another code golf-type interview question

compress.scala

// compress a string by counting its repeating characters
def compress(a: String): String = a.foldLeft("") { (acc, l) =>
  acc.lastOption match {
    case Some(c) =>
      if (c == l) => {
        acc.slice(acc.lastIndexOf(l) - 1, acc.lastIndexOf(l)).headOption match {
          case Some(d) => c match {
            case l => if (Character.isDigit(d)) {
              s"${acc.slice(0, acc.lastIndexOf(l) - 1)}${Character.digit(d, 10) + 1}" :+ l // add 1 to repeating characters we have seen
            } else {
              s"${acc.slice(0, acc.lastIndexOf(l))}2" :+ l // initialize a character counter at 2 for an unseen repeating character
            }
          }
          case None => s"2${l}" // rare case: the first character of the input string repeats
        }
      } else acc :+ l // the last character of the accumulator is not equal to the current character, append it
    case None => l.toString // empty accumulator, we're just getting started!
  }
}

val result = compress("ttestttiiiinggggg") == "2tes3t4in5g"

println(s"${result}") // true