/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Interpreter 1
 */

package eval;

import parser.*;

/**
 * <code>EQUALSP</code> represents the '=' function in Lisp.
 */
public class EQUALSP extends LispFunction {

    public SExpression call(Cons argList) {
        // make sure we have received at least one argument
        if (argList.nullp()) {
            Cons originalSExpr = new Cons(new Symbol("="), argList);

            throw new RuntimeException("too few arguments given to =: " +
                                       originalSExpr);
        }

        SExpression firstArg = argList.getCar();
        Cons argRest = (Cons) argList.getCdr();

        // make sure that the first argument is a number
        if (firstArg.numberp()) {
            LispNumber num1 = (LispNumber) firstArg;

            if (argRest.nullp()) {
                return Symbol.T;
            }

            SExpression secondArg = argRest.getCar();

            // make sure that the second argument is a number as well
            if (secondArg.numberp()) {
                LispNumber num2 = (LispNumber) secondArg;

                if (num1.getValue() == num2.getValue()) {
                    return call(argRest);
                }

                return Nil.getUniqueInstance();
            }
                
            throw new RuntimeException("=: " + secondArg + " is not a number");
        }

        throw new RuntimeException("=: " + firstArg + " is not a number");
    }

}