package expression;

/**
 * @author Bogdan Timchenko
 */
public class CheckedParser implements Parser{
    private String unparse;
    private int left;
    private int len;

    public TripleExpression parse(String str) throws RuntimeException {
        unparse = str;
        left = 0;
        len = unparse.length();
        TripleExpression exp;
        exp = PlusMinus();
        unparse = "";
        len = 0;
        left = 0;
        return exp;
    }

    private TripleExpression PlusMinus() throws RuntimeException {
        TripleExpression acc = MulDiv();
        TripleExpression res;
        while (len > left) {
            delSpaces();
            char sign = unparse.charAt(left);
            if (!(sign == '+' || sign == '-')) {
                break;
            }
            left++;
            res = MulDiv();
            if (sign == '+') {
                acc = new CheckedAdd(acc, res);
            } else {
                acc = new CheckedSubtract(acc, res);
            }
        }
        if (len > left) {
            throw new RuntimeException("Cannot parse: " + '"' + unparse.substring(left) + '"');
        }
        return acc;
    }

    private TripleExpression MulDiv() throws RuntimeException {
        TripleExpression acc = Bracket();
        TripleExpression res;
        while (true) {
            delSpaces();
            if (left == len) {
                return acc;
            }
            char sign = unparse.charAt(left);
            if (!(sign == '*' || sign == '/')) {
                return acc;
            }
            left++;
            res = Bracket();
            if (sign == '*') {
                acc = new CheckedMultiply(acc, res);
            } else {
                acc = new CheckedDivide(acc, res);
            }
        }
    }

    private TripleExpression Bracket() throws RuntimeException {
        delSpaces();
        char firstCh = unparse.charAt(left);
        if (firstCh == '(') {
            left++;
            TripleExpression res = PlusMinus();
            if (unparse.charAt(left) != ')') {
                throw new RuntimeException("Missing ')'");
            }
            left++;
            return res;
        }
        return ConstVar();
    }

    private TripleExpression ConstVar() throws RuntimeException {
        delSpaces();
        String var = unparse.substring(left, left + 1);
        if (var.equals("x") || var.equals("y") || var.equals("z")) {
            left++;
            return new Variable(var);
        }
        return Minus();
    }

    private TripleExpression Minus() throws RuntimeException {
        delSpaces();
        if (unparse.charAt(left) == '-') {
            if (len - left > 1 && !Character.isDigit(unparse.charAt(left + 1))) {
                left++;
                return new CheckedNegate(Bracket());
            }
        }
        return Num();
    }

    private TripleExpression Num() throws RuntimeException {
        delSpaces();
        int i = 0;
        if (unparse.charAt(left) == '-') {
            i++;
        }
        while (i < len - left && Character.isDigit(unparse.charAt(left + i))) {
            i++;
        }
        if (i == 0) {
            throw new RuntimeException("Unexpected symbol: '" + unparse.charAt(left) + "'");
        }
        int num = Integer.parseInt(unparse.substring(left, left + i));
        left += i;
        return new Const(num);
    }

    private void delSpaces() {
        while (left < len) {
            if (!Character.isWhitespace(unparse.charAt(left))) {
                break;
            }
            left++;
        }
    }
}