Swift 4 - Pairwise Sequence

Pairwise+Sequence.swift

public struct PairwiseIterator<T>: IteratorProtocol {
	private var iterator: AnyIterator<T>
	private var lastElement: T?
	
	public init(_ iterator: AnyIterator<T>) {
		self.iterator = iterator
		self.lastElement = self.iterator.next()
	}
	
	public mutating func next() -> (first: T, second: T)? {
		guard let element = lastElement, let currentElement = iterator.next() else {
			return nil
		}
		
		lastElement = currentElement
		return (first: element, second: currentElement)
	}
}

public extension Sequence {
	public var pairwise: AnySequence<(first: Self.Element, second: Self.Element)> {
		let seq = AnySequence { PairwiseIterator(AnyIterator(self.makeIterator())) }
		
		return seq
	}
}