shortlog: support arbitrary commit format `--group`s

In addition to generating a shortlog based on committer, author, or the
identity in one or more specified trailers, it can be useful to generate
a shortlog based on an arbitrary commit format.

This can be used, for example, to generate a distribution of commit
activity over time, like so:

    $ git shortlog --group='%cd' --date='format:%Y-%m' -s v2.37.0..
       117  2022-06
       274  2022-07
       324  2022-08
       263  2022-09
         7  2022-10

Arbitrary commit formats can be used. In fact, `git shortlog`'s default
behavior (to count by commit authors) can be emulated as follows:

    $ git shortlog --group='%aN <%aE>' ...

and future patches will make the default behavior (as well as
`--committer`, and `--group=trailer:<trailer>`) special cases of the
more flexible `--group` option.

Note also that the SHORTLOG_GROUP_FORMAT enum value is used only to
designate that `--group:<format>` is in use when in stdin mode to
declare that the combination is invalid.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -50,7 +50,7 @@ OPTIONS
 --date=<format>::
 	Show dates formatted according to the given date string. (See
 	the `--date` option in the "Commit Formatting" section of
-	linkgit:git-log[1]).
+	linkgit:git-log[1]). Useful with `--group=format:<format>`.
 
 --group=<type>::
 	Group commits based on `<type>`. If no `--group` option is
@@ -64,6 +64,9 @@ OPTIONS
    example, if your project uses `Reviewed-by` trailers, you might want
    to see who has been reviewing with
    `git shortlog -ns --group=trailer:reviewed-by`.
+ - `format:<format>`, any string accepted by the `--format` option of
+   'git log'. (See the "PRETTY FORMATS" section of
+   linkgit:git-log[1].)
 +
 Note that commits that do not include the trailer will not be counted.
 Likewise, commits with multiple trailers (e.g., multiple signoffs) may

builtin/shortlog.c

@@ -133,6 +133,8 @@ static void read_from_stdin(struct shortlog *log)
 		break;
 	case SHORTLOG_GROUP_TRAILER:
 		die(_("using %s with stdin is not supported"), "--group=trailer");
+	case SHORTLOG_GROUP_FORMAT:
+		die(_("using %s with stdin is not supported"), "--group=format");
 	default:
 		BUG("unhandled shortlog group");
 	}
@@ -203,6 +205,32 @@ static void insert_records_from_trailers(struct shortlog *log,
 	unuse_commit_buffer(commit, commit_buffer);
 }
 
+static int shortlog_needs_dedup(const struct shortlog *log)
+{
+	return HAS_MULTI_BITS(log->groups) || log->format.nr > 1 || log->trailers.nr;
+}
+
+static void insert_records_from_format(struct shortlog *log,
+				       struct strset *dups,
+				       struct commit *commit,
+				       struct pretty_print_context *ctx,
+				       const char *oneline)
+{
+	struct strbuf buf = STRBUF_INIT;
+	struct string_list_item *item;
+
+	for_each_string_list_item(item, &log->format) {
+		strbuf_reset(&buf);
+
+		format_commit_message(commit, item->string, &buf, ctx);
+
+		if (!shortlog_needs_dedup(log) || strset_add(dups, buf.buf))
+			insert_one_record(log, buf.buf, oneline);
+	}
+
+	strbuf_release(&buf);
+}
+
 void shortlog_add_commit(struct shortlog *log, struct commit *commit)
 {
 	struct strbuf ident = STRBUF_INIT;
@@ -244,6 +272,7 @@ void shortlog_add_commit(struct shortlog *log, struct commit *commit)
 			insert_one_record(log, ident.buf, oneline_str);
 	}
 	insert_records_from_trailers(log, &dups, commit, &ctx, oneline_str);
+	insert_records_from_format(log, &dups, commit, &ctx, oneline_str);
 
 	strset_clear(&dups);
 	strbuf_release(&ident);
@@ -315,6 +344,7 @@ static int parse_group_option(const struct option *opt, const char *arg, int uns
 	if (unset) {
 		log->groups = 0;
 		string_list_clear(&log->trailers, 0);
+		string_list_clear(&log->format, 0);
 	} else if (!strcasecmp(arg, "author"))
 		log->groups |= SHORTLOG_GROUP_AUTHOR;
 	else if (!strcasecmp(arg, "committer"))
@@ -322,8 +352,15 @@ static int parse_group_option(const struct option *opt, const char *arg, int uns
 	else if (skip_prefix(arg, "trailer:", &field)) {
 		log->groups |= SHORTLOG_GROUP_TRAILER;
 		string_list_append(&log->trailers, field);
-	} else
+	} else if (skip_prefix(arg, "format:", &field)) {
+		log->groups |= SHORTLOG_GROUP_FORMAT;
+		string_list_append(&log->format, field);
+	} else if (strchr(arg, '%')) {
+		log->groups |= SHORTLOG_GROUP_FORMAT;
+		string_list_append(&log->format, arg);
+	} else {
 		return error(_("unknown group type: %s"), arg);
+	}
 
 	return 0;
 }
@@ -341,6 +378,7 @@ void shortlog_init(struct shortlog *log)
 	log->in2 = DEFAULT_INDENT2;
 	log->trailers.strdup_strings = 1;
 	log->trailers.cmp = strcasecmp;
+	log->format.strdup_strings = 1;
 }
 
 int cmd_shortlog(int argc, const char **argv, const char *prefix)
@@ -481,4 +519,5 @@ void shortlog_output(struct shortlog *log)
 	log->list.strdup_strings = 1;
 	string_list_clear(&log->list, 1);
 	clear_mailmap(&log->mailmap);
+	string_list_clear(&log->format, 0);
 }

shortlog.h

@@ -22,8 +22,10 @@ struct shortlog {
 		SHORTLOG_GROUP_AUTHOR = (1 << 0),
 		SHORTLOG_GROUP_COMMITTER = (1 << 1),
 		SHORTLOG_GROUP_TRAILER = (1 << 2),
+		SHORTLOG_GROUP_FORMAT = (1 << 3),
 	} groups;
 	struct string_list trailers;
+	struct string_list format;
 
 	int email;
 	struct string_list mailmap;

t/t4201-shortlog.sh

@@ -244,6 +244,26 @@ test_expect_success 'shortlog --group=trailer:signed-off-by' '
 	test_cmp expect actual
 '
 
+test_expect_success 'shortlog --group=format' '
+	git shortlog -s --date="format:%Y" --group="format:%cN (%cd)" \
+		HEAD >actual &&
+	cat >expect <<-\EOF &&
+	     4	C O Mitter (2005)
+	     1	Sin Nombre (2005)
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'shortlog --group=<format> DWIM' '
+	git shortlog -s --date="format:%Y" --group="%cN (%cd)" HEAD >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'shortlog bogus --group' '
+	test_must_fail git shortlog --group=bogus HEAD 2>err &&
+	grep "unknown group type" err
+'
+
 test_expect_success 'trailer idents are split' '
 	cat >expect <<-\EOF &&
 	     2	C O Mitter
@@ -326,6 +346,18 @@ test_expect_success 'shortlog can match multiple groups' '
 	test_cmp expect actual
 '
 
+test_expect_success 'shortlog can match multiple format groups' '
+	GIT_COMMITTER_NAME="$GIT_AUTHOR_NAME" \
+		git commit --allow-empty -m "identical names" &&
+	test_tick &&
+	cat >expect <<-\EOF &&
+	     2	A U Thor
+	     1	C O Mitter
+	EOF
+	git shortlog -ns --group="%cn" --group="%an" -2 HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'set up option selection tests' '
 	git commit --allow-empty -F - <<-\EOF
 	subject