Opposite collections

NMF often faces scenarios in which adding an element to a collection should raise side effects such as adding the parent element to another collection or setting a back reference. To support such use cases, NMF has a range of opposite collections, OppositeList, OppositeSet and OppositeOrderedSet. These implementations are very similar but implement different kinds of collections:

• OppositeList implements an array list, i.e., a collection with insertion order, duplicates and fast enumeration. The operations Contains and Remove are linear time, but Add is deterministically O(1) amortized.
• OppositeSet has the characteristics of a hash set, meaning it is a unique collection without a defined order. Here, Contains, Remove and Add are all O(1) but only under the assumption of a perfect hash function.
• OppositeOrderedSet is an ordered set that combines a unique collection with insertion order and fast enumeration.

All opposite collections have a constructor that takes a parent element as a parameter and an abstract method SetOpposite that is used to execute the hook for the referenced element. This method accepts two parameters: the item and the new parent. The new parent could either be null or the parent element. The task of SetOpposite is to set the opposite reference.

Note that the opposite collection classes do not ensure a specific order of operations. That means, it should not matter in which order a user performs a given operation. Hence, ideally operations performed within SetOpposite should be idempotent.