From: Steve Howell <showell30@...>
Date: 2010-10-15T14:40:24+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.

Ok, I found and fixed a bug (*) and reworked the example to
(hopefully) make the implied intent more clear.

  rows = [
    {:name => "al",      :salary => 40},
    {:name => 'charlie', :salary => 100},
    {:name => "al",      :salary => 50},
    {:name => 'diane',   :salary => 1000, :commission => 40},
    {:name => 'diane',   :salary => 1000, :commission => 30},
    {:name => 'ed',      :salary => 20},
  ]

  def do_sql_like_sort(rows)
    def asc(field)
      [Proc.new { |row| row[field] }, :asc ]
    end
    def desc(field)
      [Proc.new { |row| row[field] }, :desc ]
    end
    # The semantics for sort_by_multi are similar to
    # SQL.
    rows.sort_by_multi(
      asc(:name),
      desc(:salary),
      desc(:commission)
    )
  end

  expected_result = [
    {:name=>"al",        :salary=>50},
    {:name=>"al",        :salary=>40},
    {:name=>"charlie",   :salary=>100},
    {:name=>"diane",     :salary=>1000, :commission => 40},
    {:name=>"diane",     :salary=>1000, :commission => 30},
    {:name=>"ed",        :salary=>20},
  ]


  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
          x, y = (key_methods[i][1] == :desc) ? [b, a] : [a, b]
          result = (x[0][i] <=> y[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

  sorted_rows = do_sql_like_sort(rows)
  if sorted_rows != expected_result
    raise 'fail'
  end

* - The previous iteration of this code was swapping a and b instead
of making copies before swapping.  This broke the case where two of
the fields were to be sorted in descending order.