Efficient way to count how many bits are set in a word

Asked May 14 '11 at 20:29

Active May 14 '11 at 20:29

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Possible Duplicate:
Best algorithm to count the number of set bits in a 32-bit integer?

This is a nice one:

What is the the most efficient way you know (in C) to count how many bits are set in a word?

In case of 8bit, you can use lookup table, but what about 128bit? more?

edited May 23 '17 at 12:19

Community

asked May 14 '11 at 20:29

Amir

http://graphics.stanford.edu/~seander/bithacks.html for some interesting ways to count bits. – Bart May 14 '11 at 20:32
The keyword you should search for is "popcount" or "hamming weight". – R.. GitHub STOP HELPING ICE May 14 '11 at 21:51

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