feat: add happy number algorithm

Maths/HappyNumber.js

@@ -0,0 +1,33 @@
+/**
+ * @function isHappyNumber
+ * @description A happy number is defined by the following process:
+ *   Starting with any positive integer, replace the number by the sum
+ *   of the squares of its digits, and repeat until the number equals 1
+ *   (where it will stay), or it loops endlessly in a cycle that does
+ *   not include 1.
+ * @param {number} n - A positive integer
+ * @returns {boolean} - True if n is a happy number, false otherwise
+ * @see https://en.wikipedia.org/wiki/Happy_number
+ * @example isHappyNumber(19) => true
+ * @example isHappyNumber(7) => true
+ * @example isHappyNumber(4) => false
+ */
+const isHappyNumber = (n) => {
+  if (!Number.isInteger(n) || n < 1) {
+    throw new TypeError('Input must be a positive integer')
+  }
+  const sumOfSquares = (num) =>
+    String(num)
+      .split('')
+      .reduce((sum, d) => sum + Number(d) ** 2, 0)
+  const seen = new Set()
+  let current = n
+  while (current !== 1) {
+    if (seen.has(current)) return false
+    seen.add(current)
+    current = sumOfSquares(current)
+  }
+  return true
+}
+
+export { isHappyNumber }

Maths/test/HappyNumber.test.js

@@ -0,0 +1,51 @@
+import { isHappyNumber } from '../HappyNumber.js'
+
+describe('isHappyNumber', () => {
+  test('1 is a happy number', () => {
+    expect(isHappyNumber(1)).toBe(true)
+  })
+
+  test('7 is a happy number', () => {
+    expect(isHappyNumber(7)).toBe(true)
+  })
+
+  test('19 is a happy number', () => {
+    expect(isHappyNumber(19)).toBe(true)
+  })
+
+  test('23 is a happy number', () => {
+    expect(isHappyNumber(23)).toBe(true)
+  })
+
+  test('100 is a happy number', () => {
+    expect(isHappyNumber(100)).toBe(true)
+  })
+
+  test('4 is NOT a happy number', () => {
+    expect(isHappyNumber(4)).toBe(false)
+  })
+
+  test('11 is NOT a happy number', () => {
+    expect(isHappyNumber(11)).toBe(false)
+  })
+
+  test('20 is NOT a happy number', () => {
+    expect(isHappyNumber(20)).toBe(false)
+  })
+
+  test('36 is NOT a happy number', () => {
+    expect(isHappyNumber(36)).toBe(false)
+  })
+
+  test('throws TypeError for zero', () => {
+    expect(() => isHappyNumber(0)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for negative input', () => {
+    expect(() => isHappyNumber(-19)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for float input', () => {
+    expect(() => isHappyNumber(19.5)).toThrow(TypeError)
+  })
+})