如何使用位掩码从缓冲区访问六位

Question

Let's says I have a buffer and I need to get 6 bits. Four bits from one element in the buffer and two bits from the next element. I think I know how to access the correct bits from each element but I'm not sure how to combine the bits. Here's some example code.

FILE * pFile;
long lSize;
unsigned char * buffer;
//int16_t * buffer;
size_t result;

pFile = fopen ( "TestFile.jpg" , "rb" );
if (pFile==NULL) {fputs ("File error",stderr); exit (1);}

// obtain file size:
fseek (pFile , 0 , SEEK_END);
lSize = ftell (pFile);
//lSize = (ftell (pFile))/2;
rewind (pFile);

// allocate memory to contain the whole file:
buffer = (unsigned char*) malloc (sizeof(unsigned char)*lSize);
//buffer = (int16_t*) malloc (sizeof(int16_t)*lSize);
if (buffer == NULL) {fputs ("Memory error",stderr); exit (2);}

// copy the file into the buffer:
result = fread (buffer,1,lSize,pFile);
//result = fread (buffer,2,lSize,pFile);
if (result != lSize) {fputs ("Reading error",stderr); exit (3);}

/* the whole file is now loaded in the memory buffer. */

// I think this should get the last four bits of this element (buffer[i] >> 4) & 0xF)

// I think this should get the first two bits from the next element (buffer[i+1] & 0x3)

if (32 == ( ( (buffer[i] >> 4) & 0xF) & (buffer[i+1] & 0x3) ) ){
/*Do something.*/
}

Answer 1

I'm not sure which order you want to combine the bits in, but perhaps you want something like this:

unsigned char lastFourBits = (buffer[i] >> 4) & 0xF;         // 0000xxxx
unsigned char firstTwoBits = (buffer[i+1] & 0x3);            // 000000yy
unsigned char combined = (lastFourBits << 2) | firstTwoBits; // 00xxxxyy

or alternatively:

unsigned char combined = (firstTwoBits << 4) | lastFourBits; // 00yyxxxx 

Answer 2

The bit shift you are performing here (buffer[i] >> 4) is actually losing the 4 bits that you want. (buffer[i] & 0xF) will capture the low order four bits of the first byte. For the second byte, you want the two high order bits (buffer[i+1] & 0xC0) captures those bytes.

To concatenate:

value = ((buffer[i] & 0x0F) << 2) + ((buffer[i+1] & 0xC0) >> 6);

An example: If the data stream is 0xABCD, or 1010 1011 1100 1101, this equation captures 101111.