string-util: add strrstr() helper

src/basic/string-util.c

@@ -1519,3 +1519,26 @@ ssize_t strlevenshtein(const char *x, const char *y) {
 
         return t1[yl];
 }
+
+char *strrstr(const char *haystack, const char *needle) {
+        const char *f = NULL;
+        size_t l;
+
+        /* Like strstr() but returns the last rather than the first occurence of "needle" in "haystack". */
+
+        if (!haystack || !needle)
+                return NULL;
+
+        l = strlen(needle);
+
+        /* Special case: for the empty string we return the very last possible occurence, i.e. *after* the
+         * last char, not before. */
+        if (l == 0)
+                return strchr(haystack, 0);
+
+        for (const char *p = haystack; *p; p++)
+                if (strncmp(p, needle, l) == 0)
+                        f = p;
+
+        return (char*) f;
+}

src/basic/string-util.h

@@ -322,3 +322,5 @@ static inline int strdup_or_null(const char *s, char **ret) {
         *ret = c;
         return 1;
 }
+
+char *strrstr(const char *haystack, const char *needle);

src/test/test-string-util.c

@@ -1324,4 +1324,27 @@ TEST(strlevenshtein) {
         assert_se(strlevenshtein("sunday", "saturday") == 3);
 }
 
+TEST(strrstr) {
+        assert_se(!strrstr(NULL, NULL));
+        assert_se(!strrstr("foo", NULL));
+        assert_se(!strrstr(NULL, "foo"));
+
+        const char *p = "foo";
+        assert_se(strrstr(p, "foo") == p);
+        assert_se(strrstr(p, "fo") == p);
+        assert_se(strrstr(p, "f") == p);
+        assert_se(strrstr(p, "oo") == p + 1);
+        assert_se(strrstr(p, "o") == p + 2);
+        assert_se(strrstr(p, "") == p + strlen(p));
+        assert_se(!strrstr(p, "bar"));
+
+        p = "xoxoxox";
+        assert_se(strrstr(p, "") == p + strlen(p));
+        assert_se(strrstr(p, "x") == p + 6);
+        assert_se(strrstr(p, "ox") == p + 5);
+        assert_se(strrstr(p, "xo") == p + 4);
+        assert_se(strrstr(p, "xox") == p + 4);
+        assert_se(!strrstr(p, "xx"));
+}
+
 DEFINE_TEST_MAIN(LOG_DEBUG);