I found this function and it seemed reasonable, but some of my users reported that I was somtimes returning the wrong sign. Turns out that the function above has inconsistent behavior on negative numbers:

gcf(-5, 5) = 5
gcf(5, -5) = -5
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when they should really both be 5.

14 years late, but here's a fix. I changed my code so that gcf() ends with

return abs $x;