#include <u.h>
#include <libc.h>
#include <draw.h>
uchar p[256];
#define SKEW 0.211324865405 /* (3 - √3) / 6 */
#define UNSKEW 0.366025403784 /* (√3 - 1) / 2 */
double
dot(int h, double x, double y){
	double t;
	t = 0.5 - x * x - y * y;
	if(t > 0.0){
		t *= t *= t;
		switch(h & 3){
		case 0: return t * (x + y);
		case 1: return t * (y - x);
		case 2: return t * (x - y);
		case 3: return t * (-x - y);
		}
	}
	return 0.0;
}
double
simplex(double x, double y){
	double t;
	int xgty, i, j;
	t = (x + y) * UNSKEW;
	i = (int)(x + t);
	j = (int)(y + t);
	t = (i + j) * SKEW;
	x += t - i;
	y += t - j;
	xgty = x > y;
	return 70.0 * (dot(p[i + p[j & 255] & 255], x, y)
		+ dot(p[i + xgty + p[j + !xgty & 255] & 255],
			x + SKEW - xgty, y + SKEW - !xgty)
		+ dot(p[i + 1 + p[j + 1 & 255] & 255],
			x + SKEW + SKEW - 1.0, y + SKEW + SKEW - 1.0));
}
void
loadgrey(Image *i, double t){
	uchar bgr[3];
	bgr[1] = bgr[2] = 128 + 128 * t;
	bgr[0] = 220 + 35 * t;
	loadimage(i, i->r, bgr, 3);
}
void
shuffle(void){
	int i, j, k;
	srand(truerand());
	for(i = 0; i < 256; i++)
		p[i] = i;
	while(i){
		j = nrand(i--);
		k = p[j];
		p[j] = p[i];
		p[i] = k;
	}
}
void
main(int, char**){
	Image *px;
	Rectangle r;
	double x, y;
	if(initdraw(nil, nil, nil) < 0)
		sysfatal("initdraw: %r");
	px = allocimage(display, Rect(0, 0, 1, 1), RGB24, 1, DNofill);
	if(px == nil)
		sysfatal("allocimage: %r");
	shuffle();
	y = 0.0;
	r.min.y = screen->r.min.y;
	while(r.min.y < screen->r.max.y){
		r.max.y = r.min.y + 1;
		x = 0.0;
		r.min.x = screen->r.min.x;
		while(r.min.x < screen->r.max.x){
			r.max.x = r.min.x + 1;
			loadgrey(px, 
				simplex(x, y) * 0.5
				+ simplex(x * 2.0, y * 2.0) * 0.25
				+ simplex(x * 4.0, y * 4.0) * 0.125
				+ simplex(x * 8.0, y * 8.0) * 0.0625
			);
			draw(screen, r, px, nil, ZP);
			r.min.x = r.max.x;
			x += 0.003;
		}
		r.min.y = r.max.y;
		y += 0.003;
	}
	flushimage(display, 1);
	sleep(999999);
}