Scala solution to TopCoder practice problem "Decipher": http://community.topcoder.com/stat?c=problem_statement&pm=4674&rd=7227

Decipher.scala

import scala.collection.SortedSet
import scala.collection.SortedMap

// Attempts to decipher an encripted message by matching the frequency of letters in the message vs an ordered frequency key 
object Decipher {
  // calculates a decoding table in the form (encoded->decoded)
  def codex(encoded:String, freq: String):Map[Char,Char] = {
    
    // Transforms the encoded string in a frequency map char->frequecy(int). Note that white spaces are removed
    val charFreqMap = encoded.filter(_!=' ').foldLeft(Map[Char,Int]())((x,y) => {val freq = x.getOrElse(y,0)+1; x + ((y,freq))})
    
    // Transforms the character frequency map into a Map of frequencies into the order set of chars with that frequency
    // E.g. 2->('A','B'), 3->'Z',...
    val freqMap = charFreqMap.foldLeft(Map[Int,SortedSet[Char]]())((map,elem) => elem match {case (k,v) => {val sameCharFreq = map.getOrElse(v,SortedSet[Char]()) + k; map + ((v, sameCharFreq))}})
    
    //Creates a sequence of characters ordered by their frequency in descendent order. 
    //Characters with the same frequency are ordered alphabetically (by virtue of the SortedSet they where collected into)
    val orderedByFrequency = freqMap.keys.toList.sortWith(_>_).map(freqMap).flatten
    
    //Returns a decoding map by matching the ordered-by-frequency char sequence with the provided key. 
    //Space support is added to this map
    (orderedByFrequency zip freq).toMap + (' ' -> ' ')
  }
    
  def decipher(encoded:Array[String], frequencyOrder:String):Array[String] = {
    // Creates a single string concatenating the String array provided. Used to calculate the decoding function
    val stringConcat = encoded.mkString("")
    
    // Creates a decoding map from the single string and the frequency key (also a string)
    val decoder = codex(stringConcat, frequencyOrder)
    
    // Decode every string in the input array with the calculated decoding map
    encoded.map(st => st.map(decoder))
  }
  
  val encoded = Array("RAZVLHAR KNW CNR", "HEA HNFMNSAR NFAK")
  decipher( encoded ,"EORTPNFHSCDIWG")  // res0: Array[String] = Array(REGISTER NOW FOR, THE TOPCODER OPEN)
  
}