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!
Population Count - Forth routine
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