ForeverSequence.swift

/* My first attempt at building a sequence that will make a collection loop forever.

  I had a need where I wanted to loop over two lists but instead of truncating the 
  longer list to the shorter one, I wanted the shorter one repeat until the longer 
  one completed.
  
  FE:
  let c = ["Z", "Y", "X", "W", "V", "U", "T"]
  let d = [0,9,7]
  let i = ForeverSequence(vals: d)
  for (k,l) in zip(c,i) {
      print("\(k) - \(l)")
  }
 */

struct ForeverSequence<C : CollectionType> : SequenceType {
    let vals: C
    func generate() -> AnyGenerator<C.Generator.Element> {
        guard vals.count > 0 else {
            return AnyGenerator { nil }
        }
        var gen = vals.generate()
        return AnyGenerator<C.Generator.Element> {
            if let r = gen.next() {
                return r
            } else {
                // at end, start over.
                gen = self.vals.generate() // reset generator
                return gen.next()! // From the guard above, we know that there is atleast one element here.
            }
        }
    }
}

Used ideas from @kevlario (see http://swiftlang.ng.bluemix.net/#/repl/38e113adcb713529895a4ed9360c5a169dd965ebf68784d4d520781231129bb9 )