Question

I have a big lump of binary data in a char[] array which I need to interpret as an array of packed 6-bit values.

I could sit down and write some code to do this but I’m thinking there has to be a good extant class or function somebody has written already.

What I need is something like:

int get_bits(char* data, unsigned bitOffset, unsigned numBits); 

so I could get the 7th 6-bit character in the data by calling:

const unsigned BITSIZE = 6; char ch = static_cast<char>(get_bits(data, 7 * BITSIZE, BITSIZE)); 

Answer 1

This may not work for sizes greater than 8, depending on endian system. It’s basically what Marco posted, though I’m not entirely sure why he’d gather one bit at a time.

int get_bits(char* data, unsigned int bitOffset, unsigned int numBits) {     numBits = pow(2,numBits) - 1; //this will only work up to 32 bits, of course     data += bitOffset/8;     bitOffset %= 8;     return (*((int*)data) >> bitOffset) & numBits;  //little endian     //return (flip(data[0]) >> bitOffset) & numBits; //big endian }  //flips from big to little or vice versa int flip(int x) {     char temp, *t = (char*)&x;     temp = t[0];     t[0] = t[3];     t[3] = temp;     temp = t[1];     t[1] = t[2];     t[2] = temp;     return x; }