A simple CSV parser

Application.scala

object Application {
  case class CsvLine(values: List[String] = Nil, inQuote: Boolean = false, buff: List[Char] = Nil)

  @tailrec
  def parseCsvLine(line: CsvLine, remains: List[Char]): List[String] = (line, remains) match {
    case (CsvLine(values, false, buff), Nil) =>
      values :+ buff.mkString

    case (CsvLine(values, true, buff), '"' :: Nil) =>
      values :+ buff.mkString

    case (CsvLine(values, false, buff), ',' :: tail) =>
      parseCsvLine(CsvLine(values :+ buff.mkString, inQuote = false, Nil), tail)

    case (CsvLine(values, false, Nil), '"' :: tail) =>
      parseCsvLine(CsvLine(values, inQuote = true, Nil), tail)

    case (CsvLine(values, true, buff), '"' :: ',' :: tail) =>
      parseCsvLine(CsvLine(values :+ buff.mkString, inQuote = false, Nil), tail)

    case (CsvLine(values, true, buff), '"' :: '"' :: tail) =>
      parseCsvLine(CsvLine(values, inQuote = true, buff :+ '"'), tail)

    case (CsvLine(values, inQuote, buff), c :: tail) =>
      parseCsvLine(CsvLine(values, inQuote, buff :+ c), tail)

    case v =>
      throw new Error(s"error state: $v")
  }

  def processString(input: String): List[String] = {
    parseCsvLine(CsvLine(), input.toList)
  }
}

ApplicationSpec.scala

import org.scalatest.{Matchers, WordSpec}

class ApplicationSpec extends WordSpec with Matchers {
  "Application.processString" should {
    "process csv line correctly" in {
      Application.processString("a,b,c") shouldEqual Seq("a", "b", "c")
      Application.processString("aa,bb,cc") shouldEqual Seq("aa", "bb", "cc")
      Application.processString("aa,,cc") shouldEqual Seq("aa", "", "cc")
      Application.processString("aa,,") shouldEqual Seq("aa", "", "")
      Application.processString(",,") shouldEqual Seq("", "", "")
      Application.processString("") shouldEqual Seq("")

      Application.processString("\"a\",b,c") shouldEqual Seq("a", "b", "c")
      Application.processString("\"a\"\"\",b,c") shouldEqual Seq("a\"", "b", "c")

      Application.processString("""a,"b, d",c""") shouldEqual Seq("a", "b, d", "c")

      Application.processString("""a,"b, "","" d",c""") shouldEqual Seq("a", "b, \",\" d", "c")

      Application.processString("""a,b,"c"""") shouldEqual Seq("a", "b", "c")
      Application.processString("""a,b,"c, """") shouldEqual Seq("a", "b", "c, ")
      Application.processString("""a,b,"c,"" """") shouldEqual Seq("a", "b", "c,\" ")
    }
  }
}