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commit: use generations in paint_down_to_common()

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Define compare_commits_by_gen_then_commit_date(), which uses generation
numbers as a primary comparison and commit date to break ties (or as a
comparison when both commits do not have computed generation numbers).

Since the commit-graph file is closed under reachability, we know that
all commits in the file have generation at most GENERATION_NUMBER_MAX
which is less than GENERATION_NUMBER_INFINITY.

This change does not affect the number of commits that are walked during
the execution of paint_down_to_common(), only the order that those
commits are inspected. In the case that commit dates violate topological
order (i.e. a parent is "newer" than a child), the previous code could
walk a commit twice: if a commit is reached with the PARENT1 bit, but
later is re-visited with the PARENT2 bit, then that PARENT2 bit must be
propagated to its parents. Using generation numbers avoids this extra
effort, even if it is somewhat rare.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Derrick Stolee
2018-05-01 12:47:11 +00:00
committed by Junio C Hamano
parent 3258c66332
commit 3afc679b3c
2 changed files with 20 additions and 1 deletions
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commit.h
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@ -341,6 +341,7 @@ extern int remove_signature(struct strbuf *buf);
extern int check_commit_signature(const struct commit *commit, struct signature_check *sigc);
int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused);
int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused);
LAST_ARG_MUST_BE_NULL
extern int run_commit_hook(int editor_is_used, const char *index_file, const char *name, ...);