read a directory contents, parse the words in each file and store the result as a Map(word -> Set(documentName))

fileToMap.scala

import java.io.File
import scala.io.Source

def getFileTree(f: File): Stream[File] =
  f #:: (if (f.isDirectory) f.listFiles().toStream.flatMap(getFileTree)
  else Stream.empty)


def createIndex(path: String): Map[String, Set[String]] = {
  val f = new File(path)

  val fileWordPairs = for {
    file <- getFileTree(f) if file.isFile
    line <- Source fromFile file getLines()
    word <- line.split(" ")
  } yield (file.getName, word.trim)

  val index = fileWordPairs.foldLeft(Map.empty[String, Set[String]]) {
    case (acc, (k,v)) => acc + (v -> (acc.getOrElse(v,Set.empty[String] + k)))
  }
  index
}

def search(word: String, index: Map[String,List[String]]) = {
  index(word)
}