Bi-directional multimap?

[orj]

Recently I encountered an issue where I needed a data structure that provided an efficient way of storing multiple values per key and being able to query for all keys that had a value.

A one way multi-value map can be implemented using Dictionary<Key, Set<Value>> but to get the keys that match values requires maintaining another Dictionary<Value, Set<Key>>. It would be nice if Swift had an efficient Bidirectional Multimap.