ordering = builder.ordering();
this.minHeap = new Heap(ordering);
this.maxHeap = new Heap(ordering.reverse());
minHeap.otherHeap = maxHeap;
maxHeap.otherHeap = minHeap;
this.maximumSize = builder.maximumSize;
// TODO(kevinb): pad?
this.queue = new Object[queueSize];
}
@Override
public int size() {
return size;
}
/**
* Adds the given element to this queue. If this queue has a maximum size, after adding {@code
* element} the queue will automatically evict its greatest element (according to its comparator),
* which may be {@code element} itself.
*
* @return {@code true} always
*/
@CanIgnoreReturnValue
@Override
public boolean add(E element) {
offer(element);
return true;
}
@CanIgnoreReturnValue
@Override
public boolean addAll(Collection newElements) {
boolean modified = false;
for (E element : newElements) {
offer(element);
modified = true;
}
return modified;
}
/**
* Adds the given element to this queue. If this queue has a maximum size, after adding {@code
* element} the queue will automatically evict its greatest element (according to its comparator),
* which may be {@code element} itself.
*/
@CanIgnoreReturnValue
@Override
public boolean offer(E element) {
checkNotNull(element);
modCount++;
int insertIndex = size++;
growIfNeeded();
// Adds the element to the end of the heap and bubbles it up to the correct
// position.
heapForIndex(insertIndex).bubbleUp(insertIndex, element);
return size <= maximumSize || pollLast() != element;
}
@CanIgnoreReturnValue
@Override
@CheckForNull
public E poll() {
return isEmpty() ? null : removeAndGet(0);
}
@SuppressWarnings("unchecked") // we must carefully only allow Es to get in
E elementData(int index) {
/*
* requireNonNull is safe as long as we're careful to call this method only with populated
* indexes.
*/
return (E) requireNonNull(queue[index]);
}
@Override
@CheckForNull
public E peek() {
return isEmpty() ? null : elementData(0);
}
/** Returns the index of the max element. */
private int getMaxElementIndex() {
switch (size) {
case 1:
return 0; // The lone element in the queue is the maximum.
case 2:
return 1; // The lone element in the maxHeap is the maximum.
default:
// The max element must sit on the first level of the maxHeap. It is
// actually the *lesser* of the two from the maxHeap's perspective.
return (maxHeap.compareElements(1, 2) <= 0) ? 1 : 2;
}
}
/**
* Removes and returns the least element of this queue, or returns {@code null} if the queue is
* empty.
*/
@CanIgnoreReturnValue
@CheckForNull
public E pollFirst() {
return poll();
}
/**
* Removes and returns the least element of this queue.
*
* @throws NoSuchElementException if the queue is empty
*/
@CanIgnoreReturnValue
public E removeFirst() {
return remove();
}
/**
* Retrieves, but does not remove, the least element of this queue, or returns {@code null} if the
* queue is empty.
*/
@CheckForNull
public E peekFirst() {
return peek();
}
/**
* Removes and returns the greatest element of this queue, or returns {@code null} if the queue is
* empty.
*/
@CanIgnoreReturnValue
@CheckForNull
public E pollLast() {
return isEmpty() ? null : removeAndGet(getMaxElementIndex());
}
/**
* Removes and returns the greatest element of this queue.
*
* @throws NoSuchElementException if the queue is empty
*/
@CanIgnoreReturnValue
public E removeLast() {
if (isEmpty()) {
throw new NoSuchElementException();
}
return removeAndGet(getMaxElementIndex());
}
/**
* Retrieves, but does not remove, the greatest element of this queue, or returns {@code null} if
* the queue is empty.
*/
@CheckForNull
public E peekLast() {
return isEmpty() ? null : elementData(getMaxElementIndex());
}
/**
* Removes the element at position {@code index}.
*
* Normally this method leaves the elements at up to {@code index - 1}, inclusive, untouched.
* Under these circumstances, it returns {@code null}.
*
*
Occasionally, in order to maintain the heap invariant, it must swap a later element of the
* list with one before {@code index}. Under these circumstances it returns a pair of elements as
* a {@link MoveDesc}. The first one is the element that was previously at the end of the heap and
* is now at some position before {@code index}. The second element is the one that was swapped
* down to replace the element at {@code index}. This fact is used by iterator.remove so as to
* visit elements during a traversal once and only once.
*/
@VisibleForTesting
@CanIgnoreReturnValue
@CheckForNull
MoveDesc removeAt(int index) {
checkPositionIndex(index, size);
modCount++;
size--;
if (size == index) {
queue[size] = null;
return null;
}
E actualLastElement = elementData(size);
int lastElementAt = heapForIndex(size).swapWithConceptuallyLastElement(actualLastElement);
if (lastElementAt == index) {
// 'actualLastElement' is now at 'lastElementAt', and the element that was at 'lastElementAt'
// is now at the end of queue. If that's the element we wanted to remove in the first place,
// don't try to (incorrectly) trickle it. Instead, just delete it and we're done.
queue[size] = null;
return null;
}
E toTrickle = elementData(size);
queue[size] = null;
MoveDesc changes = fillHole(index, toTrickle);
if (lastElementAt < index) {
// Last element is moved to before index, swapped with trickled element.
if (changes == null) {
// The trickled element is still after index.
return new MoveDesc<>(actualLastElement, toTrickle);
} else {
// The trickled element is back before index, but the replaced element
// has now been moved after index.
return new MoveDesc<>(actualLastElement, changes.replaced);
}
}
// Trickled element was after index to begin with, no adjustment needed.
return changes;
}
@CheckForNull
private MoveDesc fillHole(int index, E toTrickle) {
Heap heap = heapForIndex(index);
// We consider elementData(index) a "hole", and we want to fill it
// with the last element of the heap, toTrickle.
// Since the last element of the heap is from the bottom level, we
// optimistically fill index position with elements from lower levels,
// moving the hole down. In most cases this reduces the number of
// comparisons with toTrickle, but in some cases we will need to bubble it
// all the way up again.
int vacated = heap.fillHoleAt(index);
// Try to see if toTrickle can be bubbled up min levels.
int bubbledTo = heap.bubbleUpAlternatingLevels(vacated, toTrickle);
if (bubbledTo == vacated) {
// Could not bubble toTrickle up min levels, try moving
// it from min level to max level (or max to min level) and bubble up
// there.
return heap.tryCrossOverAndBubbleUp(index, vacated, toTrickle);
} else {
return (bubbledTo < index) ? new MoveDesc(toTrickle, elementData(index)) : null;
}
}
// Returned from removeAt() to iterator.remove()
static class MoveDesc {
final E toTrickle;
final E replaced;
MoveDesc(E toTrickle, E replaced) {
this.toTrickle = toTrickle;
this.replaced = replaced;
}
}
/** Removes and returns the value at {@code index}. */
private E removeAndGet(int index) {
E value = elementData(index);
removeAt(index);
return value;
}
private Heap heapForIndex(int i) {
return isEvenLevel(i) ? minHeap : maxHeap;
}
private static final int EVEN_POWERS_OF_TWO = 0x55555555;
private static final int ODD_POWERS_OF_TWO = 0xaaaaaaaa;
@VisibleForTesting
static boolean isEvenLevel(int index) {
int oneBased = ~~(index + 1); // for GWT
checkState(oneBased > 0, "negative index");
return (oneBased & EVEN_POWERS_OF_TWO) > (oneBased & ODD_POWERS_OF_TWO);
}
/**
* Returns {@code true} if the MinMax heap structure holds. This is only used in testing.
*
* TODO(kevinb): move to the test class?
*/
@VisibleForTesting
boolean isIntact() {
for (int i = 1; i < size; i++) {
if (!heapForIndex(i).verifyIndex(i)) {
return false;
}
}
return true;
}
/**
* Each instance of MinMaxPriorityQueue encapsulates two instances of Heap: a min-heap and a
* max-heap. Conceptually, these might each have their own array for storage, but for efficiency's
* sake they are stored interleaved on alternate heap levels in the same array (MMPQ.queue).
*/
@WeakOuter
class Heap {
final Ordering ordering;
@SuppressWarnings("nullness:initialization.field.uninitialized")
@Weak
Heap otherHeap; // always initialized immediately after construction
Heap(Ordering ordering) {
this.ordering = ordering;
}
int compareElements(int a, int b) {
return ordering.compare(elementData(a), elementData(b));
}
/**
* Tries to move {@code toTrickle} from a min to a max level and bubble up there. If it moved
* before {@code removeIndex} this method returns a pair as described in {@link #removeAt}.
*/
@CheckForNull
MoveDesc tryCrossOverAndBubbleUp(int removeIndex, int vacated, E toTrickle) {
int crossOver = crossOver(vacated, toTrickle);
if (crossOver == vacated) {
return null;
}
// Successfully crossed over from min to max.
// Bubble up max levels.
E parent;
// If toTrickle is moved up to a parent of removeIndex, the parent is
// placed in removeIndex position. We must return that to the iterator so
// that it knows to skip it.
if (crossOver < removeIndex) {
// We crossed over to the parent level in crossOver, so the parent
// has already been moved.
parent = elementData(removeIndex);
} else {
parent = elementData(getParentIndex(removeIndex));
}
// bubble it up the opposite heap
if (otherHeap.bubbleUpAlternatingLevels(crossOver, toTrickle) < removeIndex) {
return new MoveDesc<>(toTrickle, parent);
} else {
return null;
}
}
/** Bubbles a value from {@code index} up the appropriate heap if required. */
void bubbleUp(int index, E x) {
int crossOver = crossOverUp(index, x);
Heap heap;
if (crossOver == index) {
heap = this;
} else {
index = crossOver;
heap = otherHeap;
}
heap.bubbleUpAlternatingLevels(index, x);
}
/**
* Bubbles a value from {@code index} up the levels of this heap, and returns the index the
* element ended up at.
*/
@CanIgnoreReturnValue
int bubbleUpAlternatingLevels(int index, E x) {
while (index > 2) {
int grandParentIndex = getGrandparentIndex(index);
E e = elementData(grandParentIndex);
if (ordering.compare(e, x) <= 0) {
break;
}
queue[index] = e;
index = grandParentIndex;
}
queue[index] = x;
return index;
}
/**
* Returns the index of minimum value between {@code index} and {@code index + len}, or {@code
* -1} if {@code index} is greater than {@code size}.
*/
int findMin(int index, int len) {
if (index >= size) {
return -1;
}
checkState(index > 0);
int limit = Math.min(index, size - len) + len;
int minIndex = index;
for (int i = index + 1; i < limit; i++) {
if (compareElements(i, minIndex) < 0) {
minIndex = i;
}
}
return minIndex;
}
/** Returns the minimum child or {@code -1} if no child exists. */
int findMinChild(int index) {
return findMin(getLeftChildIndex(index), 2);
}
/** Returns the minimum grand child or -1 if no grand child exists. */
int findMinGrandChild(int index) {
int leftChildIndex = getLeftChildIndex(index);
if (leftChildIndex < 0) {
return -1;
}
return findMin(getLeftChildIndex(leftChildIndex), 4);
}
/**
* Moves an element one level up from a min level to a max level (or vice versa). Returns the
* new position of the element.
*/
int crossOverUp(int index, E x) {
if (index == 0) {
queue[0] = x;
return 0;
}
int parentIndex = getParentIndex(index);
E parentElement = elementData(parentIndex);
if (parentIndex != 0) {
/*
* This is a guard for the case of the childless aunt node. Since the end of the array is
* actually the middle of the heap, a smaller childless aunt node can become a child of x
* when we bubble up alternate levels, violating the invariant.
*/
int grandparentIndex = getParentIndex(parentIndex);
int auntIndex = getRightChildIndex(grandparentIndex);
if (auntIndex != parentIndex && getLeftChildIndex(auntIndex) >= size) {
E auntElement = elementData(auntIndex);
if (ordering.compare(auntElement, parentElement) < 0) {
parentIndex = auntIndex;
parentElement = auntElement;
}
}
}
if (ordering.compare(parentElement, x) < 0) {
queue[index] = parentElement;
queue[parentIndex] = x;
return parentIndex;
}
queue[index] = x;
return index;
}
// About the term "aunt node": it's better to leave gender out of it, but for this the English
// language has nothing for us. Except for the whimsical neologism "pibling" (!) which we
// obviously could not expect to increase anyone's understanding of the code.
/**
* Swap {@code actualLastElement} with the conceptually correct last element of the heap.
* Returns the index that {@code actualLastElement} now resides in.
*
* Since the last element of the array is actually in the middle of the sorted structure, a
* childless aunt node could be smaller, which would corrupt the invariant if this element
* becomes the new parent of the aunt node. In that case, we first switch the last element with
* its aunt node, before returning.
*/
int swapWithConceptuallyLastElement(E actualLastElement) {
int parentIndex = getParentIndex(size);
if (parentIndex != 0) {
int grandparentIndex = getParentIndex(parentIndex);
int auntIndex = getRightChildIndex(grandparentIndex);
if (auntIndex != parentIndex && getLeftChildIndex(auntIndex) >= size) {
E auntElement = elementData(auntIndex);
if (ordering.compare(auntElement, actualLastElement) < 0) {
queue[auntIndex] = actualLastElement;
queue[size] = auntElement;
return auntIndex;
}
}
}
return size;
}
/**
* Crosses an element over to the opposite heap by moving it one level down (or up if there are
* no elements below it).
*
*
Returns the new position of the element.
*/
int crossOver(int index, E x) {
int minChildIndex = findMinChild(index);
// TODO(kevinb): split the && into two if's and move crossOverUp so it's
// only called when there's no child.
if ((minChildIndex > 0) && (ordering.compare(elementData(minChildIndex), x) < 0)) {
queue[index] = elementData(minChildIndex);
queue[minChildIndex] = x;
return minChildIndex;
}
return crossOverUp(index, x);
}
/**
* Fills the hole at {@code index} by moving in the least of its grandchildren to this position,
* then recursively filling the new hole created.
*
* @return the position of the new hole (where the lowest grandchild moved from, that had no
* grandchild to replace it)
*/
int fillHoleAt(int index) {
int minGrandchildIndex;
while ((minGrandchildIndex = findMinGrandChild(index)) > 0) {
queue[index] = elementData(minGrandchildIndex);
index = minGrandchildIndex;
}
return index;
}
private boolean verifyIndex(int i) {
if ((getLeftChildIndex(i) < size) && (compareElements(i, getLeftChildIndex(i)) > 0)) {
return false;
}
if ((getRightChildIndex(i) < size) && (compareElements(i, getRightChildIndex(i)) > 0)) {
return false;
}
if ((i > 0) && (compareElements(i, getParentIndex(i)) > 0)) {
return false;
}
if ((i > 2) && (compareElements(getGrandparentIndex(i), i) > 0)) {
return false;
}
return true;
}
// These would be static if inner classes could have static members.
private int getLeftChildIndex(int i) {
return i * 2 + 1;
}
private int getRightChildIndex(int i) {
return i * 2 + 2;
}
private int getParentIndex(int i) {
return (i - 1) / 2;
}
private int getGrandparentIndex(int i) {
return getParentIndex(getParentIndex(i)); // (i - 3) / 4
}
}
/**
* Iterates the elements of the queue in no particular order.
*
*
If the underlying queue is modified during iteration an exception will be thrown.
*/
private class QueueIterator implements Iterator {
private int cursor = -1;
private int nextCursor = -1;
private int expectedModCount = modCount;
// The same element is not allowed in both forgetMeNot and skipMe, but duplicates are allowed in
// either of them, up to the same multiplicity as the queue.
@CheckForNull private Queue forgetMeNot;
@CheckForNull private List skipMe;
@CheckForNull private E lastFromForgetMeNot;
private boolean canRemove;
@Override
public boolean hasNext() {
checkModCount();
nextNotInSkipMe(cursor + 1);
return (nextCursor < size()) || ((forgetMeNot != null) && !forgetMeNot.isEmpty());
}
@Override
public E next() {
checkModCount();
nextNotInSkipMe(cursor + 1);
if (nextCursor < size()) {
cursor = nextCursor;
canRemove = true;
return elementData(cursor);
} else if (forgetMeNot != null) {
cursor = size();
lastFromForgetMeNot = forgetMeNot.poll();
if (lastFromForgetMeNot != null) {
canRemove = true;
return lastFromForgetMeNot;
}
}
throw new NoSuchElementException("iterator moved past last element in queue.");
}
@Override
public void remove() {
checkRemove(canRemove);
checkModCount();
canRemove = false;
expectedModCount++;
if (cursor < size()) {
MoveDesc moved = removeAt(cursor);
if (moved != null) {
// Either both are null or neither is, but we check both to satisfy the nullness checker.
if (forgetMeNot == null || skipMe == null) {
forgetMeNot = new ArrayDeque<>();
skipMe = new ArrayList<>(3);
}
if (!foundAndRemovedExactReference(skipMe, moved.toTrickle)) {
forgetMeNot.add(moved.toTrickle);
}
if (!foundAndRemovedExactReference(forgetMeNot, moved.replaced)) {
skipMe.add(moved.replaced);
}
}
cursor--;
nextCursor--;
} else { // we must have set lastFromForgetMeNot in next()
checkState(removeExact(requireNonNull(lastFromForgetMeNot)));
lastFromForgetMeNot = null;
}
}
/** Returns true if an exact reference (==) was found and removed from the supplied iterable. */
private boolean foundAndRemovedExactReference(Iterable elements, E target) {
for (Iterator it = elements.iterator(); it.hasNext(); ) {
E element = it.next();
if (element == target) {
it.remove();
return true;
}
}
return false;
}
/** Removes only this exact instance, not others that are equals() */
private boolean removeExact(Object target) {
for (int i = 0; i < size; i++) {
if (queue[i] == target) {
removeAt(i);
return true;
}
}
return false;
}
private void checkModCount() {
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
/**
* Advances nextCursor to the index of the first element after {@code c} that is not in {@code
* skipMe} and returns {@code size()} if there is no such element.
*/
private void nextNotInSkipMe(int c) {
if (nextCursor < c) {
if (skipMe != null) {
while (c < size() && foundAndRemovedExactReference(skipMe, elementData(c))) {
c++;
}
}
nextCursor = c;
}
}
}
/**
* Returns an iterator over the elements contained in this collection, in no particular
* order.
*
* The iterator is fail-fast: If the MinMaxPriorityQueue is modified at any time after
* the iterator is created, in any way except through the iterator's own remove method, the
* iterator will generally throw a {@link ConcurrentModificationException}. Thus, in the face of
* concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary,
* non-deterministic behavior at an undetermined time in the future.
*
*
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally
* speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent
* modification. Fail-fast iterators throw {@code ConcurrentModificationException} on a
* best-effort basis. Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: the fail-fast behavior of iterators should be used only to
* detect bugs.
*
* @return an iterator over the elements contained in this collection
*/
@Override
public Iterator iterator() {
return new QueueIterator();
}
@Override
public void clear() {
for (int i = 0; i < size; i++) {
queue[i] = null;
}
size = 0;
}
@Override
@J2ktIncompatible // Incompatible return type change. Use inherited (unoptimized) implementation
public Object[] toArray() {
Object[] copyTo = new Object[size];
System.arraycopy(queue, 0, copyTo, 0, size);
return copyTo;
}
/**
* Returns the comparator used to order the elements in this queue. Obeys the general contract of
* {@link PriorityQueue#comparator}, but returns {@link Ordering#natural} instead of {@code null}
* to indicate natural ordering.
*/
public Comparator comparator() {
return minHeap.ordering;
}
@VisibleForTesting
int capacity() {
return queue.length;
}
// Size/capacity-related methods
private static final int DEFAULT_CAPACITY = 11;
@VisibleForTesting
static int initialQueueSize(
int configuredExpectedSize, int maximumSize, Iterable initialContents) {
// Start with what they said, if they said it, otherwise DEFAULT_CAPACITY
int result =
(configuredExpectedSize == Builder.UNSET_EXPECTED_SIZE)
? DEFAULT_CAPACITY
: configuredExpectedSize;
// Enlarge to contain initial contents
if (initialContents instanceof Collection) {
int initialSize = ((Collection) initialContents).size();
result = Math.max(result, initialSize);
}
// Now cap it at maxSize + 1
return capAtMaximumSize(result, maximumSize);
}
private void growIfNeeded() {
if (size > queue.length) {
int newCapacity = calculateNewCapacity();
Object[] newQueue = new Object[newCapacity];
System.arraycopy(queue, 0, newQueue, 0, queue.length);
queue = newQueue;
}
}
/** Returns ~2x the old capacity if small; ~1.5x otherwise. */
private int calculateNewCapacity() {
int oldCapacity = queue.length;
int newCapacity =
(oldCapacity < 64) ? (oldCapacity + 1) * 2 : IntMath.checkedMultiply(oldCapacity / 2, 3);
return capAtMaximumSize(newCapacity, maximumSize);
}
/** There's no reason for the queueSize to ever be more than maxSize + 1 */
private static int capAtMaximumSize(int queueSize, int maximumSize) {
return Math.min(queueSize - 1, maximumSize) + 1; // don't overflow
}
}