package function.builtin.special;

import static function.builtin.EVAL.eval;
import static sexpression.Nil.NIL;

import function.*;
import sexpression.*;

@FunctionNames({ "COND" })
public class COND extends LispSpecialFunction {

    private ArgumentValidator argumentValidator;

    public COND() {
        this.argumentValidator = new ArgumentValidator("COND");
        this.argumentValidator.setEveryArgumentExpectedType(Cons.class);
        this.argumentValidator.setEveryArgumentExcludedType(Nil.class);
    }

    public SExpression call(Cons argumentList) {
        argumentValidator.validate(argumentList);

        return callTailRecursive(argumentList);
    }

    private SExpression callTailRecursive(Cons argumentList) {
        if (argumentList.isNull())
            return NIL;

        Cons clause = (Cons) argumentList.getFirst();
        Cons remainingClauses = (Cons) argumentList.getRest();
        SExpression test = eval(clause.getFirst());

        if (isTestSuccessful(test))
            return evaluateConsequents(clause.getRest(), test);

        return callTailRecursive(remainingClauses);
    }

    private boolean isTestSuccessful(SExpression test) {
        return test != NIL;
    }

    private SExpression evaluateConsequents(SExpression consequentList, SExpression test) {
        SExpression lastConsequentValue = test;

        for (; consequentList.isCons(); consequentList = advanceCons(consequentList))
            lastConsequentValue = eval(getFirst(consequentList));

        return lastConsequentValue;
    }

    private SExpression advanceCons(SExpression knownCons) {
        return ((Cons) knownCons).getRest();
    }

    private SExpression getFirst(SExpression knownCons) {
        return ((Cons) knownCons).getFirst();
    }

}