Bidirectional Iterator

BidirectionalCollection+BidirectionalIterator.swift

extension BidirectionalCollection {

    func makeBidirectionalIterator() -> BidirectionalIterator<Self, Index> {
        return BidirectionalIterator<Self, Index>(collection: self)
    }

}

let a = [1, 2, 3, 4]

var i = a.makeBidirectionalIterator()

i.next() // => 1?
i.next() // => 2?
i.previous() // => 1?
i.next() // => 2?
i.next() // => 3?
i.next() // => 4?
i.next() // => nil
i.next() // => nil
i.previous() // => 4?
i.previous() // => 3?
i.previous() // => 2?
i.previous() // => 1?
i.previous() // => nil

BidirectionalIterator.swift

struct BidirectionalIterator<B, I> where B: BidirectionalCollection, B.Index == I {

    let collection: B

    var lastGivenIndex: I? = nil

    init(collection: B) {
        self.collection = collection
    }

    mutating func next() -> B.Iterator.Element? {
        guard let lastGivenIndex = lastGivenIndex else {
            self.lastGivenIndex = collection.startIndex
            return collection.first
        }

        if lastGivenIndex >= collection.index(before: collection.endIndex) {
            self.lastGivenIndex = collection.endIndex
            return nil
        }

        self.lastGivenIndex = collection.index(after: lastGivenIndex)
        return self.lastGivenIndex.map({ collection[$0] })
    }

    mutating func previous() -> B.Iterator.Element? {
        guard let lastGivenIndex = lastGivenIndex else {
            return nil
        }

        if lastGivenIndex <= collection.startIndex {
            self.lastGivenIndex = collection.startIndex
            return nil
        }

        self.lastGivenIndex = collection.index(before: lastGivenIndex)
        return self.lastGivenIndex.map({ collection[$0] })
    }

}