How do I extract a bit in a more optimal way?

Question

I had a interview today where they asked me to write two "C" functions, one to to extract a single bit and other to extract a range of bits from a character. I took a while and came up with these methods.

int extractBit(char byte, int pos) {
    assert( (pos >= 0) && (pos < 8) );
    return ( ( byte & (1<<pos) ) >> pos);
}
char extractBitRange(char byte, int startingPos, int offset) {
   assert( ( (startingPos + offset) >= 0) && ( (startingPos + offset) < 8) );
   return ( byte >> startingPos ) & ~(0xff << (offset + 1));
}

But the interviewer kept asking me if I could speed up the code further (in terms of cpu cycles) and if there is any scope of optimization that I could do to achieve it. I was clearly out of sorts and I am curious to know how would you do this?

Answer 1

In extractBit, if you shift first, you can mask with 1 instead of (1<<pos). Considering that pos is an argument of the function, that saves a computation.

return (byte >> pos) & 1;

In the second function, I would assert that startingPos and offset are both positive instead of asserting that their sum is positive, it makes more sense that way.