CyclicSequence.swift

struct CyclicSequence<Base: Sequence>: Sequence {

    let base: Base

    init(base: Base) {
        self.base = base
    }

    func makeIterator() -> CyclicIterator<Base> {
        return CyclicIterator(base: base)
    }

}

struct CyclicIterator<Base: Sequence>: IteratorProtocol {

    let base: Base

    var currentIterator: Base.Iterator

    init(base: Base) {
        self.base = base
        self.currentIterator = base.makeIterator()
    }

    mutating func next() -> Base.Element? {
        if let next = currentIterator.next() {
            return next
        }
        currentIterator = base.makeIterator()
        return currentIterator.next()
    }
}

extension Sequence {
    func cycle() -> CyclicSequence<Self> {
        return CyclicSequence(base: self)
    }
}