[PATCH] shortlog: use a stable sort

[PATCH] shortlog: use a stable sort

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When sorting the output of `git shortlog` by count, a list of authors in
alphabetical order is then sorted by contribution count. Obviously, the
idea is to maintain the alphabetical order for items with identical
contribution count.

At the moment, this job is performed by `qsort()`. As that function is
not guaranteed to implement a stable sort algorithm, this can lead to
inconsistent and/or surprising behavior: items with identical
contribution count could lose their alphabetical sub-order.

The `qsort()` in MS Visual C's runtime does _not_ implement a stable
sort algorithm, and under certain circumstances this even causes a test
failure in t4201.21 "shortlog can match multiple groups", where two
authors both are listed with 2 contributions, and are listed in inverse
alphabetical order.

Let's instead use the stable sort provided by `git_stable_qsort()` to
avoid this inconsistency.

This is a companion to 2049b8dc65 (diffcore_rename(): use a stable sort,
2019-09-30).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    shortlog: use a stable sort
    
    This is another fall-out from validating Git for Windows v2.37.1 via the
    reinstated Azure Pipeline.
    
    The most valid question a reviewer might have about this patch is: "But
    why does our current CI not fail in the vs-test job?". The answer is
    surprisingly trivial: Because our CMake-based definition universally
    defines INTERNAL_QSORT
    [https://github.com/git/git/blob/v2.37.1/contrib/buildsystems/CMakeLists.txt#L227],
    which should actually not even be necessary, while the Azure Pipeline I
    used still calls make vcxproj to generate the Visual Studio build
    definition and does not define that flag.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1290%2Fdscho%2Fshortlog-requires-stable-sort-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1290/dscho/shortlog-requires-stable-sort-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1290

 builtin/shortlog.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/builtin/shortlog.c b/builtin/shortlog.c
index 35825f075e3..086dfee45aa 100644
--- a/builtin/shortlog.c
+++ b/builtin/shortlog.c
@@ -443,7 +443,7 @@ void shortlog_output(struct shortlog *log)
 	struct strbuf sb = STRBUF_INIT;
 
 	if (log->sort_by_number)
-		QSORT(log->list.items, log->list.nr,
+		STABLE_QSORT(log->list.items, log->list.nr,
 		      log->summary ? compare_by_counter : compare_by_list);
 	for (i = 0; i < log->list.nr; i++) {
 		const struct string_list_item *item = &log->list.items[i];

base-commit: bbea4dcf42b28eb7ce64a6306cdde875ae5d09ca
-- 
gitgitgadget

Re: [PATCH] shortlog: use a stable sort

[Junio C Hamano]

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> When sorting the output of `git shortlog` by count, a list of authors in
> alphabetical order is then sorted by contribution count. Obviously, the
> idea is to maintain the alphabetical order for items with identical
> contribution count.
> ...
>  	if (log->sort_by_number)
> -		QSORT(log->list.items, log->list.nr,
> +		STABLE_QSORT(log->list.items, log->list.nr,

Makes perfect sense.  Will queue.

Thanks.

Re: [PATCH] shortlog: use a stable sort

[Junio C Hamano]

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
writes:

>     why does our current CI not fail in the vs-test job?". The answer is
>     surprisingly trivial: Because our CMake-based definition universally
>     defines INTERNAL_QSORT
>     [https://github.com/git/git/blob/v2.37.1/contrib/buildsystems/CMakeLists.txt#L227],
>     which should actually not even be necessary, while the Azure Pipeline I
>     used still calls make vcxproj to generate the Visual Studio build
>     definition and does not define that flag.

Diversity in test environments is very good.