Look at Bit Twiddling Hacks
- Counting bits set
- Counting bits set, naive way
- Counting bits set by lookup table
- Counting bits set, Brian Kernighan's way
- Counting bits set in 12, 24, or 32-bit words using 64-bit instructions
- Counting bits set, in parallel
- Count bits set (rank) from the most-significant bit upto a given position
- Select the bit position (from the most-significant bit) with the given count (rank)
Edit As for the 'bit position bucket accumulation' (bit_counter[]) I have a feeling that this might be a good case for valarrays + masking. That'd be a fair bit of coding+testing+profiling though. Let me know if you are really interested.
You could, these days, come very close to valarray behaviour using tied tuples (TR1, boost or C++11); I have a feeling it would come out being simpler to read and slower to compile.
