hash_map.rb

module Enumerable
  # operates like Enumerable#map, but returns a hash
  def hmap
    inject Hash.new do |new_hash, (k,v)|
      new_k, new_v = yield k, v
      new_hash[new_k] = new_v
      new_hash
    end
  end
end

h = {a: 1, b: 2}
h.hmap{|k,v| [v, k]}