Bit Hack Miscs


Power of 2
http://www.mytechinterviews.com/power-of-2
isPower = (x !=0 && !(x & (x-1)))

Given two integers, m and n, please calculate the number of bits in binary that need to be modified to change m to n.
int bitsToModify(int number1, int number2) {
    int temp = number1 ^ number2;

    // the number of 1 bits in temp
    int bits = 0;
    while(temp != 0) {
        ++bits;
        temp = (temp - 1) & temp;
    }

    return bits;
}
http://liuwenyu2126.lofter.com/post/1d3abccc_715bcfb
(7)整数的平均值 
对于两个整数x,y,如果用 (x+y)/2 求平均值,会产生溢出,因为 x+y 可能会大于INT_MAX,但是我们知道它们的平均值是肯定不会溢出的,我们用如下算法: 
int average(int x, int y)   //返回X,Y 的平均值 
{    
     return (x&y)+((x^y)>>1); 

ans = a + (b - a)/2
(8)判断一个整数是不是2的幂,对于一个数 x >= 0,判断他是不是2的幂 
boolean power2(int x) 

    return ((x&(x-1))==0)&&(x!=0); 

(9)不用temp交换两个整数 
void swap(int x , int y) 

    x ^= y; 
    y ^= x; 
    x ^= y; 

(10)计算绝对值 
int abs( int x ) 

int y ; 
y = x >> 31 ; 
return (x^y)-y ;        //or: (x+y)^y 

(11)取模运算转化成位运算 (在不产生溢出的情况下) 
         a % (2^n) 等价于 a & (2^n - 1) 
(12)乘法运算转化成位运算 (在不产生溢出的情况下) 
         a * (2^n) 等价于 a<< n 
(13)除法运算转化成位运算 (在不产生溢出的情况下) 
         a / (2^n) 等价于 a>> n 
        例: 12/8 == 12>>3 
(14) a % 2 等价于 a & 1        
(15) if (x == a) x= b; 
            else x= a; 
        等价于 x= a ^ b ^ x; 

(16) x 的 相反数 表示为 (~x+1)

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