rank_by.rb

# returns a ranking as a hash { elem => rank }
# ranks based on the sort order of the values the given block yields
# elements yielding the same value are ranked the same
module Enumerable
  def rank_by
    inject(Hash.new {|h, k| h[k] = []}) { |h, e| h[yield(e)] << e; h }
      .sort.each_with_index
      .inject({}) { |h,((_, es), i)| es.each { |e| h.update e => i + 1 }; h }
  end
end

if __FILE__ == $0
  require 'rspec/autorun'
  describe 'rank_by' do
    it 'should rank' do
      [1, 2, 3].rank_by(&:to_i).should == { 1 => 1, 2 => 2, 3 => 3 }
      [1, 2, 3].rank_by { |i| -i }.should == { 3 => 1, 2 => 2, 1 => 3 }
      
      %w(1 123 2 234 12345).rank_by(&:length).should == {
        "1" => 1,
        "123" => 2,
        "2" => 1,
        "234" => 2,
        "12345" => 3
      }
      
      %w(wat wat duck).rank_by(&:to_s).should == { 'duck' => 1, 'wat' => 2 }
    end
  end
end