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linked_list.dart
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linked_list.dart
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// Copyright (c) 2013, 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 specialized double-linked list of elements that extends [LinkedListEntry].
///
/// This is not a generic data structure. It only accepts elements that *extend*
/// the [LinkedListEntry] class. See the [Queue] implementations for generic
/// collections that allow constant time adding and removing at the ends.
///
/// This is not a [List] implementation. Despite its name, this class does not
/// implement the [List] interface. It does not allow constant time lookup by
/// index.
///
/// Because the elements themselves contain the links of this linked list,
/// each element can be in only one list at a time. To add an element to another
/// list, it must first be removed from its current list (if any).
/// For the same reason, the [remove] and [contains] methods
/// are based on *identity*, even if the [LinkedListEntry] chooses
/// to override [Object.==].
///
/// In return, each element knows its own place in the linked list, as well as
/// which list it is in. This allows constant time
/// [LinkedListEntry.insertAfter], [LinkedListEntry.insertBefore] and
/// [LinkedListEntry.unlink] operations when all you have is the element.
///
/// A `LinkedList` also allows constant time adding and removing at either end,
/// and a constant time length getter.
///
/// Example:
/// ```dart
/// class EntryItem extends LinkedListEntry<EntryItem> {
/// final int id;
/// final String text;
/// EntryItem(this.id, this.text);
///
/// @override
/// String toString() {
/// return '$id : $text';
/// }
/// }
///
/// void main(){
/// final linkedList = LinkedList<EntryItem>();
/// linkedList.addAll(
/// [EntryItem(1, 'A'), EntryItem(2, 'B'), EntryItem(3, 'C')]);
/// print(linkedList.first); // 1 : A
/// print(linkedList.last); // 3 : C
///
/// // Add new item after first item.
/// linkedList.first.insertAfter(EntryItem(15, 'E'));
/// // Add new item before last item.
/// linkedList.last.insertBefore(EntryItem(10, 'D'));
/// // Iterate items.
/// for (var entry in linkedList) {
/// print(entry);
/// // 1 : A
/// // 15 : E
/// // 2 : B
/// // 10 : D
/// // 3 : C
/// }
///
/// // Remove item using index from list.
/// linkedList.elementAt(2).unlink();
/// print(linkedList); // (1 : A, 15 : E, 10 : D, 3 : C)
/// // Remove first item.
/// linkedList.first.unlink();
/// print(linkedList); // (15 : E, 10 : D, 3 : C)
/// // Remove last item from list.
/// linkedList.remove(linkedList.last);
/// print(linkedList); // (15 : E, 10 : D)
/// // Remove all items.
/// linkedList.clear();
/// print(linkedList.length); // 0
/// print(linkedList.isEmpty); // true
/// }
/// ```
class LinkedList<E extends LinkedListEntry<E>> extends Iterable<E> {
int _modificationCount = 0;
int _length = 0;
E? _first;
/// Constructs a new empty linked list.
LinkedList();
/// Adds [entry] to the beginning of the linked list.
void addFirst(E entry) {
_insertBefore(_first, entry, updateFirst: true);
_first = entry;
}
/// Adds [entry] to the end of the linked list.
void add(E entry) {
_insertBefore(_first, entry, updateFirst: false);
}
/// Adds [entries] to the end of the linked list.
void addAll(Iterable<E> entries) {
entries.forEach(add);
}
/// Removes [entry] from the linked list.
///
/// Returns false and does nothing if [entry] is not in this linked list.
///
/// This is equivalent to calling `entry.unlink()` if the entry is in this
/// list.
bool remove(E entry) {
if (entry._list != this) return false;
_unlink(entry); // Unlink will decrement length.
return true;
}
/// Whether [entry] is a [LinkedListEntry] belonging to this list.
///
/// The [entry] is considered as belonging to this list if
/// its [LinkedListEntry.list] is this list.
bool contains(Object? entry) =>
entry is LinkedListEntry && identical(this, entry.list);
Iterator<E> get iterator => _LinkedListIterator<E>(this);
int get length => _length;
/// Remove all elements from this linked list.
void clear() {
_modificationCount++;
if (isEmpty) return;
E next = _first!;
do {
E entry = next;
next = entry._next!;
entry._next = entry._previous = entry._list = null;
} while (!identical(next, _first));
_first = null;
_length = 0;
}
E get first {
if (isEmpty) {
throw StateError('No such element');
}
return _first!;
}
E get last {
if (isEmpty) {
throw StateError('No such element');
}
return _first!._previous!;
}
E get single {
if (isEmpty) {
throw StateError('No such element');
}
if (_length > 1) {
throw StateError('Too many elements');
}
return _first!;
}
/// Call [action] with each entry in this linked list.
///
/// It's an error if [action] modifies the linked list.
void forEach(void action(E entry)) {
int modificationCount = _modificationCount;
if (isEmpty) return;
E current = _first!;
do {
action(current);
if (modificationCount != _modificationCount) {
throw ConcurrentModificationError(this);
}
current = current._next!;
} while (!identical(current, _first));
}
bool get isEmpty => _length == 0;
/// Inserts [newEntry] as last entry of the list.
///
/// If [updateFirst] is true and [entry] is the first entry in the list,
/// updates the [_first] field to point to the [newEntry] as first entry.
void _insertBefore(E? entry, E newEntry, {required bool updateFirst}) {
if (newEntry.list != null) {
throw StateError('LinkedListEntry is already in a LinkedList');
}
_modificationCount++;
newEntry._list = this;
if (isEmpty) {
assert(entry == null);
newEntry._previous = newEntry._next = newEntry;
_first = newEntry;
_length++;
return;
}
E predecessor = entry!._previous!;
E successor = entry;
newEntry._previous = predecessor;
newEntry._next = successor;
predecessor._next = newEntry;
successor._previous = newEntry;
if (updateFirst && identical(entry, _first)) {
_first = newEntry;
}
_length++;
}
void _unlink(E entry) {
_modificationCount++;
entry._next!._previous = entry._previous;
E? next = entry._previous!._next = entry._next;
_length--;
entry._list = entry._next = entry._previous = null;
if (isEmpty) {
_first = null;
} else if (identical(entry, _first)) {
_first = next;
}
}
}
class _LinkedListIterator<E extends LinkedListEntry<E>> implements Iterator<E> {
final LinkedList<E> _list;
final int _modificationCount;
E? _current;
E? _next;
bool _visitedFirst;
_LinkedListIterator(LinkedList<E> list)
: _list = list,
_modificationCount = list._modificationCount,
_next = list._first,
_visitedFirst = false;
E get current => _current as E;
bool moveNext() {
if (_modificationCount != _list._modificationCount) {
throw ConcurrentModificationError(this);
}
if (_list.isEmpty || (_visitedFirst && identical(_next, _list.first))) {
_current = null;
return false;
}
_visitedFirst = true;
_current = _next;
_next = _next!._next;
return true;
}
}
/// An object that can be an element in a [LinkedList].
///
/// All elements of a `LinkedList` must extend this class.
/// The class provides the internal links that link elements together
/// in the `LinkedList`, and a reference to the linked list itself
/// that an element is currently part of.
///
/// An entry can be in at most one linked list at a time.
/// While an entry is in a linked list, the [list] property points to that
/// linked list, and otherwise the `list` property is `null`.
///
/// When created, an entry is not in any linked list.
abstract class LinkedListEntry<E extends LinkedListEntry<E>> {
LinkedList<E>? _list;
E? _next;
E? _previous;
/// The linked list containing this element.
///
/// The value is `null` if this entry is not currently in any list.
LinkedList<E>? get list => _list;
/// Unlink the element from its linked list.
///
/// The entry must currently be in a linked list when this method is called.
void unlink() {
_list!._unlink(this as E);
}
/// The successor of this element in its linked list.
///
/// The value is `null` if there is no successor in the linked list,
/// or if this entry is not currently in any list.
E? get next {
if (_list == null || identical(_list!.first, _next)) return null;
return _next;
}
/// The predecessor of this element in its linked list.
///
/// The value is `null` if there is no predecessor in the linked list,
/// or if this entry is not currently in any list.
E? get previous {
if (_list == null || identical(this, _list!.first)) return null;
return _previous;
}
/// Insert an element after this element in this element's linked list.
///
/// This entry must be in a linked list when this method is called.
/// The [entry] must not be in a linked list.
void insertAfter(E entry) {
_list!._insertBefore(_next, entry, updateFirst: false);
}
/// Insert an element before this element in this element's linked list.
///
/// This entry must be in a linked list when this method is called.
/// The [entry] must not be in a linked list.
void insertBefore(E entry) {
_list!._insertBefore(this as E, entry, updateFirst: true);
}
}