commit-reach: introduce struct paint_state with per-side counters

Add a paint_state struct for use by paint_down_to_common() that
wraps a prio_queue with per-side commit counters. Each non-stale
queued commit occupies exactly one counter bucket based on its
paint flags: PARENT1-only, PARENT2-only, or both sides (a pending
merge-base candidate).

The counters are maintained by paint_count_update() which adjusts
the appropriate bucket by a signed delta. An exhaustive switch on
the paint+stale bits documents all valid flag combinations in one
place.

Convert paint_down_to_common() to use paint_state. The loop now
drains the queue via paint_queue_get() which returns NULL when all
counters reach zero, replacing the old pointer-based termination
(max_nonstale). This is equivalent behavior -- both conditions
detect that no non-stale entries remain.

paint_queue_get() uses a "pop first" form: it dequeues a commit,
then checks the counters. This means the loop exits one iteration
earlier than the old code in some topologies (the popped stale
commit is never processed), so a few step counts drop by one.

The existing nonstale_queue is left in place for ahead_behind(),
though nonstale_queue_put_dedup() and nonstale_queue_get_dedup()
became unused and are removed.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/paint-down-to-common.adoc

@@ -94,15 +94,12 @@ re-enqueued is bounded by the number of flag transitions.
 Termination
 -----------
 
-The walk uses a `nonstale_queue` wrapper around `prio_queue` that
-tracks `max_nonstale`: the lowest-priority non-stale commit enqueued
-so far. Once that commit is dequeued, every remaining entry is known
-to be STALE and the loop terminates. Specifically, the main loop
+The walk tracks the number of commits of each type in the queue
+(PARENT1-only, PARENT2-only, pending merge-base). The main loop
 ends when one of the following conditions holds:
 
   1. The queue is empty.
-  2. `max_nonstale` has been dequeued, meaning the queue only contains
-     STALE entries.
+  2. The queue contains only stale entries.
   3. Generation cutoff: the dequeued commit's generation is below
      a caller-supplied `min_generation` threshold.
   4. Single result: the caller only needs one merge base, one has

commit-reach.c

@@ -79,21 +79,73 @@ static void clear_nonstale_queue(struct nonstale_queue *queue)
 	queue->max_nonstale = NULL;
 }
 
-static void nonstale_queue_put_dedup(struct nonstale_queue *queue,
-				     struct commit *c)
+/*
+ * Priority queue with per-side commit counters for paint_down_to_common().
+ * Each non-stale queued commit occupies exactly one bucket: PARENT1-only,
+ * PARENT2-only, or both (a pending merge-base candidate).
+ */
+struct paint_state {
+	struct prio_queue queue;
+	size_t parent1_count;
+	size_t parent2_count;
+	size_t mb_candidate_count;
+	int gen_ordered;
+};
+
+static void paint_count_update(struct paint_state *state,
+			       unsigned flags, int delta)
 {
-	if (c->object.flags & ENQUEUED)
-		return;
-	c->object.flags |= ENQUEUED;
-	nonstale_queue_put(queue, c);
+	switch (flags & (PARENT1 | PARENT2 | STALE)) {
+	case PARENT1:
+		state->parent1_count += delta;
+		break;
+
+	case PARENT2:
+		state->parent2_count += delta;
+		break;
+
+	case PARENT1 | PARENT2:
+		state->mb_candidate_count += delta;
+		break;
+
+	case PARENT1 | PARENT2 | STALE:
+		break;
+
+	default:
+		BUG("unexpected paint state");
+	}
 }
 
-static struct commit *nonstale_queue_get_dedup(struct nonstale_queue *queue)
+static void paint_queue_put(struct paint_state *state,
+			    struct commit *c, unsigned add_flags)
 {
-	struct commit *commit = nonstale_queue_get(queue);
+	unsigned old_flags = c->object.flags;
+	c->object.flags |= add_flags;
 
-	if (commit)
-		commit->object.flags &= ~ENQUEUED;
+	if (old_flags & ENQUEUED) {
+		paint_count_update(state, old_flags, -1);
+		paint_count_update(state, c->object.flags, 1);
+	} else {
+		c->object.flags |= ENQUEUED;
+		prio_queue_put(&state->queue, c);
+		paint_count_update(state, c->object.flags, 1);
+	}
+}
+
+static struct commit *paint_queue_get(struct paint_state *state)
+{
+	struct commit *commit = prio_queue_get(&state->queue);
+
+	if (!commit)
+		return NULL;
+
+	commit->object.flags &= ~ENQUEUED;
+
+	if (!state->parent1_count && !state->parent2_count &&
+	    !state->mb_candidate_count)
+		return NULL;
+
+	paint_count_update(state, commit->object.flags, -1);
 	return commit;
 }
 
@@ -109,18 +161,19 @@ static int paint_down_to_common(struct repository *r,
 				enum merge_base_flags mb_flags,
 				struct commit_list **result)
 {
-	struct nonstale_queue queue = {
-		{ compare_commits_by_gen_then_commit_date }
+	struct paint_state state = {
+		.queue = { compare_commits_by_gen_then_commit_date },
+		.gen_ordered = 1,
 	};
+	struct commit *commit;
 	int i;
-	int gen_ordered = 1;
 	int steps = 0;
 	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
 	if (!min_generation && !corrected_commit_dates_enabled(r)) {
-		queue.pq.compare = compare_commits_by_commit_date;
-		gen_ordered = 0;
+		state.queue.compare = compare_commits_by_commit_date;
+		state.gen_ordered = 0;
 	}
 
 	one->object.flags |= PARENT1;
@@ -128,15 +181,12 @@ static int paint_down_to_common(struct repository *r,
 		commit_list_append(one, result);
 		return 0;
 	}
-	nonstale_queue_put_dedup(&queue, one);
+	paint_queue_put(&state, one, 0);
 
-	for (i = 0; i < n; i++) {
-		twos[i]->object.flags |= PARENT2;
-		nonstale_queue_put_dedup(&queue, twos[i]);
-	}
+	for (i = 0; i < n; i++)
+		paint_queue_put(&state, twos[i], PARENT2);
 
-	while (queue.max_nonstale) {
-		struct commit *commit = nonstale_queue_get_dedup(&queue);
+	while ((commit = paint_queue_get(&state))) {
 		struct commit_list *parents;
 		int flags;
 		timestamp_t generation = commit_graph_generation(commit);
@@ -162,7 +212,7 @@ static int paint_down_to_common(struct repository *r,
 				 * descendant of this one.
 				 */
 				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
-				    gen_ordered &&
+				    state.gen_ordered &&
 				    generation < GENERATION_NUMBER_INFINITY)
 					break;
 			}
@@ -176,7 +226,7 @@ static int paint_down_to_common(struct repository *r,
 			if ((p->object.flags & flags) == flags)
 				continue;
 			if (repo_parse_commit(r, p)) {
-				clear_nonstale_queue(&queue);
+				clear_prio_queue(&state.queue);
 				commit_list_free(*result);
 				*result = NULL;
 				/*
@@ -191,12 +241,11 @@ static int paint_down_to_common(struct repository *r,
 				return error(_("could not parse commit %s"),
 					     oid_to_hex(&p->object.oid));
 			}
-			p->object.flags |= flags;
-			nonstale_queue_put_dedup(&queue, p);
+			paint_queue_put(&state, p, flags);
 		}
 	}
 
-	clear_nonstale_queue(&queue);
+	clear_prio_queue(&state.queue);
 	trace2_data_intmax("paint_down_to_common", r,
 			   "steps", steps);
 	commit_list_sort_by_date(result);

t/t6600-test-reach.sh

@@ -366,7 +366,7 @@ test_expect_success 'get_merge_bases_many:pending-stale' '
 		git rev-parse ps-B
 	} >expect &&
 	test_all_modes get_merge_bases_many &&
-	test_paint_down_steps 6 6 6 6
+	test_paint_down_steps 5 5 5 5
 '
 
 test_expect_success 'get_merge_bases_many:infinity-both-sides' '
@@ -381,7 +381,7 @@ test_expect_success 'get_merge_bases_many:infinity-both-sides' '
 		git rev-parse pi-B
 	} >expect &&
 	test_all_modes get_merge_bases_many &&
-	test_paint_down_steps 5 5 5 5
+	test_paint_down_steps 5 4 5 5
 '
 
 test_expect_success 'setup mixed finite/INFINITY topology' '
@@ -438,7 +438,7 @@ test_expect_success 'merge-base --all with clock skew and redundant ancestor (si
 	>input &&
 	git rev-parse se2-MB1 >expect &&
 	run_all_modes git merge-base --all se2-A se2-B &&
-	test_paint_down_steps 8 7 8 8
+	test_paint_down_steps 8 6 8 8
 '
 
 test_expect_success 'reduce_heads' '