C/C++ Code to treat a character array as a bitstream

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

Answer 1

I think something in the line of the following 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've not debugged nor tested it, but you can use it as a start point.

Also, take into account bit order. 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 bit array has been generated.