feat(maths): add Factorial (#38)

Maths/Factorial.ts

@@ -0,0 +1,16 @@
+/**
+ * @function Factorial
+ * @description Calculate the factorial of a natural number.
+ * @param {number} num - A natural number.
+ * @return {number} - The factorial.
+ * @see https://en.wikipedia.org/wiki/Factorial
+ * @example Factorial(0) = 1
+ * @example Factorial(3) = 6
+ */
+export const Factorial = (num: number): number => {
+  if (num < 0 || !Number.isInteger(num)) {
+    throw new Error("only natural numbers are supported");
+  }
+
+  return num === 0 ? 1 : num * Factorial(num - 1);
+};

Maths/test/Factorial.test.ts

@@ -0,0 +1,23 @@
+import { Factorial } from "../Factorial";
+
+describe("Factorial", () => {
+  test.each([-0.1, -1, -2, -42, 0.01, 0.42, 0.5, 1.337])(
+    "should throw an error for non natural number %d",
+    (num) => {
+      expect(() => Factorial(num)).toThrowError(
+        "only natural numbers are supported",
+      );
+    },
+  );
+
+  test.each([[1, 1], [3, 6], [5, 120], [10, 3628800]])(
+    "of %i should be %i",
+    (num, expected) => {
+      expect(Factorial(num)).toBe(expected);
+    },
+  );
+
+  test("of 1 should be 0 by definition", () => {
+    expect(Factorial(0)).toBe(1);
+  });
+});