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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.Arrays;
import java.util.Comparator;
import static org.apache.cassandra.utils.btree.BTree.*;
/**
* Supports two basic operations for moving around a BTree, either forwards or backwards:
* moveOne(), and seekTo()
*
* These two methods, along with movement to the start/end, permit us to construct any desired
* movement around a btree, without much cognitive burden.
*
* This TreeCursor is (and has its methods) package private. This is to avoid polluting the BTreeSearchIterator
* that extends it, and uses its functionality. If this class is useful for wider consumption, a public extension
* class can be provided, that just makes all of its methods public.
*/
class TreeCursor extends NodeCursor
{
// TODO: spend some time optimising compiler inlining decisions: many of these methods have only one primary call-site
NodeCursor cur;
TreeCursor(Comparator comparator, Object[] node)
{
super(node, null, comparator);
}
/**
* Move the cursor to either the first or last item in the btree
* @param start true if should move to the first item; false moves to the last
*/
void reset(boolean start)
{
cur = root();
root().inChild = false;
// this is a corrupt position, but we ensure we never use it except to start our search from
root().position = start ? -1 : getKeyEnd(root().node);
}
/**
* move the Cursor one item, either forwards or backwards
* @param forwards direction of travel
* @return false iff the cursor is exhausted in the direction of travel
*/
int moveOne(boolean forwards)
{
NodeCursor cur = this.cur;
if (cur.isLeaf())
{
// if we're a leaf, we try to step forwards inside ourselves
if (cur.advanceLeafNode(forwards))
return cur.globalLeafIndex();
// if we fail, we just find our bounding parent
this.cur = cur = moveOutOfLeaf(forwards, cur, root());
return cur.globalIndex();
}
// otherwise we descend directly into our next child
if (forwards)
++cur.position;
cur = cur.descend();
// and go to its first item
NodeCursor next;
while ( null != (next = cur.descendToFirstChild(forwards)) )
cur = next;
this.cur = cur;
return cur.globalLeafIndex();
}
/**
* seeks from the current position, forwards or backwards, for the provided key
* while the direction could be inferred (or ignored), it is required so that (e.g.) we do not infinitely loop on bad inputs
* if there is no such key, it moves to the key that would naturally follow/succeed it (i.e. it behaves as ceil when ascending; floor when descending)
*/
boolean seekTo(K key, boolean forwards, boolean skipOne)
{
NodeCursor cur = this.cur;
/**
* decide if we will "try one" value by itself, as a sequential access;
* we actually *require* that we try the "current key" for any node before we call seekInNode on it.
*
* if we are already on a value, we just check it irregardless of if it is a leaf or not;
* if we are not, we have already excluded it (as we have consumed it), so:
* if we are on a branch we consider that good enough;
* otherwise, we move onwards one, and we try the new value
*
*/
boolean tryOne = !skipOne;
if ((!tryOne & cur.isLeaf()) && !(tryOne = (cur.advanceLeafNode(forwards) || (cur = moveOutOfLeaf(forwards, cur, null)) != null)))
{
// we moved out of the tree; return out-of-bounds
this.cur = root();
return false;
}
if (tryOne)
{
// we're presently on a value we can (and *must*) cheaply test
K test = cur.value();
int cmp;
if (key == test) cmp = 0; // check object identity first, since we utilise that in some places and it's very cheap
else cmp = comparator.compare(test, key); // order of provision matters for asymmetric comparators
if (forwards ? cmp >= 0 : cmp <= 0)
{
// we've either matched, or excluded the value from being present
this.cur = cur;
return cmp == 0;
}
}
// if we failed to match with the cheap test, first look to see if we're even in the correct sub-tree
while (cur != root())
{
NodeCursor bound = cur.boundIterator(forwards);
if (bound == null)
break; // we're all that's left
int cmpbound = comparator.compare(bound.bound(forwards), key); // order of provision matters for asymmetric comparators
if (forwards ? cmpbound > 0 : cmpbound < 0)
break; // already in correct sub-tree
// bound is on-or-before target, so ascend to that bound and continue looking upwards
cur = bound;
cur.safeAdvanceIntoBranchFromChild(forwards);
if (cmpbound == 0) // it was an exact match, so terminate here
{
this.cur = cur;
return true;
}
}
// we must now be able to find our target in the sub-tree rooted at cur
boolean match;
while (!(match = cur.seekInNode(key, forwards)) && !cur.isLeaf())
{
cur = cur.descend();
cur.position = forwards ? -1 : getKeyEnd(cur.node);
}
if (!match)
cur = ensureValidLocation(forwards, cur);
this.cur = cur;
assert !cur.inChild;
return match;
}
/**
* ensures a leaf node we have seeked in, is not positioned outside of its bounds,
* by moving us into its parents (if any); if it is the root, we're permitted to be out-of-bounds
* as this indicates exhaustion
*/
private NodeCursor ensureValidLocation(boolean forwards, NodeCursor cur)
{
assert cur.isLeaf();
int position = cur.position;
// if we're out of bounds of the leaf, move once in direction of travel
if ((position < 0) | (position >= getLeafKeyEnd(cur.node)))
cur = moveOutOfLeaf(forwards, cur, root());
return cur;
}
/**
* move out of a leaf node that is currently out of (its own) bounds
* @return null if we're now out-of-bounds of the whole tree
*/
private NodeCursor moveOutOfLeaf(boolean forwards, NodeCursor cur, NodeCursor ifFail)
{
while (true)
{
cur = cur.parent;
if (cur == null)
{
root().inChild = false;
return ifFail;
}
if (cur.advanceIntoBranchFromChild(forwards))
break;
}
cur.inChild = false;
return cur;
}
/**
* resets the cursor and seeks to the specified position; does not assume locality or take advantage of the cursor's current position
*/
void seekTo(int index)
{
if ((index < 0) | (index >= BTree.size(rootNode())))
{
if ((index < -1) | (index > BTree.size(rootNode())))
throw new IndexOutOfBoundsException(index + " not in range [0.." + BTree.size(rootNode()) + ")");
reset(index == -1);
return;
}
NodeCursor cur = root();
assert cur.nodeOffset == 0;
while (true)
{
int relativeIndex = index - cur.nodeOffset; // index within subtree rooted at cur
Object[] node = cur.node;
if (cur.isLeaf())
{
assert relativeIndex < getLeafKeyEnd(node);
cur.position = relativeIndex;
this.cur = cur;
return;
}
int[] sizeMap = getSizeMap(node);
int boundary = Arrays.binarySearch(sizeMap, relativeIndex);
if (boundary >= 0)
{
// exact match, in this branch node
assert boundary < sizeMap.length - 1;
cur.position = boundary;
cur.inChild = false;
this.cur = cur;
return;
}
cur.inChild = true;
cur.position = -1 -boundary;
cur = cur.descend();
}
}
private NodeCursor root()
{
return this;
}
Object[] rootNode()
{
return this.node;
}
K currentValue()
{
return cur.value();
}
}