TheNextNumber.scala

object TheNextNumber extends App {

  def nextNumber(number: String): String = {

    @annotation.tailrec
    def cutSuffix(number: String, acc: List[Char]): (String, List[Char]) =
      if (number.isEmpty) ("0", acc)
      else if (number.last < acc.head) (number, acc)
      else cutSuffix(number.init, number.last :: acc)

    val (front, back) = cutSuffix(number.init, number.last :: Nil)
    val digit = back.filter(_ > front.last).min

    front.init + digit ++ (front.last :: back diff digit :: Nil).sorted.mkString
  }

  def process(lineIn: Iterator[String])(lineOut: String => Unit) =
    for (i <- 1 to lineIn.next().toInt) lineOut(s"Case #$i: ${nextNumber(lineIn.next())}")

  val writer = new java.io.PrintWriter("b.large.out")
  try {
    process(io.Source.fromFile("B-large-practice.in").getLines) { s =>
      writer.println(s)
      writer.flush()
    }
  } finally {
    writer.flush(); writer.close()
  }

}

TheNextNumberTest.scala

import org.scalatest._

import TheNextNumber._

class TheNextNumberTest extends FunSuite {

  test("sample #1") {
    assert(nextNumber("115")  === "151" )
  }
  
  test("sample #2") {
    assert(nextNumber("1051")  === "1105" )
  }
  
  test("sample #3") {
    assert(nextNumber("6233")  === "6323" )
  }
  
  test("sample case") {
    val input = """3
115
1051
6233""".lines

    val expected = """Case #1: 151
Case #2: 1105
Case #3: 6323""".lines

    lineComparison(input, expected)
  }

  test("full small case") {
    val input = io.Source.fromFile("B-small-practice.in").getLines()
    val expected = io.Source.fromFile("b.small.out.ref").getLines()

    lineComparison(input, expected)
  }

  test("full large case") {
    val input = io.Source.fromFile("B-large-practice.in").getLines()
    val expected = io.Source.fromFile("b.large.out.ref").getLines()

    lineComparison(input, expected)
  }

  def lineComparison(input: Iterator[String], expected: Iterator[String]) {
    process(input) { s =>
      for (line <- s.lines) assert(line.trim === expected.next().trim)
    }
    assert(expected.hasNext === false, "Finished too fast.")
  }

}