Add countSetBits (trekhleb#152)

Author: tapaswenipathak

src/algorithms/math/bits/README.md

@@ -116,6 +116,17 @@ by `4` bits (`x << 4`).
 
 > See `multiplyUnsigned` function for further details.
 
+#### Count Set Bits
+
+This method counts the number of set bits in a number using bitwise operators.
+
+``
+Number: 5 = (0101)_2
+Count Set Bits = 2
+``
+
+> See `countSetBits` function for further details.
+
 ## References
 
 - [Bit Manipulation on YouTube](https://www.youtube.com/watch?v=NLKQEOgBAnw&t=0s&index=28&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)

src/algorithms/math/bits/__test__/countSetBits.test.js

@@ -0,0 +1,8 @@
+import countSetBits from '../countSetBits';
+
+describe('countSetBits', () => {
+  it('Should return number of set bits', () => {
+      expect(countSetBits(5)).toBe(2);
+      expect(countSetBits(1)).toBe(1);
+    });
+});

src/algorithms/math/bits/countSetBits.js

@@ -0,0 +1,13 @@
+/**
+ * @param {number} number
+ * @return {number}
+ */
+export default function countSetBits(number) {
+  let count = 0;
+  let num = number; // eslint error https://eslint.org/docs/rules/no-param-reassign
+  while (num) {
+    count += num & 1;
+    num >>= 1;
+  }
+  return count;
+}