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e8f91e3df8
If two items are added to a prio_queue and compare equal, they currently come out in an apparently random order (this order is deterministic for a particular sequence of insertions and removals, but does not necessarily match the insertion order). This makes it unlike using a date-ordered commit_list, which is one of the main types we would like to replace with it (because prio_queue does not suffer from O(n) insertions). We can make the priority queue stable by keeping an insertion counter for each element, and using it to break ties. This does increase the memory usage of the structure (one int per element), but in practice it does not seem to affect runtime. A best-of-five "git rev-list --topo-order" on linux.git showed less than 1% difference (well within the run-to-run noise). In an ideal world, we would offer both stable and unstable priority queues (the latter to try to maximize performance). However, given the lack of a measurable performance difference, it is not worth the extra code. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
91 lines
1.9 KiB
C
91 lines
1.9 KiB
C
#include "cache.h"
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#include "prio-queue.h"
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static inline int compare(struct prio_queue *queue, int i, int j)
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{
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int cmp = queue->compare(queue->array[i].data, queue->array[j].data,
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queue->cb_data);
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if (!cmp)
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cmp = queue->array[i].ctr - queue->array[j].ctr;
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return cmp;
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}
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static inline void swap(struct prio_queue *queue, int i, int j)
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{
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struct prio_queue_entry tmp = queue->array[i];
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queue->array[i] = queue->array[j];
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queue->array[j] = tmp;
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}
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void prio_queue_reverse(struct prio_queue *queue)
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{
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int i, j;
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if (queue->compare != NULL)
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die("BUG: prio_queue_reverse() on non-LIFO queue");
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for (i = 0; i <= (j = (queue->nr - 1) - i); i++)
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swap(queue, i, j);
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}
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void clear_prio_queue(struct prio_queue *queue)
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{
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free(queue->array);
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queue->nr = 0;
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queue->alloc = 0;
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queue->array = NULL;
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queue->insertion_ctr = 0;
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}
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void prio_queue_put(struct prio_queue *queue, void *thing)
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{
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int ix, parent;
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/* Append at the end */
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ALLOC_GROW(queue->array, queue->nr + 1, queue->alloc);
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queue->array[queue->nr].ctr = queue->insertion_ctr++;
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queue->array[queue->nr].data = thing;
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queue->nr++;
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if (!queue->compare)
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return; /* LIFO */
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/* Bubble up the new one */
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for (ix = queue->nr - 1; ix; ix = parent) {
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parent = (ix - 1) / 2;
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if (compare(queue, parent, ix) <= 0)
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break;
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swap(queue, parent, ix);
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}
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}
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void *prio_queue_get(struct prio_queue *queue)
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{
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void *result;
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int ix, child;
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if (!queue->nr)
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return NULL;
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if (!queue->compare)
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return queue->array[--queue->nr].data; /* LIFO */
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result = queue->array[0].data;
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if (!--queue->nr)
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return result;
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queue->array[0] = queue->array[queue->nr];
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/* Push down the one at the root */
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for (ix = 0; ix * 2 + 1 < queue->nr; ix = child) {
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child = ix * 2 + 1; /* left */
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if (child + 1 < queue->nr &&
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compare(queue, child, child + 1) >= 0)
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child++; /* use right child */
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if (compare(queue, ix, child) <= 0)
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break;
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swap(queue, child, ix);
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}
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return result;
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}
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