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

/* FractionCalculator.java
** 
** Author: R. McCloskey
** Date: November 2018
**
** Java application that provides a simple fractions calculator.
** That is, in response to the user entering an expression such as 
**
**                       3/8 + 15/35
**
** it will evaluate the expression and display the result.  This is
** repeated until the user enters "quit".
**
** Only expressions of this form (i.e., two fractions separated by an
** operator) are handled.  Supported operators are +, -, *, /, and =,
** the last being the relational "equals" operator.  In the case of
** the = operator, the displayed result is 0/1 if the expression 
** evaluates to true and 1/1 if the expression evaluates to false.
** Spaces may not appear within a fraction, but the operator must be 
** preceded and followed by at least one space.
**
** To make the program read from a file (rather than the keyboard),
** provide a command-line argument (or what jGrasp calls a "run argument")
** that is the name of that file.
*/


public class FractionCalculator {

   private static final char ADD_OP = '+';
   private static final char SUBTRACT_OP = '-';
   private static final char MULTIPLY_OP = '*';
   private static final char DIVIDE_OP = '/';
   private static final char EQUALS_OP = '=';
   private static final Fraction ZERO = new Fraction(0,1);
   private static final Fraction ONE = new Fraction(1,1);

   private static Scanner input;
   private static boolean echo;

   public static void main(String[] args) {
      establishScanner(args);
      System.out.println("Welcome to the Fractions Calculator!");

      boolean keepGoing = true;
      while (keepGoing) {
         System.out.print("\n> ");
         String line = input.nextLine().trim();
         if (echo) { System.out.println(line); }
         if (line.length() == 0) {
            // ignore a blank line
         }
         else if (line.toLowerCase().equals("quit")) {
            keepGoing = false;
         }
         else {
            try {
               System.out.println(evaluate(line));
            }
            catch (Exception e) {
               e.printStackTrace(System.out);
            }
         }
      }
      System.out.println("Goodbye.");
   }

   /* Evaluates the given expression and returns the result, in the
   ** form of a String.
   ** An IllegalArgumentException is thrown if it is determined that
   ** the given expression is malformed.
   */
   public static String evaluate(String expr) {
      Scanner exprScanner = new Scanner(expr);
      String frac1Str = exprScanner.next(); 
      String operatorStr = exprScanner.next();
      if (operatorStr.length() != 1) {
         throw new IllegalArgumentException("Operator is invalid");
      }
      char operator = operatorStr.charAt(0);

      String frac2Str = exprScanner.next(); 

      Fraction frac1 = new Fraction(frac1Str);
      Fraction frac2 = new Fraction(frac2Str);

      String result;
      if (operator == EQUALS_OP) {
         if (frac1.equals(frac2)) { result = "true"; }
         else { result = "false"; }
      }
      else {  // operator is something other than =
         if (operator == ADD_OP) {
            frac1.addTo(frac2);  result = frac1.toString();
         }
         else if (operator == SUBTRACT_OP) {
            frac1.subtractFrom(frac2);  result = frac1.toString();
         }
         else if (operator == MULTIPLY_OP) {
            frac1.multiplyBy(frac2);  result = frac1.toString();
         }
         else if (operator == DIVIDE_OP) {
            frac1.divideBy(frac2);  result = frac1.toString();
         }
         else {
            throw new IllegalArgumentException("Invalid operator.");
         }
      } 
      return result;
   }

   /* Establishes the source of input, which is the file whose
   ** name is args[0] unless args has length zero, in which case
   ** it is the keyboard.
   */
   private static void establishScanner(String[] args) {
      if (args.length == 0) {
         input = new Scanner(System.in); 
         echo = false;
      }
      else {
         String fileName = args[0];
         try {
            input = new Scanner(new File(fileName));
         }
         catch (FileNotFoundException e) {
            System.out.printf("No file named \"%s\"; program aborting.\n",
                              fileName); 
            System.exit(0);
         }
         echo = true;
      }
   }
   
}