ReversedCollectionProtocol.swift

/* Minimalist implementation of ReversedCollectionProtocol */

public protocol ReversedCollectionProtocol: BidirectionalCollection {
  associatedtype Base: BidirectionalCollection
  var _base: Base { get }
}

extension ReversedCollection: ReversedCollectionProtocol {}

extension LazyCollectionProtocol
  where Self: BidirectionalCollection, Elements: ReversedCollectionProtocol {

  /// Reversing a lazy reversed collection returns a lazy representation of the original collection.
  public func reversed() -> LazyCollection<Elements.Base> {
    return elements._base.lazy
  }
}

// as an alternative, reversed() optimization for lazy collections using
// parametrized extensions, which not available in Swift 4.2 or older
// see: https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#parameterized-extensions

extension<T: BidirectionalCollection> LazyCollectionProtocol 
  where Self: BidirectionalCollection, Elements == ReversedCollection<T> {

  /// Reversing a lazy reversed collection returns a lazy representation of the original collection.
    public func reversed() -> LazyCollection<T> {
      return elements._base.lazy
    }
  }
}