gistfile1.rb

require 'rubygems'
require 'active_support'
require "rbench"

class Hash
  
  def fast_reverse_merge( other_hash )
    self.merge( other_hash ){|k,o,n| o }
  end
  
  def fast_reverse_merge!( other_hash )
    self.merge!( other_hash ){|k,o,n| o }
  end
  
end

first = { :one => 1, :two => 2 }
second = { :three => 3, :four => 4 }
 
RBench.run(10_000) do
  
  report "Hash#reverse_merge!" do
    first.reverse_merge!( second )
  end

  report "Hash#fast_reverse_merge!" do
    first.fast_reverse_merge!( second )
  end

  report "Hash#reverse_merge" do
    first.reverse_merge( second )
  end

  report "Hash#fast_reverse_merge" do
    first.fast_reverse_merge( second )
  end
  
end  

=begin
                                 Results |
------------------------------------------
Hash#reverse_merge!                0.035 |
Hash#fast_reverse_merge!           0.018 |
Hash#reverse_merge                 0.020 |
Hash#fast_reverse_merge            0.031 |
=end