PRNG.swift

// Algorithm from https://en.wikipedia.org/wiki/Permuted_congruential_generator

struct PRNG: RandomNumberGenerator {

  var state: UInt64 = 0x4d595df4d0f33173
  let multiplier: UInt64 = 6364136223846793005
  let increment: UInt64 = 1442695040888963407

  init(seed: UInt64) {
    state = seed &+ increment
    _ = next()
  }

  private func rotr32(x: UInt32, r: UInt32) -> UInt32 {
    (x &>> r) | x &<< ((~r &+ 1) & 31)
  }

  @inlinable mutating func next32() -> UInt32 {
    var x = state
    let count = UInt32(x &>> 59)
    state = x &* multiplier &+ increment
    x ^= x &>> 18
    return rotr32(x: UInt32(truncatingIfNeeded: x &>> 27),
                            r: count)
  }

  mutating func next() -> UInt64 {
    UInt64(next32()) << 32 | UInt64(next32())
  }
}

var randomSource = PRNG(seed: 1234)
                   // SystemRandomNumberGenerator()

for _ in 1...10 {
  print(Int.random(in: 1...10, using: &randomSource))
}

var prng2 = PRNG(seed: 12346)
for _ in 1...10 {
  print(Int.random(in: 1...10, using: &randomSource))
}