From: "Bartosz Dziewoński" <matma.rex@...>
Date: 2013-02-28T02:19:30+09:00
Subject: Re: Please, help (GCD) greatest common divisor.

Replying to the gist.

> ### II. "inject"-part
> # Breaking it down with [6,8]
> gcd[[6,8]]      #=> 2
> gcd=->(_){_.sort_by(&:-@).inject{|p,q|q.nonzero?&&gcd[[q,p-q]]||--p}}
> # ( I am sure that I am missing something here )
> # 1st iteration: [8,6] ->                       6 && gcd[[6,2]] || --8
> # 2nd iteration: [6,2] ->                  6 && 2 && gcd[[2,4]] || --6
> # 3rd iteration: [4,2] ->             6 && 2 && 2 && gcd[[2,2]] || --4
> # 4th iteration: [2,2] ->        6 && 2 && 2 && 2 && gcd[[2,0]] || --2
> # 5th iteration: [2,0] -> 6 && 2 && 2 && 2 && nil && gcd[[0,2]] || --2
> # On the last iteration everything evaluates to nil before the ||.# (One remaining question: why --p?)

#inject on a two-element array is essentially the same as splatting it in two separate variables. The following two snippets do the same thing:

     result = [1, 2].inject{|a, b| do_stuff_with(a, b) }

     a, b = *[1, 2]
     result = do_stuff_with(a, b)

"a && b || c" is the same as "if a; b; else c; end" (or "a ? b : c"). This is a little abuse of the behavior of boolean operators :)

"--p" is just for laughs. While this looks like decrementation in C / C++, it parses as "-(-p)" as is the same as simply "p".

-- 
Matma Rex