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queue.dart
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// Copyright (c) 2011, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
part of dart.collection;
/// A [Queue] is a collection that can be manipulated at both ends. One
/// can iterate over the elements of a queue through [forEach] or with
/// an [Iterator].
///
/// It is generally not allowed to modify the queue (add or remove entries)
/// while an operation in the queue is being performed, for example during a
/// call to [forEach].
/// Modifying the queue while it is being iterated will most likely break the
/// iteration.
/// This goes both for using the [iterator] directly, or for iterating an
/// `Iterable` returned by a method like [map] or [where].
///
/// Example:
/// ```dart
/// final queue = Queue<int>(); // ListQueue() by default
/// print(queue.runtimeType); // ListQueue
///
/// // Adding items to queue
/// queue.addAll([1, 2, 3]);
/// queue.addFirst(0);
/// queue.addLast(10);
/// print(queue); // {0, 1, 2, 3, 10}
///
/// // Removing items from queue
/// queue.removeFirst();
/// queue.removeLast();
/// print(queue); // {1, 2, 3}
/// ```
abstract interface class Queue<E> implements EfficientLengthIterable<E> {
/// Creates a queue.
factory Queue() = ListQueue<E>;
/// Creates a queue containing all [elements].
///
/// The element order in the queue is as if the elements were added using
/// [addLast] in the order provided by [elements].iterator.
///
/// All the [elements] should be instances of [E].
/// The `elements` iterable itself may have any element type, so this
/// constructor can be used to down-cast a `Queue`, for example as:
/// ```dart
/// Queue<SuperType> superQueue = ...;
/// Queue<SubType> subQueue =
/// Queue<SubType>.from(superQueue.whereType<SubType>());
/// ```
factory Queue.from(Iterable elements) = ListQueue<E>.from;
/// Creates a queue from [elements].
///
/// The element order in the queue is as if the elements were added using
/// [addLast] in the order provided by [elements].iterator.
factory Queue.of(Iterable<E> elements) = ListQueue<E>.of;
/// Adapts [source] to be a `Queue<T>`.
///
/// Any time the queue would produce an element that is not a [T],
/// the element access will throw.
///
/// When a [T] value is stored into the adapted queue,
/// the operation will throw unless the value is also an instance of [S].
///
/// If all accessed elements of [source] are actually instances of [T],
/// and if all elements stored into the returned queue are actually instances
/// of [S],
/// then the returned queue can be used as a `Queue<T>`.
///
/// Methods which accept `Object?` as argument, like [contains] and [remove],
/// will pass the argument directly to this queue's method
/// without any checks.
static Queue<T> castFrom<S, T>(Queue<S> source) => CastQueue<S, T>(source);
/// Provides a view of this queue as a queue of [R] instances, if necessary.
///
/// If this queue contains only instances of [R], all read operations
/// will work correctly. If any operation tries to access an element
/// that is not an instance of [R], the access will throw instead.
///
/// Elements added to the queue (e.g., by using [addFirst] or [addAll])
/// must be instances of [R] to be valid arguments to the adding function,
/// and they must also be instances of [E] to be accepted by
/// this queue as well.
///
/// Methods which accept `Object?` as argument, like [contains] and [remove],
/// will pass the argument directly to the this queue's method
/// without any checks.
/// That means that you can do `queueOfStrings.cast<int>().remove("a")`
/// successfully, even if it looks like it shouldn't have any effect.
Queue<R> cast<R>();
/// Removes and returns the first element of this queue.
///
/// The queue must not be empty when this method is called.
E removeFirst();
/// Removes and returns the last element of the queue.
///
/// The queue must not be empty when this method is called.
E removeLast();
/// Adds [value] at the beginning of the queue.
void addFirst(E value);
/// Adds [value] at the end of the queue.
void addLast(E value);
/// Adds [value] at the end of the queue.
void add(E value);
/// Removes a single instance of [value] from the queue.
///
/// Returns `true` if a value was removed, or `false` if the queue
/// contained no element equal to [value].
bool remove(Object? value);
/// Adds all elements of [iterable] at the end of the queue. The
/// length of the queue is extended by the length of [iterable].
void addAll(Iterable<E> iterable);
/// Removes all elements matched by [test] from the queue.
///
/// The `test` function must not throw or modify the queue.
void removeWhere(bool test(E element));
/// Removes all elements not matched by [test] from the queue.
///
/// The `test` function must not throw or modify the queue.
void retainWhere(bool test(E element));
/// Removes all elements in the queue. The size of the queue becomes zero.
void clear();
}
/// Interface and base class for the link classes used by [DoubleLinkedQueue].
///
/// Both the [_DoubleLinkedQueueElement] and [_DoubleLinkedQueueSentinel]
/// implement this interface.
abstract class _DoubleLinkedQueueEntry<E> {
_DoubleLinkedQueueEntry<E>? _previousLink;
_DoubleLinkedQueueEntry<E>? _nextLink;
void _link(
_DoubleLinkedQueueEntry<E>? previous, _DoubleLinkedQueueEntry<E>? next) {
_nextLink = next;
_previousLink = previous;
previous?._nextLink = this;
next?._previousLink = this;
}
void _unlink() {
_previousLink?._nextLink = _nextLink;
_nextLink?._previousLink = _previousLink;
_previousLink = _nextLink = null;
}
_DoubleLinkedQueueElement<E>? _asNonSentinelEntry();
void _append(E element, DoubleLinkedQueue<E>? queue) {
_DoubleLinkedQueueElement<E>(element, queue)._link(this, _nextLink);
}
void _prepend(E element, DoubleLinkedQueue<E>? queue) {
_DoubleLinkedQueueElement<E>(element, queue)._link(_previousLink, this);
}
E _remove();
E get element;
}
/// Linked list entry used by the [DoubleLinkedQueue] to hold an element.
///
/// These entry objects are also exposed by [DoubleLinkedQueue.firstEntry],
/// [DoubleLinkedQueue.lastEntry] and [DoubleLinkedQueue.forEachEntry].
///
/// The entry contains both the [element] (which is mutable to anyone with
/// access to the entry object) and a reference to the queue, allowing
/// [append]/[prepend] to update the list length.
///
/// When an entry is removed from its queue, the [_queue] is set to `null`
/// and will never change again. You can still use the unlinked entry
/// to create a new list, by calling [append] and [prepend], but it won't
/// be part of any [DoubleLinkedQueue].
class _DoubleLinkedQueueElement<E> extends _DoubleLinkedQueueEntry<E>
implements DoubleLinkedQueueEntry<E> {
DoubleLinkedQueue<E>? _queue;
E element;
_DoubleLinkedQueueElement(this.element, this._queue);
void append(E e) {
_append(e, _queue);
_queue?._elementCount++;
}
void prepend(E e) {
_prepend(e, _queue);
_queue?._elementCount++;
}
E _remove() {
_queue = null;
_unlink();
return element;
}
E remove() {
_queue?._elementCount--;
return _remove();
}
_DoubleLinkedQueueElement<E> _asNonSentinelEntry() => this;
DoubleLinkedQueueEntry<E>? previousEntry() =>
_previousLink?._asNonSentinelEntry();
DoubleLinkedQueueEntry<E>? nextEntry() => _nextLink?._asNonSentinelEntry();
}
/// A header object used to hold the two ends of a double linked queue.
///
/// A [DoubleLinkedQueue] has exactly one sentinel,
/// which is the only entry when the list is constructed.
///
/// Initially, a sentinel has its next and previous entries point to itself.
/// Its next and previous links are never `null` after creation, and
/// the entries linked always form a circular structure with the next link
/// pointing to the first element of the queue, and the previous link
/// pointing to the last element of the queue, or both pointing to itself
/// again if the queue becomes empty.
///
/// Implements [_DoubleLinkedQueueEntry._remove] and
/// [_DoubleLinkedQueueEntry.element] as throwing because
/// it makes it simple to implement members like [Queue.removeFirst]
/// or [Queue.first] as throwing on an empty queue.
///
/// A sentinel does not contain any user element.
class _DoubleLinkedQueueSentinel<E> extends _DoubleLinkedQueueEntry<E> {
_DoubleLinkedQueueSentinel() {
_previousLink = this;
_nextLink = this;
}
Null _asNonSentinelEntry() => null;
/// Hit by, e.g., [DoubleLinkedQueue.removeFirst] if the queue is empty.
E _remove() {
throw IterableElementError.noElement();
}
/// Hit by, e.g., [DoubleLinkedQueue.first] if the queue is empty.
E get element {
throw IterableElementError.noElement();
}
}
/// A [Queue] implementation based on a double-linked list.
///
/// Allows constant time add, remove-at-ends and peek operations.
final class DoubleLinkedQueue<E> extends Iterable<E> implements Queue<E> {
final _DoubleLinkedQueueSentinel<E> _sentinel =
_DoubleLinkedQueueSentinel<E>();
int _elementCount = 0;
DoubleLinkedQueue();
/// Creates a double-linked queue containing all [elements].
///
/// The element order in the queue is as if the elements were added using
/// [addLast] in the order provided by [elements].iterator.
///
/// All the [elements] should be instances of [E].
/// The [elements] iterable itself may have any element type, so this
/// constructor can be used to down-cast a [Queue], for example as:
/// ```dart
/// Queue<SuperType> superQueue = ...;
/// Queue<SubType> subQueue =
/// DoubleLinkedQueue<SubType>.from(superQueue.whereType<SubType>());
/// ```
factory DoubleLinkedQueue.from(Iterable<dynamic> elements) {
DoubleLinkedQueue<E> list = DoubleLinkedQueue<E>();
for (final e in elements) {
list.addLast(e as E);
}
return list;
}
/// Creates a double-linked queue from [elements].
///
/// The element order in the queue is as if the elements were added using
/// [addLast] in the order provided by [elements].iterator.
factory DoubleLinkedQueue.of(Iterable<E> elements) =>
DoubleLinkedQueue<E>()..addAll(elements);
Queue<R> cast<R>() => Queue.castFrom<E, R>(this);
int get length => _elementCount;
void addLast(E value) {
_sentinel._prepend(value, this);
_elementCount++;
}
void addFirst(E value) {
_sentinel._append(value, this);
_elementCount++;
}
void add(E value) {
_sentinel._prepend(value, this);
_elementCount++;
}
void addAll(Iterable<E> iterable) {
for (final E value in iterable) {
_sentinel._prepend(value, this);
_elementCount++;
}
}
E removeLast() {
// Hits sentinel's `_remove` if queue is empty.
E result = _sentinel._previousLink!._remove();
_elementCount--;
return result;
}
E removeFirst() {
// Hits sentinel's `_remove` if queue is empty.
E result = _sentinel._nextLink!._remove();
_elementCount--;
return result;
}
bool remove(Object? o) {
_DoubleLinkedQueueEntry<E> entry = _sentinel._nextLink!;
while (true) {
var elementEntry = entry._asNonSentinelEntry();
if (elementEntry == null) return false;
bool equals = (elementEntry.element == o);
if (!identical(this, elementEntry._queue)) {
// Entry must still be in the queue.
throw ConcurrentModificationError(this);
}
if (equals) {
entry._remove();
_elementCount--;
return true;
}
entry = entry._nextLink!;
}
}
void _filter(bool test(E element), bool removeMatching) {
_DoubleLinkedQueueEntry<E> entry = _sentinel._nextLink!;
while (true) {
var elementEntry = entry._asNonSentinelEntry();
if (elementEntry == null) return;
bool matches = test(elementEntry.element);
if (!identical(this, elementEntry._queue)) {
// Entry must still be in the queue.
throw ConcurrentModificationError(this);
}
var next = entry._nextLink!; // Cannot be null while entry is in queue.
if (identical(removeMatching, matches)) {
elementEntry._remove();
_elementCount--;
}
entry = next;
}
}
void removeWhere(bool test(E element)) {
_filter(test, true);
}
void retainWhere(bool test(E element)) {
_filter(test, false);
}
// Hits sentinel's `get element` if no element in queue.
E get first => _sentinel._nextLink!.element;
// Hits sentinel's `get element` if no element in queue.
E get last => _sentinel._previousLink!.element;
E get single {
// Note that this throws correctly if the queue is empty
// because reading the element of the sentinel throws.
if (identical(_sentinel._nextLink, _sentinel._previousLink)) {
return _sentinel._nextLink!.element;
}
throw IterableElementError.tooMany();
}
/// The entry object of the first element in the queue.
///
/// Each element of the queue has an associated [DoubleLinkedQueueEntry].
///
/// Returns the entry object corresponding to the first element of the queue,
/// or `null` if the queue is empty.
///
/// The entry objects can also be accessed using [lastEntry],
/// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry] and
/// [DoubleLinkedQueueEntry.previousEntry].
DoubleLinkedQueueEntry<E>? firstEntry() =>
_sentinel._nextLink!._asNonSentinelEntry();
/// The entry object of the last element in the queue.
///
/// Each element of the queue has an associated [DoubleLinkedQueueEntry].
///
/// Returns the entry object corresponding to the last element of the queue,
/// or `null` if the queue is empty.
///
/// The entry objects can also be accessed using [firstEntry],
/// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry] and
/// [DoubleLinkedQueueEntry.previousEntry].
DoubleLinkedQueueEntry<E>? lastEntry() =>
_sentinel._previousLink!._asNonSentinelEntry();
bool get isEmpty => identical(_sentinel._nextLink, _sentinel);
void clear() {
var cursor = _sentinel._nextLink!;
while (true) {
var entry = cursor._asNonSentinelEntry();
if (entry == null) break;
cursor = cursor._nextLink!;
entry
.._nextLink = null
.._previousLink = null
.._queue = null;
}
_sentinel._nextLink = _sentinel;
_sentinel._previousLink = _sentinel;
_elementCount = 0;
}
/// Calls [action] for each entry object of this double-linked queue.
///
/// Each element of the queue has an associated [DoubleLinkedQueueEntry].
/// This method iterates the entry objects from first to last and calls
/// [action] with each object in turn.
///
/// The entry objects can also be accessed using [firstEntry] and [lastEntry],
/// and iterated using [DoubleLinkedQueueEntry.nextEntry()] and
/// [DoubleLinkedQueueEntry.previousEntry()].
///
/// The [action] function can use methods on [DoubleLinkedQueueEntry] to
/// remove the entry or it can insert elements before or after the entry.
/// If the current entry is removed, iteration continues with the entry that
/// was following the current entry when [action] was called. Any elements
/// inserted after the current element before it is removed will not be
/// visited by the iteration.
void forEachEntry(void action(DoubleLinkedQueueEntry<E> element)) {
var cursor = _sentinel._nextLink!;
while (true) {
var element = cursor._asNonSentinelEntry();
if (element == null) break;
if (!identical(element._queue, this)) {
throw ConcurrentModificationError(this);
}
cursor = cursor._nextLink!;
// Remember both element and element._nextLink (as cursor).
// If someone calls `element.remove()` we continue from `next`.
// Otherwise we use the value of element._nextLink which may have been
// updated.
action(element);
if (identical(this, element._queue)) {
cursor = element._nextLink!;
}
}
}
_DoubleLinkedQueueIterator<E> get iterator {
return _DoubleLinkedQueueIterator<E>(this);
}
String toString() => Iterable.iterableToFullString(this, '{', '}');
}
class _DoubleLinkedQueueIterator<E> implements Iterator<E> {
/// Queue being iterated. Used for concurrent modification checks.
DoubleLinkedQueue<E>? _queue;
/// Next entry to visit. Set to null when hitting the sentinel.
_DoubleLinkedQueueEntry<E>? _nextEntry;
/// Current element value, when valid.
E? _current;
_DoubleLinkedQueueIterator(DoubleLinkedQueue<E> this._queue)
: _nextEntry = _queue._sentinel._nextLink;
bool moveNext() {
var nextElement = _nextEntry?._asNonSentinelEntry();
if (nextElement == null) {
// Clear everything to not unnecessarily keep values alive.
_current = null;
_nextEntry = null;
_queue = null;
return false;
}
if (!identical(_queue, nextElement._queue)) {
throw ConcurrentModificationError(_queue);
}
_current = nextElement.element;
_nextEntry = nextElement._nextLink;
return true;
}
E get current => _current as E;
}
/// List based [Queue].
///
/// Keeps a cyclic buffer of elements, and grows to a larger buffer when
/// it fills up. This guarantees constant time peek and remove operations, and
/// amortized constant time add operations.
///
/// The structure is efficient for any queue or stack usage.
///
/// Example:
/// ```dart
/// final queue = ListQueue<int>();
/// ```
/// To add objects to a queue, use [add], [addAll], [addFirst] or[addLast].
/// ```dart continued
/// queue.add(5);
/// queue.addFirst(0);
/// queue.addLast(10);
/// queue.addAll([1, 2, 3]);
/// print(queue); // {0, 5, 10, 1, 2, 3}
/// ```
/// To check if the queue is empty, use [isEmpty] or [isNotEmpty].
/// To find the number of queue entries, use [length].
/// ```dart continued
/// final isEmpty = queue.isEmpty; // false
/// final queueSize = queue.length; // 6
/// ```
/// To get first or last item from queue, use [first] or [last].
/// ```dart continued
/// final first = queue.first; // 0
/// final last = queue.last; // 3
/// ```
/// To get item value using index, use [elementAt].
/// ```dart continued
/// final itemAt = queue.elementAt(2); // 10
/// ```
/// To convert queue to list, call [toList].
/// ```dart continued
/// final numbers = queue.toList();
/// print(numbers); // [0, 5, 10, 1, 2, 3]
/// ```
/// To remove item from queue, call [remove], [removeFirst] or [removeLast].
/// ```dart continued
/// queue.remove(10);
/// queue.removeFirst();
/// queue.removeLast();
/// print(queue); // {5, 1, 2}
/// ```
/// To remove multiple elements at the same time, use [removeWhere].
/// ```dart continued
/// queue.removeWhere((element) => element == 1);
/// print(queue); // {5, 2}
/// ```
/// To remove all elements in this queue that do not meet a condition,
/// use [retainWhere].
/// ```dart continued
/// queue.retainWhere((element) => element < 4);
/// print(queue); // {2}
/// ```
/// To remove all items and empty the set, use [clear].
/// ```dart continued
/// queue.clear();
/// print(queue.isEmpty); // true
/// print(queue); // {}
/// ```
final class ListQueue<E> extends ListIterable<E> implements Queue<E> {
static const int _INITIAL_CAPACITY = 8;
List<E?> _table;
int _head;
int _tail;
int _modificationCount = 0;
/// Create an empty queue.
///
/// If [initialCapacity] is given, prepare the queue for at least that many
/// elements.
ListQueue([int? initialCapacity])
: _head = 0,
_tail = 0,
_table = List<E?>.filled(_calculateCapacity(initialCapacity), null);
static int _calculateCapacity(int? initialCapacity) {
if (initialCapacity == null || initialCapacity < _INITIAL_CAPACITY) {
return _INITIAL_CAPACITY;
} else if (!_isPowerOf2(initialCapacity)) {
return _nextPowerOf2(initialCapacity);
}
assert(_isPowerOf2(initialCapacity));
return initialCapacity;
}
/// Create a `ListQueue` containing all [elements].
///
/// The elements are added to the queue, as by [addLast], in the order given
/// by `elements.iterator`.
///
/// All the [elements] should be instances of [E].
/// The `elements` iterable itself may have any element type, so this
/// constructor can be used to down-cast a `Queue`, for example as:
/// ```dart
/// Queue<SuperType> superQueue = ...;
/// Queue<SubType> subQueue =
/// ListQueue<SubType>.from(superQueue.whereType<SubType>());
/// ```
/// Example:
/// ```dart
/// final numbers = <num>[10, 20, 30];
/// final queue = ListQueue<int>.from(numbers);
/// print(queue); // {10, 20, 30}
/// ```
factory ListQueue.from(Iterable<dynamic> elements) {
if (elements is List<dynamic>) {
int length = elements.length;
ListQueue<E> queue = ListQueue<E>(length + 1);
assert(queue._table.length > length);
for (int i = 0; i < length; i++) {
queue._table[i] = elements[i] as E;
}
queue._tail = length;
return queue;
} else {
int capacity = _INITIAL_CAPACITY;
if (elements is EfficientLengthIterable) {
capacity = elements.length;
}
ListQueue<E> result = ListQueue<E>(capacity);
for (final element in elements) {
result.addLast(element as E);
}
return result;
}
}
/// Create a `ListQueue` from [elements].
///
/// The elements are added to the queue, as by [addLast], in the order given
/// by `elements.iterator`.
/// Example:
/// ```dart
/// final baseQueue = ListQueue.of([1.0, 2.0, 3.0]); // A ListQueue<double>
/// final numQueue = ListQueue<num>.of(baseQueue);
/// print(numQueue); // {1.0, 2.0, 3.0}
/// ```
factory ListQueue.of(Iterable<E> elements) =>
ListQueue<E>()..addAll(elements);
// Iterable interface.
Queue<R> cast<R>() => Queue.castFrom<E, R>(this);
Iterator<E> get iterator => _ListQueueIterator<E>(this);
void forEach(void f(E element)) {
int modificationCount = _modificationCount;
for (int i = _head; i != _tail; i = (i + 1) & (_table.length - 1)) {
f(_table[i] as E);
_checkModification(modificationCount);
}
}
bool get isEmpty => _head == _tail;
int get length => (_tail - _head) & (_table.length - 1);
E get first {
if (_head == _tail) throw IterableElementError.noElement();
return _table[_head] as E;
}
E get last {
if (_head == _tail) throw IterableElementError.noElement();
return _table[(_tail - 1) & (_table.length - 1)] as E;
}
E get single {
if (_head == _tail) throw IterableElementError.noElement();
if (length > 1) throw IterableElementError.tooMany();
return _table[_head] as E;
}
E elementAt(int index) {
IndexError.check(index, length, indexable: this);
return _table[(_head + index) & (_table.length - 1)] as E;
}
List<E> toList({bool growable = true}) {
int mask = _table.length - 1;
int length = (_tail - _head) & mask;
if (length == 0) return List<E>.empty(growable: growable);
var list = List<E>.filled(length, first, growable: growable);
for (int i = 0; i < length; i++) {
list[i] = _table[(_head + i) & mask] as E;
}
return list;
}
// Collection interface.
void add(E value) {
_add(value);
}
void addAll(Iterable<E> elements) {
if (elements is List<E>) {
List<E> list = elements;
int addCount = list.length;
int length = this.length;
if (length + addCount >= _table.length) {
_preGrow(length + addCount);
// After preGrow, all elements are at the start of the list.
_table.setRange(length, length + addCount, list, 0);
_tail += addCount;
} else {
// Adding addCount elements won't reach _head.
int endSpace = _table.length - _tail;
if (addCount < endSpace) {
_table.setRange(_tail, _tail + addCount, list, 0);
_tail += addCount;
} else {
int preSpace = addCount - endSpace;
_table.setRange(_tail, _tail + endSpace, list, 0);
_table.setRange(0, preSpace, list, endSpace);
_tail = preSpace;
}
}
_modificationCount++;
} else {
for (E element in elements) _add(element);
}
}
bool remove(Object? value) {
for (int i = _head; i != _tail; i = (i + 1) & (_table.length - 1)) {
E? element = _table[i];
if (element == value) {
_remove(i);
_modificationCount++;
return true;
}
}
return false;
}
void _filterWhere(bool test(E element), bool removeMatching) {
int modificationCount = _modificationCount;
int i = _head;
while (i != _tail) {
E element = _table[i] as E;
bool remove = identical(removeMatching, test(element));
_checkModification(modificationCount);
if (remove) {
i = _remove(i);
modificationCount = ++_modificationCount;
} else {
i = (i + 1) & (_table.length - 1);
}
}
}
/// Remove all elements matched by [test].
///
/// This method is inefficient since it works by repeatedly removing single
/// elements, each of which can take linear time.
void removeWhere(bool test(E element)) {
_filterWhere(test, true);
}
/// Remove all elements not matched by [test].
///
/// This method is inefficient since it works by repeatedly removing single
/// elements, each of which can take linear time.
void retainWhere(bool test(E element)) {
_filterWhere(test, false);
}
void clear() {
if (_head != _tail) {
for (int i = _head; i != _tail; i = (i + 1) & (_table.length - 1)) {
_table[i] = null;
}
_head = _tail = 0;
_modificationCount++;
}
}
String toString() => Iterable.iterableToFullString(this, "{", "}");
// Queue interface.
void addLast(E value) {
_add(value);
}
void addFirst(E value) {
_head = (_head - 1) & (_table.length - 1);
_table[_head] = value;
if (_head == _tail) _grow();
_modificationCount++;
}
E removeFirst() {
if (_head == _tail) throw IterableElementError.noElement();
_modificationCount++;
E result = _table[_head] as E;
_table[_head] = null;
_head = (_head + 1) & (_table.length - 1);
return result;
}
E removeLast() {
if (_head == _tail) throw IterableElementError.noElement();
_modificationCount++;
_tail = (_tail - 1) & (_table.length - 1);
E result = _table[_tail] as E;
_table[_tail] = null;
return result;
}
// Internal helper functions.
/// Whether [number] is a power of two.
///
/// Only works for positive numbers.
static bool _isPowerOf2(int number) => (number & (number - 1)) == 0;
/// Rounds [number] up to the nearest power of 2.
///
/// If [number] is a power of 2 already, it is returned.
///
/// Only works for positive numbers.
static int _nextPowerOf2(int number) {
assert(number > 0);
number = (number << 1) - 1;
for (;;) {
int nextNumber = number & (number - 1);
if (nextNumber == 0) return number;
number = nextNumber;
}
}
/// Check if the queue has been modified during iteration.
void _checkModification(int expectedModificationCount) {
if (expectedModificationCount != _modificationCount) {
throw ConcurrentModificationError(this);
}
}
/// Adds element at end of queue. Used by both [add] and [addAll].
void _add(E element) {
_table[_tail] = element;
_tail = (_tail + 1) & (_table.length - 1);
if (_head == _tail) _grow();
_modificationCount++;
}
/// Removes the element at [offset] into [_table].
///
/// Removal is performed by linearly moving elements either before or after
/// [offset] by one position.
///
/// Returns the new offset of the following element. This may be the same
/// offset or the following offset depending on how elements are moved
/// to fill the hole.
int _remove(int offset) {
int mask = _table.length - 1;
int startDistance = (offset - _head) & mask;
int endDistance = (_tail - offset) & mask;
if (startDistance < endDistance) {
// Closest to start.
int i = offset;
while (i != _head) {
int prevOffset = (i - 1) & mask;
_table[i] = _table[prevOffset];
i = prevOffset;
}
_table[_head] = null;
_head = (_head + 1) & mask;
return (offset + 1) & mask;
} else {
_tail = (_tail - 1) & mask;
int i = offset;
while (i != _tail) {
int nextOffset = (i + 1) & mask;
_table[i] = _table[nextOffset];
i = nextOffset;
}
_table[_tail] = null;
return offset;
}
}
/// Grow the table when full.
void _grow() {
List<E?> newTable = List<E?>.filled(_table.length * 2, null);
int split = _table.length - _head;
newTable.setRange(0, split, _table, _head);
newTable.setRange(split, split + _head, _table, 0);
_head = 0;
_tail = _table.length;
_table = newTable;
}
int _writeToList(List<E?> target) {
assert(target.length >= length);
if (_head <= _tail) {
int length = _tail - _head;
target.setRange(0, length, _table, _head);
return length;
} else {
int firstPartSize = _table.length - _head;
target.setRange(0, firstPartSize, _table, _head);
target.setRange(firstPartSize, firstPartSize + _tail, _table, 0);
return _tail + firstPartSize;
}
}
/// Grows the table even if it is not full.
void _preGrow(int newElementCount) {
assert(newElementCount >= length);
// Add some extra room to ensure that there's room for more elements after
// expansion.
newElementCount += newElementCount >> 1;
int newCapacity = _nextPowerOf2(newElementCount);
List<E?> newTable = List<E?>.filled(newCapacity, null);
_tail = _writeToList(newTable);
_table = newTable;
_head = 0;
}
}
/// Iterator for a [ListQueue].
///
/// Considers any add or remove operation a concurrent modification.
class _ListQueueIterator<E> implements Iterator<E> {
final ListQueue<E> _queue;
final int _end;
final int _modificationCount;
int _position;
E? _current;
_ListQueueIterator(ListQueue<E> queue)
: _queue = queue,
_end = queue._tail,
_modificationCount = queue._modificationCount,
_position = queue._head;
E get current => _current as E;
bool moveNext() {
_queue._checkModification(_modificationCount);
if (_position == _end) {
_current = null;
return false;
}
_current = _queue._table[_position];
_position = (_position + 1) & (_queue._table.length - 1);
return true;
}
}