#include <stdio.h>
#include <math.h>
#include <unistd.h>

const char *shades = ".:!*oe&#%@";

double light[3] = { -50, 0, 50 };
void normalize(double * v)
{
	double len = sqrt(v[0]*v[0] + v[1]*v[1] + v[2]*v[2]);
	v[0] /= len; v[1] /= len; v[2] /= len;
}

double dot(double *x, double *y)
{
	double d = x[0]*y[0] + x[1]*y[1] + x[2]*y[2];
	return d < 0 ? -d : 0;
}

typedef struct { double cx, cy, cz, r; } sphere_t;

/* positive shpere and negative sphere */
sphere_t pos = { 20, 20, 0, 20 }, neg = { 1, 1, -6, 20 };

/* check if a ray (x,y, -inf)->(x, y, inf) hits a sphere; if so, return
   the intersecting z values.  z1 is closer to the eye */
int hit_sphere(sphere_t *sph, double x, double y, double *z1, double *z2)
{
	double zsq;
	x -= sph->cx;
	y -= sph->cy;
	zsq = sph->r * sph->r - (x * x + y * y);
	if (zsq < 0) return 0;
	zsq = sqrt(zsq);
	*z1 = sph->cz - zsq;
	*z2 = sph->cz + zsq;
	return 1;
}

void draw_sphere(double k, double ambient)
{
	int i, j, intensity, hit_result;
	double b;
	double vec[3], x, y, zb1, zb2, zs1, zs2;
	for (i = floor(pos.cy - pos.r); i <= ceil(pos.cy + pos.r); i++) {
		y = i + .5;
		for (j = floor(pos.cx - 2 * pos.r); j <= ceil(pos.cx + 2 * pos.r); j++) {
			x = (j - pos.cx) / 2. + .5 + pos.cx;

			/* ray lands in blank space, draw bg */
			if (!hit_sphere(&pos, x, y, &zb1, &zb2))
				hit_result = 0;

			/* ray hits pos sphere but not neg, draw pos sphere surface */
			else if (!hit_sphere(&neg, x, y, &zs1, &zs2))
				hit_result = 1;

			/* ray hits both, but pos front surface is closer */
			else if (zs1 > zb1) hit_result = 1;

			/* pos sphere surface is inside neg sphere, show bg */
			else if (zs2 > zb2) hit_result = 0;

			/* back surface on neg sphere is inside pos sphere,
			   the only place where neg sphere surface will be shown */
			else if (zs2 > zb1) hit_result = 2;
			else		    hit_result = 1;

			switch(hit_result) {
			case 0:
				putchar('+');
				continue;
			case 1:
				vec[0] = x - pos.cx;
				vec[1] = y - pos.cy;
				vec[2] = zb1 - pos.cz;
				break;
			default:
				vec[0] = neg.cx - x;
				vec[1] = neg.cy - y;
				vec[2] = neg.cz - zs2;
			}

			normalize(vec);
			b = pow(dot(light, vec), k) + ambient;
			intensity = (1 - b) * (sizeof(shades) - 1);
			if (intensity < 0) intensity = 0;
			if (intensity >= sizeof(shades) - 1)
				intensity = sizeof(shades) - 2;
			putchar(shades[intensity]);
		}
		putchar('\n');
	}
}

int main()
{
	double ang = 0;

	while (1) {
		printf("\033[H");
		light[1] = cos(ang * 2);
		light[2] = cos(ang);
		light[0] = sin(ang);
		normalize(light);
		ang += .05;

		draw_sphere(2, .3);
		usleep(100000);
	}
	return 0;
}