make get_short_ref a public function

Often we want to shorten a full ref name to something "prettier"
to show a user. For example, "refs/heads/master" is often shown
simply as "master", or "refs/remotes/origin/master" is shown as
"origin/master".

Many places in the code use a very simple formula: skip common
prefixes like refs/heads, refs/remotes, etc. This is codified in
the prettify_ref function.

for-each-ref has a more correct (but more expensive) approach:
consider the ref lookup rules, and try shortening as much as
possible while remaining unambiguous.

This patch makes the latter strategy globally available as
shorten_unambiguous_ref.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-for-each-ref.c

@@ -545,109 +545,6 @@ static void grab_values(struct atom_value *val, int deref, struct object *obj, v
 	}
 }
 
-/*
- * generate a format suitable for scanf from a ref_rev_parse_rules
- * rule, that is replace the "%.*s" spec with a "%s" spec
- */
-static void gen_scanf_fmt(char *scanf_fmt, const char *rule)
-{
-	char *spec;
-
-	spec = strstr(rule, "%.*s");
-	if (!spec || strstr(spec + 4, "%.*s"))
-		die("invalid rule in ref_rev_parse_rules: %s", rule);
-
-	/* copy all until spec */
-	strncpy(scanf_fmt, rule, spec - rule);
-	scanf_fmt[spec - rule] = '\0';
-	/* copy new spec */
-	strcat(scanf_fmt, "%s");
-	/* copy remaining rule */
-	strcat(scanf_fmt, spec + 4);
-
-	return;
-}
-
-/*
- * Shorten the refname to an non-ambiguous form
- */
-static char *get_short_ref(const char *ref)
-{
-	int i;
-	static char **scanf_fmts;
-	static int nr_rules;
-	char *short_name;
-
-	/* pre generate scanf formats from ref_rev_parse_rules[] */
-	if (!nr_rules) {
-		size_t total_len = 0;
-
-		/* the rule list is NULL terminated, count them first */
-		for (; ref_rev_parse_rules[nr_rules]; nr_rules++)
-			/* no +1 because strlen("%s") < strlen("%.*s") */
-			total_len += strlen(ref_rev_parse_rules[nr_rules]);
-
-		scanf_fmts = xmalloc(nr_rules * sizeof(char *) + total_len);
-
-		total_len = 0;
-		for (i = 0; i < nr_rules; i++) {
-			scanf_fmts[i] = (char *)&scanf_fmts[nr_rules]
-					+ total_len;
-			gen_scanf_fmt(scanf_fmts[i], ref_rev_parse_rules[i]);
-			total_len += strlen(ref_rev_parse_rules[i]);
-		}
-	}
-
-	/* bail out if there are no rules */
-	if (!nr_rules)
-		return xstrdup(ref);
-
-	/* buffer for scanf result, at most ref must fit */
-	short_name = xstrdup(ref);
-
-	/* skip first rule, it will always match */
-	for (i = nr_rules - 1; i > 0 ; --i) {
-		int j;
-		int short_name_len;
-
-		if (1 != sscanf(ref, scanf_fmts[i], short_name))
-			continue;
-
-		short_name_len = strlen(short_name);
-
-		/*
-		 * check if the short name resolves to a valid ref,
-		 * but use only rules prior to the matched one
-		 */
-		for (j = 0; j < i; j++) {
-			const char *rule = ref_rev_parse_rules[j];
-			unsigned char short_objectname[20];
-			char refname[PATH_MAX];
-
-			/*
-			 * the short name is ambiguous, if it resolves
-			 * (with this previous rule) to a valid ref
-			 * read_ref() returns 0 on success
-			 */
-			mksnpath(refname, sizeof(refname),
-				 rule, short_name_len, short_name);
-			if (!read_ref(refname, short_objectname))
-				break;
-		}
-
-		/*
-		 * short name is non-ambiguous if all previous rules
-		 * haven't resolved to a valid ref
-		 */
-		if (j == i)
-			return short_name;
-	}
-
-	free(short_name);
-	return xstrdup(ref);
-}
-
-
 /*
  * Parse the object referred by ref, and grab needed value.
  */
@@ -704,7 +601,7 @@ static void populate_value(struct refinfo *ref)
 		if (formatp) {
 			formatp++;
 			if (!strcmp(formatp, "short"))
-				refname = get_short_ref(refname);
+				refname = shorten_unambiguous_ref(refname);
 			else
 				die("unknown %.*s format %s",
 				    (int)(formatp - name), name, formatp);

refs.c

@@ -1652,3 +1652,102 @@ struct ref *find_ref_by_name(const struct ref *list, const char *name)
 			return (struct ref *)list;
 	return NULL;
 }
+
+/*
+ * generate a format suitable for scanf from a ref_rev_parse_rules
+ * rule, that is replace the "%.*s" spec with a "%s" spec
+ */
+static void gen_scanf_fmt(char *scanf_fmt, const char *rule)
+{
+	char *spec;
+
+	spec = strstr(rule, "%.*s");
+	if (!spec || strstr(spec + 4, "%.*s"))
+		die("invalid rule in ref_rev_parse_rules: %s", rule);
+
+	/* copy all until spec */
+	strncpy(scanf_fmt, rule, spec - rule);
+	scanf_fmt[spec - rule] = '\0';
+	/* copy new spec */
+	strcat(scanf_fmt, "%s");
+	/* copy remaining rule */
+	strcat(scanf_fmt, spec + 4);
+
+	return;
+}
+
+char *shorten_unambiguous_ref(const char *ref)
+{
+	int i;
+	static char **scanf_fmts;
+	static int nr_rules;
+	char *short_name;
+
+	/* pre generate scanf formats from ref_rev_parse_rules[] */
+	if (!nr_rules) {
+		size_t total_len = 0;
+
+		/* the rule list is NULL terminated, count them first */
+		for (; ref_rev_parse_rules[nr_rules]; nr_rules++)
+			/* no +1 because strlen("%s") < strlen("%.*s") */
+			total_len += strlen(ref_rev_parse_rules[nr_rules]);
+
+		scanf_fmts = xmalloc(nr_rules * sizeof(char *) + total_len);
+
+		total_len = 0;
+		for (i = 0; i < nr_rules; i++) {
+			scanf_fmts[i] = (char *)&scanf_fmts[nr_rules]
+					+ total_len;
+			gen_scanf_fmt(scanf_fmts[i], ref_rev_parse_rules[i]);
+			total_len += strlen(ref_rev_parse_rules[i]);
+		}
+	}
+
+	/* bail out if there are no rules */
+	if (!nr_rules)
+		return xstrdup(ref);
+
+	/* buffer for scanf result, at most ref must fit */
+	short_name = xstrdup(ref);
+
+	/* skip first rule, it will always match */
+	for (i = nr_rules - 1; i > 0 ; --i) {
+		int j;
+		int short_name_len;
+
+		if (1 != sscanf(ref, scanf_fmts[i], short_name))
+			continue;
+
+		short_name_len = strlen(short_name);
+
+		/*
+		 * check if the short name resolves to a valid ref,
+		 * but use only rules prior to the matched one
+		 */
+		for (j = 0; j < i; j++) {
+			const char *rule = ref_rev_parse_rules[j];
+			unsigned char short_objectname[20];
+			char refname[PATH_MAX];
+
+			/*
+			 * the short name is ambiguous, if it resolves
+			 * (with this previous rule) to a valid ref
+			 * read_ref() returns 0 on success
+			 */
+			mksnpath(refname, sizeof(refname),
+				 rule, short_name_len, short_name);
+			if (!read_ref(refname, short_objectname))
+				break;
+		}
+
+		/*
+		 * short name is non-ambiguous if all previous rules
+		 * haven't resolved to a valid ref
+		 */
+		if (j == i)
+			return short_name;
+	}
+
+	free(short_name);
+	return xstrdup(ref);
+}

refs.h

@@ -80,6 +80,7 @@ extern int for_each_reflog(each_ref_fn, void *);
 extern int check_ref_format(const char *target);
 
 extern const char *prettify_ref(const struct ref *ref);
+extern char *shorten_unambiguous_ref(const char *ref);
 
 /** rename ref, return 0 on success **/
 extern int rename_ref(const char *oldref, const char *newref, const char *logmsg);