[swift-users] Set-uniqueness of instances with distinct hashValue

[milos at milos-and-slavica.net]

Given an array of instances of a `Hashable` value type, all equal 
according to `Equatable` protocol, but with distinct `hashValue`s, I 
would expect that initialising a set with that array would preserve all 
the instances. Instead, running the code below in an iOS playground on 
Xcode 8.0 (8A218a), results in a behaviour that I cannot explain. If 
anyone can, I’d be very grateful!

     struct X: Hashable {
         let x: Int
         let hashValue: Int

         static func == (lhs: X, rhs: X) -> Bool {
             return lhs.x == rhs.x
         }
     }

     let array: [X] = (1...100).map{ X(x: 7, hashValue: $0) } // unique 
hash values
     array.count //→ 100

     let set = Set(array)
     set.count //→ 43

     let set2 = Set(Array(set))
     set2.count //→ 30

     let set3 = Set(Array(set2))
     set3.count //→ 30

     set3.sorted{ $0.hashValue < $1.hashValue }
         .forEach{ print(String($0.hashValue, radix: 2), "=", 
$0.hashValue) } /*→
            1 = 1
           11 = 3
          101 = 5
         1111 = 15
        10010 = 18
        10011 = 19
        11000 = 24
        11101 = 29
        11111 = 31
       100000 = 32
       100011 = 35
       100100 = 36
       100110 = 38
       101000 = 40
       101001 = 41
       101011 = 43
       101111 = 47
       110010 = 50
       110101 = 53
       111101 = 61
      1000010 = 66
      1001011 = 75
      1001101 = 77
      1001111 = 79
      1010000 = 80
      1011001 = 89
      1011100 = 92
      1011110 = 94
      1011111 = 95
      1100010 = 98
     */

Many thanks,

milos