add append and compiler flag

Author: vaeng

exercises/practice/reverse-string/.docs/instructions.append.md

@@ -0,0 +1,14 @@
+# Additional Instructions
+
+## Grapheme Clusters
+
+Unicode supports a lot of characters and languages.
+The displayed 'graphemes' are constructed from code points.
+Their combination and _order_ is the basis of the resulting grapheme.
+Imagine, a simple algorithm that might flip the 'dots' from one place to another during reversal: `Noël` becomes `leöN`.
+Now consider more [complex constructions][grapheme-construction] like the Bengali word `ঝিল্লি`, which consists of six code points.
+
+As graphemes make the string reversal process a lot more complicated, we made the respective test cases optional.
+You can try them out by setting the `TEST_GRAPHEMES` compiler flag or editing the test file accordingly.
+
+[grapheme-construction]: https://r12a.github.io/uniview/?charlist=%E0%A6%9D%E0%A6%BF%E0%A6%B2%E0%A7%8D%E0%A6%B2%E0%A6%BF

exercises/practice/reverse-string/reverse_string_test.cpp

@@ -36,6 +36,7 @@ TEST_CASE("an even-sized word", "[b9e7dec1-c6df-40bd-9fa3-cd7ded010c4c]") {
 	REQUIRE("reward" == reverse_string::reverse_string("drawer"));
 }
 
+#if defined(TEST_GRAPHEMES)
 TEST_CASE("wide characters", "[1bed0f8a-13b0-4bd3-9d59-3d0593326fa2]") {
 	REQUIRE("猫子" == reverse_string::reverse_string("子猫"));
 }
@@ -47,5 +48,6 @@ TEST_CASE("grapheme cluster with pre-combined form", "[93d7e1b8-f60f-4f3c-9559-4
 TEST_CASE("grapheme clusters", "[1028b2c1-6763-4459-8540-2da47ca512d9]") {
 	REQUIRE("มรกแรปโนยขีเผู้" == reverse_string::reverse_string("ผู้เขียนโปรแกรม"));
 }
+#endif
 
 #endif