ruby hash invert

gistfile1.txt

require './hash_inverse'

describe Hash do
  describe "when empty and asked for the inverse" do  
    it "returns an empty hash" do 
      {}.inverse.must_equal Hash.new
    end
  end

  describe "when mapping are unique and asked for the inverse" do  
    it "returns the inverse hash" do 
      {a: 1, b: 2, c: 3}.inverse.must_equal({1=>[:a], 2=>[:b], 3=>[:c]})
    end
  end

  describe "when mapping are not unique and asked for the inverse" do  
    it "returns the one to many key to values" do 
      {a: 1, b: 1, c: 2}.inverse.must_equal({1=>[:a, :b], 2=>[:c]})
    end
  end
end

hash_inverse.rb

class Hash
  # like invert but not lossy
  #{1=>2,3=>2,2=>4}.inverse => {2=>[1, 3], 4=>[2]} 
  def inverse
    self.each_with_object( {} ) { |(key, value), out| ( out[value] ||= [] ) << key }
  end
end

Shorter:

self.each_with_object({}){|(k,v),o| (o[v] ||=[]) << k}

or with reduce:

self.reduce({}){|h,(k,v)| (h[v] ||=[]) << k; h}