深入理解计算机系统第二版课后习题2.65

Write code to implement the following functions:

 /*Return 1 when x contains an odd number of 1s; 0 otherwise.

   Assume w = 32. */

int odd_ones(unsigned x);

Your function should follow the bit-level integer coding rules(page 120),except that you may assume that data type int has w = 32 bits.

       Your code should contain a total of at most 12 arithmetic, bit-wise, and logical operations.

解答：

用到的主要原理：1.       1^1 = 1       0^0 = 0       1^0 = 1        0^1=1

                                2.        奇数 - 偶数 = 奇数，偶数 - 偶数 = 偶数

如图所示，对于unsigned x，高16位与低16位异或，因为1^1 = 0，得到的16bit中含1的个数恰好是x中1的个数减去偶数个。以此类推，最后异或后得到一位，如