nmerge.rb

class Hash
  def self.nmerge(*hashes, &block)
    keys = hashes.map(&:keys).flatten.uniq
    rhashes = hashes.reverse
    keys.inject({ }) do |output, key|
      if block
        output[key] = block.call(key, hashes.map{ |x| x[key]})
      else
        rhashes.each do |hash|
          if hash[key]
            output[key] = hash[key]
            break
          end
        end
      end
      output
    end
  end
  def self.mmerge(*hashes)
    hashes.inject { |a,b| a.merge b }
  end
  def self.lmerge(*hashes)
    hashes.inject({}) { |a,b| a.merge! b }
  end
  def self.tmerge(*hashes)
    {}.tap{|blank| hashes.each{|hash| blank.merge!(hash)}}
  end
end

require 'benchmark'

one   = {:one   => 1, :two   => 1}
two   = {:two   => 2, :three => 2}
three = {:three => 3, :four  => 3}

puts Hash.nmerge(one, two, three).inspect
puts Hash.mmerge(one, two, three).inspect
puts Hash.lmerge(one, two, three).inspect
puts Hash.tmerge(one, two, three).inspect

Benchmark.bmbm do |x|
  x.report('nmerge') { 100000.times { Hash.nmerge one, two, three } }
  x.report('mmerge') { 100000.times { Hash.mmerge one, two, three } }
  x.report('lmerge') { 100000.times { Hash.lmerge one, two, three } }
  x.report('tmerge') { 100000.times { Hash.tmerge one, two, three } }
end

output.txt

>> ruby nmerge.rb
{:one=>1, :two=>2, :three=>3, :four=>3}
{:one=>1, :two=>2, :three=>3, :four=>3}
{:one=>1, :two=>2, :three=>3, :four=>3}
{:one=>1, :two=>2, :three=>3, :four=>3}
Rehearsal ------------------------------------------
nmerge   1.120000   0.000000   1.120000 (  1.124069)
mmerge   0.480000   0.010000   0.490000 (  0.485693)
lmerge   0.320000   0.000000   0.320000 (  0.379063)
tmerge   0.270000   0.000000   0.270000 (  0.275908)
--------------------------------- total: 2.200000sec

             user     system      total        real
nmerge   1.120000   0.010000   1.130000 (  1.128885)
mmerge   0.470000   0.000000   0.470000 (  0.477016)
lmerge   0.280000   0.000000   0.280000 (  0.287000)
tmerge   0.270000   0.000000   0.270000 (  0.269606)

>> ruby -v
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin10.8.0]