mfbt/LinkedList.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 /* A type-safe doubly-linked list class. */
   7
   8 /*
   9  * The classes LinkedList<T> and LinkedListElement<T> together form a
  10  * convenient, type-safe doubly-linked list implementation.
  11  *
  12  * The class T which will be inserted into the linked list must inherit from
  13  * LinkedListElement<T>.  A given object may be in only one linked list at a
  14  * time.
  15  *
  16  * A LinkedListElement automatically removes itself from the list upon
  17  * destruction, and a LinkedList will fatally assert in debug builds if it's
  18  * non-empty when it's destructed.
  19  *
  20  * For example, you might use LinkedList in a simple observer list class as
  21  * follows.
  22  *
  23  *   class Observer : public LinkedListElement<Observer>
  24  *   {
  25  *     public:
  26  *       void observe(char* topic) { ... }
  27  *   };
  28  *
  29  *   class ObserverContainer
  30  *   {
  31  *     private:
  32  *       LinkedList<Observer> list;
  33  *
  34  *     public:
  35  *       void addObserver(Observer* observer) {
  36  *         // Will assert if |observer| is part of another list.
  37  *         list.insertBack(observer);
  38  *       }
  39  *
  40  *       void removeObserver(Observer* observer) {
  41  *         // Will assert if |observer| is not part of some list.
  42  *         observer.remove();
  43  *         // Or, will assert if |observer| is not part of |list| specifically.
  44  *         // observer.removeFrom(list);
  45  *       }
  46  *
  47  *       void notifyObservers(char* topic) {
  48  *         for (Observer* o = list.getFirst(); o != NULL; o = o->getNext())
  49  *           o->Observe(topic);
  50  *       }
  51  *   };
  52  *
  53  */
  54
  55 #ifndef mozilla_LinkedList_h_
  56 #define mozilla_LinkedList_h_
  57
  58 #include "mozilla/Assertions.h"
  59 #include "mozilla/Attributes.h"
  60
  61 #ifdef __cplusplus
  62
  63 namespace mozilla {
  64
  65 template<typename T>
  66 class LinkedList;
  67
  68 template<typename T>
  69 class LinkedListElement
  70 {
  71     /*
  72      * It's convenient that we return NULL when getNext() or getPrevious() hits
  73      * the end of the list, but doing so costs an extra word of storage in each
  74      * linked list node (to keep track of whether |this| is the sentinel node)
  75      * and a branch on this value in getNext/getPrevious.
  76      *
  77      * We could get rid of the extra word of storage by shoving the "is
  78      * sentinel" bit into one of the pointers, although this would, of course,
  79      * have performance implications of its own.
  80      *
  81      * But the goal here isn't to win an award for the fastest or slimmest
  82      * linked list; rather, we want a *convenient* linked list.  So we won't
  83      * waste time guessing which micro-optimization strategy is best.
  84      *
  85      *
  86      * Speaking of unnecessary work, it's worth addressing here why we wrote
  87      * mozilla::LinkedList in the first place, instead of using stl::list.
  88      *
  89      * The key difference between mozilla::LinkedList and stl::list is that
  90      * mozilla::LinkedList stores the prev/next pointers in the object itself,
  91      * while stl::list stores the prev/next pointers in a list element which
  92      * itself points to the object being stored.
  93      *
  94      * mozilla::LinkedList's approach makes it harder to store an object in more
  95      * than one list.  But the upside is that you can call next() / prev() /
  96      * remove() directly on the object.  With stl::list, you'd need to store a
  97      * pointer to its iterator in the object in order to accomplish this.  Not
  98      * only would this waste space, but you'd have to remember to update that
  99      * pointer every time you added or removed the object from a list.
 100      *
 101      * In-place, constant-time removal is a killer feature of doubly-linked
 102      * lists, and supporting this painlessly was a key design criterion.
 103      */
 104
 105   private:
 106     LinkedListElement* next;
 107     LinkedListElement* prev;
 108     const bool isSentinel;
 109
 110   public:
 111     LinkedListElement()
 112       : next(MOZ_THIS_IN_INITIALIZER_LIST()),
 113         prev(MOZ_THIS_IN_INITIALIZER_LIST()),
 114         isSentinel(false)
 115     { }
 116
 117     ~LinkedListElement() {
 118       if (!isSentinel && isInList())
 119         remove();
 120     }
 121
 122     /*
 123      * Get the next element in the list, or NULL if this is the last element in
 124      * the list.
 125      */
 126     T* getNext() {
 127       return next->asT();
 128     }
 129     const T* getNext() const {
 130       return next->asT();
 131     }
 132
 133     /*
 134      * Get the previous element in the list, or NULL if this is the first element
 135      * in the list.
 136      */
 137     T* getPrevious() {
 138       return prev->asT();
 139     }
 140     const T* getPrevious() const {
 141       return prev->asT();
 142     }
 143
 144     /*
 145      * Insert elem after this element in the list.  |this| must be part of a
 146      * linked list when you call setNext(); otherwise, this method will assert.
 147      */
 148     void setNext(T* elem) {
 149         MOZ_ASSERT(isInList());
 150         setNextUnsafe(elem);
 151     }
 152
 153     /*
 154      * Insert elem before this element in the list.  |this| must be part of a
 155      * linked list when you call setPrevious(); otherwise, this method will
 156      * assert.
 157      */
 158     void setPrevious(T* elem) {
 159       MOZ_ASSERT(isInList());
 160       setPreviousUnsafe(elem);
 161     }
 162
 163     /*
 164      * Remove this element from the list which contains it.  If this element is
 165      * not currently part of a linked list, this method asserts.
 166      */
 167     void remove() {
 168       MOZ_ASSERT(isInList());
 169
 170       prev->next = next;
 171       next->prev = prev;
 172       next = this;
 173       prev = this;
 174     }
 175
 176     /*
 177      * Identical to remove(), but also asserts in debug builds that this element
 178      * is in list.
 179      */
 180     void removeFrom(const LinkedList<T>& list) {
 181       list.assertContains(asT());
 182       remove();
 183     }
 184
 185     /*
 186      * Return true if |this| part is of a linked list, and false otherwise.
 187      */
 188     bool isInList() const {
 189       MOZ_ASSERT((next == this) == (prev == this));
 190       return next != this;
 191     }
 192
 193   private:
 194     friend class LinkedList<T>;
 195
 196     enum NodeKind {
 197       NODE_KIND_NORMAL,
 198       NODE_KIND_SENTINEL
 199     };
 200
 201     LinkedListElement(NodeKind nodeKind)
 202       : next(MOZ_THIS_IN_INITIALIZER_LIST()),
 203         prev(MOZ_THIS_IN_INITIALIZER_LIST()),
 204         isSentinel(nodeKind == NODE_KIND_SENTINEL)
 205     { }
 206
 207     /*
 208      * Return |this| cast to T* if we're a normal node, or return NULL if we're
 209      * a sentinel node.
 210      */
 211     T* asT() {
 212       if (isSentinel)
 213         return NULL;
 214
 215       return static_cast<T*>(this);
 216     }
 217     const T* asT() const {
 218       if (isSentinel)
 219         return NULL;
 220
 221       return static_cast<const T*>(this);
 222     }
 223
 224     /*
 225      * Insert elem after this element, but don't check that this element is in
 226      * the list.  This is called by LinkedList::insertFront().
 227      */
 228     void setNextUnsafe(T* elem) {
 229       LinkedListElement *listElem = static_cast<LinkedListElement*>(elem);
 230       MOZ_ASSERT(!listElem->isInList());
 231
 232       listElem->next = this->next;
 233       listElem->prev = this;
 234       this->next->prev = listElem;
 235       this->next = listElem;
 236     }
 237
 238     /*
 239      * Insert elem before this element, but don't check that this element is in
 240      * the list.  This is called by LinkedList::insertBack().
 241      */
 242     void setPreviousUnsafe(T* elem) {
 243       LinkedListElement<T>* listElem = static_cast<LinkedListElement<T>*>(elem);
 244       MOZ_ASSERT(!listElem->isInList());
 245
 246       listElem->next = this;
 247       listElem->prev = this->prev;
 248       this->prev->next = listElem;
 249       this->prev = listElem;
 250     }
 251
 252   private:
 253     LinkedListElement& operator=(const LinkedList<T>& other) MOZ_DELETE;
 254     LinkedListElement(const LinkedList<T>& other) MOZ_DELETE;
 255 };
 256
 257 template<typename T>
 258 class LinkedList
 259 {
 260   private:
 261     LinkedListElement<T> sentinel;
 262
 263   public:
 264     LinkedList() : sentinel(LinkedListElement<T>::NODE_KIND_SENTINEL) { }
 265
 266     ~LinkedList() {
 267       MOZ_ASSERT(isEmpty());
 268     }
 269
 270     /*
 271      * Add elem to the front of the list.
 272      */
 273     void insertFront(T* elem) {
 274       /* Bypass setNext()'s this->isInList() assertion. */
 275       sentinel.setNextUnsafe(elem);
 276     }
 277
 278     /*
 279      * Add elem to the back of the list.
 280      */
 281     void insertBack(T* elem) {
 282       sentinel.setPreviousUnsafe(elem);
 283     }
 284
 285     /*
 286      * Get the first element of the list, or NULL if the list is empty.
 287      */
 288     T* getFirst() {
 289       return sentinel.getNext();
 290     }
 291     const T* getFirst() const {
 292       return sentinel.getNext();
 293     }
 294
 295     /*
 296      * Get the last element of the list, or NULL if the list is empty.
 297      */
 298     T* getLast() {
 299       return sentinel.getPrevious();
 300     }
 301     const T* getLast() const {
 302       return sentinel.getPrevious();
 303     }
 304
 305     /*
 306      * Get and remove the first element of the list.  If the list is empty,
 307      * return NULL.
 308      */
 309     T* popFirst() {
 310       T* ret = sentinel.getNext();
 311       if (ret)
 312         static_cast<LinkedListElement<T>*>(ret)->remove();
 313       return ret;
 314     }
 315
 316     /*
 317      * Get and remove the last element of the list.  If the list is empty,
 318      * return NULL.
 319      */
 320     T* popLast() {
 321       T* ret = sentinel.getPrevious();
 322       if (ret)
 323         static_cast<LinkedListElement<T>*>(ret)->remove();
 324       return ret;
 325     }
 326
 327     /*
 328      * Return true if the list is empty, or false otherwise.
 329      */
 330     bool isEmpty() const {
 331       return !sentinel.isInList();
 332     }
 333
 334     /*
 335      * Remove all the elements from the list.
 336      *
 337      * This runs in time linear to the list's length, because we have to mark
 338      * each element as not in the list.
 339      */
 340     void clear() {
 341       while (popFirst())
 342         continue;
 343     }
 344
 345     /*
 346      * In a debug build, make sure that the list is sane (no cycles, consistent
 347      * next/prev pointers, only one sentinel).  Has no effect in release builds.
 348      */
 349     void debugAssertIsSane() const {
 350 #ifdef DEBUG
 351       const LinkedListElement<T>* slow;
 352       const LinkedListElement<T>* fast1;
 353       const LinkedListElement<T>* fast2;
 354
 355       /*
 356        * Check for cycles in the forward singly-linked list using the
 357        * tortoise/hare algorithm.
 358        */
 359       for (slow = sentinel.next,
 360            fast1 = sentinel.next->next,
 361            fast2 = sentinel.next->next->next;
 362            slow != sentinel && fast1 != sentinel && fast2 != sentinel;
 363            slow = slow->next, fast1 = fast2->next, fast2 = fast1->next)
 364       {
 365         MOZ_ASSERT(slow != fast1);
 366         MOZ_ASSERT(slow != fast2);
 367       }
 368
 369       /* Check for cycles in the backward singly-linked list. */
 370       for (slow = sentinel.prev,
 371            fast1 = sentinel.prev->prev,
 372            fast2 = sentinel.prev->prev->prev;
 373            slow != sentinel && fast1 != sentinel && fast2 != sentinel;
 374            slow = slow->prev, fast1 = fast2->prev, fast2 = fast1->prev)
 375       {
 376         MOZ_ASSERT(slow != fast1);
 377         MOZ_ASSERT(slow != fast2);
 378       }
 379
 380       /*
 381        * Check that |sentinel| is the only node in the list with
 382        * isSentinel == true.
 383        */
 384       for (const LinkedListElement<T>* elem = sentinel.next;
 385            elem != sentinel;
 386            elem = elem->next)
 387       {
 388         MOZ_ASSERT(!elem->isSentinel);
 389       }
 390
 391       /* Check that the next/prev pointers match up. */
 392       const LinkedListElement<T>* prev = sentinel;
 393       const LinkedListElement<T>* cur = sentinel.next;
 394       do {
 395           MOZ_ASSERT(cur->prev == prev);
 396           MOZ_ASSERT(prev->next == cur);
 397
 398           prev = cur;
 399           cur = cur->next;
 400       } while (cur != sentinel);
 401 #endif /* ifdef DEBUG */
 402     }
 403
 404   private:
 405     friend class LinkedListElement<T>;
 406
 407     void assertContains(const T* t) const {
 408 #ifdef DEBUG
 409       for (const T* elem = getFirst();
 410            elem;
 411            elem = elem->getNext())
 412       {
 413         if (elem == t)
 414           return;
 415       }
 416       MOZ_CRASH("element wasn't found in this list!");
 417 #endif
 418     }
 419
 420     LinkedList& operator=(const LinkedList<T>& other) MOZ_DELETE;
 421     LinkedList(const LinkedList<T>& other) MOZ_DELETE;
 422 };
 423
 424 } /* namespace mozilla */
 425
 426 #endif /* ifdef __cplusplus */
 427 #endif /* ifdef mozilla_LinkedList_h_ */