Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * pairingheap.c
4 : * A Pairing Heap implementation
5 : *
6 : * A pairing heap is a data structure that's useful for implementing
7 : * priority queues. It is simple to implement, and provides amortized O(1)
8 : * insert and find-min operations, and amortized O(log n) delete-min.
9 : *
10 : * The pairing heap was first described in this paper:
11 : *
12 : * Michael L. Fredman, Robert Sedgewick, Daniel D. Sleator, and Robert E.
13 : * Tarjan. 1986.
14 : * The pairing heap: a new form of self-adjusting heap.
15 : * Algorithmica 1, 1 (January 1986), pages 111-129. DOI: 10.1007/BF01840439
16 : *
17 : * Portions Copyright (c) 2012-2024, PostgreSQL Global Development Group
18 : *
19 : * IDENTIFICATION
20 : * src/backend/lib/pairingheap.c
21 : *
22 : *-------------------------------------------------------------------------
23 : */
24 :
25 : #include "postgres.h"
26 :
27 : #include "lib/pairingheap.h"
28 :
29 : static pairingheap_node *merge(pairingheap *heap, pairingheap_node *a,
30 : pairingheap_node *b);
31 : static pairingheap_node *merge_children(pairingheap *heap,
32 : pairingheap_node *children);
33 :
34 : /*
35 : * pairingheap_allocate
36 : *
37 : * Returns a pointer to a newly-allocated heap, with the heap property defined
38 : * by the given comparator function, which will be invoked with the additional
39 : * argument specified by 'arg'.
40 : */
41 : pairingheap *
42 9208 : pairingheap_allocate(pairingheap_comparator compare, void *arg)
43 : {
44 : pairingheap *heap;
45 :
46 9208 : heap = (pairingheap *) palloc(sizeof(pairingheap));
47 9208 : heap->ph_compare = compare;
48 9208 : heap->ph_arg = arg;
49 :
50 9208 : heap->ph_root = NULL;
51 :
52 9208 : return heap;
53 : }
54 :
55 : /*
56 : * pairingheap_free
57 : *
58 : * Releases memory used by the given pairingheap.
59 : *
60 : * Note: The nodes in the heap are not freed!
61 : */
62 : void
63 0 : pairingheap_free(pairingheap *heap)
64 : {
65 0 : pfree(heap);
66 0 : }
67 :
68 : /*
69 : * A helper function to merge two subheaps into one.
70 : *
71 : * The subheap with smaller value is put as a child of the other one (assuming
72 : * a max-heap).
73 : *
74 : * The next_sibling and prev_or_parent pointers of the input nodes are
75 : * ignored. On return, the returned node's next_sibling and prev_or_parent
76 : * pointers are garbage.
77 : */
78 : static pairingheap_node *
79 23111254 : merge(pairingheap *heap, pairingheap_node *a, pairingheap_node *b)
80 : {
81 23111254 : if (a == NULL)
82 5321396 : return b;
83 17789858 : if (b == NULL)
84 0 : return a;
85 :
86 : /* swap 'a' and 'b' so that 'a' is the one with larger value */
87 17789858 : if (heap->ph_compare(a, b, heap->ph_arg) < 0)
88 : {
89 : pairingheap_node *tmp;
90 :
91 2166888 : tmp = a;
92 2166888 : a = b;
93 2166888 : b = tmp;
94 : }
95 :
96 : /* and put 'b' as a child of 'a' */
97 17789858 : if (a->first_child)
98 7108532 : a->first_child->prev_or_parent = b;
99 17789858 : b->prev_or_parent = a;
100 17789858 : b->next_sibling = a->first_child;
101 17789858 : a->first_child = b;
102 :
103 17789858 : return a;
104 : }
105 :
106 : /*
107 : * pairingheap_add
108 : *
109 : * Adds the given node to the heap in O(1) time.
110 : */
111 : void
112 18891538 : pairingheap_add(pairingheap *heap, pairingheap_node *node)
113 : {
114 18891538 : node->first_child = NULL;
115 :
116 : /* Link the new node as a new tree */
117 18891538 : heap->ph_root = merge(heap, heap->ph_root, node);
118 18891538 : heap->ph_root->prev_or_parent = NULL;
119 18891538 : heap->ph_root->next_sibling = NULL;
120 18891538 : }
121 :
122 : /*
123 : * pairingheap_first
124 : *
125 : * Returns a pointer to the first (root, topmost) node in the heap without
126 : * modifying the heap. The caller must ensure that this routine is not used on
127 : * an empty heap. Always O(1).
128 : */
129 : pairingheap_node *
130 3022242 : pairingheap_first(pairingheap *heap)
131 : {
132 : Assert(!pairingheap_is_empty(heap));
133 :
134 3022242 : return heap->ph_root;
135 : }
136 :
137 : /*
138 : * pairingheap_remove_first
139 : *
140 : * Removes the first (root, topmost) node in the heap and returns a pointer to
141 : * it after rebalancing the heap. The caller must ensure that this routine is
142 : * not used on an empty heap. O(log n) amortized.
143 : */
144 : pairingheap_node *
145 6749034 : pairingheap_remove_first(pairingheap *heap)
146 : {
147 : pairingheap_node *result;
148 : pairingheap_node *children;
149 :
150 : Assert(!pairingheap_is_empty(heap));
151 :
152 : /* Remove the root, and form a new heap of its children. */
153 6749034 : result = heap->ph_root;
154 6749034 : children = result->first_child;
155 :
156 6749034 : heap->ph_root = merge_children(heap, children);
157 6749034 : if (heap->ph_root)
158 : {
159 1427904 : heap->ph_root->prev_or_parent = NULL;
160 1427904 : heap->ph_root->next_sibling = NULL;
161 : }
162 :
163 6749034 : return result;
164 : }
165 :
166 : /*
167 : * Remove 'node' from the heap. O(log n) amortized.
168 : */
169 : void
170 18323534 : pairingheap_remove(pairingheap *heap, pairingheap_node *node)
171 : {
172 : pairingheap_node *children;
173 : pairingheap_node *replacement;
174 : pairingheap_node *next_sibling;
175 : pairingheap_node **prev_ptr;
176 :
177 : /*
178 : * If the removed node happens to be the root node, do it with
179 : * pairingheap_remove_first().
180 : */
181 18323534 : if (node == heap->ph_root)
182 : {
183 6213132 : (void) pairingheap_remove_first(heap);
184 6213132 : return;
185 : }
186 :
187 : /*
188 : * Before we modify anything, remember the removed node's first_child and
189 : * next_sibling pointers.
190 : */
191 12110402 : children = node->first_child;
192 12110402 : next_sibling = node->next_sibling;
193 :
194 : /*
195 : * Also find the pointer to the removed node in its previous sibling, or
196 : * if this is the first child of its parent, in its parent.
197 : */
198 12110402 : if (node->prev_or_parent->first_child == node)
199 12085880 : prev_ptr = &node->prev_or_parent->first_child;
200 : else
201 24522 : prev_ptr = &node->prev_or_parent->next_sibling;
202 : Assert(*prev_ptr == node);
203 :
204 : /*
205 : * If this node has children, make a new subheap of the children and link
206 : * the subheap in place of the removed node. Otherwise just unlink this
207 : * node.
208 : */
209 12110402 : if (children)
210 : {
211 1952 : replacement = merge_children(heap, children);
212 :
213 1952 : replacement->prev_or_parent = node->prev_or_parent;
214 1952 : replacement->next_sibling = node->next_sibling;
215 1952 : *prev_ptr = replacement;
216 1952 : if (next_sibling)
217 1752 : next_sibling->prev_or_parent = replacement;
218 : }
219 : else
220 : {
221 12108450 : *prev_ptr = next_sibling;
222 12108450 : if (next_sibling)
223 2853482 : next_sibling->prev_or_parent = node->prev_or_parent;
224 : }
225 : }
226 :
227 : /*
228 : * Merge a list of subheaps into a single heap.
229 : *
230 : * This implements the basic two-pass merging strategy, first forming pairs
231 : * from left to right, and then merging the pairs.
232 : */
233 : static pairingheap_node *
234 6750986 : merge_children(pairingheap *heap, pairingheap_node *children)
235 : {
236 : pairingheap_node *curr,
237 : *next;
238 : pairingheap_node *pairs;
239 : pairingheap_node *newroot;
240 :
241 6750986 : if (children == NULL || children->next_sibling == NULL)
242 6244726 : return children;
243 :
244 : /* Walk the subheaps from left to right, merging in pairs */
245 506260 : next = children;
246 506260 : pairs = NULL;
247 : for (;;)
248 : {
249 2747494 : curr = next;
250 :
251 2747494 : if (curr == NULL)
252 262752 : break;
253 :
254 2484742 : if (curr->next_sibling == NULL)
255 : {
256 : /* last odd node at the end of list */
257 243508 : curr->next_sibling = pairs;
258 243508 : pairs = curr;
259 243508 : break;
260 : }
261 :
262 2241234 : next = curr->next_sibling->next_sibling;
263 :
264 : /* merge this and the next subheap, and add to 'pairs' list. */
265 :
266 2241234 : curr = merge(heap, curr, curr->next_sibling);
267 2241234 : curr->next_sibling = pairs;
268 2241234 : pairs = curr;
269 : }
270 :
271 : /*
272 : * Merge all the pairs together to form a single heap.
273 : */
274 506260 : newroot = pairs;
275 506260 : next = pairs->next_sibling;
276 2484742 : while (next)
277 : {
278 1978482 : curr = next;
279 1978482 : next = curr->next_sibling;
280 :
281 1978482 : newroot = merge(heap, newroot, curr);
282 : }
283 :
284 506260 : return newroot;
285 : }
286 :
287 : /*
288 : * A debug function to dump the contents of the heap as a string.
289 : *
290 : * The 'dumpfunc' callback appends a string representation of a single node
291 : * to the StringInfo. 'opaque' can be used to pass more information to the
292 : * callback.
293 : */
294 : #ifdef PAIRINGHEAP_DEBUG
295 : static void
296 : pairingheap_dump_recurse(StringInfo buf,
297 : pairingheap_node *node,
298 : void (*dumpfunc) (pairingheap_node *node, StringInfo buf, void *opaque),
299 : void *opaque,
300 : int depth,
301 : pairingheap_node *prev_or_parent)
302 : {
303 : while (node)
304 : {
305 : Assert(node->prev_or_parent == prev_or_parent);
306 :
307 : appendStringInfoSpaces(buf, depth * 4);
308 : dumpfunc(node, buf, opaque);
309 : appendStringInfoChar(buf, '\n');
310 : if (node->first_child)
311 : pairingheap_dump_recurse(buf, node->first_child, dumpfunc, opaque, depth + 1, node);
312 : prev_or_parent = node;
313 : node = node->next_sibling;
314 : }
315 : }
316 :
317 : char *
318 : pairingheap_dump(pairingheap *heap,
319 : void (*dumpfunc) (pairingheap_node *node, StringInfo buf, void *opaque),
320 : void *opaque)
321 : {
322 : StringInfoData buf;
323 :
324 : if (!heap->ph_root)
325 : return pstrdup("(empty)");
326 :
327 : initStringInfo(&buf);
328 :
329 : pairingheap_dump_recurse(&buf, heap->ph_root, dumpfunc, opaque, 0, NULL);
330 :
331 : return buf.data;
332 : }
333 : #endif
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