Zip2Collection.swift

struct Zip2Collection<C1: Collection, C2: Collection> : Collection {
  enum Index: Comparable {
    case index(C1.Index, C2.Index)
    case end
    
    static func <(lhs: Index, rhs: Index) -> Bool {
      switch (lhs, rhs) {
      case (.end, _): return false
      case (_, .end): return true
      case let (.index(l, _), .index(r, _)):
        return l < r
      }
    }
    
    static func ==(lhs: Index, rhs: Index) -> Bool {
      switch (lhs, rhs) {
      case (.end, .end): return true
      case (.end, _), (_, .end): return false
      case let (.index(l, _), .index(r, _)):
        return l == r
      }
    }
  }
  
  var startIndex: Index {
    return .index(c1.startIndex, c2.startIndex)
  }
  
  var endIndex: Index {
    return .end
  }
  
  subscript(index: Index) -> (C1.Element, C2.Element) {
    guard case let .index(i1, i2) = index else {
      fatalError("Can't subscript with endIndex")
    }
    return (c1[i1], c2[i2])
  }
  
  func index(after i: Index) -> Index {
    guard case let .index(i1, i2) = i else {
      fatalError("Can't advance beyond endIndex")
    }
    let (r1, r2) = (c1.index(after: i1), c2.index(after: i2))
    if r1 == c1.endIndex || r2 == c2.endIndex {
      return .end
    }
    return .index(r1, r2)
  }
  
  let c1: C1
  let c2: C2
}

func zip<C1, C2>(_ c1: C1, _ c2: C2) -> Zip2Collection<C1, C2> {
  return Zip2Collection(c1: c1, c2: c2)
}