[再帰ドリル](https://github.com/kazu-yamamoto/recursion-drill) を Scala で書いてみる．

DrillOne.scala

package recfun

object DrillOne {
  // sum of arithmetic progression
  def soap(x: Int): Int =
    if (x == 0) 0
    else soap(x - 1) + x

  // factorial
  def fact(x: Int): Int =
    if (x == 1) 1
    else fact(x - 1) * x

  // multiply
  def mul(m: Int, n: Int): Int =
    if (n == 1) m
    else mul(m, n - 1) + m

  // plus
  def plus(m: Int, n: Int): Int =
    if (n == 0) m
    else plus(m, n - 1) + 1

  // minus
  def minus(m: Int, n : Int): Int =
    if (n == 0) m
    else minus(m, n - 1) - 1

  // power
  def power(m: Int, n: Int): Int =
    if (n == 1) m
    else power(m, n - 1) * m
}

DrillThree.scala

package recfun

import scala.annotation.tailrec

object DrillThree {
  // less than
  @tailrec
  def lt(m: Int, n: Int): Boolean = (m, n) match {
      case (_, 0) => false
      case (0, _) => true
      case (m, n) => lt(m - 1, n - 1)
    }

  // less than or eqaul to
  @tailrec
  def lteq(m: Int, n: Int): Boolean = (m, n) match {
      case (0, _) => true
      case (_, 0) => false
      case (m, n) => lteq(m - 1, n - 1)
    }

  // even number
  @tailrec
  def isEven(n: Int): Boolean = n match {
      case 0 => true
      case 1 => false
      case n => isEven(n - 2)
    }

  // divide
  def divide(m: Int, n: Int): Int =
    if (m < n) 0
    else divide(m - n, n) + 1

  // greater than
  @tailrec
  def gt(m: Int, n: Int): Boolean = (m, n) match {
      case (0, _) => false
      case (_, 0) => true
      case (m, n) => gt(m - 1, n - 1)
    }

  // greater than or equal to
  @tailrec
  def gteq(m: Int, n: Int): Boolean = (m, n) match {
      case (_, 0) => true
      case (0, _) => false
      case (m, n) => gteq(m - 1, n - 1)
    }

  // odd number
  @tailrec
  def isOdd(n: Int): Boolean = n match {
      case 0 => false
      case 1 => true
      case n => isOdd(n - 2)
    }

  // remainder
  @tailrec
  def remainder(m: Int, n: Int): Int =
    if (m < n) m
    else remainder(m - n, n)

  // recursive version of divide
  def divide2(m: Int, n: Int): Int = {
    @tailrec
    def loop(acc: Int, m: Int): Int =
      if (m < n) acc
      else loop(acc + 1, m - n)
    loop(0, m)
  }
}

DrillTwo.scala

package recfun

import scala.annotation.tailrec

object DrillTwo {
  // sum of arithmetic progression
  def soap(n: Int): Int = {
    @tailrec
    def loop(acc: Int, n: Int): Int =
      if (n == 0) acc
      else loop(acc + n, n - 1)
    loop(0, n)
  }

  // factorial
  def fact(n: Int): Int = {
    @tailrec
    def loop(acc: Int, n: Int): Int =
      if (n == 1) acc
      else loop(acc * n, n - 1)
    loop(1, n)
  }

  // multiply
  def mul(m: Int, n: Int): Int = {
    @tailrec
    def loop(acc: Int, n: Int): Int =
      if (n == 0) acc
      else loop(acc + m, n - 1)
    loop(0, n)
  }

  // plus
  def plus(m: Int, n: Int): Int = {
    @tailrec
    def loop(acc: Int, n: Int): Int =
      if (n == 0) acc
      else loop(acc + 1, n - 1)
    loop(m, n)
  }

  // minus
  def minus(m: Int, n: Int): Int = {
    @tailrec
    def loop(acc: Int, n: Int): Int =
      if (n == 0) acc
      else loop(acc - 1, n - 1)
    loop(m, n)
  }

  // power
  def power(m: Int, n: Int): Int = {
    @tailrec
    def loop(acc: Int, n: Int): Int =
      if (n == 0) acc
      else loop(acc * m, n - 1)
    loop(1, n)
  }
}