Pair.swift

public struct Pair <Type1, Type2> {
 public let first: Type1
 public let second: Type2

 public init (_ first: Type1, _ second: Type2) {
   (self.first, self.second) = (first, second)
 }
}

extension Pair {
 func fill <WrappedType1, WrappedType2> (_ first: WrappedType1, _ second: WrappedType2) -> Pair<WrappedType1, WrappedType2> where Type1 == Optional<WrappedType1>, Type2 == Optional<WrappedType2> {
   return Pair<WrappedType1, WrappedType2>(self.first ?? first, self.second ?? second)
 }
}

extension Pair where Type1: Sequence, Type2: Sequence  {
 public enum Zip {
   public struct LongestSequence: Sequence {
     internal let sequence1: Type1
     internal let sequence2: Type2

     internal init (_ sequence1: Type1, _ sequence2: Type2) {
       (self.sequence1, self.sequence2) = (sequence1, sequence2)
     }

     public struct Iterator: IteratorProtocol {
       internal var iterator1: Type1.Iterator
       internal var iterator2: Type2.Iterator
       internal var reachedEnd = false

       internal init (_ iterator1: Type1.Iterator, _ iterator2: Type2.Iterator) {
         (self.iterator1, self.iterator2) = (iterator1, iterator2)
       }

       public mutating func next() -> (Type1.Iterator.Element?, Type2.Iterator.Element?)? {
         if self.reachedEnd {
           return nil
         }

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

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

           return nil
         }

         return (element1, element2)
       }
     }

     public func makeIterator () -> Iterator {
       return Iterator(self.sequence1.makeIterator(), self.sequence2.makeIterator())
     }
   }

   public struct ShortestSequence: Sequence {
     internal let sequence1: Type1
     internal let sequence2: Type2

     internal init (_ sequence1: Type1, _ sequence2: Type2) {
       (self.sequence1, self.sequence2) = (sequence1, sequence2)
     }

     public struct Iterator: IteratorProtocol {
       internal var iterator1: Type1.Iterator
       internal var iterator2: Type2.Iterator
       internal var reachedEnd = false

       internal init (_ iterator1: Type1.Iterator, _ iterator2: Type2.Iterator) {
         (self.iterator1, self.iterator2) = (iterator1, iterator2)
       }

       public mutating func next() -> (Type1.Iterator.Element, Type2.Iterator.Element)? {
         if self.reachedEnd {
           return nil
         }

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

           return nil
         }

         return (element1, element2)
       }
     }

     public func makeIterator () -> Iterator {
       return Iterator(self.sequence1.makeIterator(), self.sequence2.makeIterator())
     }
   }
 }

 public var zip: (shortest: Zip.ShortestSequence, longest: Zip.LongestSequence) {
   return (shortest: Zip.ShortestSequence(self.first, self.second), longest: Zip.LongestSequence(self.first, self.second))
 }
}