001/*
002 * Copyright (C) 2009-2011 Mathias Doenitz
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package org.parboiled.matchers;
018
019import org.parboiled.MatcherContext;
020import org.parboiled.Rule;
021import org.parboiled.matchervisitors.MatcherVisitor;
022
023import static org.parboiled.common.Preconditions.checkArgNotNull;
024
025/**
026 * A {@link Matcher} that tries its submatcher once against the input and always succeeds.
027 */
028public class OptionalMatcher extends CustomDefaultLabelMatcher<OptionalMatcher> {
029    public final Matcher subMatcher;
030
031    public OptionalMatcher(Rule subRule) {
032        super(checkArgNotNull(subRule, "subRule"), "Optional");
033        this.subMatcher = getChildren().get(0);
034    }
035
036    public boolean match(MatcherContext context) {
037        subMatcher.getSubContext(context).runMatcher();
038        context.createNode();
039        return true;
040    }
041
042    public <R> R accept(MatcherVisitor<R> visitor) {
043        checkArgNotNull(visitor, "visitor");
044        return visitor.visit(this);
045    }
046}