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I have a string "abbbbccdddd" and the function should return "a1b4c2d4".

This is what I have written in Scala.

Iterative-version

def compress(str: String) = {
 val chars = List(str).flatten.map(_.toString) ++ List(null)
 var result: String = ""
 var lookBack: String = chars.head
 var occurance: Int = 0
 chars.foreach { c =>
 if (c != lookBack) {
 result = result + lookBack.toString + occurance
 occurance = 0
 }
 occurance = occurance + 1
 lookBack = c
 }
 
 result
}

Recursive-version

def compress(str: String): String = {
 def compressHandler(str: String, lookBack: String, occurance: Int, result: String): String = {
 if(str.isEmpty) {
 result
 } else if(str.head.toString == lookBack) {
 compressHandler(str.drop(1), str.head.toString, occurance + 1, result)
 } else {
 compressHandler(str, str.head.toString, 0, result + lookBack + occurance)
 }
 }
 
 compressHandler(str + "0", str.head.toString, 0, "")
}

Scala — being a functional language should have much better solutions!

How to improve second (by somehow using map/reduce/fold) and how to do the first following concept of immutability (purely functional)?