计算一个无符号整数的二进制表示中1的个数

计算一个无符号整数的二进制表示中1的个数，这个问题又被称作 Hamming weight
Java中有支持的API:
Integer.bitCount
Long.bitCount

    public static int bitCount(int i) {
        // HD, Figure 5-2
	i = i - ((i >>> 1) & 0x55555555);
	i = (i & 0x33333333) + ((i >>> 2) & 0x33333333);
	i = (i + (i >>> 4)) & 0x0f0f0f0f;
	i = i + (i >>> 8);
	i = i + (i >>> 16);
	return i & 0x3f;
    }

http://stackoverflow.com/questions/1458314/number-of-1s-in-32-bit-number
http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
http://aggregate.ee.engr.uky.edu/MAGIC/#Population%20Count%20%28Ones%20Count%29