gecko-dev/mfbt/LinkedList.h

486 lines
13 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);
* }
* }
* };
*
*/
#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:
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;
}
}
/*
* 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;
};
} /* namespace mozilla */
#endif /* __cplusplus */
#endif /* mozilla_LinkedList_h */