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package org.scalacheck.rng

/**
 * Simple RNG by Bob Jenkins:
 *
 * http://burtleburtle.net/bob/rand/smallprng.html
 */
sealed abstract class Seed {
  protected val a: Long
  protected val b: Long
  protected val c: Long
  protected val d: Long

  /** Generate the next seed in the RNG's sequence. */
  def next: Seed = {
    import java.lang.Long.rotateLeft
    val e = a - rotateLeft(b, 7)
    val a1 = b ^ rotateLeft(c, 13)
    val b1 = c + rotateLeft(d, 37)
    val c1 = d + e
    val d1 = e + a
    Seed(a1, b1, c1, d1)
  }

  /** Reseed the RNG using the given Long value. */
  def reseed(n: Long): Seed = {
    val n0 = ((n >>> 32) & 0xffffffff)
    val n1 = (n & 0xffffffff)
    var i = 0
    var seed: Seed = Seed(a ^ n0, b ^ n1, c, d)
    while(i < 16) { seed = seed.next; i += 1 }
    seed
  }

  /**
   * Generates a Long value.
   *
   * The values will be uniformly distributed. */
  def long: (Long, Seed) = (d, next)

  /**
   * Generates a Double value.
   *
   * The values will be uniformly distributed, and will be contained
   * in the interval [0.0, 1.0). */
  def double: (Double, Seed) = ((d >>> 11) * 1.1102230246251565e-16, next)
}

object Seed {

  private case class apply(a: Long, b: Long, c: Long, d: Long) extends Seed

  /** Generate a deterministic seed. */
  def apply(s: Long): Seed = {
    var i = 0
    var seed: Seed = Seed(0xf1ea5eed, s, s, s)
    while (i < 20) { seed = seed.next; i += 1 }
    seed
  }

  /** Generate a random seed. */
  def random(): Seed = apply(scala.util.Random.nextLong)

}