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Message: [QUOTE="James Harris, post: 3126667"] Since others are going for speed how about the following. (The C may not be completely correct.) It combines fast lookup with a small table. There is a reasonable chance that the looked up value will be in cache (which would be better if there were some way to align the array). int nibble_bits[] = { /* The number of bits in each nibble 0 to 15 */ 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 } int bitcount (int i) { int bits_set = 0; for (; i; i >>= 4) { bits_set += nibble_bits[i & 0xf]; } return bits_set; } [/QUOTE]