DirectionItertion.swift

internal struct DirectionalSequence<Elements : CollectionType where Elements.Index : Strideable> {

    private let elements:Elements
    private var reversed:Bool

    // MARK: -

    internal init(_ elements:Elements, reversed:Bool) {
        (self.elements, self.reversed) = (elements, reversed)
    }
}

// MARK: -

extension DirectionalSequence {
    private var distance:Elements.Index.Stride { return reversed ? -1 : 1 }
    private var startIndex:Elements.Index { return reversed ? elements.endIndex - 1 : elements.startIndex }
    private var endIndex:Elements.Index { return reversed ? elements.startIndex - 1 : elements.endIndex }
    private var stride:StrideTo<Elements.Index> { return startIndex.stride(to:endIndex, by:distance) }
}

// MARK: -

extension DirectionalSequence : SequenceType {
    func generate() -> DirectionalIterator<Elements> {
        return DirectionalIterator(elements, stride)
    }
}



internal struct DirectionalIterator<Elements : CollectionType where Elements.Index : Strideable> {

    private let elements:Elements
    private var iterator:StrideToGenerator<Elements.Index>

    // MARK: -

    private init(_ elements:Elements, _ stride:StrideTo<Elements.Index>) {
        self.iterator = stride.generate()
        self.elements = elements
    }
}

// MARK: -

extension DirectionalIterator : GeneratorType {
    mutating func next() -> Elements.Generator.Element? {
        guard let index = iterator.next() else { return nil }
        return elements[index]
    }
}