BLI_string: add BLI_string_elem_split_by_delim

Add a utility function to check if one string contains another
when split by a delimiter.

source/blender/blenlib/BLI_string.h

@@ -562,6 +562,27 @@ int BLI_string_find_split_words(const char *str,
                                 int r_words[][2],
                                 int words_max) ATTR_WARN_UNUSED_RESULT ATTR_NONNULL(1, 4);
 
+/**
+ * A version of `STR_ELEM(..)` that treats `haystack` as multiple elements split by `delim`.
+ * Return true when `needle` is found in `haystack`.
+ *
+ * \param haystack: The string to search in.
+ * \param delim: The delimiter which divides haystack.
+ * \param needle: The string to search for
+ * (must not contain `delim` or the result will never be true).
+ *
+ * The following guarantees are made:
+ * - Only an exact match returns true, requiring the strings length and case be match.
+ * - Successive delimiters are supported.
+ * - An empty needle will match against:
+ *   - A blank string.
+ *   - Delimiters at the beginning or end of the string
+ *   - Two successive delimiters.
+ */
+bool BLI_string_elem_split_by_delim(const char *haystack,
+                                    const char delim,
+                                    const char *needle) ATTR_WARN_UNUSED_RESULT ATTR_NONNULL(1, 3);
+
 /* -------------------------------------------------------------------- */
 /** \name String Copy/Format Macros
  * Avoid repeating destination with `sizeof(..)`.

source/blender/blenlib/intern/string.c

@@ -1117,6 +1117,24 @@ int BLI_string_find_split_words(
   return n;
 }
 
+bool BLI_string_elem_split_by_delim(const char *haystack, const char delim, const char *needle)
+{
+  /* May be zero, returns true when an empty span exists. */
+  const size_t needle_len = strlen(needle);
+  const char *p = haystack;
+  while (true) {
+    const char *p_next = BLI_strchr_or_end(p, delim);
+    if (((size_t)(p_next - p) == needle_len) && (memcmp(p, needle, needle_len) == 0)) {
+      return true;
+    }
+    if (*p_next == '\0') {
+      break;
+    }
+    p = p_next + 1;
+  }
+  return false;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/blenlib/tests/BLI_string_test.cc

@@ -1085,6 +1085,56 @@ TEST_F(StringFindSplitWords, LimitChars)
 
 /** \} */
 
+/* -------------------------------------------------------------------- */
+/** \name String Element
+ * \{ */
+
+/* #BLI_string_elem_split_by_delim */
+
+TEST(string, StringElemEmpty)
+{
+  EXPECT_FALSE(BLI_string_elem_split_by_delim("A", ':', ""));
+
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("", ':', ""));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim(":", ':', ""));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("::", ':', ""));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("A:", ':', ""));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim(":A", ':', ""));
+}
+
+TEST(string, StringElemSingle)
+{
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("A", ':', "A"));
+  EXPECT_FALSE(BLI_string_elem_split_by_delim("A", ':', "B"));
+
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("B", 'A', "B"));
+  EXPECT_FALSE(BLI_string_elem_split_by_delim("A", 'A', "A"));
+}
+
+TEST(string, StringElemComplex)
+{
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("TEST", ':', "TEST"));
+
+  EXPECT_TRUE(BLI_string_elem_split_by_delim(":TEST", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("TEST:", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim(":TEST:", ':', "TEST"));
+
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("::TEST", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("TEST::", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("::TEST::", ':', "TEST"));
+
+  EXPECT_FALSE(BLI_string_elem_split_by_delim(":TEST ", ':', "TEST"));
+  EXPECT_FALSE(BLI_string_elem_split_by_delim(" TEST:", ':', "TEST"));
+  EXPECT_FALSE(BLI_string_elem_split_by_delim(": TEST :", ':', "TEST"));
+
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("A:B:TEST", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("TEST:A:B", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim("A:TEST:B", ':', "TEST"));
+  EXPECT_TRUE(BLI_string_elem_split_by_delim(":A:TEST:B:", ':', "TEST"));
+}
+
+/** \} */
+
 /* -------------------------------------------------------------------- */
 /** \name String Search (Case Insensitive)
  * \{ */