each_cons from Ruby in Swift as a function on Sequence

Array+EachCons.swift

extension Collection {
    func eachConsecutive(_ size: Int) -> Array<SubSequence> {
        let droppedIndices = indices.dropFirst(size - 1)
        return zip(indices, droppedIndices)
            .map { return self[$0...$1] }
    }
}

Sequence+EachPair.swift

extension Sequence {

    typealias Pair = (Element, Element)

    // you can do zip(self, self.dropFirst), but this slightly
    // more complex version will work for single pass sequences as well
    func eachPair() -> AnySequence<Pair> {
        var iterator = self.makeIterator()
        guard var previous = iterator.next() else { return AnySequence([]) }
        return AnySequence({ () -> AnyIterator<Pair> in
            return AnyIterator({
                guard let next = iterator.next() else { return nil }
                defer { previous = next }
                return (previous, next)
            })
        })
    }
}