &b->pos, &distance);

	dist = vector_abs(&distance);
	vector_div(&distance, dist, &distance);

	f = a->mass * b->mass / (dist * dist + 5) * time;

	acc = f / b->mass;
	vector_scale(&distance, acc, &sum);
	vector_add(&b->speed, &sum, &b->speed);

	acc = f / a->mass;
	vector_scale(&distance, acc, &sum);
	vector_sub(&a->speed, &sum, &a->speed);
}

void move_planet(struct planet *p, const double time)
{
	struct vector tmp;
	vector_scale(&p->speed, time, &tmp);
	vector_add(&p->pos, &tmp, &p->pos);
}

void print_planet(const struct planet *p)
{
	printf("pos: (%f.%f), speed: (%f.%f), mass: %f\n",
	       p->pos.x, p->pos.y, p->speed.x, p->speed.y, p->mass);
}