feat: Added Double Factorial Iterative Implementation

DIRECTORY.md

@@ -9,9 +9,11 @@
 ## Bit Manipulation
   * [Is Power Of 2](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/bit_manipulation/is_power_of_2.ts)
   * [Is Power Of 4](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/bit_manipulation/is_power_of_4.ts)
+  * [Log Two](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/bit_manipulation/log_two.ts)
   * Test
     * [Is Power Of 2.Test](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/bit_manipulation/test/is_power_of_2.test.ts)
     * [Is Power Of 4.Test](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/bit_manipulation/test/is_power_of_4.test.ts)
+    * [Log Two.Test](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/bit_manipulation/test/log_two.test.ts)
 
 ## Ciphers
   * [Xor Cipher](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/ciphers/xor_cipher.ts)
@@ -71,6 +73,7 @@
 
 ## Dynamic Programming
   * [Knapsack](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/dynamic_programming/knapsack.ts)
+  * [Lcs](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/dynamic_programming/lcs.ts)
 
 ## Graph
   * [Bellman Ford](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/graph/bellman_ford.ts)
@@ -97,6 +100,7 @@
   * [Calculate Median](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/calculate_median.ts)
   * [Degrees To Radians](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/degrees_to_radians.ts)
   * [Digit Sum](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/digit_sum.ts)
+  * [Double Factorial Iterative](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/double_factorial_iterative.ts)
   * [Factorial](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/factorial.ts)
   * [Factors](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/factors.ts)
   * [Fibonacci](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/maths/fibonacci.ts)
@@ -154,3 +158,4 @@
   * [Quick Sort](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/sorts/quick_sort.ts)
   * [Selection Sort](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/sorts/selection_sort.ts)
   * [Shell Sort](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/sorts/shell_sort.ts)
+  * [Swap Sort](https://github.com/TheAlgorithms/TypeScript/blob/HEAD/sorts/swap_sort.ts)

maths/double_factorial_iterative.ts

@@ -0,0 +1,30 @@
+/**
+ * @function DoubleFactorialIterative
+ * @description Calculate the double factorial of a number (iterative implementation)
+ * @summary In mathematics, double factorial of a number n is denoted by n!!. 
+ * It is not to be confused with (n!)!, which is the factorial function iterated twice.  
+ * The double factorial is the product of all positive integers upto n that have the same parity (odd or even)
+ * as n.  
+ * Therefore,  
+ * 9!! = 9 . 7 . 5 . 3 . 1  
+ * 10!! = 10 . 8 . 6 . 4 . 2
+ * 
+ * Please note that for factorials of even numbers, the series ends at 2.
+ * @see [Wikipedia](https://en.wikipedia.org/wiki/Double_factorial)
+ * @see [Mathworld](https://mathworld.wolfram.com/DoubleFactorial.html)
+ * @see [GeeksForGeeks](https://www.geeksforgeeks.org/double-factorial/)
+ * @example DoubleFactorialIterative(4) = 8
+ * @example DoubleFactorialIterative(5) = 15
+ */
+const DoubleFactorialIterative = (n: number) => {
+  if(n < 0) throw new RangeError("The number needs to be non-negative")
+  if(n === 0) return 1;
+  let doubleFactorial = 1;
+
+  for(let i = n; i > 0; i -= 2)
+    doubleFactorial *= i;
+
+  return doubleFactorial;
+}
+
+export { DoubleFactorialIterative };

maths/test/double_factorial_iterative.test.ts

@@ -0,0 +1,7 @@
+import { DoubleFactorialIterative } from "../double_factorial_iterative";
+
+describe("Double Factorial", () => {
+	test.each([[4, 8], [5, 15], [10, 3840]])("%i!! = %i", (n, expected) => {
+		expect(DoubleFactorialIterative(n)).toBe(expected)
+	})
+})