HashSet.swift

public struct HashSet<T: Hashable> {
  private var dictionary = Dictionary<T, Bool>()
  
  public mutating func insert(element: T) {
    dictionary[element] = true
  }
  
  public mutating func remove(element: T) {
    dictionary[element] = nil
  }
  
  public func contains(element: T) -> Bool {
    return dictionary[element] != nil
  }
  
  public func allElements() -> [T] {
    return Array(dictionary.keys)
  }
  
  public var count: Int {
    return dictionary.count
  }
  
  public var isEmpty: Bool {
    return dictionary.isEmpty
  }
}

extension HashSet {
  public func union(otherSet: HashSet<T>) -> HashSet<T> {
    var combined = HashSet<T>()
    for obj in dictionary.keys {
      combined.insert(obj)
    }
    for obj in otherSet.dictionary.keys {
      combined.insert(obj)
    }
    return combined
  }

  public func intersect(otherSet: HashSet<T>) -> HashSet<T> {
    var common = HashSet<T>()
    for obj in dictionary.keys {
      if otherSet.contains(obj) {
        common.insert(obj)
      }
    }
    return common
  }

  public func difference(otherSet: HashSet<T>) -> HashSet<T> {
    var diff = HashSet<T>()
    for obj in dictionary.keys {
      if !otherSet.contains(obj) {
        diff.insert(obj)
      }
    }
    return diff
  }
}