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package scala.util

/** A utility object to support command line parsing for @main methods */
object CommandLineParser {

  /** An exception raised for an illegal command line
    *  @param idx  The index of the argument that's faulty (starting from 0)
    *  @param msg  The error message
    */
  class ParseError(val idx: Int, val msg: String) extends Exception

  /** Parse command line argument `s`, which has index `n`, as a value of type `T`
   *  @throws ParseError if argument cannot be converted to type `T`.
   */
  def parseString[T](str: String, n: Int)(using fs: FromString[T]): T = {
    try fs.fromString(str)
    catch {
      case ex: IllegalArgumentException => throw ParseError(n, ex.toString)
    }
  }

  /** Parse `n`'th argument in `args` (counting from 0) as a value of type `T`
   *  @throws ParseError if argument does not exist or cannot be converted to type `T`.
   */
  def parseArgument[T](args: Array[String], n: Int)(using fs: FromString[T]): T =
    if n < args.length then parseString(args(n), n)
    else throw ParseError(n, "more arguments expected")

  /** Parse all arguments from `n`'th one (counting from 0) as a list of values of type `T`
   *  @throws ParseError if some of the arguments cannot be converted to type `T`.
   */
  def parseRemainingArguments[T](args: Array[String], n: Int)(using fs: FromString[T]): List[T] =
    if n < args.length then parseString(args(n), n) :: parseRemainingArguments(args, n + 1)
    else Nil

  /** Print error message explaining given ParserError */
  def showError(err: ParseError): Unit = {
    val where =
      if err.idx == 0 then ""
      else if err.idx == 1 then " after first argument"
      else s" after ${err.idx} arguments"
    println(s"Illegal command line$where: ${err.msg}")
  }
}