Subset Iteration with Bitmasks

Iterate all subsets of a mask:

sub = mask
while sub > 0:
    // process subset 'sub'
    sub = (sub - 1) & mask
// don't forget empty subset

Why it works: Decrementing flips trailing $1$s and sets next $0$ to $1$. The & mask keeps only bits from the original mask.

Example: mask $= 1011$, subsets: $1011, 1010, 1001, 1000, 0011, 0010, 0001, 0000$.

Time: $O(3^n)$ for iterating all subsets of all masks from $0$ to $2^n - 1$.