cassandra-trunk/src/java/org/apache/cassandra/db/tries/MergeTrie.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.cassandra.db.tries;

import com.google.common.collect.Iterables;

/**
 * A merged view of two tries.
 *
 * This is accomplished by walking the two cursors in parallel; the merged cursor takes the position and features of the
 * smaller and advances with it; when the two cursors are equal, both are advanced.
 *
 * Crucial for the efficiency of this is the fact that when they are advanced like this, we can compare cursors'
 * positions by their depth descending and then incomingTransition ascending.
 *
 * See Trie.md for further details.
 */
class MergeTrie<T> extends Trie<T>
{
    private final MergeResolver<T> resolver;
    protected final Trie<T> t1;
    protected final Trie<T> t2;

    MergeTrie(MergeResolver<T> resolver, Trie<T> t1, Trie<T> t2)
    {
        this.resolver = resolver;
        this.t1 = t1;
        this.t2 = t2;
    }

    @Override
    protected Cursor<T> cursor()
    {
        return new MergeCursor<>(resolver, t1, t2);
    }

    static class MergeCursor<T> implements Cursor<T>
    {
        private final MergeResolver<T> resolver;
        private final Cursor<T> c1;
        private final Cursor<T> c2;

        boolean atC1;
        boolean atC2;

        MergeCursor(MergeResolver<T> resolver, Trie<T> t1, Trie<T> t2)
        {
            this.resolver = resolver;
            this.c1 = t1.cursor();
            this.c2 = t2.cursor();
            assert c1.depth() == 0;
            assert c2.depth() == 0;
            atC1 = atC2 = true;
        }

        @Override
        public int advance()
        {
            return checkOrder(atC1 ? c1.advance() : c1.depth(),
                              atC2 ? c2.advance() : c2.depth());
        }

        @Override
        public int skipChildren()
        {
            return checkOrder(atC1 ? c1.skipChildren() : c1.depth(),
                              atC2 ? c2.skipChildren() : c2.depth());
        }

        @Override
        public int advanceMultiple(TransitionsReceiver receiver)
        {
            // While we are on a shared position, we must descend one byte at a time to maintain the cursor ordering.
            if (atC1 && atC2)
                return checkOrder(c1.advance(), c2.advance());

            // If we are in a branch that's only covered by one of the sources, we can use its advanceMultiple as it is
            // only different from advance if it takes multiple steps down, which does not change the order of the
            // cursors.
            // Since it might ascend, we still have to check the order after the call.
            if (atC1)
                return checkOrder(c1.advanceMultiple(receiver), c2.depth());
            else // atC2
                return checkOrder(c1.depth(), c2.advanceMultiple(receiver));
        }

        private int checkOrder(int c1depth, int c2depth)
        {
            if (c1depth > c2depth)
            {
                atC1 = true;
                atC2 = false;
                return c1depth;
            }
            if (c1depth < c2depth)
            {
                atC1 = false;
                atC2 = true;
                return c2depth;
            }
            // c1depth == c2depth
            int c1trans = c1.incomingTransition();
            int c2trans = c2.incomingTransition();
            atC1 = c1trans <= c2trans;
            atC2 = c1trans >= c2trans;
            return c1depth;
        }

        @Override
        public int depth()
        {
            return atC1 ? c1.depth() : c2.depth();
        }

        @Override
        public int incomingTransition()
        {
            return atC1 ? c1.incomingTransition() : c2.incomingTransition();
        }

        public T content()
        {
            T mc = atC2 ? c2.content() : null;
            T nc = atC1 ? c1.content() : null;
            if (mc == null)
                return nc;
            else if (nc == null)
                return mc;
            else
                return resolver.resolve(nc, mc);
        }
    }

    /**
     * Special instance for sources that are guaranteed (by the caller) distinct. The main difference is that we can
     * form unordered value list by concatenating sources.
     */
    static class Distinct<T> extends MergeTrie<T>
    {
        Distinct(Trie<T> input1, Trie<T> input2)
        {
            super(throwingResolver(), input1, input2);
        }

        @Override
        public Iterable<T> valuesUnordered()
        {
            return Iterables.concat(t1.valuesUnordered(), t2.valuesUnordered());
        }
    }
}
