Given a string representation of a number, returns the number of variations of said input that is divisible by 3

DivBy3.scala

/* Divisible by 3
   --------------

    Given a string representation of a number, returns the number of variations of said input that is divisible by 3. The variations consist of changing one digit for each variation. 
    For example, "21"'s variations are:
    
    01
    11
    21
    31
    41
    51
    61
    71
    81
    91
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    
    Of those, 
    
    21, 24, 27, 81, 51
    
    are divisible by 3. So, the method should return its cardinality or 5.
*/
object DivBy3 {
  def soln(s: String): Int = {
    val n = s.length
    (0 until n).foldLeft((0, Set.empty[String])) { case ((nDivs0, done0), i) =>
      val prefix = s.substring(0, i)
      val suffix = s.substring(i + 1, n)
      (0 to 9).foldLeft((nDivs0, done0)) { case ((nDivs, done), d) =>
        val int = prefix + d + suffix
        if (done(int))
          (nDivs, done)
        else {
          val nDivs1 = nDivs + (if (divisibleBy3(int)) 1 else 0)
          (nDivs1, done + int)
        }
      }
    }._1
  }

  private def divisibleBy3(int: String) = int.toInt % 3 == 0
}

DivBy3Spec.scala

import org.scalatest.flatspec.AnyFlatSpec

class DivBy3Spec extends AnyFlatSpec {
  "0081" should "11" in {
    assert(DivBy3.soln("0081") === 11)
  }
}