Make UTF-8 checking into a unit test with Catch

text.cpp

@@ -0,0 +1,52 @@
+#include "text.hpp"
+#include <stdio.h>
+
+/**
+ * Returns an empty string if `s` is valid utf8;
+ * otherwise returns an error message.
+ */
+std::string check_utf8(std::string s) {
+	for (size_t i = 0; i < s.size(); i++) {
+		int fail = 0;
+
+		if ((s[i] & 0x80) == 0x80) {
+			if ((s[i] & 0xE0) == 0xC0) {
+				if (i + 1 >= s.size() || (s[i + 1] & 0xC0) != 0x80) {
+					fail = 2;
+				} else {
+					i += 1;
+				}
+			} else if ((s[i] & 0xF0) == 0xE0) {
+				if (i + 2 >= s.size() || (s[i + 1] & 0xC0) != 0x80 || (s[i + 2] & 0xC0) != 0x80) {
+					fail = 3;
+				} else {
+					i += 2;
+				}
+			} else if ((s[i] & 0xF8) == 0xF0) {
+				if (i + 3 >= s.size() || (s[i + 1] & 0xC0) != 0x80 || (s[i + 2] & 0xC0) != 0x80 || (s[i + 3] & 0xC0) != 0x80) {
+					fail = 4;
+				} else {
+					i += 3;
+				}
+			} else {
+				fail = 1;
+			}
+		}
+
+		if (fail != 0) {
+			std::string out = "\"" + s + "\" is not valid UTF-8 (";
+			for (size_t j = 0; j < fail && i + j < s.size(); j++) {
+				if (j != 0) {
+					out += " ";
+				}
+				char tmp[6];
+				sprintf(tmp, "0x%02X", s[i + j] & 0xFF);
+				out += std::string(tmp);
+			}
+			out += ")";
+			return out;
+		}
+	}
+
+	return "";
+}