diff --git a/engine/lib/mulberry32.test.ts b/engine/lib/mulberry32.test.ts
new file mode 100644
index 0000000..3cb57a8
--- /dev/null
+++ b/engine/lib/mulberry32.test.ts
@@ -0,0 +1,14 @@
+import { expect, test } from "vitest";
+import { mulberry32 } from "./mulberry32.ts";
+
+test("mulberry32", () => {
+	const nextA = mulberry32(1);
+	const nextA2 = mulberry32(1);
+	const nextB = mulberry32(2);
+	const a1 = nextA();
+
+	expect(a1).toBe(0.6270739405881613);
+	expect(nextA2()).toBe(a1);
+	expect(nextA()).not.toBe(a1);
+	expect(nextB()).not.toBe(a1);
+});
diff --git a/engine/lib/mulberry32.ts b/engine/lib/mulberry32.ts
index 0cc4db4..b1ecb57 100644
--- a/engine/lib/mulberry32.ts
+++ b/engine/lib/mulberry32.ts
@@ -1,7 +1,7 @@
 /**
  * @see https://github.com/cprosche/mulberry32
  */
-export function mulberry32(a: number) {
+export function mulberry32(a: number): () => number {
 	return () => {
 		a |= 0;
 		a = (a + 0x6d2b79f5) | 0;