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

package eval;

import parser.*;

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

    // The number of arguments that EQ takes.
    private static final int NUM_ARGS = 2;

    public SExpression call(Cons argList) {
        // retrieve the number of arguments passed to EQ
        int argListLength = LENGTH.getLength(argList);

        // make sure we have received the proper number of arguments
        if (argListLength != NUM_ARGS) {
            Cons originalSExpr = new Cons(new Symbol("EQ"), argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to EQ: " + originalSExpr;

            throw new RuntimeException(errMsg);
        }

        SExpression argOne = argList.getCar();  // first argument
        Cons cdr = (Cons) argList.getCdr();
        SExpression argTwo = cdr.getCar();      // second argumnet

        if (argOne.atomp() && argTwo.atomp()) {
            return ((argOne.toString().equals(argTwo.toString()))
                    ? Symbol.T : Nil.getUniqueInstance());
        }

        return ((argOne == argTwo) ? Symbol.T : Nil.getUniqueInstance());
    }

}