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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>
55 lines
1.5 KiB
C
55 lines
1.5 KiB
C
#ifndef PRIO_QUEUE_H
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#define PRIO_QUEUE_H
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/*
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* A priority queue implementation, primarily for keeping track of
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* commits in the 'date-order' so that we process them from new to old
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* as they are discovered, but can be used to hold any pointer to
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* struct. The caller is responsible for supplying a function to
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* compare two "things".
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*
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* Alternatively, this data structure can also be used as a LIFO stack
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* by specifying NULL as the comparison function.
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*/
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/*
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* Compare two "things", one and two; the third parameter is cb_data
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* in the prio_queue structure. The result is returned as a sign of
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* the return value, being the same as the sign of the result of
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* subtracting "two" from "one" (i.e. negative if "one" sorts earlier
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* than "two").
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*/
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typedef int (*prio_queue_compare_fn)(const void *one, const void *two, void *cb_data);
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struct prio_queue_entry {
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unsigned ctr;
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void *data;
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};
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struct prio_queue {
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prio_queue_compare_fn compare;
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unsigned insertion_ctr;
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void *cb_data;
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int alloc, nr;
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struct prio_queue_entry *array;
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};
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/*
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* Add the "thing" to the queue.
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*/
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extern void prio_queue_put(struct prio_queue *, void *thing);
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/*
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* Extract the "thing" that compares the smallest out of the queue,
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* or NULL. If compare function is NULL, the queue acts as a LIFO
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* stack.
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*/
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extern void *prio_queue_get(struct prio_queue *);
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extern void clear_prio_queue(struct prio_queue *);
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/* Reverse the LIFO elements */
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extern void prio_queue_reverse(struct prio_queue *);
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#endif /* PRIO_QUEUE_H */
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