public func zip <Sequence1: Sequence, Sequence2: Sequence> (_ sequence1: Sequence1, _ sequence2: Sequence2) ->
    AnySequence<(Sequence1.Element, Sequence2.Element)> {
  return AnySequence { () -> AnyIterator<(Sequence1.Element, Sequence2.Element)> in
    var iterator1 = sequence1.makeIterator()
    var iterator2 = sequence2.makeIterator()
    var reachedEnd = false

    return AnyIterator { () -> (Sequence1.Element, Sequence2.Element)? in
      if reachedEnd {
        return nil
      }

      guard let element1 = iterator1.next(), let element2 = iterator2.next() else {
        reachedEnd.toggle()

        return nil
      }

      return (element1, element2)
    }
  }
}

public func zipLongest <Sequence1: Sequence, Sequence2: Sequence> (_ sequence1: Sequence1, _ sequence2: Sequence2) ->
    AnySequence<(Sequence1.Element?, Sequence2.Element?)> {
  return AnySequence { () -> AnyIterator<(Sequence1.Element?, Sequence2.Element?)> in
    var iterator1 = sequence1.makeIterator()
    var iterator2 = sequence2.makeIterator()
    var reachedEnd = false

    return AnyIterator { () -> (Sequence1.Element?, Sequence2.Element?)? in
      if reachedEnd {
        return nil
      }

      let element1 = iterator1.next()
      let element2 = iterator2.next()

      if element1 == nil && element2 == nil {
        reachedEnd.toggle()

        return nil
      }

      return (element1, element2)
    }
  }
}

public func unzip <Sequence1: Sequence, Type1, Type2> (_ sequence: Sequence1) -> ([Type1], [Type2]) where
    Sequence1.Element == (Type1, Type2) {
  return sequence.reduce(into: ([Type1](), [Type2]())) {
    $0.0.append($1.0)
    $0.1.append($1.1)
  }
}

public typealias Zip2Sequence<Sequence1, Sequence2> = AnySequence<(Sequence1.Element, Sequence2.Element)> where
  Sequence1: Sequence, Sequence2: Sequence
public typealias Zip2Iterator<Sequence1, Sequence2> = AnySequence<(Sequence1.Element, Sequence2.Element)>.Iterator
  where Sequence1: Sequence, Sequence2: Sequence