Tail recursive exponentiation by squaring

Pow.scala

def pow(x: Int, n: Int) = {
  def pow(y: Int, x: Int, n: Int): Int =
    n match {
      case 0 => 1
      case 1 => x * y
      case _ if n % 2 == 0 => pow(y, x * x, n / 2)
      case _ => pow(x * y, x * x, (n - 1) / 2)
    }

  if (n < 0) sys.error(s"pow: negative exponent: $n") else pow(1, x, n)
}