A fork of http://rubyforge.org/projects/deepmerge/ with 1.9 fixes and ActiveSupport compatability
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DeepMerge Overview

Deep Merge is a simple set of utility functions for Hash. It permits
you to merge elements inside a hash together recursively. The manner
by which it does this is somewhat arbitrary (since there is no defining 
standard for this) but it should end up being pretty intuitive and do what
you expect.

You can learn a lot more about this by reading the test file. It's pretty
well documented and has many examples of various merges from very simple
to pretty complex.

The primary need that caused me to write this library is the merging of elements
coming from HTTP parameters and related stored parameters in session. This lets
a user build up a set of parameters over time, modifying individual items.

Deep Merge Core Documentation
 deep_merge! method permits merging of arbitrary child elements. The two top level
 elements must be hashes. These hashes can contain unlimited (to stack limit) levels
 of child elements. These child elements to not have to be of the same types.
 Where child elements are of the same type, deep_merge will attempt to merge them together.
 Where child elements are not of the same type, deep_merge will skip or optionally overwrite
 the destination element with the contents of the source element at that level.
 So if you have two hashes like this:
   source = {:x => [1,2,3], :y => 2}
   dest =   {:x => [4,5,'6'], :y => [7,8,9]}
   Results: {:x => [1,2,3,4,5,'6'], :y => 2}
 By default, "deep_merge!" will overwrite any unmergeables and merge everything else.
 To avoid this, use "deep_merge" (no bang/exclamation mark)
   Options are specified in the last parameter passed, which should be in hash format:
   hash.deep_merge!({:x => [1,2]}, {:knockout_prefix => '--'})
   :preserve_unmergeables  DEFAULT: false
      Set to true to skip any unmergeable elements from source
   :knockout_prefix        DEFAULT: nil
      Set to string value to signify prefix which deletes elements from existing element
   :sort_merged_arrays     DEFAULT: false
      Set to true to sort all arrays that are merged together
   :unpack_arrays          DEFAULT: nil
      Set to string value to run "Array::join" then "String::split" against all arrays
   :merge_debug            DEFAULT: false
      Set to true to get console output of merge process for debugging

 Selected Options Details:
 :knockout_prefix => The purpose of this is to provide a way to remove elements 
   from existing Hash by specifying them in a special way in incoming hash
    source = {:x => ['--1', '2']}
    dest   = {:x => ['1', '3']}
    Results: {:x => ['2','3']}
   Additionally, if the knockout_prefix is passed alone as a string, it will cause
   the entire element to be removed:
    source = {:x => '--'}
    dest   = {:x => [1,2,3]}
    Results: {:x => ""}
 :unpack_arrays => The purpose of this is to permit compound elements to be passed
   in as strings and to be converted into discrete array elements
   irsource = {:x => ['1,2,3', '4']}
   dest   = {:x => ['5','6','7,8']}
   dest.deep_merge!(source, {:unpack_arrays => ','})
   Results: {:x => ['1','2','3','4','5','6','7','8'}
   Why: If receiving data from an HTML form, this makes it easy for a checkbox 
    to pass multiple values from within a single HTML element
 There are many tests for this library - and you can learn more about the features
 and usages of deep_merge! by just browsing the test examples

Simple Example Code

require 'deep_merge'
x = {:x => [3,4,5]}
y = {:x => [1,2,3]}
# results: y = {:x => [1,2,3,4,5]}

SVN Repo here:  http://trac.misuse.org/science/wiki/DeepMerge
Contact author: http://www.misuse.org/science

Copyright (c) 2008 Steve Midgley, released under the MIT license