分享两个C库源码中的移位函数

杨源鑫

发布于 2020-07-01 15:04:29

1K0

发布于 2020-07-01 15:04:29

以下源代码是之前阅读C库代码获取的，分享一下：

_lrotr()将一个无符号长整形数左循环移位的函数
原形：unsigned long _lrotr(unsigned long value,int count)
功能：将value向左循环移动count位。
返回值：将value向左循环移动count位后的值。
头文件：stdlib.h

unsigned _rotl (unsigned val,int shift)
{
        register unsigned hibit;        /* non-zero means hi bit set */
        register unsigned num = val;    /* number to rotate */
        shift &= 0x1f;                  /* modulo 32 -- this will also make
                                           negative shifts work */
        while (shift--) {
                hibit = num & 0x80000000;  /* get high bit */
                num <<= 1;              /* shift left one bit */
                if (hibit)
                        num |= 1;       /* set lo bit if hi bit was set */
        }
        return num;
}

//这个函数就是将_rotl函数做再次封装
unsigned long _lrotl (unsigned long val,int shift)
{
        return( (unsigned long) _rotl((unsigned) val, shift) );
}

_lrotr()将一个无符号长整形数右循环移位的函数
原形：unsigned long _lrotr(unsigned long value,int count)
功能：将value向右循环移动count位。
返回值：将value向右循环移动count位后的值。
头文件：stdlib.h

unsigned _rotr (unsigned val,int shift)
{
        register unsigned lobit;        /* non-zero means lo bit set */
        register unsigned num = val;    /* number to rotate */
 
        shift &= 0x1f;                  /* modulo 32 -- this will also make
                                           negative shifts work */
        while (shift--) {
                lobit = num & 1;        /* get high bit */
                num >>= 1;              /* shift right one bit */
                if (lobit)
                        num |= 0x80000000;  /* set hi bit if lo bit was set */
        }
        return num;
}
//对右移函数做再次封装
unsigned long _lrotr (unsigned long val,int shift)
{
        return( (unsigned long) _rotr((unsigned) val, shift) );
}

简单对以上两个核心函数进行测试，测试环境基于DevC++:

#include <stdio.h>
unsigned _rotl (unsigned val,int shift)
{
        register unsigned hibit;        /* non-zero means hi bit set */
        register unsigned num = val;    /* number to rotate */
        shift &= 0x1f;                  /* modulo 32 -- this will also make
                                           negative shifts work */
        while (shift--) {
                hibit = num & 0x80000000;  /* get high bit */
                num <<= 1;              /* shift left one bit */
                if (hibit)
                        num |= 1;       /* set lo bit if hi bit was set */
        }
        return num;
}
 
//这个函数就是将_rotl函数做再次封装
unsigned long _lrotl (unsigned long val,int shift)
{
        return( (unsigned long) _rotl((unsigned) val, shift) );
}

unsigned _rotr (unsigned val,int shift)
{
        register unsigned lobit;        /* non-zero means lo bit set */
        register unsigned num = val;    /* number to rotate */
 
        shift &= 0x1f;                  /* modulo 32 -- this will also make
                                           negative shifts work */
        while (shift--) {
                lobit = num & 1;        /* get high bit */
                num >>= 1;              /* shift right one bit */
                if (lobit)
                        num |= 0x80000000;  /* set hi bit if lo bit was set */
        }
        return num;
}
//对右移函数做再次封装
unsigned long _lrotr (unsigned long val,int shift)
{
        return( (unsigned long) _rotr((unsigned) val, shift) );
}
 
int main(void)
{
    unsigned long val = 2;
    unsigned long ret = _lrotl(val , 4) ;
    printf("%d\n",ret);
    unsigned long r = _lrotr(ret,4);
    printf("%d\n",r);
    return 0 ;
}

运行结果：