Not thoroughly tested, needs update for some Random changes in Swift

xoshiroPRNG.swift

import Foundation

// http://xoshiro.di.unimi.it/xoshiro256starstar.c

protocol PseudoRandomNumberGenerator {
  associatedtype ReturnValue
  mutating func next() -> ReturnValue
}

struct Arc4Random: PseudoRandomNumberGenerator {
  typealias ReturnValue = UInt32

  mutating func next() -> UInt32 {
    return arc4random()
  }
}

struct XoShiro256: PseudoRandomNumberGenerator {
  typealias ReturnValue = UInt64
  private var seed: [UInt64]

  // initialize with a new seed
  init(withSeed seed: [UInt64]) {
    precondition(seed.count == 4)
    self.seed = seed
  }

  // initialize with arc4random seed values
  init() {
    seed = [
      UInt64(upper: arc4random(), lower: arc4random()),
      UInt64(upper: arc4random(), lower: arc4random()),
      UInt64(upper: arc4random(), lower: arc4random()),
      UInt64(upper: arc4random(), lower: arc4random())
    ]
  }

  mutating func next() -> UInt64 {
    let result_starstar = rotl(x: seed[1] * 5, k: 7) * 9
    let scrambler = seed[1] << 17;

    seed[2] ^= seed[0]
    seed[3] ^= seed[1]
    seed[1] ^= seed[2]
    seed[0] ^= seed[3]

    seed[2] ^= scrambler

    seed[3] = rotl(x: seed[3], k: 45)

    return result_starstar
  }

  // todo: implement jump

  @inline(__always)
  private func rotl(x: UInt64, k: UInt64) -> UInt64 {
    return (x << k) | (x >> (64 - k))
  }

  private static let jump_const: [UInt64] = [ 0x180ec6d33cfd0aba, 0xd5a61266f0c9392c, 0xa9582618e03fc9aa, 0x39abdc4529b1661c ]
}

extension UInt64 {
  // create a 64-bit integer from two 32-bit integers
  init(upper: UInt32, lower: UInt32) {
    self.init(upper << 32 | lower)
  }
}

var rand1 = XoShiro256()
print("xoshiro256")
print(rand1.next())
print(rand1.next())
print(rand1.next())

var rand2 = Arc4Random()
print("arc4random")
print(rand2.next())
print(rand2.next())
print(rand2.next())

// print(rand.next())
// print(rand.next())