gecko/mfbt/LinkedList.h
Steve Fink 0fce9b0d8f Backed out 5 changesets (bug 1212624) for breaking stuff.
Backed out changeset cf5ffa45a4a2
Backed out changeset 0d7a968d2d64 (bug 1212624)
Backed out changeset 379edefa8e47 (bug 1212624)
Backed out changeset f73fca35daad (bug 1212624)
Backed out changeset 4f499d30a0e0 (bug 1212624)
2015-10-14 16:49:12 -07:00

534 lines
14 KiB
C++

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A type-safe doubly-linked list class. */
/*
* The classes LinkedList<T> and LinkedListElement<T> together form a
* convenient, type-safe doubly-linked list implementation.
*
* The class T which will be inserted into the linked list must inherit from
* LinkedListElement<T>. A given object may be in only one linked list at a
* time.
*
* A LinkedListElement automatically removes itself from the list upon
* destruction, and a LinkedList will fatally assert in debug builds if it's
* non-empty when it's destructed.
*
* For example, you might use LinkedList in a simple observer list class as
* follows.
*
* class Observer : public LinkedListElement<Observer>
* {
* public:
* void observe(char* aTopic) { ... }
* };
*
* class ObserverContainer
* {
* private:
* LinkedList<Observer> list;
*
* public:
* void addObserver(Observer* aObserver)
* {
* // Will assert if |aObserver| is part of another list.
* list.insertBack(aObserver);
* }
*
* void removeObserver(Observer* aObserver)
* {
* // Will assert if |aObserver| is not part of some list.
* aObserver.remove();
* // Or, will assert if |aObserver| is not part of |list| specifically.
* // aObserver.removeFrom(list);
* }
*
* void notifyObservers(char* aTopic)
* {
* for (Observer* o = list.getFirst(); o != nullptr; o = o->getNext()) {
* o->observe(aTopic);
* }
* }
* };
*
* Additionally, the class AutoCleanLinkedList<T> is a LinkedList<T> that will
* remove and delete each element still within itself upon destruction. Note
* that because each element is deleted, elements must have been allocated
* using |new|.
*/
#ifndef mozilla_LinkedList_h
#define mozilla_LinkedList_h
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/Move.h"
#ifdef __cplusplus
namespace mozilla {
template<typename T>
class LinkedList;
template<typename T>
class LinkedListElement
{
/*
* It's convenient that we return nullptr when getNext() or getPrevious()
* hits the end of the list, but doing so costs an extra word of storage in
* each linked list node (to keep track of whether |this| is the sentinel
* node) and a branch on this value in getNext/getPrevious.
*
* We could get rid of the extra word of storage by shoving the "is
* sentinel" bit into one of the pointers, although this would, of course,
* have performance implications of its own.
*
* But the goal here isn't to win an award for the fastest or slimmest
* linked list; rather, we want a *convenient* linked list. So we won't
* waste time guessing which micro-optimization strategy is best.
*
*
* Speaking of unnecessary work, it's worth addressing here why we wrote
* mozilla::LinkedList in the first place, instead of using stl::list.
*
* The key difference between mozilla::LinkedList and stl::list is that
* mozilla::LinkedList stores the mPrev/mNext pointers in the object itself,
* while stl::list stores the mPrev/mNext pointers in a list element which
* itself points to the object being stored.
*
* mozilla::LinkedList's approach makes it harder to store an object in more
* than one list. But the upside is that you can call next() / prev() /
* remove() directly on the object. With stl::list, you'd need to store a
* pointer to its iterator in the object in order to accomplish this. Not
* only would this waste space, but you'd have to remember to update that
* pointer every time you added or removed the object from a list.
*
* In-place, constant-time removal is a killer feature of doubly-linked
* lists, and supporting this painlessly was a key design criterion.
*/
private:
LinkedListElement* mNext;
LinkedListElement* mPrev;
const bool mIsSentinel;
public:
LinkedListElement()
: mNext(this),
mPrev(this),
mIsSentinel(false)
{ }
LinkedListElement(LinkedListElement<T>&& other)
: mIsSentinel(other.mIsSentinel)
{
if (!other.isInList()) {
mNext = this;
mPrev = this;
return;
}
MOZ_ASSERT(other.mNext->mPrev == &other);
MOZ_ASSERT(other.mPrev->mNext == &other);
/*
* Initialize |this| with |other|'s mPrev/mNext pointers, and adjust those
* element to point to this one.
*/
mNext = other.mNext;
mPrev = other.mPrev;
mNext->mPrev = this;
mPrev->mNext = this;
/*
* Adjust |other| so it doesn't think it's in a list. This makes it
* safely destructable.
*/
other.mNext = &other;
other.mPrev = &other;
}
~LinkedListElement()
{
if (!mIsSentinel && isInList()) {
remove();
}
}
/*
* Get the next element in the list, or nullptr if this is the last element
* in the list.
*/
T* getNext() { return mNext->asT(); }
const T* getNext() const { return mNext->asT(); }
/*
* Get the previous element in the list, or nullptr if this is the first
* element in the list.
*/
T* getPrevious() { return mPrev->asT(); }
const T* getPrevious() const { return mPrev->asT(); }
/*
* Insert aElem after this element in the list. |this| must be part of a
* linked list when you call setNext(); otherwise, this method will assert.
*/
void setNext(T* aElem)
{
MOZ_ASSERT(isInList());
setNextUnsafe(aElem);
}
/*
* Insert aElem before this element in the list. |this| must be part of a
* linked list when you call setPrevious(); otherwise, this method will
* assert.
*/
void setPrevious(T* aElem)
{
MOZ_ASSERT(isInList());
setPreviousUnsafe(aElem);
}
/*
* Remove this element from the list which contains it. If this element is
* not currently part of a linked list, this method asserts.
*/
void remove()
{
MOZ_ASSERT(isInList());
mPrev->mNext = mNext;
mNext->mPrev = mPrev;
mNext = this;
mPrev = this;
}
/*
* Identical to remove(), but also asserts in debug builds that this element
* is in aList.
*/
void removeFrom(const LinkedList<T>& aList)
{
aList.assertContains(asT());
remove();
}
/*
* Return true if |this| part is of a linked list, and false otherwise.
*/
bool isInList() const
{
MOZ_ASSERT((mNext == this) == (mPrev == this));
return mNext != this;
}
private:
friend class LinkedList<T>;
enum NodeKind {
NODE_KIND_NORMAL,
NODE_KIND_SENTINEL
};
explicit LinkedListElement(NodeKind nodeKind)
: mNext(this),
mPrev(this),
mIsSentinel(nodeKind == NODE_KIND_SENTINEL)
{ }
/*
* Return |this| cast to T* if we're a normal node, or return nullptr if
* we're a sentinel node.
*/
T* asT()
{
return mIsSentinel ? nullptr : static_cast<T*>(this);
}
const T* asT() const
{
return mIsSentinel ? nullptr : static_cast<const T*>(this);
}
/*
* Insert aElem after this element, but don't check that this element is in
* the list. This is called by LinkedList::insertFront().
*/
void setNextUnsafe(T* aElem)
{
LinkedListElement *listElem = static_cast<LinkedListElement*>(aElem);
MOZ_ASSERT(!listElem->isInList());
listElem->mNext = this->mNext;
listElem->mPrev = this;
this->mNext->mPrev = listElem;
this->mNext = listElem;
}
/*
* Insert aElem before this element, but don't check that this element is in
* the list. This is called by LinkedList::insertBack().
*/
void setPreviousUnsafe(T* aElem)
{
LinkedListElement<T>* listElem = static_cast<LinkedListElement<T>*>(aElem);
MOZ_ASSERT(!listElem->isInList());
listElem->mNext = this;
listElem->mPrev = this->mPrev;
this->mPrev->mNext = listElem;
this->mPrev = listElem;
}
private:
LinkedListElement& operator=(const LinkedListElement<T>& aOther) = delete;
LinkedListElement(const LinkedListElement<T>& aOther) = delete;
};
template<typename T>
class LinkedList
{
private:
LinkedListElement<T> sentinel;
public:
class Iterator {
T* mCurrent;
public:
explicit Iterator(T* aCurrent) : mCurrent(aCurrent) {}
T* operator *() const {
return mCurrent;
}
const Iterator& operator++() {
mCurrent = mCurrent->getNext();
return *this;
}
bool operator!=(Iterator& aOther) const {
return mCurrent != aOther.mCurrent;
}
};
LinkedList() : sentinel(LinkedListElement<T>::NODE_KIND_SENTINEL) { }
LinkedList(LinkedList<T>&& aOther)
: sentinel(mozilla::Move(aOther.sentinel))
{ }
~LinkedList() { MOZ_ASSERT(isEmpty()); }
/*
* Add aElem to the front of the list.
*/
void insertFront(T* aElem)
{
/* Bypass setNext()'s this->isInList() assertion. */
sentinel.setNextUnsafe(aElem);
}
/*
* Add aElem to the back of the list.
*/
void insertBack(T* aElem)
{
sentinel.setPreviousUnsafe(aElem);
}
/*
* Get the first element of the list, or nullptr if the list is empty.
*/
T* getFirst() { return sentinel.getNext(); }
const T* getFirst() const { return sentinel.getNext(); }
/*
* Get the last element of the list, or nullptr if the list is empty.
*/
T* getLast() { return sentinel.getPrevious(); }
const T* getLast() const { return sentinel.getPrevious(); }
/*
* Get and remove the first element of the list. If the list is empty,
* return nullptr.
*/
T* popFirst()
{
T* ret = sentinel.getNext();
if (ret) {
static_cast<LinkedListElement<T>*>(ret)->remove();
}
return ret;
}
/*
* Get and remove the last element of the list. If the list is empty,
* return nullptr.
*/
T* popLast()
{
T* ret = sentinel.getPrevious();
if (ret) {
static_cast<LinkedListElement<T>*>(ret)->remove();
}
return ret;
}
/*
* Return true if the list is empty, or false otherwise.
*/
bool isEmpty() const
{
return !sentinel.isInList();
}
/*
* Remove all the elements from the list.
*
* This runs in time linear to the list's length, because we have to mark
* each element as not in the list.
*/
void clear()
{
while (popFirst()) {
continue;
}
}
/*
* Allow range-based iteration:
*
* for (MyElementType* elt : myList) { ... }
*/
Iterator begin() {
return Iterator(getFirst());
}
Iterator end() {
return Iterator(nullptr);
}
/*
* Measures the memory consumption of the list excluding |this|. Note that
* it only measures the list elements themselves. If the list elements
* contain pointers to other memory blocks, those blocks must be measured
* separately during a subsequent iteration over the list.
*/
size_t sizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const
{
size_t n = 0;
for (const T* t = getFirst(); t; t = t->getNext()) {
n += aMallocSizeOf(t);
}
return n;
}
/*
* Like sizeOfExcludingThis(), but measures |this| as well.
*/
size_t sizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
{
return aMallocSizeOf(this) + sizeOfExcludingThis(aMallocSizeOf);
}
/*
* In a debug build, make sure that the list is sane (no cycles, consistent
* mNext/mPrev pointers, only one sentinel). Has no effect in release builds.
*/
void debugAssertIsSane() const
{
#ifdef DEBUG
const LinkedListElement<T>* slow;
const LinkedListElement<T>* fast1;
const LinkedListElement<T>* fast2;
/*
* Check for cycles in the forward singly-linked list using the
* tortoise/hare algorithm.
*/
for (slow = sentinel.mNext,
fast1 = sentinel.mNext->mNext,
fast2 = sentinel.mNext->mNext->mNext;
slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
slow = slow->mNext, fast1 = fast2->mNext, fast2 = fast1->mNext) {
MOZ_ASSERT(slow != fast1);
MOZ_ASSERT(slow != fast2);
}
/* Check for cycles in the backward singly-linked list. */
for (slow = sentinel.mPrev,
fast1 = sentinel.mPrev->mPrev,
fast2 = sentinel.mPrev->mPrev->mPrev;
slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
slow = slow->mPrev, fast1 = fast2->mPrev, fast2 = fast1->mPrev) {
MOZ_ASSERT(slow != fast1);
MOZ_ASSERT(slow != fast2);
}
/*
* Check that |sentinel| is the only node in the list with
* mIsSentinel == true.
*/
for (const LinkedListElement<T>* elem = sentinel.mNext;
elem != &sentinel;
elem = elem->mNext) {
MOZ_ASSERT(!elem->mIsSentinel);
}
/* Check that the mNext/mPrev pointers match up. */
const LinkedListElement<T>* prev = &sentinel;
const LinkedListElement<T>* cur = sentinel.mNext;
do {
MOZ_ASSERT(cur->mPrev == prev);
MOZ_ASSERT(prev->mNext == cur);
prev = cur;
cur = cur->mNext;
} while (cur != &sentinel);
#endif /* ifdef DEBUG */
}
private:
friend class LinkedListElement<T>;
void assertContains(const T* aValue) const
{
#ifdef DEBUG
for (const T* elem = getFirst(); elem; elem = elem->getNext()) {
if (elem == aValue) {
return;
}
}
MOZ_CRASH("element wasn't found in this list!");
#endif
}
LinkedList& operator=(const LinkedList<T>& aOther) = delete;
LinkedList(const LinkedList<T>& aOther) = delete;
};
template <typename T>
class AutoCleanLinkedList : public LinkedList<T>
{
public:
~AutoCleanLinkedList()
{
while (T* element = this->popFirst()) {
delete element;
}
}
};
} /* namespace mozilla */
#endif /* __cplusplus */
#endif /* mozilla_LinkedList_h */