algorithm: pronic number (#82)

* Added implementation for Pronic Number 😁

* Update DIRECTORY.md

Co-authored-by: autoprettier <[email protected]>

Author: SpiderMath

DIRECTORY.md

@@ -26,6 +26,7 @@
   * [Is Odd](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/is_odd.ts)
   * [Lowest Common Multiple](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/lowest_common_multiple.ts)
   * [Perfect Square](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/perfect_square.ts)
+  * [Pronic Number](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/pronic_number.ts)
   * [Radians To Degrees](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/radians_to_degrees.ts)
   * Series
     * [Hexagonal Numbers](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/series/hexagonal_numbers.ts)

maths/pronic_number.ts

@@ -0,0 +1,24 @@
+/**
+ * @function PronicNumber
+ * @description Checks whether a given number is a pronic number or not
+ * @summary Pronic numbers, or oblong numbers as they are often referred to as,
+ * are numbers which are the product of two consecutive integers. That is,
+ * they are numbers of the form n*(n+1)
+ * 
+ * For example, 20 is a pronic number since 20 = 4 * 5
+ * @param num The number to check for being pronic
+ * @returns {boolean} Whether the number is pronic or not
+ * @see [Wikipedia](https://en.wikipedia.org/wiki/Pronic_number)
+ * @example PronicNumber(20) = true
+ * @example PronicNumber(30) = true
+ * @example PronicNumber(49) = false
+ */
+const PronicNumber = (n: number) => {
+  if (isNaN(n)) throw new Error('The input needs to be a number')
+  if (!Number.isInteger(n) || n < 0) throw new Error('The input needs to be a non-negative integer')
+  if (n === 0) return true
+
+  return !Number.isInteger(Math.sqrt(n)) && Math.floor(Math.sqrt(n)) * Math.ceil(Math.sqrt(n)) === n
+}
+
+export { PronicNumber }

maths/test/pronic_number.test.ts

@@ -0,0 +1,5 @@
+import { PronicNumber } from '../pronic_number'
+
+test.each([[0, true], [10, false], [30, true], [69, false], [420, true]])('Pronic Number', (number, result) => {
+  expect(PronicNumber(number)).toBe(result)
+})