A sequence type that iterates over pairs of the underlying sequence

PairSequence.swift

import Foundation

struct PairGenerator<T: GeneratorType> : GeneratorType {
    typealias Pair = (first: T.Element?, second: T.Element?)
    var baseGenerator: T
    var previousPair: Pair
    init(_ baseGenerator: T) {
        self.baseGenerator = baseGenerator
        self.previousPair = (nil, nil)
    }
    mutating func next() -> Pair? {
        let pair = (first: previousPair.second, second: baseGenerator.next())
        previousPair = pair
        if pair.first == nil && pair.second == nil {
            return nil
        } else {
            return pair
        }
    }
}

extension SequenceType {
    func pairs() -> GeneratorSequence<PairGenerator<Self.Generator>> {
        return GeneratorSequence(PairGenerator(generate()))
    }
}

Given any SequenceType this adds a method pairs() which returns another kind of SequenceType. It lets you iterate over pairs in a sequence, like so:

let words = ["good", "morning", "swift"]
for (a, b) in words.pairs() {
    print("\(a), \(b)")
    // nil, "good"
    // "good", "morning"
    // "morning", "swift"
    // "swift", nil
}