Various ways to implement factorial in Scala

factorial.scala

#!/bin/sh
exec scala "$0" "$@"
!#
// vim: set ts=2 sw=2 et fileencoding=utf-8:

import java.lang.reflect.Method
import scala.annotation.tailrec
import scala.language.postfixOps

/**
 * Various ways to implement factorial in Scala
 */
object Factorial {
  val runnableNameRX = "^factorial\\d+$".r pattern


  def isRunnableName(f: Method) = runnableNameRX matcher(f getName) matches

  def methods = this.getClass getMethods

  def methodsToRun = methods filter isRunnableName

  def methodName(f: Method) = f getName

  //////////////////////////////////////////////////////////////////////////////

  def factorial0(n: Int): Int = {
    var a: Int = 1;
    var i: Int = 2;
    while (i <= n) {
      a *= i;
      i += 1;
    }
    return a;
  }

  def factorial1(n: Int) = {
    @tailrec
    def inner(n: Int, retval: Int = 1): Int = n match {
      case i if i < 2 ⇒
        retval
      case i ⇒
        inner(i - 1, i * retval)
    }
    inner(n)
  }

  def factorial2(n: Int) = Range(2, n + 1).foldLeft(1)((a, v) ⇒ a * v)

  def factorial3(n: Int) = (1 /: (2 to n)) (_ * _)

  def factorial4(n: Int) = 1 to n product

  //////////////////////////////////////

  implicit class IntDecorator(val n: Int) {
    def ! = factorial4(n)
  }

  def factorial5(n: Int) = n!

  def factorial6(n: Int) = IntDecorator(n).!

  //////////////////////////////////////////////////////////////////////////////

  def main(args: Array[String]): Unit = {
    val fact_n = if (args.length > 0) args(0) toInt else 14

    (methodsToRun sortBy methodName) foreach { f ⇒
      val result = f.invoke(this, fact_n.asInstanceOf[AnyRef])
      println(s"${f getName}($fact_n) == $result")
    }
  }
}