Split a hash into multiple equal-sized hashes

split_into.rb

# Extend hashes by adding one method that allows you to split a hash into an array of equal-sized hashes.
# Filter items to be included in the result by adding a selecting block that returns true 
# if the key/value is to be kept.
# Per the usage example, return values are not ordered unless you're using Ruby 1.9

# usage: 
#  a = {1 => 'a', 2 => 'b', 3 => 'c', 4 => 'd', 5 => 'e'}

#  a.split_into(3) {|k,v| k.instance_of?(Integer)}

#  => [{5=>"e", 3=>"c"}, {1=>"a", 4=>"d"}, {2=>"b"}]

Hash.class_eval do
  def split_into(divisions, &:block)
    count = 0
    inject([]) do |final, key_value|
      if !block_given? || yield(key_value[0], key_value[1])
        final[count%divisions] ||= {}
        final[count%divisions].merge!({key_value[0] => key_value[1]})
        count += 1
      end
      final
    end
  end
end