From: Brian Candler <B.Candler@...>
Date: 2004-10-10T00:47:11+09:00
Subject: Re: A sort_by descending sort

On Sat, Oct 09, 2004 at 12:55:14AM +0900, Michael Gaunnac wrote:
> # sort                         ascending      descending  year
> descending  month/day                          descending hour:minute
> arr = arr.sort_by {|i| [[i.slice(0,1)],
> [i.slice(7,4).tr('0-9','9876543210')],
> [i.slice(1,5).tr('/0-9','/9876543210')],
> [i.slice(11,5).tr(':0-9',':9876543210')]]}
> 
> arr.each {|i| print i, "\n"}
> 
> 
> <challenge>
> Can this sort be simplified (with Ruby of course)?
> </challenge>

Arguably cheating, and very specific to this particular example, but:

arr2 = arr.sort_by { |i|
  [-i[0], i[7..10], i[4..5], i[1..2], i[11..15]]
}.reverse!

For a more general pattern, how about:

module Rev
  def <=>(other); -super; end
end

arr3 = arr.sort_by { |i|
  [i[0..0], i[7..10].extend(Rev), i[4..5].extend(Rev),
  i[1..2].extend(Rev), i[11..15].extend(Rev)] }

or more compactly,

arr4 = arr.sort_by { |i|
  [i[0], (i[7..10]+i[4..5]+i[1..2]+i[11..15]).extend(Rev)]
}

To be really cute, let's define 'negative strings':

class String
  def -@
    if kind_of? Rev
      String.new(self)
    else
      extend Rev
    end
  end
end

arr5 = arr.sort_by { |i|
  [i[0], -(i[7..10]+i[4..5]+i[1..2]+i[11..15])]
}

Regards,

Brian.