unzip collection

unzip2collection.swift

/// Provides a view of a collection of tuples as multiple collections.
public func unzip<C, TA, TB>(_ collection: C) -> Unzip2Collection<C, TA, TB> where C: Collection {
  return Unzip2Collection(collection)
}

/// Provides a view of a collection of tuples as multiple collections.
public struct Unzip2Collection<C, TupleA, TupleB> where C: Collection, C.Iterator.Element == (TupleA, TupleB) {
  private let _col: C
  public init(_ c: C) { _col = c }

  public struct FirstCollection: Collection {
    fileprivate let _col: C
    public var count: Int { return _col.count }
    public var startIndex: C.Index    { return _col.startIndex }
    public var endIndex: C.Index      { return _col.endIndex }
    public func index(after i: C.Index) -> C.Index { return _col.index(after: i) }
    public func index(_ i: C.Index, offsetBy n: Int) -> C.Index { return _col.index(i, offsetBy: n) }
    public subscript(index: C.Index) -> TupleA     { return _col[index].0 }
  }
  public struct SecondCollection: Collection {
    fileprivate let _col: C
    public var count: Int { return _col.count }
    public var startIndex: C.Index    { return _col.startIndex }
    public var endIndex: C.Index      { return _col.endIndex }
    public func index(after i: C.Index) -> C.Index { return _col.index(after: i) }
    public func index(_ i: C.Index, offsetBy n: Int) -> C.Index { return _col.index(i, offsetBy: n) }
    public subscript(index: C.Index) -> TupleB     { return _col[index].1 }
  }

  public var first:  FirstCollection  { return FirstCollection(_col: _col)  }
  public var second: SecondCollection { return SecondCollection(_col: _col) }
}

extension Unzip2Collection.FirstCollection: BidirectionalCollection where C: BidirectionalCollection {
  public func index(before i: C.Index) -> C.Index {
    return _col.index(before: i)
  }
}
extension Unzip2Collection.SecondCollection: BidirectionalCollection where C: BidirectionalCollection {
  public func index(before i: C.Index) -> C.Index {
    return _col.index(before: i)
  }
}

extension Unzip2Collection.FirstCollection: RandomAccessCollection where C: RandomAccessCollection {
}
extension Unzip2Collection.SecondCollection: RandomAccessCollection where C: RandomAccessCollection {
}