tcpl_ex-2-9.c

/*
 * Exercise 2-9. In a two's complement number system, x &= (x-1) deletes the
 * right most 1-bit in x. Explain why. Use this observation to write a faster 
 * version of bitcount. 
 */
#include <stdio.h>

int bitcount(unsigned int x);

int bitcount(unsigned int x) {
	int i = 0;
	while (x != 0) {
		i++;
		x = x & (x - 1);
	}
	return i;

}

int main(void) {
	unsigned int x = 0xff;
	int r = 8;
	if (r == bitcount(x))
		printf("assert success!\n");
	else
		printf("assert fail!\n");

	return 0;
}