feat: Added signum (#81)

Author: SpiderMath

DIRECTORY.md

@@ -32,6 +32,7 @@
     * Test
       * [Hexagonal Numbers.Test](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/series/test/hexagonal_numbers.test.ts)
   * [Sieve Of Eratosthenes](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/sieve_of_eratosthenes.ts)
+  * [Signum](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/signum.ts)
 
 ## Other
   * [Parse Nested Brackets](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/other/parse_nested_brackets.ts)

maths/signum.ts

@@ -0,0 +1,22 @@
+/**
+ * @function Signum
+ * @description Returns the sign of a number
+ * @summary The signum function is an odd mathematical function, which returns the 
+ * sign of the provided real number.
+ * It can return 3 values: 1 for values greater than zero, 0 for zero itself,
+ * and -1 for values less than zero
+ * @param {Number} input
+ * @returns {-1 | 0 | 1 | NaN} sign of input (and NaN if the input is not a number)
+ * @see [Wikipedia](https://en.wikipedia.org/wiki/Sign_function)
+ * @example Signum(10) = 1
+ * @example Signum(0) = 0
+ * @example Signum(-69) = -1
+ * @example Signum("hello world") = NaN
+ */
+export const Signum = (num: number) => {
+  if (num === 0) return 0
+  if (num > 0) return 1
+  if (num < 0) return -1
+
+  return NaN
+}

maths/test/signum.test.ts

@@ -0,0 +1,5 @@
+import { Signum } from "../signum";
+
+test.each([[10, 1], [0, 0], [-69, -1], [NaN, NaN]])("The sign of %i is %i", (num, expected) => {
+  expect(Signum(num)).toBe(expected)
+})