Population count, or popcount, is counting the number of set bits in a byte or word (or whatever your computer uses). It is also known as Hamming Weigth (see below for a link). There is a surprisingly large range of algorithms where a popcount is usefull. For instance it is handy when handling interrupts, or when indexing used file blocks on a storage medium, or in cryptology. And chess-programs seem to use it in the valuation of a board. Anyhow, the common factor in most of these uses is that speed is essential and so most modern processors have an opcode for doing a popcount.

For processors which lack such an opcode, and for Forths implementations which lack a popcount primitive, here an algorithms to quickly get the count of set bits.

The following program is around 10 times faster than a simple counting routine using a loop. Working out how this short program works is a very interesting excercise and is left to the user. Lets not spoil all the fun!

hex
: popcount ( number -- no_of_bits_in_number )
    dup  1 rshift 55555555 and -
    dup  33333333 and
    swap 2 rshift 33333333 and +
    dup  4 rshift +
    f0f0f0f and
    1010101 *
    18 rshift
;
decimal

Population count on Wikipedia: Wikipedia - Hamming Weight