/*
 *  Primitive Collections for Java.
 *  Copyright (C) 2003  Søren Bak
 *
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
import bak.pcj.list.DoubleStack;
import bak.pcj.list.DoubleArrayStack;

/**
 *  This class implements an evaluator for postfix notation
 *  expressions. The class demonstrates the use of primitive
 *  stacks.
 *
 *  @author     Søren Bak
 *  @version    1.1     2003/17/2
 */
public class Calculator {

    // ---------------------------------------------------------------
    //      Tokenization
    // ---------------------------------------------------------------

    //  Lexemes for tokens
    private static final int NUM        = 0;
    private static final int MUL        = 1;
    private static final int DIV        = 2;
    private static final int ADD        = 3;
    private static final int SUB        = 4;
    private static final int END        = 5;

    //  Thrown to indicate a malformed expression
    private static class ParserException extends Exception {
        ParserException(String s) {
            super(s);
        }
    }

    //  Tokenizer for expressions
    private static class Tokenizer {
        private char nextChar;
        private int nextToken;
        private double nextValue;
        private int ptr;
        private String input;
        StringBuffer num = new StringBuffer();

        Tokenizer(String input) throws ParserException {
            setInput(input);
        }

        void setInput(String input) throws ParserException {
            this.input = input;
            ptr = 0;
            nextChar();
            advance();
        }

        //  Advance to next character in input
        private void nextChar() {
            nextChar = ptr == input.length() ? '\0' : input.charAt(ptr++);
        }

        //  Skip whitespace from input
        private void skipws() {
            while (nextChar == ' ' || nextChar == '\t')
                nextChar();
        }

        //  Advance to next token in input
        void advance() throws ParserException {
            skipws();
            if (nextChar == '*') {
                nextChar();
                nextToken = MUL;
            } else if (nextChar == '/') {
                nextChar();
                nextToken = DIV;
            } else if (nextChar == '+') {
                nextChar();
                nextToken = ADD;
            } else if (nextChar == '-') {
                nextChar();
                nextToken = SUB;
            } else if (nextChar >= '0' && nextChar <= '9') {
                num.setLength(0);
                num.append(nextChar);
                nextChar();
                while ((nextChar >= '0' && nextChar <= '9') || nextChar == '.') {
                    num.append(nextChar);
                    nextChar();
                }
                try {
                    nextValue = Double.parseDouble(num.toString());
                    nextToken = NUM;
                } catch (NumberFormatException e) {
                    throw new ParserException("Illegal number: " + num.toString());
                }
            } else if (nextChar == '\0') {
                nextToken = END;
            } else {
                throw new ParserException("Illegal character: " + nextChar);
            }
        }

        //  Returns next token in input
        int next() 
        { return nextToken; }

        //  Returns double value of next NUM token in input
        double value() 
        { return nextValue; }
        
    }

    // ---------------------------------------------------------------
    //      Evaluation
    // ---------------------------------------------------------------

    /**
     *  Evaluates a postfix expression.
     *
     *  @param      input
     *              a character sequence representation of the
     *              expression to evaluate.
     *
     *  @return     the value of the expression.
     *
     *  @throws     NullPointerException
     *              if <tt>input</tt> is <tt>null</tt>.
     *
     *  @throws     ParserException
     *              if the expression is malformed.
     */
    public double evaluate(String input) throws ParserException {
        return evaluate(new Tokenizer(input));
    }

    private double evaluate(Tokenizer lex) throws ParserException {
        try {
            DoubleStack stack = new DoubleArrayStack();
            int token;
            while ((token = lex.next()) != END) {
                if (token == NUM)
                    stack.push(lex.value());
                else if (token == MUL) {
                    double v2 = stack.pop();
                    double v1 = stack.pop();
                    stack.push(v1*v2);
                } else if (token == DIV) {
                    double v2 = stack.pop();
                    double v1 = stack.pop();
                    stack.push(v1/v2);
                } else if (token == ADD) {
                    double v2 = stack.pop();
                    double v1 = stack.pop();
                    stack.push(v1+v2);
                } else if (token == SUB) {
                    double v2 = stack.pop();
                    double v1 = stack.pop();
                    stack.push(v1-v2);
                }
                lex.advance();
            }
            double result = stack.pop();
            if (!stack.isEmpty())
                throw new ParserException("Stack is not empty");
            return result;
        } catch (IndexOutOfBoundsException e) { //  Thrown when stack is empty
            throw new ParserException("Stack was empty");
        }
    }

    public static void main(String[] args) {
        try {
            Calculator calc = new Calculator();
            System.out.println(calc.evaluate(args[0]));
        } catch (ParserException pe) {
            System.out.println("Syntax error: " + pe.getMessage());
        } catch (ArithmeticException ae) {
            System.out.println("Arithmetic error: " + ae.getMessage());
        }
    }

}