Formula engine: Substring function now accepts negative size

This results in counting backwards from the given offset.
Size of 1 and -1 have the same effect.

src/formula_function.cpp

@@ -395,31 +395,19 @@ class substring_function
 		if(offset < 0) {
 			offset += result.size();
 			if(offset < 0) {
-				WRN_SF << "[substring] Offset '"
-						<< args()[1]->evaluate(variables, fdb).as_int()
-						<< "' results in a negative start in string '"
-						<< result
-						<< "' and is reset at the beginning of the string.\n";
-
 				offset = 0;
 			}
 		} else {
 			if(static_cast<size_t>(offset) >= result.size()) {
-				WRN_SF << "[substring] Offset '" << offset
-						<< "' is larger than the size of '" << result
-						<< "' and results in an empty string.\n";
-
 				return variant(std::string());
 			}
 		}
 
 		if(args().size() > 2) {
-			const int size = args()[2]->evaluate(variables, fdb).as_int();
+			int size = args()[2]->evaluate(variables, fdb).as_int();
 			if(size < 0) {
-				ERR_SF << "[substring] Size is negative an "
-						<< "empty string is returned.\n";
-
-				return variant(std::string());
+				size = -size;
+				offset = std::max(0, offset - size + 1);
 			}
 			return variant(result.substr(offset, size));
 		} else {

src/tests/test_formula_function.cpp

@@ -73,12 +73,10 @@ BOOST_AUTO_TEST_CASE(test_formula_function_substring)
 				.evaluate().as_string()
 			, "");
 
-	lg::set_log_domain_severity("scripting/formula", lg::err.get_severity() - 1); // Don't log anything
-
 	BOOST_CHECK_EQUAL(
 			game_logic::formula("substring('hello world', 0, -1)")
 				.evaluate().as_string()
-			, "");
+			, "h");
 
 	lg::set_log_domain_severity("scripting/formula", lg::debug);
 
@@ -97,6 +95,16 @@ BOOST_AUTO_TEST_CASE(test_formula_function_substring)
 				.evaluate().as_string()
 			, "ello worl");
 
+	BOOST_CHECK_EQUAL(
+			game_logic::formula("substring('hello world', -1, -5)")
+				.evaluate().as_string()
+			, "world");
+
+	BOOST_CHECK_EQUAL(
+			game_logic::formula("substring('hello world', 4, -5)")
+				.evaluate().as_string()
+			, "hello");
+
 }
 
 BOOST_AUTO_TEST_CASE(test_formula_function_length)