Scala for impatients, chapter 2, 10th assignment

gistfile1.scala

def pow( x: Double, p: Int ): Double = { 
  if ( p == 1 ) x 
  else if ( p == 0 ) 1 
  else if ( p < 0 ) 1 / pow( x, -p ) 
  else if ( p % 2 == 0 ) { 
    val y = pow( x, p / 2) 
    y * y 
  } 
  else pow( x, p - 1) * x 
}