2.66
/*
* leftmost-one.c
*/
#include <stdio.h>
#include <assert.h>
/*
* Generate mask indicating leftmost 1 in x. Assume w=32
* For example, 0xFF00 -> 0x8000, and 0x6000 -> 0x4000.
* If x = 0, then return 0
*/
int leftmost_one(unsigned x) {
/*
* first, generate a mask that all bits after leftmost one are one
* e.g. 0xFF00 -> 0xFFFF, and 0x6000 -> 0x7FFF
* If x = 0, get 0
*/
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
/*
* then, do (mask >> 1) + (mask && 1), in which mask && 1 deals with case x = 0, reserve leftmost bit one
* that's we want
*/
return (x >> 1) + (x && 1);
}
int main(int argc, char* argv[]) {
assert(leftmost_one(0xFF00) == 0x8000);
assert(leftmost_one(0x6000) == 0x4000);
assert(leftmost_one(0x0) == 0x0);
assert(leftmost_one(0x80000000) == 0x80000000);
return 0;
}