#### Count of Set Bits

```/**
* @file
* @brief Implementation to [count number of set bits of a number]
* (https://www.geeksforgeeks.org/count-set-bits-in-an-integer/) in an
* integer.
*
* @details
* We are given an integer number. We need to calculate the number of set bits
* in it.
*
* A binary number consists of two digits. They are 0 & 1. Digit 1 is known as
* set bit in computer terms.
* Worst Case Time Complexity: O(log n)
* Space complexity: O(1)
* @author [Swastika Gupta](https://github.com/Swastyy)
* @author [Prashant Thakur](https://github.com/prashant-th18)
*/
#include <cassert>   /// for assert
#include <iostream>  /// for IO operations
/**
* @namespace bit_manipulation
* @brief Bit manipulation algorithms
*/
namespace bit_manipulation {
/**
* @namespace count_of_set_bits
* @brief Functions for the [count sets
* bits](https://www.geeksforgeeks.org/count-set-bits-in-an-integer/)
* implementation
*/
namespace count_of_set_bits {
/**
* @brief The main function implements set bit count
* @param n is the number whose set bit will be counted
* @returns total number of set-bits in the binary representation of number `n`
*/
std::uint64_t countSetBits(
std ::int64_t n) {  // int64_t is preferred over int so that
// no Overflow can be there.

int count = 0;  // "count" variable is used to count number of set-bits('1')
// in binary representation of number 'n'
while (n != 0) {
++count;
n = (n & (n - 1));
}
return count;
// Why this algorithm is better than the standard one?
// Because this algorithm runs the same number of times as the number of
// set-bits in it. Means if my number is having "3" set bits, then this
// while loop will run only "3" times!!
}
}  // namespace count_of_set_bits
}  // namespace bit_manipulation

static void test() {
// n = 4 return 1
assert(bit_manipulation::count_of_set_bits::countSetBits(4) == 1);
// n = 6 return 2
assert(bit_manipulation::count_of_set_bits::countSetBits(6) == 2);
// n = 13 return 3
assert(bit_manipulation::count_of_set_bits::countSetBits(13) == 3);
// n = 9 return 2
assert(bit_manipulation::count_of_set_bits::countSetBits(9) == 2);
// n = 15 return 4
assert(bit_manipulation::count_of_set_bits::countSetBits(15) == 4);
// n = 25 return 3
assert(bit_manipulation::count_of_set_bits::countSetBits(25) == 3);
// n = 97 return 3
assert(bit_manipulation::count_of_set_bits::countSetBits(97) == 3);
// n = 31 return 5
assert(bit_manipulation::count_of_set_bits::countSetBits(31) == 5);
std::cout << "All test cases successfully passed!" << std::endl;
}
/**
* @brief Main function
* @returns 0 on exit
*/
int main() {
test();  // run self-test implementations
return 0;
}
```  