Yeah, but I fired the last guy who billed me for that time he needed to re-discover the quadratic equation.
An interview should evaluate the skills one needs for the job, not puzzles that make you feel smart. Were I interviewing someone, I would happily accept an answer something along the lines of "that's been solved already, I would use an existing library so I can move on and solve my real problem" with regard to bitwise operators.
The goal is obviously to figure out how good people will be at solving real problems. But it's not realistic, practical, or fair to come up with an unsolved problem for each interview.
An interview should evaluate the skills one needs for the job, not puzzles that make you feel smart. Were I interviewing someone, I would happily accept an answer something along the lines of "that's been solved already, I would use an existing library so I can move on and solve my real problem" with regard to bitwise operators.