git/prio-queue.h
Jeff King e8f91e3df8 prio-queue: make output stable with respect to insertion
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>
2014-07-15 11:02:54 -07:00

55 lines
1.5 KiB
C

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