#include <u.h>
#include <libc.h>
#include <draw.h>
#include <event.h>

enum {N = 20};

#define dt 0.01
#define xmin -40
#define xmax 40
#define ymin -40
#define ymax 40
#define v0 0.2
#define mini(a,b) (((a)<(b))?(a):(b))

typedef struct Particle Particle;
struct Particle {
	double x, y;
	double vx, vy;
	double ax, ay;
	double prevx, prevy;
	Image* col;
};

int colors[] = {
              DBlack,
              DRed,
              DGreen,
              DBlue,
              DCyan,
              DMagenta,
              DDarkyellow,
              DDarkgreen,
              DPalegreen,
              DMedgreen,
              DDarkblue,
              DPalebluegreen,
              DPaleblue,
              DBluegreen,
              DGreygreen,
              DPalegreygreen,
              DYellowgreen,
              DMedblue,
              DGreyblue,
              DPalegreyblue,
              DPurpleblue
};

Particle A[N], B[N];
Particle *prev = A, *cur = B;

void
reset(void)
{
	int j, grid = sqrt(N)+0.5;
	Particle *p;
	draw(screen, screen->r, display->white, 0, ZP);
	for(j=0;j<N;j++) {
		p = prev+j;
		p->x = 2*(j%grid)+frand()/2;
		p->y = 2*(j/grid)+frand()/2;
		p->vx = 1.*v0*frand();
		p->vy = 1.*v0*frand();
		p->prevx = p->x - p->vx * dt;
		p->prevy = p->y - p->vy * dt;
		p->col = allocimage(display, Rect(0,0,1,1), RGB24, 1, colors[rand()%(sizeof(colors)/sizeof(int))]);
		if(!p->col) sysfatal("allocimage");
	}
}

void
main()
{
	srand(time(0));
	initdraw(0, 0, "Molecular Dynamics");
	einit(Emouse | Ekeyboard);
	
	int j;
	Particle *p, *q;
	double dx, dy, R, F;

	reset();

	while(1) {
		memset(cur, 0, sizeof(Particle) * N);
		for(p=prev;p<prev+N;p++) {
			for(q=prev;q<p;q++) {
				dx = p->x - q->x;
				dy = p->y - q->y;
				R = dx*dx + dy*dy;
				if(R >= 9) continue;
				R = 1/sqrt(R);
				double R2, R4, R6, R12;
				R2 = R * R;
				R4 = R2 * R2;
				R6 = R4 * R2;
				R12 = R6 * R6;
				F = 24*(2*R12 - R6);
				dx *= F;
				dy *= F;
				(p-prev+cur)->ax += dx;
				(p-prev+cur)->ay += dy;
				(q-prev+cur)->ax -= dx;
				(q-prev+cur)->ay -= dy;
			}
		}
		for(j=0;j<N;j++) {
			int x, y;
			p = prev+j;
			q = cur+j;
			q->x = 2*p->x - p->prevx + q->ax * dt*dt;
			q->y = 2*p->y - p->prevy + q->ay * dt*dt;
			q->vx = (q->x - p->prevx) / (2*dt);
			q->vy = (q->y - p->prevy) / (2*dt);
			q->prevx = p->x;
			q->prevy = p->y;
			q->col = p->col;
			x = (screen->r.max.x - screen->r.min.x) * (q->x - xmin) / (xmax - xmin) + screen->r.min.x;
			y = (screen->r.max.y - screen->r.min.y) * (q->y - ymin) / (ymax - ymin) + screen->r.min.y;
			draw(screen, Rect(x, y, x+1, y+1), p->col, 0, ZP);
		}

		Particle* tmp = prev;
		prev = cur;
		cur = tmp;
		flushimage(display, 0);
		
		if(ecankbd()) {
			switch(ekbd()) {
				case 'q': exits(0); break;
				case 'r': reset(); break;
				case 'f': draw(screen, screen->r, display->white, 0, ZP); break;
			}
		}
	}
}

void
eresized(int new)
{
	if(new) getwindow(display, Refnone);
}