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copied!<p>Look at <a href="http://graphics.stanford.edu/~seander/bithacks.html" rel="nofollow">Bit Twiddling Hacks</a></p> <ul> <li>Counting bits set <ul> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetNaive" rel="nofollow">Counting bits set, naive way</a></li> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetTable" rel="nofollow">Counting bits set by lookup table</a></li> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetKernighan" rel="nofollow">Counting bits set, Brian Kernighan's way</a></li> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSet64" rel="nofollow">Counting bits set in 12, 24, or 32-bit words using 64-bit instructions</a></li> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel" rel="nofollow">Counting bits set, in parallel</a></li> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#CountBitsFromMSBToPos" rel="nofollow">Count bits set (rank) from the most-significant bit upto a given position</a></li> <li><a href="http://graphics.stanford.edu/~seander/bithacks.html#SelectPosFromMSBRank" rel="nofollow">Select the bit position (from the most-significant bit) with the given count (rank)</a></li> </ul></li> </ul> <p><strong>Edit</strong> As for the 'bit position bucket accumulation' (<code>bit_counter[]</code>) 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. </p> <p>You could, these days, come very close to valarray behaviour using <em>tied tuples</em> (TR1, boost or C++11); I have a feeling it would come out being simpler to read and slower to compile.</p>

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