Straightforward implementation of CodingGame ASCII art problem in Scala

codingame-asciiart.scala

object Solution extends App {
    
    def readLetters(width: Int, height: Int): Array[Array[String]] = {
      val chars = for (i <- 0 until height) yield readLine.grouped(width).toArray
      chars.toArray
    }

    def letterIndex(c: Char): Int = if (c < 'A' || c > 'Z') 26 else c - 'A'

    val l = readInt
    val h = readInt
    val t = readLine
    
    // By (line)(char-index)
    val letters = readLetters(l, h)

    val message = for (line <- 0 until h) yield t.foldLeft("") { (prefix, letter) =>
                    prefix + letters(line)(letterIndex(letter.toUpper))
                  }

    message foreach(line => println(line.mkString))
}

One thing that I would change for production code would be to avoid building up a string in this manner, and instead build up letter chunks. Doing it this way made it a simple foldLeft and there was no extra work to keep track where the newlines go.