Add Ith Value, either early or late

addI.scala

object AddI {
def addIEarly(x: Int, i: Int): Int = {
  require(x >= 0, "addIEarly: x must be positive")
  require(i >= 0, "addIEarly: i must be positive")
  val reset = i - 1
  @scala.annotation.tailrec def go(x: Int, pos: Int, acc: Int): Int = {
    println(s"go: x-$x acc-$acc, pos:$pos")
    (x, pos) match {
      case (0, _) => acc
      case (other, 0) => go(other-1, reset, acc + other)
      case (other, pos) => go(other-1, pos -1 , acc)
    }
  }
  go(x, 0, 0)
}

addIEarly(3, 1) // Every 1 Should always equal Sum 
addIEarly(4, 2) // Add 4 + 2
addIEarly(10, 5) // 10 + 5
addIEarly(10, 2)

println()

def addILate(x: Int, i: Int): Int = {
  require(x >= 0, "addILate: x must be positive")
  require(i >= 0, "addILate: i must be positive")
  @scala.annotation.tailrec def go(x: Int, acc: Int, pos: Int): Int = {
     println(s"go: x-$x acc-$acc, pos:$pos")
     x match {
       case 0 => acc
       case other if (pos == i) => go(other-1, acc + other, 1)
       case other => go(other-1, acc, pos + 1)
     }
   }
  go(x, 0, 1)
}

addILate(3, 1) // Every 1 Should always equal Sum 
addILate(4, 2) // Add 3 + 1
addILate(10, 5) // 6 + 1
}