#include <assert.h>
#include <limits.h>

/*
 * Generate mask indicating leftmost 1 in x.  Assume w=32.
 * For example, 0xFF00 -> 0x8000, and 0x6600 --> 0x4000.
 * If x = 0, then return 0.
 */
int leftmost_one(unsigned x) {
    assert(sizeof(unsigned) == 4);
    x |= x >> 1;
    x |= x >> 2;
    x |= x >> 4;
    x |= x >> 8;
    x |= x >> 16;
    return x ^ (x >> 1);
}

int main() {
    assert(leftmost_one(0xff00) == 0x8000);
    assert(leftmost_one(0x6600) == 0x4000);
    assert(leftmost_one(INT_MIN) == INT_MIN);
    return 0;
}