Maximize a given unsigned number by swapping bits at its extreme positions in C++

Problem statement

Given a number maximize it by swapping bits at its extreme positions i.e. at first and last position, second and second last position and so on.

If the input number is 8 then its binary representation is−

00000000 00000000 00000000 00001000

After swapping bits at extreme positions number becomes −

00010000 00000000 00000000 00000000 and its decimal equivalent is: 268435456

Algorithm

1. Create a copy of the original number
2. If less significant bit is 1 and more significant bit is 0 then swap the bits in the bit from only, continue the process until less significant bit’s position is less than more significant bit’s position
3. Return new number

Example

#include <bits/stdc++.h>
#define ull unsigned long long
using namespace std;
ull getMaxNumber(ull num){
   ull origNum = num;
   int bitCnt = sizeof(ull) * 8 - 1;
   int cnt = 0;
   for(cnt = 0; cnt < bitCnt; ++cnt, --bitCnt) {
      int m = (origNum >> cnt) & 1;
      int n = (origNum >> bitCnt) & 1;
      if (m > n) {
         int x = (1 << cnt | 1 << bitCnt);
         num = num ^ x;
      }
   }
   return num;
}
int main(){
   ull num = 8;
   cout << "Maximum number = " << getMaxNumber(num) << endl;
   return 0;
}

Output

When you compile and execute the above program. It generates the following output −

Maximum number = 268435456