strbuf: introduce strbuf_addstrings() to repeatedly add a string

In a following commit we are going to port code from
"t/helper/test-sha256.c", t/helper/test-hash.c and "t/t0015-hash.sh" to
a new "t/unit-tests/t-hash.c" file using the recently added unit test
framework.

To port code like: perl -e "$| = 1; print q{aaaaaaaaaa} for 1..100000;"
we are going to need a new strbuf_addstrings() function that repeatedly
adds the same string a number of times to a buffer.

Such a strbuf_addstrings() function would already be useful in
"json-writer.c" and "builtin/submodule-helper.c" as both of these files
already have code that repeatedly adds the same string. So let's
introduce such a strbuf_addstrings() function in "strbuf.{c,h}" and use
it in both "json-writer.c" and "builtin/submodule-helper.c".

We use the "strbuf_addstrings" name as this way strbuf_addstr() and
strbuf_addstrings() would be similar for strings as strbuf_addch() and
strbuf_addchars() for characters.

Helped-by: Junio C Hamano <gitster@pobox.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-authored-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Achu Luma <ach.lumap@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -257,11 +257,9 @@ static void module_list_active(struct module_list *list)
 
 static char *get_up_path(const char *path)
 {
-	int i;
 	struct strbuf sb = STRBUF_INIT;
 
-	for (i = count_slashes(path); i; i--)
-		strbuf_addstr(&sb, "../");
+	strbuf_addstrings(&sb, "../", count_slashes(path));
 
 	/*
 	 * Check if 'path' ends with slash or not

json-writer.c

@@ -46,10 +46,7 @@ static void append_quoted_string(struct strbuf *out, const char *in)
 
 static void indent_pretty(struct json_writer *jw)
 {
-	int k;
-
-	for (k = 0; k < jw->open_stack.len; k++)
-		strbuf_addstr(&jw->json, "  ");
+	strbuf_addstrings(&jw->json, "  ", jw->open_stack.len);
 }
 
 /*

strbuf.c

@@ -313,6 +313,15 @@ void strbuf_add(struct strbuf *sb, const void *data, size_t len)
 	strbuf_setlen(sb, sb->len + len);
 }
 
+void strbuf_addstrings(struct strbuf *sb, const char *s, size_t n)
+{
+	size_t len = strlen(s);
+
+	strbuf_grow(sb, st_mult(len, n));
+	for (size_t i = 0; i < n; i++)
+		strbuf_add(sb, s, len);
+}
+
 void strbuf_addbuf(struct strbuf *sb, const struct strbuf *sb2)
 {
 	strbuf_grow(sb, sb2->len);

strbuf.h

@@ -310,6 +310,11 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s)
 	strbuf_add(sb, s, strlen(s));
 }
 
+/**
+ * Add a NUL-terminated string the specified number of times to the buffer.
+ */
+void strbuf_addstrings(struct strbuf *sb, const char *s, size_t n);
+
 /**
  * Copy the contents of another buffer at the end of the current one.
  */