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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* 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 extends Trie
{
private final MergeResolver resolver;
protected final Trie t1;
protected final Trie t2;
MergeTrie(MergeResolver resolver, Trie t1, Trie t2)
{
this.resolver = resolver;
this.t1 = t1;
this.t2 = t2;
}
@Override
protected Cursor cursor()
{
return new MergeCursor<>(resolver, t1, t2);
}
static class MergeCursor implements Cursor
{
private final MergeResolver resolver;
private final Cursor c1;
private final Cursor c2;
boolean atC1;
boolean atC2;
MergeCursor(MergeResolver resolver, Trie t1, Trie 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 extends MergeTrie
{
Distinct(Trie input1, Trie input2)
{
super(throwingResolver(), input1, input2);
}
@Override
public Iterable valuesUnordered()
{
return Iterables.concat(t1.valuesUnordered(), t2.valuesUnordered());
}
}
}