/**
 * CSI3540. Structures, techniques et normes du Web<br>
 * <br>
 * Uasage: <br>
 * In DrJava, System.out.println( RPN.eval( RPN.getRandomExpression() ) );<br>
 * From the command line:<br>
 * java RPN "9 3 / 10 2 3 * - +"
 *
 * @author Marcel Turcotte, Université d'Ottawa/University of Ottawa
 */

package test;

import java.io.IOException;
import java.io.StreamTokenizer;
import java.io.StringReader;
import java.util.Stack;
import java.util.Random;

public class RPN {
  
    public static int eval( String input ) throws IOException {
    
	Stack<Integer> s = new Stack<Integer>();
    
	StringReader sr = new StringReader( input );
	StreamTokenizer st = new StreamTokenizer( sr );
    
	st.eolIsSignificant( false ); // expressions may contain newlines
	st.ordinaryChar( '/' ); // should not be interpreted as a comment
    
	int token = st.nextToken();
	while ( token != StreamTokenizer.TT_EOF ) {
      
	    switch( token ) {
	    case StreamTokenizer.TT_NUMBER:
		s.push( (int) st.nval );
		break;
	    default:
		int r = s.pop();
		int l = s.pop();
		int res = 0;
          
		switch( st.ttype ) {
		case '+':
		    res = l + r;
		    break;
		case '-':
		    res = l - r;
		    break;
		case '*':
		    res = l * r;
		    break;
		case '/':
		    res = l / r;
		    break;
		}
		s.push( res );
	    }
	    token = st.nextToken();
	}
	return s.pop();
    }

    public static final int MAXIMUM_DEPTH = 2;
    public static final double THRESHOLD = 0.5;

    private static final Random random = new Random();

    public static String getRandomExpression() {
	return getRandomExpression( MAXIMUM_DEPTH ) + " " + getRandomExpression( MAXIMUM_DEPTH ) + " " + getRandomOperator();
    }

    private static String getRandomExpression( int depth ) {
	String expr;

	if ( depth == 0 || random.nextDouble() < THRESHOLD ) {
	    expr = getRandomTerminal();
	} else {
	    expr = getRandomExpression( depth-1 ) + " " + getRandomExpression( depth-1 ) + " " + getRandomOperator();
	}

	return expr;
    }

    private static String getRandomTerminal() {
	return Integer.toString( random.nextInt( 14 ) + 1 );
    }
  
    private static String getRandomOperator() {
	String[] ops = { "+", "-", "*", "/" };
	return ops[ random.nextInt( 3 ) ]; // avoids '/' so as to produce integer result
    }
  
    public static void main( String[] args ) throws IOException {
	System.out.println( args[ 0 ] + " = " + eval( args[ 0 ] ) );
	System.out.println( getRandomExpression() );
    }
}