package function.builtin;

import java.math.BigInteger;

import function.LispFunction;
import sexpression.*;

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

    /**
     * Returns the length of the given list.
     *
     * @param list
     *            the list to determine the length of
     * @return the length of <code>list</code>
     */
    public static int getLength(Cons list) {
        LENGTH lengthFunction = new LENGTH();
        LispNumber length = lengthFunction.call(LIST.makeList(list));

        return length.getValue().intValue(); // TODO - return BigInteger when all built-ins use
                                             // ArgumentValidator
    }

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

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

        SExpression argCar = argList.getCar();
        SExpression argCdr = argList.getCdr();

        // make sure we have received only one argument
        if (!argCdr.nullp()) {
            Cons originalSExpr = new Cons(new Symbol("LENGTH"), argList);

            throw new RuntimeException("too many arguments given to LENGTH: " + originalSExpr);
        }

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

            if (arg.nullp()) {
                return new LispNumber(BigInteger.ZERO);
            }

            Cons cdr = LIST.makeList(arg.getCdr());
            LispNumber cdrLength = call(cdr);

            return new LispNumber(BigInteger.ONE.add(cdrLength.getValue()));
        }

        throw new RuntimeException("LENGTH: a proper list must not end with " + argCar);
    }

}