位操作

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在压缩和解压数据时，常常需要在小于一个字节的数量级上进行数据操作。因此，首先必须熟悉一些对数据位进行的操作。

/* bit.h */#ifndef BIT_H#define BIT_H/* 功  能: 获取缓冲区bits中处于位置pos的位的状态.缓冲区最左边的位置为0. 返回值: 相应位置的状态:1或0.*/ int bit_get(const unsigned char *bits, int pos);/* 功  能: 设置缓冲区bits中处于位置pos的位的状态(根据state值来设置).缓冲区最左边的值为0.状态值必须为0或1. 返回值: 无.*/void bit_set(unsigned char *bits, int pos, int state);/* 功  能: 按位计算两个缓冲区bits1与bits2的异或值, 其中每个缓冲区包含size个位,然后将结果返回bitsx中. 返回值: 无.*/void bit_xor(const unsigned char *bits1, const unsigned char *bits2, unsigned char *bitsx, int size);/* 功  能: 轮转缓冲区bits(含size位),将位值向左移count位.此操作完成后,处于最左端的count位移动到缓冲区最右端,而且其它位也相应地轮转. 返回值: 无.*/void bit_rot_left(unsigned char *bits, int size, int count);#endif

/* bit.c */#include <string.h>#include "bit.h"/* 获取pos位 */intbit_get(const unsigned char *bits, int pos){    unsigned char mask;    int i;    /* Set a mask for the bit to get. */    mask = 0x80;    for (i = 0; i < (pos % 8); i++)    {        mask = mask >> 1;    }    /* Get the bis. */    return (((mask & bits[(int)(pos / 8)]) == mask) ? 1 : 0);}/* 设置pos位为state */void bit_set(unsigned char *bits, int pos, int state){    unsigned char mask;    int i;    /* Set a mask for the bit to set. */    mask = 0x80;    for (i = 0; i < (pos % 8); i++)    {        mask = mask >> 1;    }    /* Set the bit. */    if (state)    {        bits[pos / 8] = bits[pos / 8] | mask;    }    else    {        bits[pos / 8] = bits[pos / 8] & mask;    }}/* 位异或 */voidbit_xor(const unsigned char *bits1, const unsigned char *bits2, unsigned char *bitsx, int size){    int i;    /* Compute the bitwise XOR (exclusive OR) of the two buffers. */    for (i = 0; i < size; i++)    {        if (bit_get(bits1, i) != bit_get(bits2, i))        {            bit_set(bitsx, i, 1);        }         else        {            bit_set(bitsx, i, 0);        }    }}/* bit_rot_left */void bit_rot_left(unsigned char *bits, int size, int count){    int fbit, lbit, i, j;    /* Rotate the buffer to the left the specified number of bits. */    if (size > 0)    {        for (j = 0; j < count; j++)        {            for (i = 0; i <= ((size - 1) / 8); i++)            {                /* Get the bit about to be shifted off the current byte. */                lbit = bit_get(&bits[i], 0);                if (i == 0)                {                    /* Save the bit shifted off the first byte for later. */                    fbit = lbit;                }                else                {                    /* Set the rightmost bit of the previous byte to the leftmost bit about to be shifted off the current byte. */                    bit_set(&bits[i - 1], 7, lbit);                }                /* Shift the current byte to the left. */                bits[i] = bits[i] << 1;            }            /* Set the rightmost bit of the buffer to the bit shifted off the first byte. */            bit_set(bits, size - 1, fbit);        }    }}

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