gistfile1.scala

import scala.annotation.tailrec

object Crap extends App {

  val BASE = 16
  val DIGITS = "0123456789ABCDEF"

  // Original
  def encode0(number: BigInt): String = {
    require(number > 0, "number must not be negative")

    val result = new StringBuilder()
    var num = number
    while (num > 0) {
      val (value, rem) = num /% BASE
      num = value
      result.insert(0, DIGITS.charAt(rem.intValue()))
    }

    if (result.isEmpty) DIGITS.substring(0, 1) else result.toString()
  }

  // With direct tail recursion
  def encode1(num: BigInt): String = {

    @tailrec
    def helper(num: BigInt, acc: String): String =
      if (num <= 0) acc
      else {
        val (value, rem) = num /% BASE
        helper(value, DIGITS(rem.intValue) + acc)
      }

    require(num > 0, "number must not be negative")
    if (num == 0) DIGITS.substring(0, 1)
    else helper(num, "")

  }

  // With unfold

  def unfold[A, B](a: A)(f: A => Option[(A, B)]): Stream[B] =
    f(a).map { case (a, b) => b #:: unfold(a)(f) }.getOrElse(Stream.empty)

  def encode2(num: BigInt): String = {
    require(num > 0, "number must not be negative")
    if (num == 0) DIGITS.substring(0, 1)
    else unfold(num) { n =>
      if (n <= 0) None
      else Some {
        val (value, rem) = n /% BASE
        (value, DIGITS(rem.intValue))
      }
    }.reverse.mkString
  }

  println(encode0(123456789))
  println(encode1(123456789))
  println(encode2(123456789))

}