Generic primes generator

Primes.sc

package experiments

object Primes {

  class Generator[NUM](implicit numops: Integral[NUM]) {

    import annotation.tailrec
    import numops._

    private val two = one + one
    private val three = one + one + one

    private def sqrt(number: NUM) = { //https://issues.scala-lang.org/browse/SI-3739
      def next(n: NUM, i: NUM): NUM = (n + i / n) / two

      var n = one
      var n1 = next(n, number)

      while ((n1 - n).abs > one) {
        n = n1
        n1 = next(n, number)
      }

      while (n1 * n1 > number) {
        n1 -= one
      }

      n1
    }

    def isPrime(v: NUM): Boolean = {
      val upTo = sqrt(v)
      @tailrec
      def checkUpTo(from: NUM): Boolean = {
        if (v % from == 0) false
        else if (from == upTo) true else checkUpTo(from + one)
      }
      (v <= three) || checkUpTo(two)
    }

    def integers = {
      def next(cur: NUM): Stream[NUM] = cur #:: next(cur + one)
      next(one)
    }
    def candidates = integers.tail

    def primes =
      candidates
        .filter(isPrime(_))

  }


  def howlongfor[T](proc: => T): Tuple2[String, T] = {
    def now = System.currentTimeMillis
    now match {
      case start =>
        val result = proc
        (s"${now - start}ms", result)
    }
  }

  def intPrimes = (new Generator[Int]).primes
  def longPrimes = (new Generator[Long]).primes
  def bigIntPrimes = (new Generator[BigInt]).primes


  howlongfor(intPrimes.drop(25000).head)          // res0: (String, Int) = (459ms,287137)
  howlongfor(longPrimes.drop(25000).head)         // res1: (String, Long) = (1349ms,287137)
  howlongfor(bigIntPrimes.drop(25000).head)       // res2: (String, BigInt) = (4361ms,287137)
}