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Re: Count bits

On 28-Nov-05, at 5:51 AM, Bruno Causse wrote:
I seek the fastest method to count the bits has 1 in a unsigned long long

I use the following code for 32-bit unsigned int values - it is easily modified for other value sizes. It makes use of a pre-computed array 'BitsInChar' (which is independent of the data and calculated at program initialization):

// This is derived from the 'precomputed_bitcount' routine
// on the web page "Counting Number of On Bits in an Integer"
// by Gurmeet Singh Manku
// http://www-db.stanford.edu/~manku/bitcount/bitcount.html

            count = BitsInChar[(value      ) & 0xFFu]
                      +  BitsInChar[(value >>  8) & 0xFFu]
                      +  BitsInChar[(value >> 16) & 0xFFu]
                      +  BitsInChar[(value >> 24) & 0xFFu];

--
Cameron Hayne
email@hidden


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