Reusable Swift Extension to Make Any Class Hashable

ObjectIdentifierHashable.swift

protocol ObjectIdentifierHashable : class, Hashable {
}

extension ObjectIdentifierHashable {
  static func ==(lhs: Self, rhs: Self) -> Bool {
        return lhs === rhs
  }  

  public func hash(into hasher: inout Hasher) {
      hasher.combine(ObjectIdentifier(self))
  }
}

Tree.swift

class Tree : ObjectIdentifierHashable, CustomStringConvertible,  {
  var text: String
  let left: Tree?
  let right: Tree?
  var link: Tree?
  init(text: String, left: Tree? = nil, right: Tree? = nil) {
    self.text = text
    self.left = left
    self.right = right
  }
  var description: String {
    let leftDesc = left != nil ? left!.text : "null"
    let rightDesc = right != nil ? right!.text : "null"
    let linkDesc = link != nil ? link!.text : "null"
    return "text: \(text), left: \(leftDesc), right: \(rightDesc), link: \(linkDesc)"
  }
}

func copyWithLinks(_ tree: Tree?) -> Tree? {
  var copiedNodes: [Tree:Tree] = [:] // note how we use Tree as a Key ;)
  func copy(_ oldTree: Tree?) -> Tree? {
    if let oldTree = oldTree {
      let newTree = Tree(text: "c_" + oldTree.text, left: copy(oldTree.left), right: copy(oldTree.right))
      copiedNodes[oldTree] = newTree
      return newTree
    } else {
      return nil
    }
  }
  let treeCopy = copy(tree)
  for (oldTree, newTree) in copiedNodes {
    if let oldLink = oldTree.link {
      newTree.link = copiedNodes[oldLink]
    }
  }
  return treeCopy
}