multiset

https://github.com/maraigue/multiset

Unlike ordinary set(see Ruby documentation for "set" library), multiset can contain two or more same items. Set[:a,:b,:c,:b,:b,:c] # => #<Set: {:b, :c, :a}> Multiset[:a,:b,:c,:b,:b,:c] # => #<Multiset:#3 :b, #2 :c, #1 :a> Multisets are typically used for counting elements and their appearances in collections.