single_byte.scala

def decodeSingleByteXOR(cipherText: String, char: Char): String = {
  val cipherTextByteArray = decodeHex(cipherText)
  new String(cipherTextByteArray.zip(Array.fill(cipherTextByteArray.length)(char)).map((t) => t._2 ^ t._1).map(_.toByte), "UTF-8")
}

def bruteForceSingleByteXOR(cipherText: String): Array[String] = {
  (('A' to 'Z') ++ ('a' to 'z') ++ ('0' to '9')).toArray.map(decodeSingleByteXOR(cipherText,_))
}

def decodeHex(hex: String): Array[Byte] = {
  hex.replaceAll("[^0-9A-Fa-f]", "").sliding(2,2).toArray.map(Integer.parseInt(_, 16).toByte)
}

def trigramStringScoring(text: String): Int = {
  val trigramRanks = Array("the", "and", "tha", "ent", "ing", "ion", "tio", "for", "nde", "has", "nce", "edt", "tis", "oft", "sth", "men")
  text.toLowerCase.sliding(3).foldLeft(0) { (memo, sub) =>
    val score = if (trigramRanks.indexOf(sub) > -1) trigramRanks.length - trigramRanks.indexOf(sub) else 0
    memo + score
  }
}

val source = scala.io.Source.fromFile("resources/mt4.txt")
source.getLines.toArray.flatMap(bruteForceSingleByteXOR).sortBy { (line) =>
  -trigramStringScoring(line)
}.take(2).foreach { (line) =>
  println(trigramStringScoring(line) + ": " + line)
}