multiply then add numbers, in one pass

AddMulOnePass.scala

assert(eval("2+2*3+12+4*5*2+10*1") == 70)  

def eval(input: String): Int = {  
  input.splitEfficientlyOn("+").foldLeft(0)((n, str) => n + mul(str))
}

def mul(input: String): Int = {  
  input.splitEfficientlyOn("*").foldLeft(1)((n, x) => n * x.toInt)
}

implicit class StringCanSplitEfficiently(str: String) {
  def splitEfficientlyOn(delimiter: String) =
    new StringTokenizerIterator(new StringTokenizer(str, delimiter, false))

  final class StringTokenizerIterator(
    private val tokenizer: StringTokenizer
  ) extends Iterator[String] {
    def hasNext: Boolean = tokenizer.hasMoreTokens
    def next(): String = tokenizer.nextToken()
  }
}

AddMulSimple.scala

def eval(input: String): Int = {  
  input.split('+').map(mul).fold(0)(_ + _)  
}

def mul(input: String): Int = {  
  input.split('*').map(_.toInt).fold(1)(_ * _)  
}