mirror of
https://github.com/mozilla/gecko-dev.git
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466 lines
13 KiB
C++
466 lines
13 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* A type-safe doubly-linked list class. */
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/*
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* The classes LinkedList<T> and LinkedListElement<T> together form a
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* convenient, type-safe doubly-linked list implementation.
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*
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* The class T which will be inserted into the linked list must inherit from
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* LinkedListElement<T>. A given object may be in only one linked list at a
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* time.
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*
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* A LinkedListElement automatically removes itself from the list upon
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* destruction, and a LinkedList will fatally assert in debug builds if it's
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* non-empty when it's destructed.
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*
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* For example, you might use LinkedList in a simple observer list class as
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* follows.
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*
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* class Observer : public LinkedListElement<Observer>
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* {
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* public:
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* void observe(char* topic) { ... }
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* };
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*
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* class ObserverContainer
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* {
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* private:
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* LinkedList<Observer> list;
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*
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* public:
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* void addObserver(Observer* observer) {
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* // Will assert if |observer| is part of another list.
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* list.insertBack(observer);
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* }
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*
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* void removeObserver(Observer* observer) {
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* // Will assert if |observer| is not part of some list.
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* observer.remove();
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* // Or, will assert if |observer| is not part of |list| specifically.
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* // observer.removeFrom(list);
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* }
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*
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* void notifyObservers(char* topic) {
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* for (Observer* o = list.getFirst(); o != nullptr; o = o->getNext())
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* o->observe(topic);
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* }
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* };
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*
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*/
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#ifndef mozilla_LinkedList_h
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#define mozilla_LinkedList_h
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#include "mozilla/Assertions.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/Move.h"
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#include "mozilla/NullPtr.h"
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#ifdef __cplusplus
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namespace mozilla {
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template<typename T>
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class LinkedList;
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template<typename T>
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class LinkedListElement
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{
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/*
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* It's convenient that we return nullptr when getNext() or getPrevious()
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* hits the end of the list, but doing so costs an extra word of storage in
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* each linked list node (to keep track of whether |this| is the sentinel
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* node) and a branch on this value in getNext/getPrevious.
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*
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* We could get rid of the extra word of storage by shoving the "is
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* sentinel" bit into one of the pointers, although this would, of course,
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* have performance implications of its own.
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*
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* But the goal here isn't to win an award for the fastest or slimmest
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* linked list; rather, we want a *convenient* linked list. So we won't
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* waste time guessing which micro-optimization strategy is best.
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*
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*
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* Speaking of unnecessary work, it's worth addressing here why we wrote
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* mozilla::LinkedList in the first place, instead of using stl::list.
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*
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* The key difference between mozilla::LinkedList and stl::list is that
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* mozilla::LinkedList stores the prev/next pointers in the object itself,
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* while stl::list stores the prev/next pointers in a list element which
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* itself points to the object being stored.
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*
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* mozilla::LinkedList's approach makes it harder to store an object in more
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* than one list. But the upside is that you can call next() / prev() /
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* remove() directly on the object. With stl::list, you'd need to store a
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* pointer to its iterator in the object in order to accomplish this. Not
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* only would this waste space, but you'd have to remember to update that
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* pointer every time you added or removed the object from a list.
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*
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* In-place, constant-time removal is a killer feature of doubly-linked
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* lists, and supporting this painlessly was a key design criterion.
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*/
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private:
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LinkedListElement* next;
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LinkedListElement* prev;
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const bool isSentinel;
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public:
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LinkedListElement()
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: next(MOZ_THIS_IN_INITIALIZER_LIST()),
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prev(MOZ_THIS_IN_INITIALIZER_LIST()),
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isSentinel(false)
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{ }
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LinkedListElement(LinkedListElement<T>&& other)
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: isSentinel(other.isSentinel)
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{
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if (!other.isInList()) {
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next = this;
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prev = this;
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return;
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}
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MOZ_ASSERT(other.next->prev == &other);
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MOZ_ASSERT(other.prev->next == &other);
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/*
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* Initialize |this| with |other|'s prev/next pointers, and adjust those
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* element to point to this one.
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*/
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next = other.next;
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prev = other.prev;
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next->prev = this;
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prev->next = this;
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/*
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* Adjust |other| so it doesn't think it's in a list. This makes it
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* safely destructable.
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*/
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other.next = &other;
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other.prev = &other;
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}
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~LinkedListElement() {
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if (!isSentinel && isInList())
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remove();
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}
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/*
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* Get the next element in the list, or nullptr if this is the last element
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* in the list.
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*/
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T* getNext() {
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return next->asT();
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}
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const T* getNext() const {
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return next->asT();
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}
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/*
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* Get the previous element in the list, or nullptr if this is the first
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* element in the list.
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*/
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T* getPrevious() {
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return prev->asT();
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}
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const T* getPrevious() const {
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return prev->asT();
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}
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/*
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* Insert elem after this element in the list. |this| must be part of a
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* linked list when you call setNext(); otherwise, this method will assert.
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*/
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void setNext(T* elem) {
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MOZ_ASSERT(isInList());
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setNextUnsafe(elem);
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}
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/*
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* Insert elem before this element in the list. |this| must be part of a
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* linked list when you call setPrevious(); otherwise, this method will
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* assert.
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*/
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void setPrevious(T* elem) {
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MOZ_ASSERT(isInList());
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setPreviousUnsafe(elem);
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}
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/*
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* Remove this element from the list which contains it. If this element is
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* not currently part of a linked list, this method asserts.
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*/
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void remove() {
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MOZ_ASSERT(isInList());
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prev->next = next;
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next->prev = prev;
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next = this;
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prev = this;
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}
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/*
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* Identical to remove(), but also asserts in debug builds that this element
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* is in list.
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*/
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void removeFrom(const LinkedList<T>& list) {
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list.assertContains(asT());
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remove();
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}
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/*
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* Return true if |this| part is of a linked list, and false otherwise.
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*/
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bool isInList() const {
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MOZ_ASSERT((next == this) == (prev == this));
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return next != this;
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}
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private:
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friend class LinkedList<T>;
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enum NodeKind {
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NODE_KIND_NORMAL,
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NODE_KIND_SENTINEL
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};
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LinkedListElement(NodeKind nodeKind)
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: next(MOZ_THIS_IN_INITIALIZER_LIST()),
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prev(MOZ_THIS_IN_INITIALIZER_LIST()),
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isSentinel(nodeKind == NODE_KIND_SENTINEL)
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{ }
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/*
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* Return |this| cast to T* if we're a normal node, or return nullptr if
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* we're a sentinel node.
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*/
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T* asT() {
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if (isSentinel)
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return nullptr;
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return static_cast<T*>(this);
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}
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const T* asT() const {
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if (isSentinel)
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return nullptr;
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return static_cast<const T*>(this);
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}
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/*
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* Insert elem after this element, but don't check that this element is in
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* the list. This is called by LinkedList::insertFront().
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*/
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void setNextUnsafe(T* elem) {
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LinkedListElement *listElem = static_cast<LinkedListElement*>(elem);
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MOZ_ASSERT(!listElem->isInList());
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listElem->next = this->next;
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listElem->prev = this;
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this->next->prev = listElem;
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this->next = listElem;
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}
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/*
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* Insert elem before this element, but don't check that this element is in
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* the list. This is called by LinkedList::insertBack().
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*/
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void setPreviousUnsafe(T* elem) {
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LinkedListElement<T>* listElem = static_cast<LinkedListElement<T>*>(elem);
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MOZ_ASSERT(!listElem->isInList());
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listElem->next = this;
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listElem->prev = this->prev;
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this->prev->next = listElem;
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this->prev = listElem;
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}
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private:
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LinkedListElement& operator=(const LinkedListElement<T>& other) MOZ_DELETE;
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LinkedListElement(const LinkedListElement<T>& other) MOZ_DELETE;
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};
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template<typename T>
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class LinkedList
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{
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private:
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LinkedListElement<T> sentinel;
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public:
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LinkedList() : sentinel(LinkedListElement<T>::NODE_KIND_SENTINEL) { }
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LinkedList(LinkedList<T>&& other)
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: sentinel(mozilla::Move(other.sentinel))
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{ }
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~LinkedList() {
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MOZ_ASSERT(isEmpty());
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}
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/*
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* Add elem to the front of the list.
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*/
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void insertFront(T* elem) {
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/* Bypass setNext()'s this->isInList() assertion. */
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sentinel.setNextUnsafe(elem);
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}
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/*
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* Add elem to the back of the list.
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*/
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void insertBack(T* elem) {
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sentinel.setPreviousUnsafe(elem);
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}
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/*
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* Get the first element of the list, or nullptr if the list is empty.
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*/
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T* getFirst() {
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return sentinel.getNext();
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}
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const T* getFirst() const {
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return sentinel.getNext();
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}
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/*
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* Get the last element of the list, or nullptr if the list is empty.
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*/
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T* getLast() {
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return sentinel.getPrevious();
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}
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const T* getLast() const {
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return sentinel.getPrevious();
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}
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/*
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* Get and remove the first element of the list. If the list is empty,
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* return nullptr.
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*/
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T* popFirst() {
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T* ret = sentinel.getNext();
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if (ret)
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static_cast<LinkedListElement<T>*>(ret)->remove();
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return ret;
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}
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/*
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* Get and remove the last element of the list. If the list is empty,
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* return nullptr.
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*/
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T* popLast() {
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T* ret = sentinel.getPrevious();
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if (ret)
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static_cast<LinkedListElement<T>*>(ret)->remove();
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return ret;
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}
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/*
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* Return true if the list is empty, or false otherwise.
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*/
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bool isEmpty() const {
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return !sentinel.isInList();
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}
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/*
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* Remove all the elements from the list.
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*
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* This runs in time linear to the list's length, because we have to mark
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* each element as not in the list.
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*/
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void clear() {
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while (popFirst())
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continue;
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}
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/*
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* In a debug build, make sure that the list is sane (no cycles, consistent
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* next/prev pointers, only one sentinel). Has no effect in release builds.
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*/
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void debugAssertIsSane() const {
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#ifdef DEBUG
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const LinkedListElement<T>* slow;
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const LinkedListElement<T>* fast1;
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const LinkedListElement<T>* fast2;
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/*
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* Check for cycles in the forward singly-linked list using the
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* tortoise/hare algorithm.
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*/
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for (slow = sentinel.next,
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fast1 = sentinel.next->next,
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fast2 = sentinel.next->next->next;
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slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
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slow = slow->next, fast1 = fast2->next, fast2 = fast1->next)
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{
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MOZ_ASSERT(slow != fast1);
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MOZ_ASSERT(slow != fast2);
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}
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/* Check for cycles in the backward singly-linked list. */
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for (slow = sentinel.prev,
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fast1 = sentinel.prev->prev,
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fast2 = sentinel.prev->prev->prev;
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slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
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slow = slow->prev, fast1 = fast2->prev, fast2 = fast1->prev)
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{
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MOZ_ASSERT(slow != fast1);
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MOZ_ASSERT(slow != fast2);
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}
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/*
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* Check that |sentinel| is the only node in the list with
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* isSentinel == true.
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*/
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for (const LinkedListElement<T>* elem = sentinel.next;
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elem != &sentinel;
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elem = elem->next)
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{
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MOZ_ASSERT(!elem->isSentinel);
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}
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/* Check that the next/prev pointers match up. */
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const LinkedListElement<T>* prev = &sentinel;
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const LinkedListElement<T>* cur = sentinel.next;
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do {
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MOZ_ASSERT(cur->prev == prev);
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MOZ_ASSERT(prev->next == cur);
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prev = cur;
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cur = cur->next;
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} while (cur != &sentinel);
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#endif /* ifdef DEBUG */
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}
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private:
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friend class LinkedListElement<T>;
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void assertContains(const T* t) const {
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#ifdef DEBUG
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for (const T* elem = getFirst();
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elem;
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elem = elem->getNext())
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{
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if (elem == t)
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return;
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}
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MOZ_CRASH("element wasn't found in this list!");
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#endif
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}
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LinkedList& operator=(const LinkedList<T>& other) MOZ_DELETE;
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LinkedList(const LinkedList<T>& other) MOZ_DELETE;
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};
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} /* namespace mozilla */
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#endif /* __cplusplus */
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#endif /* mozilla_LinkedList_h */
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