4 /* Our doubly linked lists have two header elements: the "head"
5 just before the first element and the "tail" just after the
6 last element. The `prev' link of the front header is null, as
7 is the `next' link of the back header. Their other two links
8 point toward each other via the interior elements of the list.
10 An empty list looks like this:
13 <---| head |<--->| tail |--->
16 A list with two elements in it looks like this:
18 +------+ +-------+ +-------+ +------+
19 <---| head |<--->| 1 |<--->| 2 |<--->| tail |<--->
20 +------+ +-------+ +-------+ +------+
22 The symmetry of this arrangement eliminates lots of special
23 cases in list processing. For example, take a look at
24 list_remove(): it takes only two pointer assignments and no
25 conditionals. That's a lot simpler than the code would be
26 without header elements.
28 (Because only one of the pointers in each header element is used,
29 we could in fact combine them into a single header element
30 without sacrificing this simplicity. But using two separate
31 elements allows us to do a little bit of checking on some
32 operations, which can be valuable.) */
34 static bool is_sorted (struct list_elem *a, struct list_elem *b,
35 list_less_func *less, void *aux) UNUSED;
37 /* Returns true if ELEM is a head, false otherwise. */
39 is_head (struct list_elem *elem)
41 return elem != NULL && elem->prev == NULL && elem->next != NULL;
44 /* Returns true if ELEM is an interior element,
47 is_interior (struct list_elem *elem)
49 return elem != NULL && elem->prev != NULL && elem->next != NULL;
52 /* Returns true if ELEM is a tail, false otherwise. */
54 is_tail (struct list_elem *elem)
56 return elem != NULL && elem->prev != NULL && elem->next == NULL;
59 /* Initializes LIST as an empty list. */
61 list_init (struct list *list)
63 ASSERT (list != NULL);
64 list->head.prev = NULL;
65 list->head.next = &list->tail;
66 list->tail.prev = &list->head;
67 list->tail.next = NULL;
70 /* Returns the beginning of LIST. */
72 list_begin (struct list *list)
74 ASSERT (list != NULL);
75 return list->head.next;
78 /* Returns the element after ELEM in its list. If ELEM is the
79 last element in its list, returns the list tail. Results are
80 undefined if ELEM is itself a list tail. */
82 list_next (struct list_elem *elem)
84 ASSERT (is_head (elem) || is_interior (elem));
88 /* Returns LIST's tail.
90 list_end() is often used in iterating through a list from
91 front to back. See the big comment at the top of list.h for
94 list_end (struct list *list)
96 ASSERT (list != NULL);
100 /* Returns the LIST's reverse beginning, for iterating through
101 LIST in reverse order, from back to front. */
103 list_rbegin (struct list *list)
105 ASSERT (list != NULL);
106 return list->tail.prev;
109 /* Returns the element before ELEM in its list. If ELEM is the
110 first element in its list, returns the list head. Results are
111 undefined if ELEM is itself a list head. */
113 list_prev (struct list_elem *elem)
115 ASSERT (is_interior (elem) || is_tail (elem));
119 /* Returns LIST's head.
121 list_rend() is often used in iterating through a list in
122 reverse order, from back to front. Here's typical usage,
123 following the example from the top of list.h:
125 for (e = list_rbegin (&foo_list); e != list_rend (&foo_list);
128 struct foo *f = list_entry (e, struct foo, elem);
129 ...do something with f...
133 list_rend (struct list *list)
135 ASSERT (list != NULL);
139 /* Return's LIST's head.
141 list_head() can be used for an alternate style of iterating
142 through a list, e.g.:
144 e = list_head (&list);
145 while ((e = list_next (e)) != list_end (&list))
151 list_head (struct list *list)
153 ASSERT (list != NULL);
157 /* Return's LIST's tail. */
159 list_tail (struct list *list)
161 ASSERT (list != NULL);
165 /* Inserts ELEM just before BEFORE, which may be either an
166 interior element or a tail. The latter case is equivalent to
169 list_insert (struct list_elem *before, struct list_elem *elem)
171 ASSERT (is_interior (before) || is_tail (before));
172 ASSERT (elem != NULL);
174 elem->prev = before->prev;
176 before->prev->next = elem;
180 /* Removes elements FIRST though LAST (exclusive) from their
181 current list, then inserts them just before BEFORE, which may
182 be either an interior element or a tail. */
184 list_splice (struct list_elem *before,
185 struct list_elem *first, struct list_elem *last)
187 ASSERT (is_interior (before) || is_tail (before));
190 last = list_prev (last);
192 ASSERT (is_interior (first));
193 ASSERT (is_interior (last));
195 /* Cleanly remove FIRST...LAST from its current list. */
196 first->prev->next = last->next;
197 last->next->prev = first->prev;
199 /* Splice FIRST...LAST into new list. */
200 first->prev = before->prev;
202 before->prev->next = first;
206 /* Inserts ELEM at the beginning of LIST, so that it becomes the
209 list_push_front (struct list *list, struct list_elem *elem)
211 list_insert (list_begin (list), elem);
214 /* Inserts ELEM at the end of LIST, so that it becomes the
217 list_push_back (struct list *list, struct list_elem *elem)
219 list_insert (list_end (list), elem);
222 /* Removes ELEM from its list and returns the element that
223 followed it. Undefined behavior if ELEM is not in a list.
225 A list element must be treated very carefully after removing
226 it from its list. Calling list_next() or list_prev() on ELEM
227 will return the item that was previously before or after ELEM,
228 but, e.g., list_prev(list_next(ELEM)) is no longer ELEM!
230 The list_remove() return value provides a convenient way to
231 iterate and remove elements from a list:
233 for (e = list_begin (&list); e != list_end (&list); e = list_remove (e))
235 ...do something with e...
238 If you need to free() elements of the list then you need to be
239 more conservative. Here's an alternate strategy that works
242 while (!list_empty (&list))
244 struct list_elem *e = list_pop_front (&list);
245 ...do something with e...
249 list_remove (struct list_elem *elem)
251 ASSERT (is_interior (elem));
252 elem->prev->next = elem->next;
253 elem->next->prev = elem->prev;
257 /* Removes the front element from LIST and returns it.
258 Undefined behavior if LIST is empty before removal. */
260 list_pop_front (struct list *list)
262 struct list_elem *front = list_front (list);
267 /* Removes the back element from LIST and returns it.
268 Undefined behavior if LIST is empty before removal. */
270 list_pop_back (struct list *list)
272 struct list_elem *back = list_back (list);
277 /* Returns the front element in LIST.
278 Undefined behavior if LIST is empty. */
280 list_front (struct list *list)
282 ASSERT (!list_empty (list));
283 return list->head.next;
286 /* Returns the back element in LIST.
287 Undefined behavior if LIST is empty. */
289 list_back (struct list *list)
291 ASSERT (!list_empty (list));
292 return list->tail.prev;
295 /* Returns the number of elements in LIST.
296 Runs in O(n) in the number of elements. */
298 list_size (struct list *list)
303 for (e = list_begin (list); e != list_end (list); e = list_next (e))
308 /* Returns true if LIST is empty, false otherwise. */
310 list_empty (struct list *list)
312 return list_begin (list) == list_end (list);
315 /* Swaps the `struct list_elem *'s that A and B point to. */
317 swap (struct list_elem **a, struct list_elem **b)
319 struct list_elem *t = *a;
324 /* Reverses the order of LIST. */
326 list_reverse (struct list *list)
328 if (!list_empty (list))
332 for (e = list_begin (list); e != list_end (list); e = e->prev)
333 swap (&e->prev, &e->next);
334 swap (&list->head.next, &list->tail.prev);
335 swap (&list->head.next->prev, &list->tail.prev->next);
339 /* Returns true only if the list elements A through B (exclusive)
340 are in order according to LESS given auxiliary data AUX. */
342 is_sorted (struct list_elem *a, struct list_elem *b,
343 list_less_func *less, void *aux)
346 while ((a = list_next (a)) != b)
347 if (less (a, list_prev (a), aux))
352 /* Finds a run, starting at A and ending not after B, of list
353 elements that are in nondecreasing order according to LESS
354 given auxiliary data AUX. Returns the (exclusive) end of the
356 A through B (exclusive) must form a non-empty range. */
357 static struct list_elem *
358 find_end_of_run (struct list_elem *a, struct list_elem *b,
359 list_less_func *less, void *aux)
363 ASSERT (less != NULL);
370 while (a != b && !less (a, list_prev (a), aux));
374 /* Merges A0 through A1B0 (exclusive) with A1B0 through B1
375 (exclusive) to form a combined range also ending at B1
376 (exclusive). Both input ranges must be nonempty and sorted in
377 nondecreasing order according to LESS given auxiliary data
378 AUX. The output range will be sorted the same way. */
380 inplace_merge (struct list_elem *a0, struct list_elem *a1b0,
381 struct list_elem *b1,
382 list_less_func *less, void *aux)
385 ASSERT (a1b0 != NULL);
387 ASSERT (less != NULL);
388 ASSERT (is_sorted (a0, a1b0, less, aux));
389 ASSERT (is_sorted (a1b0, b1, less, aux));
391 while (a0 != a1b0 && a1b0 != b1)
392 if (!less (a1b0, a0, aux))
396 a1b0 = list_next (a1b0);
397 list_splice (a0, list_prev (a1b0), a1b0);
401 /* Sorts LIST according to LESS given auxiliary data AUX, using a
402 natural iterative merge sort that runs in O(n lg n) time and
403 O(1) space in the number of elements in LIST. */
405 list_sort (struct list *list, list_less_func *less, void *aux)
407 size_t output_run_cnt; /* Number of runs output in current pass. */
409 ASSERT (list != NULL);
410 ASSERT (less != NULL);
412 /* Pass over the list repeatedly, merging adjacent runs of
413 nondecreasing elements, until only one run is left. */
416 struct list_elem *a0; /* Start of first run. */
417 struct list_elem *a1b0; /* End of first run, start of second. */
418 struct list_elem *b1; /* End of second run. */
421 for (a0 = list_begin (list); a0 != list_end (list); a0 = b1)
423 /* Each iteration produces one output run. */
426 /* Locate two adjacent runs of nondecreasing elements
427 A0...A1B0 and A1B0...B1. */
428 a1b0 = find_end_of_run (a0, list_end (list), less, aux);
429 if (a1b0 == list_end (list))
431 b1 = find_end_of_run (a1b0, list_end (list), less, aux);
433 /* Merge the runs. */
434 inplace_merge (a0, a1b0, b1, less, aux);
437 while (output_run_cnt > 1);
439 ASSERT (is_sorted (list_begin (list), list_end (list), less, aux));
442 /* Inserts ELEM in the proper position in LIST, which must be
443 sorted according to LESS given auxiliary data AUX.
444 Runs in O(n) average case in the number of elements in LIST. */
446 list_insert_ordered (struct list *list, struct list_elem *elem,
447 list_less_func *less, void *aux)
451 ASSERT (list != NULL);
452 ASSERT (elem != NULL);
453 ASSERT (less != NULL);
455 for (e = list_begin (list); e != list_end (list); e = list_next (e))
456 if (less (elem, e, aux))
458 return list_insert (e, elem);
461 /* Iterates through LIST and removes all but the first in each
462 set of adjacent elements that are equal according to LESS
463 given auxiliary data AUX. If DUPLICATES is non-null, then the
464 elements from LIST are appended to DUPLICATES. */
466 list_unique (struct list *list, struct list *duplicates,
467 list_less_func *less, void *aux)
469 struct list_elem *elem, *next;
471 ASSERT (list != NULL);
472 ASSERT (less != NULL);
473 if (list_empty (list))
476 elem = list_begin (list);
477 while ((next = list_next (elem)) != list_end (list))
478 if (!less (elem, next, aux) && !less (next, elem, aux))
481 if (duplicates != NULL)
482 list_push_back (duplicates, next);
488 /* Returns the element in LIST with the largest value according
489 to LESS given auxiliary data AUX. If there is more than one
490 maximum, returns the one that appears earlier in the list. If
491 the list is empty, returns its tail. */
493 list_max (struct list *list, list_less_func *less, void *aux)
495 struct list_elem *max = list_begin (list);
496 if (max != list_end (list))
500 for (e = list_next (max); e != list_end (list); e = list_next (e))
501 if (less (max, e, aux))
507 /* Returns the element in LIST with the smallest value according
508 to LESS given auxiliary data AUX. If there is more than one
509 minimum, returns the one that appears earlier in the list. If
510 the list is empty, returns its tail. */
512 list_min (struct list *list, list_less_func *less, void *aux)
514 struct list_elem *min = list_begin (list);
515 if (min != list_end (list))
519 for (e = list_next (min); e != list_end (list); e = list_next (e))
520 if (less (e, min, aux))