org.apache.zookeeper.server.util.BitHashSet Maven / Gradle / Ivy
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* 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,
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* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.zookeeper.server.util;
import java.util.BitSet;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
/**
* Using BitSet to store all the elements, and use HashSet to cache limited
* number of elements to find a balance between memory and time complexity.
*
* Without HashSet, we need to use O(N) time to get the elements, N is
* the bit numbers in elementBits. But we need to keep the size small to make
* sure it doesn't cost too much in memory, there is a trade off between
* memory and time complexity.
*
* Previously, was deciding to dynamically switch between SparseBitSet and
* HashSet based on the memory consumption, but it will take time to copy
* data over and may have some herd effect of keep copying data from one
* data structure to anther. The current solution can do a very good job
* given most of the paths have limited number of elements.
*/
public class BitHashSet implements Iterable {
/**
* Change to SparseBitSet if we we want to optimize more, the number of
* elements on a single server is usually limited, so BitSet should be
* fine.
*/
private final BitSet elementBits = new BitSet();
/**
* HashSet is used to optimize the iterating, if there is a single
* element in this BitHashSet, but the bit is very large, without
* HashSet we need to go through all the words before return that
* element, which is not efficient.
*/
private final Set cache = new HashSet<>();
private final int cacheSize;
// To record how many elements in this set.
private int elementCount = 0;
public BitHashSet() {
this(Integer.getInteger("zookeeper.bitHashCacheSize", 10));
}
public BitHashSet(int cacheSize) {
this.cacheSize = cacheSize;
}
public synchronized boolean add(Integer elementBit) {
if (elementBit == null || elementBits.get(elementBit)) {
return false;
}
if (cache.size() < cacheSize) {
cache.add(elementBit);
}
elementBits.set(elementBit);
elementCount++;
return true;
}
/**
* Remove the watches, and return the number of watches being removed.
*/
public synchronized int remove(Set bitSet, BitSet bits) {
cache.removeAll(bitSet);
elementBits.andNot(bits);
int elementCountBefore = elementCount;
elementCount = elementBits.cardinality();
return elementCountBefore - elementCount;
}
public synchronized boolean remove(Integer elementBit) {
if (elementBit == null || !elementBits.get(elementBit)) {
return false;
}
cache.remove(elementBit);
elementBits.clear(elementBit);
elementCount--;
return true;
}
public synchronized boolean contains(Integer elementBit) {
if (elementBit == null) {
return false;
}
return elementBits.get(elementBit);
}
public synchronized int size() {
return elementCount;
}
/**
* This function is not thread-safe, need to synchronized when
* iterate through this set.
*/
@Override
public Iterator iterator() {
// sample current size at the beginning
int currentSize = size();
if (cache.size() == currentSize) {
return cache.iterator();
}
return new Iterator() {
int returnedCount = 0;
int bitIndex = 0;
@Override
public boolean hasNext() {
return returnedCount < currentSize;
}
@Override
public Integer next() {
int bit = elementBits.nextSetBit(bitIndex);
bitIndex = bit + 1;
returnedCount++;
return bit;
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
// visible for test
public synchronized int cachedSize() {
return cache.size();
}
public synchronized boolean isEmpty() {
return elementCount == 0;
}
}