strideable.swift

extension CollectionType {
    func generateWithStride(interval: Self.Index.Distance = 1) -> PermutationGenerator<Self,AnySequence<Self.Index>> {
        var index = startIndex
        let generator:  AnyGenerator<Self.Index> = anyGenerator {
        //    Was:
        //     let currentIndex = index
        //     index = currentIndex.advancedBy(interval)
        //     return currentIndex.distanceTo(self.endIndex) <= 0 ? nil : currentIndex

            // http://twitter.com/oisdk/status/643830784026103808
            // advancedBy has a limit parameter, so you can avoid the O(n) distanceTo()
            // and <. https://gist.github.com/oisdk/d5f46ba03d56c9ec0a89 
            // http://pic.twitter.com/NbjIdFL8k4
            defer { index = index.advancedBy(interval, limit: self.endIndex) }
            return index == self.endIndex ? nil : index
        }
        return PermutationGenerator(elements:self, indices: AnySequence(generator))
    }
}