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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.utils.btree;
import java.util.Comparator;
import io.netty.util.Recycler;
import static org.apache.cassandra.utils.btree.BTree.EMPTY_LEAF;
import static org.apache.cassandra.utils.btree.BTree.FAN_SHIFT;
import static org.apache.cassandra.utils.btree.BTree.POSITIVE_INFINITY;
/**
* A class for constructing a new BTree, either from an existing one and some set of modifications
* or a new tree from a sorted collection of items.
*
* This is a fairly heavy-weight object, so a Recycled instance is created for making modifications to a tree
*/
final class TreeBuilder
{
private final static Recycler builderRecycler = new Recycler()
{
protected TreeBuilder newObject(Handle handle)
{
return new TreeBuilder(handle);
}
};
public static TreeBuilder newInstance()
{
return builderRecycler.get();
}
private final Recycler.Handle recycleHandle;
private final NodeBuilder rootBuilder = new NodeBuilder();
private TreeBuilder(Recycler.Handle handle)
{
this.recycleHandle = handle;
}
/**
* At the highest level, we adhere to the classic b-tree insertion algorithm:
*
* 1. Add to the appropriate leaf
* 2. Split the leaf if necessary, add the median to the parent
* 3. Split the parent if necessary, etc.
*
* There is one important difference: we don't actually modify the original tree, but copy each node that we
* modify. Note that every node on the path to the key being inserted or updated will be modified; this
* implies that at a minimum, the root node will be modified for every update, so every root is a "snapshot"
* of a tree that can be iterated or sliced without fear of concurrent modifications.
*
* The NodeBuilder class handles the details of buffering the copied contents of the original tree and
* adding in our changes. Since NodeBuilder maintains parent/child references, it also handles parent-splitting
* (easy enough, since any node affected by the split will already be copied into a NodeBuilder).
*
* One other difference from the simple algorithm is that we perform modifications in bulk;
* we assume @param source has been sorted, e.g. by BTree.update, so the update of each key resumes where
* the previous left off.
*/
public Object[] update(Object[] btree, Comparator comparator, Iterable source, UpdateFunction updateF)
{
assert updateF != null;
NodeBuilder current = rootBuilder;
current.reset(btree, POSITIVE_INFINITY, updateF, comparator);
for (K key : source)
{
while (true)
{
if (updateF.abortEarly())
{
rootBuilder.clear();
return null;
}
NodeBuilder next = current.update(key);
if (next == null)
break;
// we were in a subtree from a previous key that didn't contain this new key;
// retry against the correct subtree
current = next;
}
}
// finish copying any remaining keys from the original btree
while (true)
{
NodeBuilder next = current.finish();
if (next == null)
break;
current = next;
}
// updating with POSITIVE_INFINITY means that current should be back to the root
assert current.isRoot();
Object[] r = current.toNode();
current.clear();
builderRecycler.recycle(this, recycleHandle);
return r;
}
public Object[] build(Iterable source, UpdateFunction updateF, int size)
{
assert updateF != null;
NodeBuilder current = rootBuilder;
// we descend only to avoid wasting memory; in update() we will often descend into existing trees
// so here we want to descend also, so we don't have lg max(N) depth in both directions
while ((size >>= FAN_SHIFT) > 0)
current = current.ensureChild();
current.reset(EMPTY_LEAF, POSITIVE_INFINITY, updateF, null);
for (K key : source)
current.addNewKey(key);
current = current.ascendToRoot();
Object[] r = current.toNode();
current.clear();
builderRecycler.recycle(this, recycleHandle);
return r;
}
}