From: Brian Mitchell <binary42@...>
Date: 2006-04-21T03:04:40+09:00
Subject: Re: Converting IP range to array of IP's

On 4/20/06, Brian Mitchell <binary42@gmail.com> wrote:
> On 4/20/06, Kris <krisleech@interkonect.com> wrote:
> > I'm looking to convert an IP string (ranges, subnet masks or wild-carded
> > IP's) in to an array of IP strings:
> >
> > eg. "192.168.1.1 - 192.168.1.5" => ["192.168.1.1", "192.168.1.2",
> > "192.168.1.3", "192.168.1.4", "192.168.1.5"]
> >
> > Before I embark on this, is there already a ruby class that does it or
> > some of this?
>
> Other's solutions should work great but just for fun and enlightenment
> I wrote my own little implementation of a simple list comprehension in
> Haskell and then ported it to Ruby. This simple piece of code allows
> you to expand more arbitrary ranges and might be good for more things
> than just IP addresses.
>
> # Condensed ruby version of this haskell code
> #
> # [[a,b,c,d] | a <- [192], b <- [168], c <- [0,1], d <- [1..10]]
> #
> # non-sugared notation:
> #   [192] >>= \a ->
> #   [168] >>= \b ->
> #   [0,1] >>= \c ->
> #   [1..10] >>= \d ->
> #   return [a,b,c,d]
>
> class Array
>   def expand(i = 0, *a)
>     return [a] if i == size
>     self[i].to_a.map {|x|
>       expand(i+1, *(a + [x]))
>     }.inject([]) {|m, a| m + a}
>   end
> end

A few redundancies to fix (my original code didn't use map but a
custom method Array#bind. The to_a is no longer needed:

class Array
  def expand(i = 0, *a)
    return [a] if i == size
    self[i].map {|x|
      expand(i+1, *(a + [x]))
    }.inject([]) {|m, x| m + x}
  end
end

This is what it used to look like:

class Array
  def bind(&blk)
    map(&blk).inject([]) {|m, x| m+x}
  end

  def expand(i = 0, *a)
    return [a] if i == size
    self[i].bind {|x| expand(i+1, *(a + [x]))}
  end
end

Note that bind could have the inject part split out into what Haskell
calls a join but that just lead to name issues so I just inlined it.

Brian.