Add rng_xy()

src/rng.c

@@ -42,6 +42,13 @@ vec3_t rng_vec3(rng_t *rng)
 	return vec3_unit(v);
 }
 
+vec3_t rng_xy(rng_t *rng)
+{
+	const double theta = 2.0 * M_PI * rng_canon(rng);
+	const double mag = rng_canon(rng);
+	return (vec3_t) { .x = mag * cos(theta), .y = mag * sin(theta) };
+}
+
 vec3_t rng_gaussian_xy(rng_t *rng, double stddev)
 {
 	const double r1 = rng_canon(rng);