tailrec.scala

import scala.annotation.tailrec

def recursiveSum(f: Int => Int)(a: Int, b: Int): Int = {
  if (a > b)
    0
  else
    f(a) + recursiveSum(f)(a + 1, b)
}

def tailRecursiveSum(f: Int => Int)(a: Int, b: Int): Int = {

  @tailrec
  def loop(curr: Int, acc: Int): Int = {
    if (curr > b)
      acc
    else
      loop(curr + 1, acc + f(curr))
  }

  loop(a, 0)
}

def recursiveProduct(f: Int => Int)(a: Int, b: Int): Int = {
  if (a > b)
    1
  else
    f(a) * recursiveProduct(f)(a + 1, b)
}

def tailRecursiveProduct(f: Int => Int)(a: Int, b: Int) = {

  @tailrec
  def loop(curr: Int, acc: Int): Int = {
    if (curr > b)
      acc
    else
      loop(curr + 1, acc * f(curr))
  }

  loop(a, 1)
}

recursiveSum((x: Int) => x)(1, 10)
tailRecursiveSum((x: Int) => x)(1, 10)

recursiveSum((x: Int) => x * x)(1, 10)
tailRecursiveSum((x: Int) => x * x)(1, 10)

recursiveProduct((x: Int) => x)(1, 4)
tailRecursiveProduct((x: Int) => x)(1, 4)

recursiveProduct((x: Int) => x * x)(1, 4)
tailRecursiveProduct((x: Int) => x * x)(1, 4)