Add tests for IsTrailUtf8CodeUnit

test/utf8_test.cpp

@@ -34,5 +34,33 @@ TEST(AppendUtf8Test, FourByteCodePoint)
 	EXPECT_EQ(s, "あ");
 }
 
+TEST(Utf8CodeUnits, ValidCodePoints)
+{
+	// Working backwards on this loop to avoid triggering signed integer overflow on platforms where char has an
+	// underlying type of signed char
+	for (char x = '\x7F'; x >= '\x00' && x <= '\x7F'; x--) {
+		EXPECT_FALSE(IsTrailUtf8CodeUnit(x)) << "Basic Latin and ASCII Control characters are not trail code units";
+	}
+
+	for (char x = '\x80'; x >= '\x80' && x <= '\xBF'; x++) {
+		EXPECT_TRUE(IsTrailUtf8CodeUnit(x)) << "Bytes in the range 0x80 to 0xBF are potentially valid trail code units";
+	}
+
+	for (char x = '\xC2'; x >= '\xC2' && x <= '\xF4'; x++) {
+		EXPECT_FALSE(IsTrailUtf8CodeUnit(x)) << "Bytes in the range 0xC2 to 0xF4 are never valid trail code units";
+	}
+}
+
+TEST(Utf8CodeUnits, InvalidCodePoints)
+{
+	for (char x = '\xC0'; x >= '\xC0' && x <= '\xC1'; x++) {
+		EXPECT_FALSE(IsTrailUtf8CodeUnit(x)) << "Bytes in the range 0xC0 to oxC1 are not trail code units";
+	}
+
+	for (char x = '\xF5'; x >= '\xF5' && x <= '\xFF'; x++) {
+		EXPECT_FALSE(IsTrailUtf8CodeUnit(x)) << "Bytes in the range 0xF5 to 0xFF are not trail code units";
+	}
+}
+
 } // namespace
 } // namespace devilution