Finding\pointing to the leftmost set bit in a binary integer

Question

Given x, a binary integer, y = x & (~x + 1) points to the rightmost set bit. For example:

x = 0000101101011000
y = 0000000000001000

Now my question is: Can we find a bitwise formula to point to the leftmost set bit? For example:

x = 0000101101011000
y = 0000100000000000

Any idea will be appreciated.

Answer 1

I have found a solution here on java

https://codeforces.com/blog/entry/10330

seems like java supports it out of the box I don't know if its implemented in C++ standard library but I could translate it into C++

code in C++ is the following:

#include <iostream>
#include <bitset>


int highestOneBit(int i) {
    i |= (i >>  1);
    i |= (i >>  2);
    i |= (i >>  4);
    i |= (i >>  8);
    i |= (i >> 16);
    return i - ((unsigned int)i >> 1);
}

int main()
{
    int x = 951;    
    std::cout << std::bitset<sizeof(int) * 8>(x) << std::endl;
    std::cout << std::bitset<sizeof(int) * 8>(highestOneBit(x)) << std::endl;
    return 0;
}