// first test 

val ls1_urban = List(("1", "a"), ("2", "a"), ("1", "c"), ("2", "a"), ("1", "c"))

val m_urban = groupById(ls1_urban, Map())

assert(m_urban.getOrElse("1", Nil).count(_ == "c") == 2)

assert(m_urban.getOrElse("1", Nil).count(_ == "a") == 1)

assert(m_urban.getOrElse("2", Nil) == List("a", "a"))

// second test

def urban_gb(ratings: List[(String, String)]) = 

  ratings.groupBy(_._1).mapValues(_.map(_._2).toSet) 

def urban_gb2(ratings: List[(String, String)]) = 

  ratings.groupBy(_._1).mapValues(_.map(_._2)) 

def urban_ck(ratings: List[(String, String)]) =

  urban_gb(ratings) == groupById(ratings, Map()).mapValues(_.toSet)

val ls2_urban = List(("1", "a"), ("1", "b"), ("2", "x"), ("3", "a"), ("2", "y"), ("3", "c"))

assert(urban_ck(ls2_urban))

// thrird test

val ls3_urban = (1 to 1000).map(_.toString).toList

val ls4_urban = ls3_urban zip ls3_urban.tail

val ls5_urban = ls4_urban ::: ls4_urban.reverse

assert(urban_ck(ls5_urban))

/*

import io.Source

import scala.util._

def urban_get_csv_file(name: String) : List[String] = {

  val csv = Source.fromFile(name)

  csv.mkString.split("\n").toList.drop(1)

}

def urban_process_ratings(lines: List[String]) : List[(String, String)] = {

  for (cols <- lines.map(_.split(",").toList); 

       if (cols(2).toFloat >= 4)) yield (cols(0), cols(1))  

}

val urban_ratings = urban_process_ratings(urban_get_csv_file("ratings.csv").take(1000))

assert(urban_ck(urban_ratings))

*/