hash_differences

class Hash
  def difference(subtrahend)
    # create a hash which contains all normalized keys
    all_pairs = (self.keys.map{|x| x.downcase} + subtrahend.keys.map{|x| x.downcase}).uniq.inject({}) do |pairs, key|
      pairs[key] = []
      pairs
    end
    #=> {"mouse"=>[], "cat"=>[], "snake"=>[], "bird"=>[], "dog"=>[]}

    # push original key value pairs into array which is the value of just created hash
    [self, subtrahend].each_with_index do |hsh, idx|
      hsh.each_pair { |k, v| all_pairs[k.downcase].push([k, v]) }
      all_pairs.each_value { |v| v.push([nil, 0]) if v.size == idx }
    end
    #=> {"mouse"=>[[nil, 0], ["Mouse", 75]], "cat"=>[["Cat", 100], ["cat", 50]], "snake"=>[["Snake", 10], ["Snake", 10]], "bird"=>[["Bird", 2], ["BIRD", 4]], "dog"=>[["Dog", 5], ["dog", 3]]}

    results = {}
    all_pairs.each_value do |values|
      diff = values[0][1] - values[1][1]
      # always take the key whose value is larger
      if diff > 0
        results[values[0][0]] = diff
      elsif diff < 0
        results[values[1][0]] = diff
      end
    end
    return results
  end
end