From e8f91e3df82a33b7e0b59c935cc4af892068baa2 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Mon, 14 Jul 2014 01:51:59 -0400 Subject: 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 Signed-off-by: Junio C Hamano --- prio-queue.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) (limited to 'prio-queue.h') diff --git a/prio-queue.h b/prio-queue.h index 9c3cd1f875..d030ec9dd6 100644 --- a/prio-queue.h +++ b/prio-queue.h @@ -21,11 +21,17 @@ */ 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; - void **array; + struct prio_queue_entry *array; }; /* -- cgit v1.2.3