LazyDropLast

LazyDropLast.swift

extension Sequence {

    func lazyDropLast(_ n: Int) -> AnySequence<Element> {
        precondition(n >= 0, "Can't drop a negative number of elements from a sequence")

        return AnySequence<Element>({ () -> AnyIterator<Element> in

            var iterator = self.makeIterator()

            if n == 0 { return AnyIterator(iterator) }

            var ringBuffer = Array<Element>()
            ringBuffer.reserveCapacity(n)

            for _ in 0..<n {
                guard let next = iterator.next() else { return AnyIterator([].makeIterator()) }
                ringBuffer.append(next)
            }

            var index = 0
            return AnyIterator({ () -> Element? in
                guard let next = iterator.next() else { return nil }
                defer { index += 1 }
                defer { ringBuffer[index % n] = next }
                return ringBuffer[index % n]
            })

        })
    }
}

print(Array((0...10).lazyDropLast(0)))
print(Array((0...10).lazyDropLast(1)))
print(Array((0...10).lazyDropLast(5)))
print(Array((0...10).lazyDropLast(11)))
print(Array((0...10).lazyDropLast(20)))