add alternate diffuse method

in_one_weekend/src/main.zig

@@ -41,7 +41,7 @@ fn rayColor(ray: Ray, world: World, rng: *Random, depth: i32) Color {
     }
 
     if (world.hit(ray, 0.001, 99999)) |hit| {
-        const target = hit.p.add(hit.normal).add(vec3.random_in_unit_sphere(rng));
+        const target = hit.p.add(vec3.random_in_hemisphere(rng, hit.normal));
         const newRay = Ray{ .origin = hit.p, .direction = target.sub(hit.p)};
         return rayColor(newRay, world, rng, depth - 1).div(2);
     }

in_one_weekend/src/vec3.zig

@@ -87,6 +87,20 @@ pub fn random_in_unit_sphere(rng: *Random) Vec3 {
     }
 }
 
+pub fn random_unit_vector(rng: *Random) Vec3 {
+    return unitVector(random_in_unit_sphere(rng));
+}
+
+pub fn random_in_hemisphere(rng: *Random, normal: Vec3) Vec3 {
+    const in_unit_sphere = random_in_unit_sphere(rng);
+    if (in_unit_sphere.dot(normal) > 0) {
+        // In the same hemisphere as the normal
+        return in_unit_sphere;
+    } else {
+        return in_unit_sphere.mul(-1);
+    }
+}
+
 pub const Point3 = Vec3;
 
 const assert = @import("std").debug.assert;