From: Steve Howell <showell30@...>
Date: 2010-10-15T10:10:19+09:00
Subject: Re: sort_by: multiple fields with reverse sort

On Oct 14, 1:25 pm, Ryan Davis <ryand-r...@zenspider.com> wrote:
> On Oct 14, 2010, at 08:35 , Steve Howell wrote:
>
> >  module Enumerable
> >    def sort_by_multi(*key_methods)
> >      def compare(a,b, key_methods)
>
> This doesn't do what you think (or at least imply that) it does.

Care to explain?

  puts a.sort_by_multi(
    [Proc.new { |a| a[0] }],
    [Proc.new { |a| a[1] }, :desc]
  ).inspect

$ ruby foo.rb

  [["archie", 20, 5], ["newton", 10, 3], ["radio", 40, 5], ["radio",
30, 5], ["radio", 20, 5]]

$ cat foo.rb
  module Enumerable
    def sort_by_multi(*key_methods)
      # allow for multiple key_methods and only
      # evaluate them when they are truly needed
      # for the sort
      def compare(a,b, key_methods)
        i = 0
        while i < key_methods.size do
          for elem in [a, b] do
            key = elem[1]
            if elem[0].size <= i
              elem[0] << key_methods[i][0].call(key)
            end
          end
          if key_methods[i][1] == :desc
            a, b = b, a
          end
          result = (a[0][i] <=> b[0][i])
          return result unless result == 0
          i += 1
        end
        return result
      end

      self.collect do |item|
        [ [], item ]
      end.sort do |a, b|
        compare(a, b, key_methods)
      end.collect do |kv|
        kv[1]
      end
    end
  end

  a = [
    ["radio", 30, 5],
    ["radio", 40, 5],
    ["radio", 20, 5],
    ["archie", 20, 5],
    ["newton", 10, 3]
  ]

  puts a.sort_by_multi(
    [Proc.new { |a| a[0] }],
    [Proc.new { |a| a[1] }, :desc]
  ).inspect