Implement crude dynamic linking of the formula evaluation function

src/formula/formula.cpp

@@ -16,13 +16,42 @@
 
 #include "formula/callable.hpp"
 #include "formula/function.hpp"
+#include "formula/string_utils.hpp"
 #include "random.hpp"
 #include "serialization/string_utils.hpp"
+#include "log.hpp"
 
 #include <cassert>
 #include <set>
 #include <sstream>
 
+// This is here only for the below initialization code.
+// If other logging is required in this file, it should use a different logdomain
+// (probably "scripting/wfl" or something)
+static lg::log_domain log_engine("engine");
+#define ERR_NG LOG_STREAM(err, log_engine)
+
+namespace utils {
+	namespace detail {
+		std::string evaluate_formula_impl(const std::string& formula) {
+			try {
+				const wfl::formula form(formula);
+				return form.evaluate().string_cast();
+			} catch(wfl::formula_error& e) {
+				ERR_NG << "Formula in WML string cannot be evaluated due to "
+					<< e.type << "\n\t--> \"";
+				return "";
+			}
+		}
+
+		struct formula_initer {
+			formula_initer() {
+				evaluate_formula = &evaluate_formula_impl;
+			}
+		} init;
+	}
+}
+
 namespace wfl
 {
 using expr_table           = std::map<std::string, expression_ptr>;

src/formula/string_utils.cpp

@@ -19,7 +19,6 @@
 
 #include "config.hpp"
 #include "log.hpp"
-//#include "formula/formula.hpp"
 #include "gettext.hpp"
 
 static lg::log_domain log_engine("engine");
@@ -30,6 +29,10 @@ static bool two_dots(char a, char b) { return a == '.' && b == '.'; }
 
 namespace utils {
 
+	namespace detail {
+		std::string(* evaluate_formula)(const std::string& formula) = nullptr;
+	}
+
 template <typename T>
 class string_map_variable_set : public variable_set
 {
@@ -110,27 +113,19 @@ static std::string do_interpolation(const std::string &str, const variable_set&
 				// TODO: support escape sequences when/if they are allowed in FormulaAI strings
 				}
 			} while(++var_end != res.end() && paren_nesting_level > 0);
-#ifdef WESNOTH_GAME
+			if(utils::detail::evaluate_formula == nullptr) {
+				WRN_NG << "Formula substitution ignored (and removed) because WFL engine is not present in the server.\n";
+				res.replace(var_begin, var_end, "");
+				continue;
+			}
 			if(paren_nesting_level > 0) {
 				ERR_NG << "Formula in WML string cannot be evaluated due to "
 					<< "a missing closing parenthesis:\n\t--> \""
 					<< std::string(var_begin, var_end) << "\"\n";
 				res.replace(var_begin, var_end, "");
 				continue;
 			}
-			try {
-				const wfl::formula form(std::string(var_begin+2, var_end-1));
-				res.replace(var_begin, var_end, form.evaluate().string_cast());
-			} catch(wfl::formula_error& e) {
-				ERR_NG << "Formula in WML string cannot be evaluated due to "
-					<< e.type << "\n\t--> \""
-					<< e.formula << "\"\n";
-				res.replace(var_begin, var_end, "");
-			}
-#else
-			WRN_NG << "Formula substitution ignored (and removed) because WFL engine is not present in the server.\n";
-			res.replace(var_begin, var_end, "");
-#endif
+			res.replace(var_begin, var_end, utils::detail::evaluate_formula(std::string(var_begin+2, var_end-1)));
 			continue;
 		}
 

src/formula/string_utils.hpp

@@ -21,6 +21,10 @@ class variable_set;
 
 namespace utils {
 
+	namespace detail {
+		extern std::string(* evaluate_formula)(const std::string& formula);
+	}
+
 /**
  * Determines if a string might contain variables to interpolate.
  * This can allow one to skip future interpolations (plural -- if there is only