Incrementing with a bitmask

Bitmasks are fun. There’s lots of little tricks you can do with them. A common situation is checking for the presence of a flag among elements in a linked list, or some similar data structure. I came across a trick a few years ago that makes it drop-dead simple.

Let’s say we needed to check for AUsefulFlag in the flags element of each node, and total how many elements in the linked list had the flag.

uint64_t count = 0;
for(Node *iter = head; iter != NULL; iter = iter->next)
    count += !!(iter->flags & AUsefulFlag);

After execution, count is the number of items which have AUsefulFlag set.

Double-not (!!) is one of those useful operations which are especially useful with bitmasks. It may require a double-take at first, but it behaves exactly how you’d think.

!! of 1 is 1. !! of 0 is 0. In fact, !! of any true value evaluates to 1, so we can use it to transform something like 0b00001000 to simply 1 and increment by that value.

From Atlas Shrugged​ by Ayn Rand:

James: What are you after?
Francisco: Money.
James: Don’t you have enough?
Francisco: In his lifetime, every one of my ancestors raised the production of d’Anconia Copper by about ten per cent. I intend to raise it by one hundred.
James: What for?
Francisco: When I die, I hope to go to heaven–whatever the hell that is–and I want to be able to afford the price of admission.
James: Virtue is the price of admission.
Francisco: That’s what I mean, James. So I want to be prepared to claim the greatest virtue of all–that I was a man who made money.
James: Any grafter can make money.
Francisco: James, you ought to discover some day that words have an exact meaning.