benchmark.rb

require "benchmark"
class Hash
  # Performs the opposite of <tt>merge</tt>, with the keys and values from the first hash taking precedence over the second.
  def reverse_merge(other_hash)
    other_hash.merge(self)
  end

  # Performs the opposite of <tt>merge</tt>, with the keys and values from the first hash taking precedence over the second.
  # Modifies the receiver in place.
  def reverse_merge!(other_hash)
    replace(reverse_merge(other_hash))
  end
end

def hash1
  Hash[*%w(a b c d e).zip([1,2,3,4,5]).flatten]
end

def hash2
  Hash[*%w(a b c d e f).zip([10,20,30,40,50,60]).flatten]
end

TESTS = 10_000
Benchmark.bmbm do |results|
  results.report("revmerge:") { TESTS.times { hash1.reverse_merge!(hash2) } }
  results.report("blomerge:") { TESTS.times { hash1.merge!(hash2) { |h,v1,v2| v1 } } }
end

rbench.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