GCD function for BinaryInteger

[NevinBR]

Added a gcd static method to BinaryInteger.

(Edit: gcd is now a free function.)

The initial implementation uses the binary GCD algorithm.

I believe that gcd is a sufficiently widespread term of art to prefer it over a more verbose spelling. However if others disagree, we could use greatestCommonDivisor instead.

[stephentyrone]

One higher-level question; should we expose a gcd operation on > 2 integers, or on collections? I'm not sure how valuable a variadic gcd would be (since there's not a great way to marshal multiple arguments like that), but is it worth having a more convenient spelling than reduce(into: 0) { $0 = gcd($0, $1) }? I think it might be worth providing because it isn't immediately obvious to most users that they can provide 0 as an initial value for the reduction.

[NevinBR]

One higher-level question; should we expose a gcd operation on > 2 integers, or on collections?

That’s a good question. We should probably hash out things like that over on issue #10.

[stephentyrone]

We should probably hash out things like that over on issue #10.

I'm not sure #10 is the right place to discuss it, but I'm perfectly happy to punt it to another PR; it would clearly be built on top of this anyway.

[NevinBR]

I'm not sure #10 is the right place to discuss it, but I'm perfectly happy to punt it to another PR; it would clearly be built on top of this anyway.

I was thinking along the lines of discussing the scope of the integer utilities modules and general shape of its APIs.

One thing I’ve found with Github is that it doesn’t really provide an obvious location for discussion, planning, and questions about a project.

[stephentyrone]

@swift-ci test

[stephentyrone]

Thanks @NevinBR! Do you want to do anything further with this before I merge it?

[stephentyrone]

@swift-ci test

[stephentyrone]

Thanks for your patience, @NevinBR!