extras/indexing/src/main/java/org/apache/rya/indexing/external/matching/TopOfQueryFilterRelocator.java

package org.apache.rya.indexing.external.matching;

/*
 * 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.
 */

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import org.eclipse.rdf4j.query.algebra.Filter;
import org.eclipse.rdf4j.query.algebra.Projection;
import org.eclipse.rdf4j.query.algebra.TupleExpr;
import org.eclipse.rdf4j.query.algebra.ValueExpr;
import org.eclipse.rdf4j.query.algebra.helpers.AbstractQueryModelVisitor;

/**
 * Class consisting of a single utility method for relocating filters.
 *
 */
public class TopOfQueryFilterRelocator {

    /**
     *
     * This method moves the Filters of a specified {@link TupleExpr} to the top
     * of the TupleExpr.
     *
     * @param query
     *            - query whose filters will be relocated
     * @return - TupleExpr with filters relocated to top
     */
    public static TupleExpr moveFiltersToTop(TupleExpr query) {

        ProjectionAndFilterGatherer fg = new ProjectionAndFilterGatherer();
        query.visit(fg);
        List<ValueExpr> filterCond = new ArrayList<>(fg.filterCond);
        Projection projection = fg.projection;

        if (filterCond.size() == 0) {
            return query;
        }

        Filter first = new Filter();
        first.setCondition(filterCond.remove(0));
        Filter current = first;
        for (ValueExpr cond : filterCond) {
            Filter filter = new Filter(null, cond);
            current.setArg(filter);
            current = filter;
        }

        TupleExpr te = projection.getArg();
        projection.setArg(first);
        current.setArg(te);

        return query;

    }

    static class ProjectionAndFilterGatherer extends AbstractQueryModelVisitor<RuntimeException> {

        Set<ValueExpr> filterCond = new HashSet<>();
        Projection projection;

        @Override
        public void meet(Projection node) {
            this.projection = node;
            node.getArg().visit(this);
        }

        @Override
        public void meet(Filter node) {
            filterCond.add(node.getCondition());
            node.replaceWith(node.getArg());
        }

    }

}