gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/FilterStep.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.tinkerpop.gremlin.process.traversal.step.filter;

import org.apache.tinkerpop.gremlin.process.traversal.GremlinTypeErrorException;
import org.apache.tinkerpop.gremlin.process.traversal.Traversal;
import org.apache.tinkerpop.gremlin.process.traversal.Traverser;
import org.apache.tinkerpop.gremlin.process.traversal.step.util.AbstractStep;
import org.apache.tinkerpop.gremlin.process.traversal.step.util.EmptyStep;

/**
 * @author Marko A. Rodriguez (http://markorodriguez.com)
 */
public abstract class FilterStep<S> extends AbstractStep<S, S> {

    public FilterStep(final Traversal.Admin traversal) {
        super(traversal);
    }

    @Override
    protected Traverser.Admin<S> processNextStart() {
        while (true) {
            try {
                final Traverser.Admin<S> traverser = this.starts.next();
                if (this.filter(traverser))
                    return traverser;
            } catch (GremlinTypeErrorException ex) {
                if (this instanceof BinaryReductionStep || getTraversal().isRoot() || !(getTraversal().getParent() instanceof FilterStep)) {
                    /*
                     * Either we are at a known reduction point (TraversalFilterStep, WhereTraversalStep), we
                     * are at the top level of the query, or our parent query is not a FilterStep and thus cannot handle
                     * a GremlinTypeErrorException. In any of these cases we do a binary reduction from
                     * ERROR -> FALSE and filter the solution quietly.
                     */
                } else {
                    // not a ternary -> binary reducer, pass the ERROR on
                    throw ex;
                }
            }
        }
    }

    protected abstract boolean filter(final Traverser.Admin<S> traverser);
}