2 * Written by Josh Dybnis and released to the public domain, as explained at
3 * http://creativecommons.org/licenses/publicdomain
5 * non thread safe skiplist
31 const datatype_t *key_type;
32 int high_water; // max level of any item in the list
35 static int random_level (void) {
36 unsigned r = nbd_rand();
37 int n = __builtin_ctz(r) / 2;
38 if (n > MAX_LEVEL) { n = MAX_LEVEL; }
42 static node_t *node_alloc (int level, map_key_t key, map_val_t val) {
43 assert(level >= 0 && level <= MAX_LEVEL);
44 size_t sz = sizeof(node_t) + level * sizeof(node_t *);
45 node_t *item = (node_t *)nbd_malloc(sz);
49 item->top_level = level;
50 TRACE("s2", "node_alloc: new node %p (%llu levels)", item, level);
54 skiplist_t *sl_alloc (const datatype_t *key_type) {
55 skiplist_t *sl = (skiplist_t *)nbd_malloc(sizeof(skiplist_t));
56 sl->key_type = key_type;
58 sl->head = node_alloc(MAX_LEVEL, 0, 0);
59 memset(sl->head->next, 0, (MAX_LEVEL+1) * sizeof(skiplist_t *));
63 void sl_free (skiplist_t *sl) {
64 node_t *item = sl->head->next[0];
66 node_t *next = item->next[0];
67 if (sl->key_type != NULL) {
68 nbd_free((void *)item->key);
75 size_t sl_count (skiplist_t *sl) {
77 node_t *item = sl->head->next[0];
85 static node_t *find_preds (node_t **preds, node_t **succs, int n, skiplist_t *sl, map_key_t key, int help_remove) {
86 node_t *pred = sl->head;
88 TRACE("s2", "find_preds: searching for key %p in skiplist (head is %p)", key, pred);
90 int start_level = sl->high_water;
91 if (EXPECT_FALSE(start_level < n)) {
95 // Traverse the levels of <sl> from the top level to the bottom
96 for (int level = start_level; level >= 0; --level) {
97 node_t *next = pred->next[level];
98 if (next == DOES_NOT_EXIST && level > n)
100 TRACE("s3", "find_preds: traversing level %p starting at %p", level, pred);
102 while (item != NULL) {
103 next = item->next[level];
105 if (EXPECT_TRUE(sl->key_type == NULL)) {
108 d = sl->key_type->cmp((void *)item->key, (void *)key);
118 TRACE("s3", "find_preds: found pred %p next %p", pred, item);
120 // The cast to unsigned is for the case when n is -1.
121 if ((unsigned)level <= (unsigned)n) {
131 // fill in empty levels
132 if (n == -1 && item != NULL && preds != NULL) {
133 assert(item->top_level <= MAX_LEVEL);
134 for (int level = start_level + 1; level <= item->top_level; ++level) {
135 preds[level] = sl->head;
140 TRACE("s2", "find_preds: found matching item %p in skiplist, pred is %p", item, pred);
143 TRACE("s2", "find_preds: found proper place for key %p in skiplist, pred is %p. returning null", key, pred);
147 static void sl_unlink (skiplist_t *sl, map_key_t key) {
148 node_t *pred = sl->head;
150 TRACE("s2", "sl_unlink: unlinking marked item with key %p", key, 0);
153 // Traverse the levels of <sl>
154 for (int level = sl->high_water; level >= 0; --level) {
155 node_t *next = pred->next[level];
156 if (next == DOES_NOT_EXIST)
158 TRACE("s3", "sl_unlink: traversing level %p starting at %p", level, pred);
160 while (item != NULL) {
161 next = item->next[level];
163 if (EXPECT_TRUE(sl->key_type == NULL)) {
166 d = sl->key_type->cmp((void *)item->key, (void *)key);
170 pred->next[level] = next;
171 TRACE("s3", "sl_unlink: unlinked item from pred %p", pred, 0);
173 next = (item != NULL) ? item->next[level] : DOES_NOT_EXIST;
183 TRACE("s3", "sl_unlink: at pred %p next %p", pred, item);
187 // Fast find that does not return the node's predecessors.
188 map_val_t sl_lookup (skiplist_t *sl, map_key_t key) {
189 TRACE("s1", "sl_lookup: searching for key %p in skiplist %p", key, sl);
190 node_t *item = find_preds(NULL, NULL, 0, sl, key, FALSE);
192 // If we found an <item> matching the <key> return its value.
194 map_val_t val = item->val;
198 TRACE("l1", "sl_lookup: no item in the skiplist matched the key", 0, 0);
199 return DOES_NOT_EXIST;
202 map_key_t sl_min_key (skiplist_t *sl) {
203 node_t *item = sl->head->next[0];
206 return DOES_NOT_EXIST;
209 map_val_t sl_cas (skiplist_t *sl, map_key_t key, map_val_t expectation, map_val_t new_val) {
210 TRACE("s1", "sl_cas: key %p skiplist %p", key, sl);
211 TRACE("s1", "sl_cas: expectation %p new value %p", expectation, new_val);
212 ASSERT((int64_t)new_val > 0);
214 node_t *preds[MAX_LEVEL+1];
215 node_t *nexts[MAX_LEVEL+1];
216 node_t *new_item = NULL;
217 int n = random_level();
218 node_t *old_item = find_preds(preds, nexts, n, sl, key, TRUE);
220 // If there is already an item in the skiplist that matches the key just update its value.
221 if (old_item != NULL) {
222 map_val_t old_val = old_item->val;
223 if (expectation == CAS_EXPECT_DOES_NOT_EXIST ||
224 (expectation != CAS_EXPECT_WHATEVER && expectation != CAS_EXPECT_EXISTS && expectation != old_val)) {
225 TRACE("s1", "update_item: found an item %p in the skiplist that matched the key. the expectation was "
226 "not met, the skiplist was not changed", item, old_val);
229 old_item->val = new_val;
233 if (EXPECT_FALSE(expectation != CAS_EXPECT_DOES_NOT_EXIST && expectation != CAS_EXPECT_WHATEVER)) {
234 TRACE("l1", "sl_cas: the expectation was not met, the skiplist was not changed", 0, 0);
235 return DOES_NOT_EXIST; // failure, the caller expected an item for the <key> to already exist
238 TRACE("s3", "sl_cas: inserting a new item between %p and %p", preds[0], nexts[0]);
240 // Create a new node and insert it into the skiplist.
241 map_key_t new_key = sl->key_type == NULL ? key : (map_key_t)sl->key_type->clone((void *)key);
242 if (n > sl->high_water) {
243 n = ++sl->high_water;
244 TRACE("s2", "sl_cas: incremented high water mark to %p", sl->high_water, 0);
246 new_item = node_alloc(n, new_key, new_val);
248 // Set <new_item>'s next pointers to their proper values
249 for (int level = 0; level <= new_item->top_level; ++level) {
250 new_item->next[level] = nexts[level];
253 // Link <new_item> into <sl>
254 for (int level = 0; level <= new_item->top_level; ++level) {
255 preds[level]->next[level] = new_item;
258 return DOES_NOT_EXIST; // success, inserted a new item
261 map_val_t sl_remove (skiplist_t *sl, map_key_t key) {
262 TRACE("s1", "sl_remove: removing item with key %p from skiplist %p", key, sl);
263 node_t *preds[MAX_LEVEL+1];
264 node_t *item = find_preds(preds, NULL, -1, sl, key, TRUE);
266 TRACE("s3", "sl_remove: remove failed, an item with a matching key does not exist in the skiplist", 0, 0);
267 return DOES_NOT_EXIST;
269 map_val_t val = item->val;
275 if (sl->key_type != NULL) {
276 nbd_free((void *)item->key);
283 void sl_print (skiplist_t *sl) {
285 printf("high water: %d levels\n", sl->high_water);
286 for (int level = MAX_LEVEL; level >= 0; --level) {
287 node_t *item = sl->head;
288 if (item->next[level] == DOES_NOT_EXIST)
290 printf("(%d) ", level);
293 node_t *next = item->next[level];
304 node_t *item = sl->head;
307 printf("%p:0x%llx ", item, (uint64_t)item->key);
308 if (item != sl->head) {
309 printf("[%d]", item->top_level);
313 for (int level = 1; level <= item->top_level; ++level) {
314 node_t *next = item->next[level];
316 if (item == sl->head && item->next[level] == DOES_NOT_EXIST)
321 item = item->next[0];
329 sl_iter_t *sl_iter_begin (skiplist_t *sl, map_key_t key) {
330 sl_iter_t *iter = (sl_iter_t *)nbd_malloc(sizeof(sl_iter_t));
331 if (key != DOES_NOT_EXIST) {
332 find_preds(NULL, &iter->next, 0, sl, key, FALSE);
334 iter->next = sl->head->next[0];
339 map_val_t sl_iter_next (sl_iter_t *iter, map_key_t *key_ptr) {
341 node_t *item = iter->next;
344 return DOES_NOT_EXIST;
346 iter->next = item->next[0];
347 if (key_ptr != NULL) {
348 *key_ptr = item->key;
353 void sl_iter_free (sl_iter_t *iter) {