options{
    //DEBUG_PARSER = true; DEBUG_TOKEN_MANAGER = true;
}

PARSER_BEGIN(Calculator)
import java.util.HashMap;

public class Calculator {

    private static HashMap variables = new HashMap();

    public static void main(String[] args) {
        Calculator parser = new Calculator(System.in);
        while (true){
            System.out.print("Type your expression: ");
            try{
                double value = parser.evaluateNextLine();
                System.out.println("Returned: " + value);
            }catch (ParseException e){
                System.out.println(e.getMessage());
                try{
                    parser.passStatement(); // wiping the tokens of the malformed expression
                }catch (ParseException ex){
                    System.out.println(ex.getMessage());
                }
            }
        }
    }
}
PARSER_END(Calculator)

SKIP : {
    " "|"\t"
}

TOKEN : {
	<EOL: "\n"|"\r">|
	<PAL: "(">|
	<PAR: ")">|
	<ADD: "+">|
	<SUB: "-">|
	<MUL: "*">|
	<DIV: "/">|
	<EQU: "=">|
	<LET: "let">|
	<END: "exit">|
    <NUM: (["0"-"9"])+("."((["0"-"9"])+))?>|
	<ID: (["a"-"z"]|["A"-"Z"])+>
}

double statement() :
{
    Token id;
    double i = 0.0;
}
{
    <END> { System.exit(0); }|
    i=expression() { return i; }|
    <LET> id=<ID> <EQU> i=expression()
        {
            variables.put(id.image,i); //Storing the variable
            return i;
        }
}

double expression() :
{
    Token op = null;
    double l = 0.0;
    double r = 0.0;
}
{
    l=term() ((op=<ADD> | op=<SUB>)  r=term()
        { // folding everything in the l variable
            if (op != null){
                if (op.image.equals("+"))
                    l += r;
                else
                    l -= r;
            }

        }
        )*
        { //returning the parsed value
            return l;
        }
}

double term() :
{
    Token op = null;
    double l = 0.0;
    double r = 0.0;
}
{
    l=factor() ((op=<DIV> | op=<MUL>)  r=factor()
        { // folding everything in the l variable
            if (op != null){
                if (op.image.equals("/"))
                    l /= r;
                else
                    l *= r;
            }

        }
        )*
        { //returning the parsed value
            return l;
        }
}

double factor() :
{
    Token t = null;
    Token id = null;
    double i = 0.0;
}
{
    (t=<NUM> | id=<ID> | <PAL> (i=expression())? <PAR>)
        {
            if(id != null){
                Object var = variables.get(id.image);
                if (var != null)
                    i = (double) var;
                else
                    throw new ParseException("'" + id.image + "' is not yet defined.");
            }else if(t != null)
                i = Double.parseDouble(t.image);
            return i;
        }|
    <SUB> (t=<NUM> | id=<ID> | <PAL> (i=expression())? <PAR>)
    {
            if(id != null){
                i = (double)variables.get(id.image);
            }else if(t != null)
                i = Double.parseDouble(t.image);
        return i*(-1);
    }
}

double evaluateNextLine() :
{
    double result = 0.0;
}
{
	(result=statement())? <EOL> {return result;}|
	<EOF>{System.out.println("\nBuffer was closed. Exiting."); System.exit(0);}
}

void passStatement() :
{}
{
    <EOL>|
	<EOF>{System.out.println("\nBuffer was closed. Exiting."); System.exit(0);}
}