feat(core): kmx_xstring: New UTF-16 to UTF-32 function 🙀

- plus test

For #7418

core/src/kmx/kmx_xstring.h

@@ -98,6 +98,39 @@ Utf32CharToUtf16(const KMX_DWORD ch32, char16_single &ch16) {
   return len;
 }
 
+/**
+ * Convert a u16 string to a u32 string. U+FFFD instead of mismatched surrogates or sliced surrogate
+ * @param source UTF-16 string
+ * @return a UTF-32 string
+ */
+inline std::u32string
+u16string_to_u32string(const std::u16string &source) {
+  std::u32string out;
+
+  for (auto ptr = source.begin(); ptr < source.end(); ptr++) {
+    const char16_t lead = *ptr;
+    if (Uni_IsSurrogate1(lead)) {
+      ptr++;
+      if (ptr == source.end()) {
+        // DebugLog("End of string during surrogate pair");
+        out.push_back(0xFFFD);  // error
+        return out;
+      }
+      const char16_t trail = *ptr;
+      if (!Uni_IsSurrogate2(trail)) {
+        out.push_back(0xFFFD);  // error, mismatched lead surrogate
+        ptr--;                  // reprocess remaining char
+      } else {
+        out.push_back(Uni_SurrogateToUTF32(lead, trail));
+      }
+    } else if (Uni_IsSurrogate2(lead)) {
+      out.push_back(0xFFFD);  // error - mismatched trail surrogate
+    } else {
+      out.push_back(lead);
+    }
+  }
+  return out;
+}
 
 } // namespace kmx
 } // namespace kbp

core/tests/unit/kmnkbd/test_kmx_xstring.cpp

@@ -1268,6 +1268,56 @@ test_utf32() {
   assert(s1 == std::u16string(u"🙀"));
 }
 
+void
+test_u16string_to_u32string() {
+  // normal cases
+  {
+    const std::u32string str = u16string_to_u32string(u"");
+    assert_equal(str.length(), 0);
+  }
+  {
+    const std::u32string str = u16string_to_u32string(u"e");
+    assert_equal(str.length(), 1);
+    assert_equal(str.at(0), 0x0065);
+  }
+  {
+    const std::u32string str = u16string_to_u32string(u"🙀");
+    assert_equal(str.length(), 1);
+    assert_equal(str.at(0), 0x0001F640);
+  }
+  {
+    const std::u32string str = u16string_to_u32string(u"Ω🙀");
+    assert_equal(str.length(), 2);
+    assert_equal(str.at(0), u'Ω');
+    assert_equal(str.at(1), 0x0001F640);
+  }
+
+  // error cases
+  {
+    std::u16string half_cat;
+    half_cat.push_back(0xD83D);  // mismatched lead surrogate
+    const std::u32string str = u16string_to_u32string(half_cat);
+    assert_equal(str.length(), 1);
+    assert_equal(str.at(0), 0xFFFD);
+  }
+  {
+    std::u16string half_cat;
+    half_cat.push_back(0xD83D);  // mismatched lead surrogate
+    half_cat.push_back(u'Ω');    // with following text
+    const std::u32string str = u16string_to_u32string(half_cat);
+    assert_equal(str.length(), 2);
+    assert_equal(str.at(0), 0xFFFD);
+    assert_equal(str.at(1), u'Ω');
+  }
+  {
+    std::u16string half_cat;
+    half_cat.push_back(0xDE40);  // mismatched trail surrogate
+    const std::u32string str = u16string_to_u32string(half_cat);
+    assert_equal(str.length(), 1);
+    assert_equal(str.at(0), 0xFFFD);
+  }
+}
+
 constexpr const auto help_str = u"\
 test_kmx_xstring [--color]\n\
 \n\
@@ -1289,6 +1339,7 @@ int main(int argc, char *argv []) {
   test_xstrlen();
   test_xstrlen_ignoreifopt();
   test_utf32();
+  test_u16string_to_u32string();
 
   return 0;
 }