feat(maths): add DigitSum (#39)

Author: skyfrk

Maths/DigitSum.ts

@@ -0,0 +1,23 @@
+/**
+ * @function DigitSum
+ * @description Calculate the sum of all digits of a natural number (number base 10).
+ * @param {number} num - A natural number.
+ * @return {number} - Sum of all digits of given natural number.
+ * @see https://en.wikipedia.org/wiki/Digit_sum
+ * @example DigitSum(12) = 3
+ * @example DigitSum(9045) = 18
+ */
+
+export const DigitSum = (num: number): number => {
+  if (num < 0 || !Number.isInteger(num)) {
+    throw new Error("only natural numbers are supported");
+  }
+
+  let sum = 0;
+  while (num != 0) {
+    sum += num % 10;
+    num = Math.floor(num / 10);
+  }
+
+  return sum;
+};

Maths/test/DigitSum.test.ts

@@ -0,0 +1,19 @@
+import { DigitSum } from "../DigitSum";
+
+describe("DigitSum", () => {
+  test.each([-42, -0.1, -1, 0.2, 3.3, NaN, -Infinity, Infinity])(
+    "should throw an error for non natural number %d",
+    (num) => {
+      expect(() => DigitSum(num)).toThrowError(
+        "only natural numbers are supported",
+      );
+    },
+  );
+
+  test.each([[0,0], [1, 1], [12, 3], [123, 6], [9045, 18], [1234567890, 45]])(
+    "of %i should be %i",
+    (num, expected) => {
+      expect(DigitSum(num)).toBe(expected);
+    },
+  );
+});