WrappedCollection.swift

struct WrappedCollection<Element>: RandomAccessCollection {
  typealias Index = Int

  var startIndex: Index { _startIndex() }
  var endIndex: Index { _endIndex() }

  subscript(position: Index) -> Element {
    _position(position)
  }

  init<BaseCollection>(_ baseCollection: BaseCollection) where BaseCollection: RandomAccessCollection, BaseCollection.Element == Element, BaseCollection.Index == Index {
    _position = { baseCollection[$0] }
    _startIndex = { baseCollection.startIndex }
    _endIndex = { baseCollection.endIndex }
  }

  private let _endIndex: () -> Index
  private let _startIndex: () -> Index
  private let _position: (Index) -> Element
}