[英]How to get and set bits for bits in char string?
通常,位操作以較小的數據寬度完成,例如int,unsigned int或wchar_t。 假設我們要以更長的格式使用位字符串,如何為char字符串中的位移位,獲取和設置位?
一種方法可能是使用常規方法進行分而治之,但是我們如何確保位保留下來呢?
特定
#define numberOfState 2000 // number of bits
#define numberOfBitsIn1Byte 8
char* record;
int numberOfCharRequiredToRepresentBits =
ceil(((float)numberOfState/(float)numberOfBitsIn1Byte));
record = (char*) malloc(sizeof(char)*numberOfCharRequiredToRepresentBits);
// record = "NAXHDKAN552ajdasdadNDfadsEBEAfA8gda5214S";
// optional : initialization by doing the set bit according to
// input from files. After which, do free(record);
我們如何進行位操作,例如
i. shift the *record
ii. get bits from a specific bit position in *record
iii. set bits from a specific bit position in *record
您的比特流本質上是一個char數組。 因此,要執行這些操作,您需要處理這些char元素。
一世。 移位操作取決於您要移位的位數。 如果數字是8的倍數,這很簡單,您只需將元素復制到左側,就跟數字是8的倍數一樣多。如果數字小於8,則對元素的每個元素執行操作。數組,但是您需要或上一個元素的溢出位。 例如,在左移時,元素i必須合並元素i + 1的溢出位,而在右移時,元素i-1的溢出位必須合並。 這兩個動作的組合可以實現您要移位的其他位數。 例如,左移18即是移16再乘以2。在任何情況下,都需要注意從位串的哪一側開始,以免丟失數據。
II。 為了獲得比特流的第n位,請訪問索引為n / 8(整數除法)的元素,並從中獲取n%8位。
III。 與ii幾乎相同。
請嘗試以下代碼:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int isLittleEndian = 1;
void checkEndian(void)
{
union
{
short inum;
char c[sizeof(short)];
} un;
un.inum=0x0102;
if(un.c[0]==1 && un.c[1]==2)
{
printf("big_endian.\n");
isLittleEndian = 0;
}
else if(un.c[0]==2 && un.c[1]==1)
{
printf("little_endian.\n");
isLittleEndian = 1;
}
}
void shift_L(char *src, char * dst, int len, int n)
{
int shiftBytes = n/8;
int shiftBits = n%8;
memset(dst, 0, len);
memcpy(dst, src + shiftBytes, len - shiftBytes);
if (shiftBits)
{
int i = 0;
unsigned short tmp = 0;
for ( i = 0; i < len; i++)
{
if (isLittleEndian)
{
tmp = *(dst+i) << 8 | *(dst+i+1);
tmp <<= shiftBits;
*(dst+i) = *((char *)&tmp + 1);
}
else
{
tmp = *(short *)(dst+i);
tmp <<= shiftBits;
*(dst+i) = *((char *)&tmp);
}
}
}
}
void shift_R(char *src, char * dst, int len, int n)
{
int shiftBytes = n/8;
int shiftBits = n%8;
memset(dst, 0, len);
memcpy(dst + shiftBytes, src, len - shiftBytes);
if (shiftBits)
{
int i = 0;
unsigned short tmp = 0;
for ( i = len -1; i >= 0; i--)
{
if (isLittleEndian)
{
tmp = *(dst+i-1) << 8 | *(dst+i);
tmp >>= shiftBits;
*(dst+i) = *((char *)&tmp);
}
else
{
tmp = *(short *)(dst+i-1);
tmp >>= shiftBits;
*(dst+i) = *((char *)&tmp+1);
}
}
}
}
int getBit(char *src, int n)
{
unsigned char tmp = *(src + n/8);
unsigned char mask = (0x1 << (8 - n%8 - 1));
int bit = 0;
bit = (tmp & mask) > 0;
printf("%d", bit);
}
void setBit(char *src, int n, int bit)
{
unsigned char * pTmp = src + n/8;
unsigned char mask = (0x1 << (8 - n%8 - 1));
if (bit)
{
*pTmp |= mask;
}
else
{
*pTmp &= ~mask;
}
}
void dumpBin(unsigned char *src, int len)
{
int i = 0;
int j = 0;
unsigned char mask = 0;
for ( i = 0; i < len; i++)
{
for ( j = 0; j < 8; j++)
{
mask = 0x1 << 8 - j - 1;
printf("%d",(*(src + i) & mask) > 0);
}
}
}
void main()
{
char *record = "NAXHDKAN552ajdasdadNDfadsEBEAfA8gda5214S";
//char *record = "NAXHDKA";
int recordLen = strlen(record);
char * buffer = NULL;
int i = 0;
checkEndian();
recordLen = recordLen + recordLen%2;
buffer = malloc(recordLen);
memcpy(buffer, record, recordLen);
printf("\n input bit stream:\n");
dumpBin(buffer, recordLen);
printf("\n bit stream from getBit:\n");
for ( i = 0; i < recordLen*8; i++)
{
getBit(buffer, i);
}
setBit(buffer, 8, 1);
setBit(buffer, 9, 0);
setBit(buffer, 10, 1);
setBit(buffer, 11, 1);
printf("\n bit stream after setBit:\n");
dumpBin(buffer, recordLen);
shift_L(record, buffer, recordLen, 1);
printf("\n bit stream after shift_L:\n");
dumpBin(buffer, recordLen);
shift_R(record, buffer, recordLen, 9);
printf("\n bit stream after shift_R:\n");
dumpBin(buffer, recordLen);
printf("\n");
free(buffer);
}
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