Add bit counter function.

Author: trekhleb

src/algorithms/math/bits/README.md

@@ -119,10 +119,12 @@ by `4` bits (`x << 4`).
 #### Count Set Bits
 
 This method counts the number of set bits in a number using bitwise operators.
+The main idea is that we shift the number right by one bit at a time and check
+the result of `&` operation that is `1` if bit is set and `0` otherwise.
 
 ``
-Number: 5 = (0101)_2
-Count Set Bits = 2
+Number: 5 = 0b0101
+Count of set bits = 2
 ``
 
 > See `countSetBits` function for further details.

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

@@ -1,8 +1,15 @@
 import countSetBits from '../countSetBits';
 
 describe('countSetBits', () => {
-  it('Should return number of set bits', () => {
-      expect(countSetBits(5)).toBe(2);
-      expect(countSetBits(1)).toBe(1);
-    });
+  it('should return number of set bits', () => {
+    expect(countSetBits(0)).toBe(0);
+    expect(countSetBits(1)).toBe(1);
+    expect(countSetBits(2)).toBe(1);
+    expect(countSetBits(3)).toBe(2);
+    expect(countSetBits(4)).toBe(1);
+    expect(countSetBits(5)).toBe(2);
+    expect(countSetBits(21)).toBe(3);
+    expect(countSetBits(255)).toBe(8);
+    expect(countSetBits(1023)).toBe(10);
+  });
 });

src/algorithms/math/bits/countSetBits.js

@@ -1,13 +1,18 @@
 /**
- * @param {number} number
+ * @param {number} originalNumber
  * @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;
+export default function countSetBits(originalNumber) {
+  let setBitsCount = 0;
+  let number = originalNumber;
+
+  while (number) {
+    // Add last bit of the number to the sum of set bits.
+    setBitsCount += number & 1;
+
+    // Shift number right by one bit to investigate other bits.
+    number >>= 1;
   }
-  return count;
+
+  return setBitsCount;
 }