TrackSelectionStrategy.swift

    class TrackSelectionStrategy {

        static let straight: TrackSelectionStrategy = Straight()
        static let shuffle: TrackSelectionStrategy = Shuffle()

        fileprivate init() {}

        func nextItem(_ sender: AudioPlayerImpl) -> AudioPlayerItem? {
            return self.nextItemIndex(sender).map({ sender._items[$0] })
        }

        func previousItem(_ sender: AudioPlayerImpl) -> AudioPlayerItem? {
            return self.previousItemIndex(sender).map({ sender._items[$0] })
        }

        func canGoForward(_ sender: AudioPlayerImpl) -> Bool {
            return self.nextItemIndex(sender) != nil
        }

        func canGoBack(_ sender: AudioPlayerImpl) -> Bool {
            return self.previousItemIndex(sender) != nil
        }

        private func nextItemIndex(_ sender: AudioPlayerImpl) -> Int? {
            return nil
        }

        private func previousItemIndex(_ sender: AudioPlayerImpl) -> Int? {
            return nil
        }

        private func currentItemIndex(_ sender: AudioPlayerImpl) -> Int? {
            let items = sender._items
            return sender.currentItem
                .flatMap({ items.index(of: $0) })
        }

        private class Straight: TrackSelectionStrategy {
            override func nextItemIndex(_ sender: AudioPlayerImpl) -> Int? {
                let items = sender._items
                return self.currentItemIndex(sender)
                    .map({ $0.advanced(by: 1) })
                    .flatMap({ $0 < items.count ? $0 : nil })
            }

            override func previousItemIndex(_ sender: AudioPlayerImpl) -> Int? {
                return self.currentItemIndex(sender)
                    .map({ $0.advanced(by: -1) })
                    .flatMap({ $0 >= 0 ? $0 : nil })
            }
        }

        private class Shuffle: TrackSelectionStrategy {
            override func nextItemIndex(_ sender: AudioPlayerImpl) -> Int? {
                return self.randomItemIndex(sender)
            }

            override func previousItemIndex(_ sender: AudioPlayerImpl) -> Int? {
                return self.randomItemIndex(sender)
            }

            private func randomItemIndex(_ sender: AudioPlayerImpl) -> Int? {
                let items = sender._items
                guard items.count > 1 else {
                    return nil
                }

                return self.currentItemIndex(sender)
                    .map({ self.generateRandomNumber(upperBound: items.count, excluding: $0) })
            }

            private func generateRandomNumber(upperBound: Int, excluding: Int) -> Int {
                let result = Int(arc4random_uniform(UInt32(upperBound)))
                return result != excluding ? result : generateRandomNumber(upperBound: upperBound, excluding: excluding)
            }
        }
    }

    var nextItem: AudioPlayerItem? {
        return self.trackSelectionStrategy.nextItem(self)
    }

    var previousItem: AudioPlayerItem? {
        return self.trackSelectionStrategy.previousItem(self)
    }