Print the Lua version at startup

Closes #692.

ChangeLog

@@ -4,6 +4,7 @@ Solarus 1.5.0 (in progress)
 _______________________________
 
 * Print when the main loop starts and stops.
+* Print the Lua version at startup (#692).
 
 _______________________________
 

include/solarus/lua/LuaContext.h

@@ -981,6 +981,7 @@ class LuaContext {
     static void do_file(lua_State* l, const std::string& script_name);
     static bool do_file_if_exists(lua_State* l, const std::string& script_name);
     void print_stack(lua_State* l);
+    void print_lua_version();
 
     // Initialization of modules.
     void register_functions(

src/lua/LuaContext.cpp

@@ -104,6 +104,8 @@ void LuaContext::initialize() {
   lua_atpanic(l, l_panic);
   luaL_openlibs(l);
 
+  print_lua_version();
+
   // Associate this LuaContext object to the lua_State pointer.
   lua_contexts[l] = this;
 
@@ -772,6 +774,45 @@ void LuaContext::print_stack(lua_State* l) {
   std::cout << std::endl;
 }
 
+/**
+ * \brief Prints the version of Lua.
+ *
+ * This detects if LuaJIT is being used.
+ */
+void LuaContext::print_lua_version() {
+
+  Debug::check_assertion(lua_gettop(l) == 0, "Non-empty Lua stack before print_lua_version()");
+
+  // _VERSION is the Lua language version, giving the same
+  // result for vanilla Lua and LuaJIT.
+  // But we want to tell the user if LuaJIT is being used.
+  // To detect this, we can check the presence of the jit table.
+  std::string version;
+                                  // -
+  lua_getglobal(l, "jit");
+                                  // jit/nil
+  if (lua_isnil(l, -1)) {
+    // Vanilla Lua.
+                                  // nil
+    lua_getglobal(l, "_VERSION");
+                                  // nil version
+    version = LuaTools::check_string(l, -1);
+    lua_pop(l, 2);
+                                  // -
+    std::cout << "LuaJIT: no (" << version << ")" << std::endl;
+  }
+  else {
+    // LuaJIT.
+                                  // jit
+    version = LuaTools::check_string_field(l, -1, "version");
+    lua_pop(l, 1);
+                                  // -
+    std::cout << "LuaJIT: yes (" << version << ")" << std::endl;
+  }
+
+  Debug::check_assertion(lua_gettop(l) == 0, "Non-empty Lua stack after print_lua_version()");
+}
+
 /**
  * \brief Defines some C++ functions into a Lua table.
  * \param module_name name of the table that will contain the functions