Hash#remap – alters a Hash's keys using a supplied mapping.

hash_remap.rb

class Hash
  # Given a hash that maps keys to the new key, this method will return a new
  # Hash whose keys are contained in the map Hash, but whose values come from
  # the Hash #remap was called on.
  #
  # Ex: { 1 => 2, 2 => 3 }.remap(1 => 100, 2 => 200) # => { 100 => 2, 200 => 3 }
  #
  def remap(mapping)
    ret = Hash.new
    mapping.each do |src, dest|
      ret[dest] = self[src]
    end
    ret
  end
end