Zip.swift

public struct Zip <Sequence1: Sequence, Sequence2: Sequence> {
  public struct ShortestSequence: Sequence {
    internal let sequence1: Sequence1
    internal let sequence2: Sequence2

    internal init (_ sequence1: Sequence1, _ sequence2: Sequence2) {
      (self.sequence1, self.sequence2) = (sequence1, sequence2)
    }

    public struct Iterator: IteratorProtocol {
      internal var iterator1: Sequence1.Iterator
      internal var iterator2: Sequence2.Iterator
      internal var reachedEnd = false

      internal init (_ iterator1: Sequence1.Iterator, _ iterator2: Sequence2.Iterator) {
        (self.iterator1, self.iterator2) = (iterator1, iterator2)
      }

      public mutating func next() -> (Sequence1.Iterator.Element, Sequence2.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 struct LongestSequence: Sequence {
    internal let sequence1: Sequence1
    internal let sequence2: Sequence2

    internal init (_ sequence1: Sequence1, _ sequence2: Sequence2) {
      (self.sequence1, self.sequence2) = (sequence1, sequence2)
    }

    public struct Iterator: IteratorProtocol {
      internal var iterator1: Sequence1.Iterator
      internal var iterator2: Sequence2.Iterator
      internal var reachedEnd = false

      internal init (_ iterator1: Sequence1.Iterator, _ iterator2: Sequence2.Iterator) {
        (self.iterator1, self.iterator2) = (iterator1, iterator2)
      }

      public mutating func next() -> (Sequence1.Iterator.Element?, Sequence2.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 func zip <Sequence1: Sequence, Sequence2: Sequence> (_ sequence1: Sequence1, _ sequence2: Sequence2) ->
    Zip<Sequence1, Sequence2>.ShortestSequence {
  return Zip.ShortestSequence(sequence1, sequence2)
}

public func zipLongest <Sequence1: Sequence, Sequence2: Sequence> (_ sequence1: Sequence1, _ sequence2: Sequence2) ->
    Zip<Sequence1, Sequence2>.LongestSequence {
  return Zip.LongestSequence(sequence1, sequence2)
}

extension Sequence {
  public func unzip <Type1, Type2> () -> ([Type1], [Type2]) where Self.Element == (Type1, Type2) {
    return self.reduce(into: ([Type1](), [Type2]())) { (result, element) in
      result.0.append(element.0)
      result.1.append(element.1)
    }
  }
}

@available(*, deprecated, renamed: "Zip.ShortestSequence")
public typealias Zip2Sequence<Sequence1, Sequence2> = Zip<Sequence1, Sequence2>.ShortestSequence where
  Sequence1: Sequence, Sequence2: Sequence
@available(*, deprecated, renamed: "Zip.ShortestSequence.Iterator")
public typealias Zip2Iterator<Sequence1, Sequence2> = Zip<Sequence1, Sequence2>.ShortestSequence.Iterator where
  Sequence1: Sequence, Sequence2: Sequence