gistfile1.rb

#!/usr/bin/env ruby

class Array
  def unique_by
    inject(Hash.new(0)) do |h, i|
      h[yield(i)] += 1
      h
    end.size == size
  end
end

puts [ 1,2,3,4,5,9 ].unique_by { |n| n % 10 }
puts [ 1,2,3,4,5,9,11 ].unique_by { |n| n % 10 }
puts [ 1,2,3,13,4,5,9 ].unique_by { |n| n % 10 }