[PATCH v4 3/9] commit-graph: refactor compute_topological_levels()

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

This patch extracts the common code used to compute topological levels
and corrected committer dates into a common routine,
compute_reachable_generation_numbers(). For ease of reading, it only
modifies compute_topological_levels() to use this new routine, leaving
compute_generation_numbers() to be modified in the next change.

This new routine dispatches to call the necessary functions to get and
set the generation number for a given commit through a vtable (the
compute_generation_info struct).

Computing the generation number itself is done in
compute_generation_from_max(), which dispatches its implementation based
on the generation version requested, or issuing a BUG() for unrecognized
generation versions. This does not use a vtable because the logic
depends only on the generation number version, not where the data is
being loaded from or being stored to. This is a subtle point that will
make more sense in a future change that modifies the in-memory
generation values instead of just preparing values for writing to a
commit-graph file.

This change looks like it adds a lot of new code. However, two upcoming
changes will be quite small due to the work being done in this change.

Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
 commit-graph.c | 106 ++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 83 insertions(+), 23 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index c11b59f28b3..4356c8c1f4b 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1446,24 +1446,52 @@ static void close_reachable(struct write_commit_graph_context *ctx)
 	stop_progress(&ctx->progress);
 }
 
-static void compute_topological_levels(struct write_commit_graph_context *ctx)
+struct compute_generation_info {
+	struct repository *r;
+	struct packed_commit_list *commits;
+	struct progress *progress;
+	int progress_cnt;
+
+	timestamp_t (*get_generation)(struct commit *c, void *data);
+	void (*set_generation)(struct commit *c, timestamp_t gen, void *data);
+	void *data;
+};
+
+static timestamp_t compute_generation_from_max(struct commit *c,
+					       timestamp_t max_gen,
+					       int generation_version)
+{
+	switch (generation_version) {
+	case 1: /* topological levels */
+		if (max_gen > GENERATION_NUMBER_V1_MAX - 1)
+			max_gen = GENERATION_NUMBER_V1_MAX - 1;
+		return max_gen + 1;
+
+	case 2: /* corrected commit date */
+		if (c->date && c->date > max_gen)
+			max_gen = c->date - 1;
+		return max_gen + 1;
+
+	default:
+		BUG("attempting unimplemented version");
+	}
+}
+
+static void compute_reachable_generation_numbers(
+			struct compute_generation_info *info,
+			int generation_version)
 {
 	int i;
 	struct commit_list *list = NULL;
 
-	if (ctx->report_progress)
-		ctx->progress = start_delayed_progress(
-					_("Computing commit graph topological levels"),
-					ctx->commits.nr);
-	for (i = 0; i < ctx->commits.nr; i++) {
-		struct commit *c = ctx->commits.list[i];
-		uint32_t level;
-
-		repo_parse_commit(ctx->r, c);
-		level = *topo_level_slab_at(ctx->topo_levels, c);
+	for (i = 0; i < info->commits->nr; i++) {
+		struct commit *c = info->commits->list[i];
+		timestamp_t gen;
+		repo_parse_commit(info->r, c);
+		gen = info->get_generation(c, info->data);
+		display_progress(info->progress, info->progress_cnt + 1);
 
-		display_progress(ctx->progress, i + 1);
-		if (level != GENERATION_NUMBER_ZERO)
+		if (gen != GENERATION_NUMBER_ZERO && gen != GENERATION_NUMBER_INFINITY)
 			continue;
 
 		commit_list_insert(c, &list);
@@ -1471,31 +1499,63 @@ static void compute_topological_levels(struct write_commit_graph_context *ctx)
 			struct commit *current = list->item;
 			struct commit_list *parent;
 			int all_parents_computed = 1;
-			uint32_t max_level = 0;
+			uint32_t max_gen = 0;
 
 			for (parent = current->parents; parent; parent = parent->next) {
-				repo_parse_commit(ctx->r, parent->item);
-				level = *topo_level_slab_at(ctx->topo_levels, parent->item);
+				repo_parse_commit(info->r, parent->item);
+				gen = info->get_generation(parent->item, info->data);
 
-				if (level == GENERATION_NUMBER_ZERO) {
+				if (gen == GENERATION_NUMBER_ZERO) {
 					all_parents_computed = 0;
 					commit_list_insert(parent->item, &list);
 					break;
 				}
 
-				if (level > max_level)
-					max_level = level;
+				if (gen > max_gen)
+					max_gen = gen;
 			}
 
 			if (all_parents_computed) {
 				pop_commit(&list);
-
-				if (max_level > GENERATION_NUMBER_V1_MAX - 1)
-					max_level = GENERATION_NUMBER_V1_MAX - 1;
-				*topo_level_slab_at(ctx->topo_levels, current) = max_level + 1;
+				gen = compute_generation_from_max(
+						current, max_gen,
+						generation_version);
+				info->set_generation(current, gen, info->data);
 			}
 		}
 	}
+}
+
+static timestamp_t get_topo_level(struct commit *c, void *data)
+{
+	struct write_commit_graph_context *ctx = data;
+	return *topo_level_slab_at(ctx->topo_levels, c);
+}
+
+static void set_topo_level(struct commit *c, timestamp_t t, void *data)
+{
+	struct write_commit_graph_context *ctx = data;
+	*topo_level_slab_at(ctx->topo_levels, c) = (uint32_t)t;
+}
+
+static void compute_topological_levels(struct write_commit_graph_context *ctx)
+{
+	struct compute_generation_info info = {
+		.r = ctx->r,
+		.commits = &ctx->commits,
+		.get_generation = get_topo_level,
+		.set_generation = set_topo_level,
+		.data = ctx,
+	};
+
+	if (ctx->report_progress)
+		info.progress = ctx->progress
+			      = start_delayed_progress(
+					_("Computing commit graph topological levels"),
+					ctx->commits.nr);
+
+	compute_reachable_generation_numbers(&info, 1);
+
 	stop_progress(&ctx->progress);
 }
 
-- 
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