BidirectionalCollection

Bidirectional collections offer traversal backward from any valid index, not including a collection’s startIndex. Bidirectional collections can therefore offer additional operations, such as a last property that provides efficient access to the last element and a reversed() method that presents the elements in reverse order. In addition, bidirectional collections have more efficient implementations of some sequence and collection methods, such as suffix(_:).

Conforming to the BidirectionalCollection Protocol

To add BidirectionalProtocol conformance to your custom types, implement the index(before:) method in addition to the requirements of the Collection protocol.

Indices that are moved forward and backward in a bidirectional collection move by the same amount in each direction. That is, for any valid index i into a bidirectional collection c:

• If i >= c.startIndex && i < c.endIndex, then c.index(before: c.index(after: i)) == i.

• If i > c.startIndex && i <= c.endIndex, then c.index(after: c.index(before: i)) == i.

Valid indices are exactly those indices that are reachable from the collection’s startIndex by repeated applications of index(after:), up to, and including, the endIndex.