coder

coder.scala

class Coder(words: List[String]) {
  def ?? = error("not implemented" )
  
  private val mnemonics = Map(
      '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL", 
      '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")
      
  /** Invert the mnemonics map to give a map from chars 'A' ... 'Z' to '2' ... '9' */
  val charCode: Map[Char, Char] = 
      for( (k,v) <- mnemonics; letter <- v ) yield  (letter -> k)
          
  /** Maps a word to the digit string it can represent, e.g. “Java” -> “5282” */
  private def wordCode(word: String): String = word.toUpperCase.map(charCode)
  
  /** A map from digit strings to the words that represent them, 
    * e,g. “5282” -> Set(“Java”, “Kata”, “Lava”, ...) */
  private val wordsForNum: Map[String, List[String]] =
    words.groupBy(wordCode)

  /** Return all ways to encode a number as a list of words */
  def encode(number: String): Set[List[String]] = {
        if(number.isEmpty) Set(Nil)
        else {
          val splits = Range(0,number.length).toSet
          for{idx <- splits
              val (prefix, rest) = number.splitAt(idx)
              if wordsForNum.isDefinedAt(prefix)
                firstWord <- wordsForNum(prefix) 
              subSolution <- encode(rest)
    
          } yield firstWord :: subSolution
        }
      }
      
  /** Maps a number to a list of all word phrases that can represent it */
  def translate(number: String): Set[String] = encode(number) map (_ mkString " ")
}