Swift 4.2+ seeding a random number generator

Question

I\'m trying to generate seeded random numbers with Swift 4.2+, with the Int.random() function, however there is no given implementation that allows for the random n

Answer 1

Looks like Swift's implementation of RandomNumberGenerator.next(using:) changed in 2019. This affects Collection.randomElement(using:) and causes it to always return the first element if your generator's next()->UInt64 implementation doesn't produce values uniformly across the domain of UInt64. The GKRandom solution provided here is therefore problematic because it's next->Int method states:

     * The value is in the range of [INT32_MIN, INT32_MAX].

Here's a solution that works for me using the RNG in Swift's TensorFlow found here:

public struct ARC4RandomNumberGenerator: RandomNumberGenerator {
  var state: [UInt8] = Array(0...255)
  var iPos: UInt8 = 0
  var jPos: UInt8 = 0

  /// Initialize ARC4RandomNumberGenerator using an array of UInt8. The array
  /// must have length between 1 and 256 inclusive.
  public init(seed: [UInt8]) {
    precondition(seed.count > 0, "Length of seed must be positive")
    precondition(seed.count <= 256, "Length of seed must be at most 256")
    var j: UInt8 = 0
    for i: UInt8 in 0...255 {
      j &+= S(i) &+ seed[Int(i) % seed.count]
      swapAt(i, j)
    }
  }

  // Produce the next random UInt64 from the stream, and advance the internal
  // state.
  public mutating func next() -> UInt64 {
    var result: UInt64 = 0
    for _ in 0.. UInt8 {
    return state[Int(index)]
  }

  // Helper to swap elements of the state.
  private mutating func swapAt(_ i: UInt8, _ j: UInt8) {
    state.swapAt(Int(i), Int(j))
  }

  // Generates the next byte in the keystream.
  private mutating func nextByte() -> UInt8 {
    iPos &+= 1
    jPos &+= S(iPos)
    swapAt(iPos, jPos)
    return S(S(iPos) &+ S(jPos))
  }
}

Hat tip to my coworkers Samuel, Noah, and Stephen who helped me get to the bottom of this.