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/*
* 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.kafka.common.utils;
import java.util.AbstractCollection;
import java.util.AbstractSequentialList;
import java.util.AbstractSet;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Set;
/**
* A memory-efficient hash set which tracks the order of insertion of elements.
*
* Like java.util.LinkedHashSet, this collection maintains a linked list of elements.
* However, rather than using a separate linked list, this collection embeds the next
* and previous fields into the elements themselves. This reduces memory consumption,
* because it means that we only have to store one Java object per element, rather
* than multiple.
*
* The next and previous fields are stored as array indices rather than pointers.
* This ensures that the fields only take 32 bits, even when pointers are 64 bits.
* It also makes the garbage collector's job easier, because it reduces the number of
* pointers that it must chase.
*
* This class uses linear probing. Unlike HashMap (but like HashTable), we don't force
* the size to be a power of 2. This saves memory.
*
* This set does not allow null elements. It does not have internal synchronization.
*/
public class ImplicitLinkedHashCollection extends AbstractCollection {
/**
* The interface which elements of this collection must implement. The prev,
* setPrev, next, and setNext functions handle manipulating the implicit linked
* list which these elements reside in inside the collection.
* elementKeysAreEqual() is the function which this collection uses to compare
* elements.
*/
public interface Element {
int prev();
void setPrev(int prev);
int next();
void setNext(int next);
default boolean elementKeysAreEqual(Object other) {
return equals(other);
}
}
/**
* A special index value used to indicate that the next or previous field is
* the head.
*/
private static final int HEAD_INDEX = -1;
/**
* A special index value used for next and previous indices which have not
* been initialized.
*/
public static final int INVALID_INDEX = -2;
/**
* The minimum new capacity for a non-empty implicit hash set.
*/
private static final int MIN_NONEMPTY_CAPACITY = 5;
/**
* A static empty array used to avoid object allocations when the capacity is zero.
*/
private static final Element[] EMPTY_ELEMENTS = new Element[0];
private static class HeadElement implements Element {
static final HeadElement EMPTY = new HeadElement();
private int prev = HEAD_INDEX;
private int next = HEAD_INDEX;
@Override
public int prev() {
return prev;
}
@Override
public void setPrev(int prev) {
this.prev = prev;
}
@Override
public int next() {
return next;
}
@Override
public void setNext(int next) {
this.next = next;
}
}
private static Element indexToElement(Element head, Element[] elements, int index) {
if (index == HEAD_INDEX) {
return head;
}
return elements[index];
}
private static void addToListTail(Element head, Element[] elements, int elementIdx) {
int oldTailIdx = head.prev();
Element element = indexToElement(head, elements, elementIdx);
Element oldTail = indexToElement(head, elements, oldTailIdx);
head.setPrev(elementIdx);
oldTail.setNext(elementIdx);
element.setPrev(oldTailIdx);
element.setNext(HEAD_INDEX);
}
private static void removeFromList(Element head, Element[] elements, int elementIdx) {
Element element = indexToElement(head, elements, elementIdx);
elements[elementIdx] = null;
int prevIdx = element.prev();
int nextIdx = element.next();
Element prev = indexToElement(head, elements, prevIdx);
Element next = indexToElement(head, elements, nextIdx);
prev.setNext(nextIdx);
next.setPrev(prevIdx);
element.setNext(INVALID_INDEX);
element.setPrev(INVALID_INDEX);
}
private class ImplicitLinkedHashCollectionIterator implements ListIterator {
private int cursor = 0;
private Element cur = head;
private int lastReturnedSlot = INVALID_INDEX;
ImplicitLinkedHashCollectionIterator(int index) {
for (int i = 0; i < index; ++i) {
cur = indexToElement(head, elements, cur.next());
cursor++;
}
}
@Override
public boolean hasNext() {
return cursor != size;
}
@Override
public boolean hasPrevious() {
return cursor != 0;
}
@Override
public E next() {
if (cursor == size) {
throw new NoSuchElementException();
}
lastReturnedSlot = cur.next();
cur = indexToElement(head, elements, cur.next());
++cursor;
@SuppressWarnings("unchecked")
E returnValue = (E) cur;
return returnValue;
}
@Override
public E previous() {
if (cursor == 0) {
throw new NoSuchElementException();
}
@SuppressWarnings("unchecked")
E returnValue = (E) cur;
cur = indexToElement(head, elements, cur.prev());
lastReturnedSlot = cur.next();
--cursor;
return returnValue;
}
@Override
public int nextIndex() {
return cursor;
}
@Override
public int previousIndex() {
return cursor - 1;
}
@Override
public void remove() {
if (lastReturnedSlot == INVALID_INDEX) {
throw new IllegalStateException();
}
if (cur == indexToElement(head, elements, lastReturnedSlot)) {
cursor--;
cur = indexToElement(head, elements, cur.prev());
}
ImplicitLinkedHashCollection.this.removeElementAtSlot(lastReturnedSlot);
lastReturnedSlot = INVALID_INDEX;
}
@Override
public void set(E e) {
throw new UnsupportedOperationException();
}
@Override
public void add(E e) {
throw new UnsupportedOperationException();
}
}
private class ImplicitLinkedHashCollectionListView extends AbstractSequentialList {
@Override
public ListIterator listIterator(int index) {
if (index < 0 || index > size) {
throw new IndexOutOfBoundsException();
}
return ImplicitLinkedHashCollection.this.listIterator(index);
}
@Override
public int size() {
return size;
}
}
private class ImplicitLinkedHashCollectionSetView extends AbstractSet {
@Override
public Iterator iterator() {
return ImplicitLinkedHashCollection.this.iterator();
}
@Override
public int size() {
return size;
}
@Override
public boolean add(E newElement) {
return ImplicitLinkedHashCollection.this.add(newElement);
}
@Override
public boolean remove(Object key) {
return ImplicitLinkedHashCollection.this.remove(key);
}
@Override
public boolean contains(Object key) {
return ImplicitLinkedHashCollection.this.contains(key);
}
@Override
public void clear() {
ImplicitLinkedHashCollection.this.clear();
}
}
private Element head;
Element[] elements;
private int size;
/**
* Returns an iterator that will yield every element in the set.
* The elements will be returned in the order that they were inserted in.
*
* Do not modify the set while you are iterating over it (except by calling
* remove on the iterator itself, of course.)
*/
@Override
final public Iterator iterator() {
return listIterator(0);
}
private ListIterator listIterator(int index) {
return new ImplicitLinkedHashCollectionIterator(index);
}
final int slot(Element[] curElements, Object e) {
return (e.hashCode() & 0x7fffffff) % curElements.length;
}
/**
* Find an element matching an example element.
*
* Using the element's hash code, we can look up the slot where it belongs.
* However, it may not have ended up in exactly this slot, due to a collision.
* Therefore, we must search forward in the array until we hit a null, before
* concluding that the element is not present.
*
* @param key The element to match.
* @return The match index, or INVALID_INDEX if no match was found.
*/
final private int findIndexOfEqualElement(Object key) {
if (key == null || size == 0) {
return INVALID_INDEX;
}
int slot = slot(elements, key);
for (int seen = 0; seen < elements.length; seen++) {
Element element = elements[slot];
if (element == null) {
return INVALID_INDEX;
}
if (element.elementKeysAreEqual(key)) {
return slot;
}
slot = (slot + 1) % elements.length;
}
return INVALID_INDEX;
}
/**
* An element e in the collection such that e.elementKeysAreEqual(key) and
* e.hashCode() == key.hashCode().
*
* @param key The element to match.
* @return The matching element, or null if there were none.
*/
final public E find(E key) {
int index = findIndexOfEqualElement(key);
if (index == INVALID_INDEX) {
return null;
}
@SuppressWarnings("unchecked")
E result = (E) elements[index];
return result;
}
/**
* Returns the number of elements in the set.
*/
@Override
final public int size() {
return size;
}
/**
* Returns true if there is at least one element e in the collection such
* that key.elementKeysAreEqual(e) and key.hashCode() == e.hashCode().
*
* @param key The object to try to match.
*/
@Override
final public boolean contains(Object key) {
return findIndexOfEqualElement(key) != INVALID_INDEX;
}
private static int calculateCapacity(int expectedNumElements) {
// Avoid using even-sized capacities, to get better key distribution.
int newCapacity = (2 * expectedNumElements) + 1;
// Don't use a capacity that is too small.
return Math.max(newCapacity, MIN_NONEMPTY_CAPACITY);
}
/**
* Add a new element to the collection.
*
* @param newElement The new element.
*
* @return True if the element was added to the collection;
* false if it was not, because there was an existing equal element.
*/
@Override
final public boolean add(E newElement) {
if (newElement == null) {
return false;
}
if (newElement.prev() != INVALID_INDEX || newElement.next() != INVALID_INDEX) {
return false;
}
if ((size + 1) >= elements.length / 2) {
changeCapacity(calculateCapacity(elements.length));
}
int slot = addInternal(newElement, elements);
if (slot >= 0) {
addToListTail(head, elements, slot);
size++;
return true;
}
return false;
}
final public void mustAdd(E newElement) {
if (!add(newElement)) {
throw new RuntimeException("Unable to add " + newElement);
}
}
/**
* Adds a new element to the appropriate place in the elements array.
*
* @param newElement The new element to add.
* @param addElements The elements array.
* @return The index at which the element was inserted, or INVALID_INDEX
* if the element could not be inserted.
*/
int addInternal(Element newElement, Element[] addElements) {
int slot = slot(addElements, newElement);
for (int seen = 0; seen < addElements.length; seen++) {
Element element = addElements[slot];
if (element == null) {
addElements[slot] = newElement;
return slot;
}
if (element.elementKeysAreEqual(newElement)) {
return INVALID_INDEX;
}
slot = (slot + 1) % addElements.length;
}
throw new RuntimeException("Not enough hash table slots to add a new element.");
}
private void changeCapacity(int newCapacity) {
Element[] newElements = new Element[newCapacity];
HeadElement newHead = new HeadElement();
int oldSize = size;
for (Iterator iter = iterator(); iter.hasNext(); ) {
Element element = iter.next();
iter.remove();
int newSlot = addInternal(element, newElements);
addToListTail(newHead, newElements, newSlot);
}
this.elements = newElements;
this.head = newHead;
this.size = oldSize;
}
/**
* Remove the first element e such that key.elementKeysAreEqual(e)
* and key.hashCode == e.hashCode.
*
* @param key The object to try to match.
* @return True if an element was removed; false otherwise.
*/
@Override
final public boolean remove(Object key) {
int slot = findElementToRemove(key);
if (slot == INVALID_INDEX) {
return false;
}
removeElementAtSlot(slot);
return true;
}
int findElementToRemove(Object key) {
return findIndexOfEqualElement(key);
}
/**
* Remove an element in a particular slot.
*
* @param slot The slot of the element to remove.
*
* @return True if an element was removed; false otherwise.
*/
private boolean removeElementAtSlot(int slot) {
size--;
removeFromList(head, elements, slot);
slot = (slot + 1) % elements.length;
// Find the next empty slot
int endSlot = slot;
for (int seen = 0; seen < elements.length; seen++) {
Element element = elements[endSlot];
if (element == null) {
break;
}
endSlot = (endSlot + 1) % elements.length;
}
// We must preserve the denseness invariant. The denseness invariant says that
// any element is either in the slot indicated by its hash code, or a slot which
// is not separated from that slot by any nulls.
// Reseat all elements in between the deleted element and the next empty slot.
while (slot != endSlot) {
reseat(slot);
slot = (slot + 1) % elements.length;
}
return true;
}
private void reseat(int prevSlot) {
Element element = elements[prevSlot];
int newSlot = slot(elements, element);
for (int seen = 0; seen < elements.length; seen++) {
Element e = elements[newSlot];
if ((e == null) || (e == element)) {
break;
}
newSlot = (newSlot + 1) % elements.length;
}
if (newSlot == prevSlot) {
return;
}
Element prev = indexToElement(head, elements, element.prev());
prev.setNext(newSlot);
Element next = indexToElement(head, elements, element.next());
next.setPrev(newSlot);
elements[prevSlot] = null;
elements[newSlot] = element;
}
/**
* Create a new ImplicitLinkedHashCollection.
*/
public ImplicitLinkedHashCollection() {
this(0);
}
/**
* Create a new ImplicitLinkedHashCollection.
*
* @param expectedNumElements The number of elements we expect to have in this set.
* This is used to optimize by setting the capacity ahead
* of time rather than growing incrementally.
*/
public ImplicitLinkedHashCollection(int expectedNumElements) {
clear(expectedNumElements);
}
/**
* Create a new ImplicitLinkedHashCollection.
*
* @param iter We will add all the elements accessible through this iterator
* to the set.
*/
public ImplicitLinkedHashCollection(Iterator iter) {
clear(0);
while (iter.hasNext()) {
mustAdd(iter.next());
}
}
/**
* Removes all of the elements from this set.
*/
@Override
final public void clear() {
clear(elements.length);
}
/**
* Removes all of the elements from this set, and resets the set capacity
* based on the provided expected number of elements.
*/
final public void clear(int expectedNumElements) {
if (expectedNumElements == 0) {
// Optimize away object allocations for empty sets.
this.head = HeadElement.EMPTY;
this.elements = EMPTY_ELEMENTS;
this.size = 0;
} else {
this.head = new HeadElement();
this.elements = new Element[calculateCapacity(expectedNumElements)];
this.size = 0;
}
}
/**
* Compares the specified object with this collection for equality. Two
* {@code ImplicitLinkedHashCollection} objects are equal if they contain the
* same elements (as determined by the element's {@code equals} method), and
* those elements were inserted in the same order. Because
* {@code ImplicitLinkedHashCollectionListIterator} iterates over the elements
* in insertion order, it is sufficient to call {@code valuesList.equals}.
*
* Note that {@link ImplicitLinkedHashMultiCollection} does not override
* {@code equals} and uses this method as well. This means that two
* {@code ImplicitLinkedHashMultiCollection} objects will be considered equal even
* if they each contain two elements A and B such that A.equals(B) but A != B and
* A and B have switched insertion positions between the two collections. This
* is an acceptable definition of equality, because the collections are still
* equal in terms of the order and value of each element.
*
* @param o object to be compared for equality with this collection
* @return true is the specified object is equal to this collection
*/
@Override
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof ImplicitLinkedHashCollection))
return false;
ImplicitLinkedHashCollection ilhs = (ImplicitLinkedHashCollection) o;
return this.valuesList().equals(ilhs.valuesList());
}
/**
* Returns the hash code value for this collection. Because
* {@code ImplicitLinkedHashCollection.equals} compares the {@code valuesList}
* of two {@code ImplicitLinkedHashCollection} objects to determine equality,
* this method uses the @{code valuesList} to compute the has code value as well.
*
* @return the hash code value for this collection
*/
@Override
public int hashCode() {
return this.valuesList().hashCode();
}
// Visible for testing
final int numSlots() {
return elements.length;
}
/**
* Returns a {@link List} view of the elements contained in the collection,
* ordered by order of insertion into the collection. The list is backed by the
* collection, so changes to the collection are reflected in the list and
* vice-versa. The list supports element removal, which removes the corresponding
* element from the collection, but does not support the {@code add} or
* {@code set} operations.
*
* The list is implemented as a circular linked list, so all index-based
* operations, such as {@code List.get}, run in O(n) time.
*
* @return a list view of the elements contained in this collection
*/
public List valuesList() {
return new ImplicitLinkedHashCollectionListView();
}
/**
* Returns a {@link Set} view of the elements contained in the collection. The
* set is backed by the collection, so changes to the collection are reflected in
* the set, and vice versa. The set supports element removal and addition, which
* removes from or adds to the collection, respectively.
*
* @return a set view of the elements contained in this collection
*/
public Set valuesSet() {
return new ImplicitLinkedHashCollectionSetView();
}
}
