Simple diff methods for Hash or other Enumerable

diffable.rb

# Author:  Erik Elmore <erik@erikelmore.com>
# License:  Public Domain

# Enable "diffing" and two-way transformations between collection objects
module Diffable
  # Calculates the changes required to transform self to the given collection.
  # @param b [Enumerable] The other collection object
  # @return [Array] The Diff: A two-element change set representing items to exclude and items to include
  def diff( b )
    a, b = to_a, b.to_a
    [a - b, b - a]
  end
  
  # Consume return value of Diffable#diff to produce a collection equal to the one used to produce the given diff.
  # @param to_drop [Enumerable] items to exclude from the target collection
  # @param to_add  [Enumerable] items to include in the target collection
  # @return [Array] New transformed collection equal to the one used to create the given change set
  def apply_diff( to_drop, to_add )
    to_a - to_drop + to_add
  end
end

if __FILE__ == $0
  # Demo: Hashes with overlapping keys and somewhat random values.
  Hash.send :include, Diffable
  rng = Random.new
  a = (:a..:q).reduce(Hash[]){|h,k| h.merge! Hash[k, rng.rand(2)] }
  b = (:i..:z).reduce(Hash[]){|h,k| h.merge! Hash[k, rng.rand(2)] }
  raise unless a == Hash[ b.apply_diff(*b.diff(a)) ] # change b to a
  raise unless b == Hash[ a.apply_diff(*a.diff(b)) ] # change a to b
  raise unless a == Hash[ a.apply_diff(*a.diff(a)) ] # change a to a
  raise unless b == Hash[ b.apply_diff(*b.diff(b)) ] # change b to b
end