PaddedZip.Swift

struct PaddedZipGenerator<G0: GeneratorType, G1: GeneratorType> : GeneratorType {
  
  typealias E0 = G0.Element
  typealias E1 = G1.Element
  
  typealias Element = (E0?, E1?)
  
  private var (g0, g1): (G0?, G1?)
  
  mutating func next() -> PaddedZipGenerator.Element? {
    let e0: E0? = g0?.next() ?? {g0 = nil; return nil}()
    let e1: E1? = g1?.next() ?? {g1 = nil; return nil}()
    return (e0 != nil || e1 != nil) ? (e0, e1) : nil
  }
}

struct PaddedZip<S0: SequenceType, S1: SequenceType> : SequenceType {
  
  typealias Generator = PaddedZipGenerator<S0.Generator, S1.Generator>
  
  private let (s0, s1): (S0, S1)
  
  func generate() -> PaddedZip.Generator {
    return PaddedZipGenerator(g0: s0.generate(), g1: s1.generate())
  }
}

func zipWithPadding<S0: SequenceType, S1: SequenceType>(s0: S0, _ s1: S1) -> PaddedZip<S0, S1> {
  return PaddedZip(s0: s0, s1: s1)
}