import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;

/* Java application that is intended to test the methods in the
** PrefixExprEvaluator class.  It repeatedly reads lines of text
** from a file (if the name of a file is provided via command line
** argument) or (if not) from the keyboard.  Each line is tested to see
** if it is a syntactically valid arithmetic expression in prefix form.
** If it is, it is evaluated.
** When a blank line is provided as input, the program terminates.
**
** Author: R. McCloskey
** Date: Oct. 2024
*/
public class PrefixExprEvalApp {

   private static Scanner input;
   private static boolean echo;

   public static void main(String[] args) {
      establishScanner(args);
      boolean keepGoing = true;
      PrefixExprEvaluator evaluator = new PrefixExprEvaluator();
      do {
         String expr = getResponse("\n> ");
         if (expr.length() == 0) 
            { keepGoing = false; }
         else if (!evaluator.isValid(expr)) { 
            System.out.println("<---- That is not a valid prefix expression");
         }
         else {
            System.out.println(evaluator.valueOf(expr));
         }
      } while (keepGoing);
      System.out.println("Goodbye.");
   }

   /* Assigns to global variable 'input' a Scanner able to read from
   ** the file whose name is args[0], assuming that args.length != 0.
   ** Otherwise, makes 'input' able to read from standard input.
   ** Also sets 'echo' to true in the former case, false in the latter,
   ** so that input data read from a file will appear in the produced
   ** output.
   */
   private static void establishScanner(String[] args) {
      if (args.length != 0) {
         try {
            input = new Scanner(new File(args[0]));
            echo = true;
         }
         catch (FileNotFoundException e) {
            e.printStackTrace(System.out);
            System.out.println("Program aborting.");
            System.exit(0);
         }
      }
      else {
         input = new Scanner(System.in);
         echo = false;
      }
   }

   /* Prints the given prompt and returns the next line read in 
   ** by the input Scanner.
   */
   private static String getResponse(String prompt) {
      System.out.print(prompt);
      String result = input.nextLine().trim();
      if (echo) { System.out.println(result); }
      return result;
   }

}