16.Answer

case class ParseError(stack: List[(Location, String)] = Nil):
  def push(loc: Location, msg: String): ParseError =
    copy(stack = (loc, msg) :: stack)

  def label(s: String): ParseError =
    ParseError(latestLoc.map((_, s)).toList)

  def latest: Option[(Location,String)] =
    stack.lastOption

  def latestLoc: Option[Location] =
    latest map (_._1)

  /**
  Display collapsed error stack - any adjacent stack elements with the
  same location are combined on one line. For the bottommost error, we
  display the full line, with a caret pointing to the column of the error.
  Example:

  1.1 file 'companies.json'; array
  5.1 object
  5.2 key-value
  5.10 ':'

  { "MSFT" ; 24,
           ^
  */
  override def toString =
    if stack.isEmpty then "no error message"
    else
      val collapsed = collapseStack(stack)
      val context =
        collapsed.lastOption.map("\n\n" + _._1.currentLine).getOrElse("") +
        collapsed.lastOption.map("\n" + _._1.columnCaret).getOrElse("")
      collapsed.map((loc, msg) => s"${formatLoc(loc)} $msg").mkString("\n") + context

  /* Builds a collapsed version of the given error stack -
   * messages at the same location have their messages merged,
   * separated by semicolons */
  def collapseStack(s: List[(Location, String)]): List[(Location, String)] =
    s.groupBy(_._1).
      view.
      mapValues(_.map(_._2).mkString("; ")).
      toList.sortBy(_._1.offset)

  def formatLoc(l: Location): String = s"${l.line}.${l.col}"