CODEWITHSUNDEEP
cgccbit
Ryan
Ryan

Reputation: 14659

Meaning of `pop` when referring to bits

Taking a look at GCC's builtins, I noticed the use of the string pop in function name (__builtin_popcount), etc. I've also seen other references to pop in other implementations.

When referring to set bits, why is pop used, and what does it mean?

Upvotes: 0

Views: 236

Answers (1)

r3mainer
r3mainer

Reputation: 24587

It's short for "population" (i.e., the number of bits that are "populated" by being set to 1), and is also known as the Hamming weight.

From Wikipedia:

The Hamming weight of a string is the number of symbols that are different from the zero-symbol of the alphabet used. It is thus equivalent to the Hamming distance from the all-zero string of the same length. For the most typical case, a string of bits, this is the number of 1's in the string. In this binary case, it is also called the population count, popcount or sideways sum. It is the digit sum of the binary representation of a given number and the ℓ₁ norm of a bit vector.

Upvotes: 3

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