Add function string_subset

README.md

@@ -5071,6 +5071,14 @@ Keep the right part of the string, after the index 'index'.
 
 Extract the medium string from offset 'index' and up to 'size' bytes.
 
+##### void string\_subset (string\_t v, const string\_t org, size\_t index, size\_t size)
+
+Extract the medium string from offset 'index' and up to 'size' bytes of 'org'
+and set it in 'v'.
+
+The subset string shall fully exist in the string org.
+The string 'v' and 'org' shall be different.
+
 ##### size\_t string\_replace\_str (string\_t v, const char str1[], const char str2[] [, size_t start])
 ##### size\_t string\_replace (string\_t v, const string\_t str1, const string\_t str2 [ , size_t start])
 

m-string.h

@@ -709,6 +709,18 @@ string_mid (string_t v, size_t index, size_t size)
   string_left(v, size);
 }
 
+/* Set the string 'v' to the substring of org from position index to size bytes */
+static inline void
+string_subset (string_t v, const string_t org, size_t index, size_t size)
+{
+  // The substring shall fully exist within the string org
+  assert(index + size <= string_size(org) );
+  assert(v != org);
+  
+  const char *str = string_get_cstr(org);
+  string_set_strn(v, &str[index], size);
+}
+
 /* Return in the string the C string str1 into the C string str2 from start
    By default, start is zero.
 */

tests/test-mstring.c

@@ -201,6 +201,9 @@ static void test0(void)
   string_mid (s1, j, 2);
   assert (string_cmp_str (s1, ", ") == 0);
 
+  string_subset (s1, s2, j, 2);
+  assert (string_cmp_str (s1, ", ") == 0);
+
   string_set (s1, s2);
   i = string_replace_str (s1, "world", "Paul");
   assert (i != STRING_FAILURE);