package function.builtin;

import function.LispFunction;
import sexpression.*;

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

    // The number of arguments that CAR takes.
    private static final int NUM_ARGS = 1;

    public SExpression call(Cons argList) {
        // retrieve the number of arguments passed to CAR
        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("CAR"), argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to CAR: " + originalSExpr;

            throw new RuntimeException(errMsg);
        }

        SExpression argCar = argList.getCar();

        // make sure that the argument is a list
        if (argCar.listp()) {
            Cons arg = (Cons) argCar;

            return arg.getCar();
        }

        // the argument is not a list
        throw new RuntimeException("CAR: " + argCar + " is not a list");
    }

}