ref-filter: modify "%(objectname:short)" to take length

Add support for %(objectname:short=<length>) which would print the
abbreviated unique objectname of given length. When no length is
specified, the length is 'DEFAULT_ABBREV'. The minimum length is
'MINIMUM_ABBREV'. The length may be exceeded to ensure that the
provided object name is unique.

Add tests and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Helped-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -110,6 +110,9 @@ objectsize::
 objectname::
 	The object name (aka SHA-1).
 	For a non-ambiguous abbreviation of the object name append `:short`.
+	For an abbreviation of the object name with desired length append
+	`:short=<length>`, where the minimum length is MINIMUM_ABBREV. The
+	length may be exceeded to ensure unique object names.
 
 upstream::
 	The name of a local ref which can be considered ``upstream''

ref-filter.c

@@ -57,7 +57,10 @@ static struct used_atom {
 			cmp_status cmp_status;
 			const char *str;
 		} if_then_else;
-		enum { O_FULL, O_SHORT } objectname;
+		struct {
+			enum { O_FULL, O_LENGTH, O_SHORT } option;
+			unsigned int length;
+		} objectname;
 	} u;
 } *used_atom;
 static int used_atom_cnt, need_tagged, need_symref;
@@ -129,10 +132,17 @@ static void contents_atom_parser(struct used_atom *atom, const char *arg)
 static void objectname_atom_parser(struct used_atom *atom, const char *arg)
 {
 	if (!arg)
-		atom->u.objectname = O_FULL;
+		atom->u.objectname.option = O_FULL;
 	else if (!strcmp(arg, "short"))
-		atom->u.objectname = O_SHORT;
-	else
+		atom->u.objectname.option = O_SHORT;
+	else if (skip_prefix(arg, "short=", &arg)) {
+		atom->u.objectname.option = O_LENGTH;
+		if (strtoul_ui(arg, 10, &atom->u.objectname.length) ||
+		    atom->u.objectname.length == 0)
+			die(_("positive value expected objectname:short=%s"), arg);
+		if (atom->u.objectname.length < MINIMUM_ABBREV)
+			atom->u.objectname.length = MINIMUM_ABBREV;
+	} else
 		die(_("unrecognized %%(objectname) argument: %s"), arg);
 }
 
@@ -606,12 +616,15 @@ static int grab_objectname(const char *name, const unsigned char *sha1,
 			   struct atom_value *v, struct used_atom *atom)
 {
 	if (starts_with(name, "objectname")) {
-		if (atom->u.objectname == O_SHORT) {
+		if (atom->u.objectname.option == O_SHORT) {
 			v->s = xstrdup(find_unique_abbrev(sha1, DEFAULT_ABBREV));
 			return 1;
-		} else if (atom->u.objectname == O_FULL) {
+		} else if (atom->u.objectname.option == O_FULL) {
 			v->s = xstrdup(sha1_to_hex(sha1));
 			return 1;
+		} else if (atom->u.objectname.option == O_LENGTH) {
+			v->s = xstrdup(find_unique_abbrev(sha1, atom->u.objectname.length));
+			return 1;
 		} else
 			die("BUG: unknown %%(objectname) option");
 	}

t/t6300-for-each-ref.sh

@@ -60,6 +60,8 @@ test_atom head objecttype commit
 test_atom head objectsize 171
 test_atom head objectname $(git rev-parse refs/heads/master)
 test_atom head objectname:short $(git rev-parse --short refs/heads/master)
+test_atom head objectname:short=1 $(git rev-parse --short=1 refs/heads/master)
+test_atom head objectname:short=10 $(git rev-parse --short=10 refs/heads/master)
 test_atom head tree $(git rev-parse refs/heads/master^{tree})
 test_atom head parent ''
 test_atom head numparent 0
@@ -99,6 +101,8 @@ test_atom tag objecttype tag
 test_atom tag objectsize 154
 test_atom tag objectname $(git rev-parse refs/tags/testtag)
 test_atom tag objectname:short $(git rev-parse --short refs/tags/testtag)
+test_atom head objectname:short=1 $(git rev-parse --short=1 refs/heads/master)
+test_atom head objectname:short=10 $(git rev-parse --short=10 refs/heads/master)
 test_atom tag tree ''
 test_atom tag parent ''
 test_atom tag numparent ''
@@ -164,6 +168,12 @@ test_expect_success 'Check invalid format specifiers are errors' '
 	test_must_fail git for-each-ref --format="%(authordate:INVALID)" refs/heads
 '
 
+test_expect_success 'arguments to %(objectname:short=) must be positive integers' '
+	test_must_fail git for-each-ref --format="%(objectname:short=0)" &&
+	test_must_fail git for-each-ref --format="%(objectname:short=-1)" &&
+	test_must_fail git for-each-ref --format="%(objectname:short=foo)"
+'
+
 test_date () {
 	f=$1 &&
 	committer_date=$2 &&