[PATCH v4 6/9] commit-graph: introduce `ensure_generations_valid()`

["Taylor Blau via GitGitGadget" <gitgitgadget@gmail.com>]

From: Taylor Blau <me@ttaylorr.com>

Use the just-introduced compute_reachable_generation_numbers_1() to
implement a function which dynamically computes topological levels (or
corrected commit dates) for out-of-graph commits.

This will be useful for the ahead-behind algorithm we are about to
introduce, which needs accurate topological levels on _all_ commits
reachable from the tips in order to avoid over-counting.

Co-authored-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
 commit-graph.c | 29 +++++++++++++++++++++++++++++
 commit-graph.h |  8 ++++++++
 2 files changed, 37 insertions(+)

diff --git a/commit-graph.c b/commit-graph.c
index 63a56483cf6..172e679db19 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1604,6 +1604,35 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 	stop_progress(&ctx->progress);
 }
 
+static void set_generation_in_graph_data(struct commit *c, timestamp_t t,
+					 void *data)
+{
+	commit_graph_data_at(c)->generation = t;
+}
+
+/*
+ * After this method, all commits reachable from those in the given
+ * list will have non-zero, non-infinite generation numbers.
+ */
+void ensure_generations_valid(struct repository *r,
+			      struct commit **commits, size_t nr)
+{
+	int generation_version = get_configured_generation_version(r);
+	struct packed_commit_list list = {
+		.list = commits,
+		.alloc = nr,
+		.nr = nr,
+	};
+	struct compute_generation_info info = {
+		.r = r,
+		.commits = &list,
+		.get_generation = get_generation_from_graph_data,
+		.set_generation = set_generation_in_graph_data,
+	};
+
+	compute_reachable_generation_numbers(&info, generation_version);
+}
+
 static void trace2_bloom_filter_write_statistics(struct write_commit_graph_context *ctx)
 {
 	trace2_data_intmax("commit-graph", ctx->r, "filter-computed",
diff --git a/commit-graph.h b/commit-graph.h
index 37faee6b66d..73e182ab2d0 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -190,4 +190,12 @@ struct commit_graph_data {
  */
 timestamp_t commit_graph_generation(const struct commit *);
 uint32_t commit_graph_position(const struct commit *);
+
+/*
+ * After this method, all commits reachable from those in the given
+ * list will have non-zero, non-infinite generation numbers.
+ */
+void ensure_generations_valid(struct repository *r,
+			      struct commit **commits, size_t nr);
+
 #endif
-- 
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