C / C ++ code for treating an array of characters as a bitstream

Question

I have a large chunk 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 think there must be a good existing class or function that someone has already written.

I need something like:

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

so that I can 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 larger than 8, depending on the system. This is basically what Marco wrote, although I'm not quite sure why he collected 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; } 

Answer 2

Boost.DynamicBitset - try it.

Answer 3

I think something in the next line might work.

 int get_bit(char *data, unsigned bitoffset) // returns the n-th bit { int c = (int)(data[bitoffset >> 3]); // X>>3 is X/8 int bitmask = 1 << (bitoffset & 7); // X&7 is X%8 return ((c & bitmask)!=0) ? 1 : 0; } int get_bits(char* data, unsigned bitOffset, unsigned numBits) { int bits = 0; for (int currentbit = bitOffset; currentbit < bitOffset + numBits; currentbit++) { bits = bits << 1; bits = bits | get_bit(data, currentbit); } return bits; } 

I have not debugged or tested it, but you can use it as a starting point.

Also consider the order of the bits. You might want to change

  int bitmask = 1 << (bitoffset & 7); // X&7 is X%8 

to

  int bitmask = 1 << (7 - (bitoffset & 7)); // X&7 is X%8 

depending on how the bitmap was created.