add red sphere

in_one_weekend/src/main.zig

@@ -16,7 +16,19 @@ const SDL_WINDOWPOS_UNDEFINED = @bitCast(c_int, sdl.c.SDL_WINDOWPOS_UNDEFINED_MA
 
 const fps = 60;
 
+fn hitSphere(center: Point3, radius: f32, ray: Ray) bool {
+    const ssr = ray.origin.sub(center); // Sphere-space ray, (A - C) in book
+    const a = Vec3.dot(ray.direction, ray.direction);
+    const b = 2.0 * Vec3.dot(ssr, ray.direction);
+    const c = Vec3.dot(ssr, ssr) - (radius * radius);
+    const discriminant = b * b - 4 * a * c;
+    return discriminant > 0;
+}
+
 fn rayColor(ray: Ray) Color {
+    if (hitSphere(Point3{ .x = 0, .y = 0, .z = -1 }, 0.5, ray)) {
+        return Color{ .x = 1, .y = 0, .z = 0 };
+    }
     const unitDirection = vec3.unitVector(ray.direction);
     const t = 0.5 * (unitDirection.y + 1.0);
     const white = Color{ .x = 1.0, .y = 1.0, .z = 1.0 };