CSAPP 实验一lab1 fitsBits

/*
* fitsBits - return 1 if x can be represented as an 
*  n-bit, two's complement integer.
*   1 <= n <= 32
*   Examples: fitsBits(5,3) = 0, fitsBits(-4,3) = 1
*   Legal ops: ! ~ & ^ | + << >>
*   Max ops: 15
*   Rating: 2
*/
int fitsBits(int x, int n) {
    int r, c;
    c = 33 + ~n;  //c = 33 + ~n = 33 + (-n - 1) = 32 - n
    r = !(((x << c)>>c)^x);
    return r;
}

参考自 http://stackoverflow.com/questions/28641540/bitwise-operations-and-shifts-problems