add a button check to disable a button when a unit is available

src/include/interface.h

@@ -405,6 +405,8 @@ extern bool ButtonCheckUnitVariable(const CUnit &unit, const ButtonAction &butto
 extern bool ButtonCheckUnitsOr(const CUnit &unit, const ButtonAction &button);
 /// Check if allowed units exists
 extern bool ButtonCheckUnitsAnd(const CUnit &unit, const ButtonAction &button);
+/// Check if units don't exist
+extern bool ButtonCheckUnitsNot(const CUnit &unit, const ButtonAction &button);
 /// Check if have network play
 extern bool ButtonCheckNetwork(const CUnit &unit, const ButtonAction &button);
 /// Check if don't have network play

src/ui/button_checks.cpp

@@ -202,6 +202,19 @@ bool ButtonCheckUnitsAnd(const CUnit &unit, const ButtonAction &button)
 	return true;
 }
 
+/**
+**  Check for button enabled, if no unit is available.
+**
+**  @param unit    Pointer to unit for button.
+**  @param button  Pointer to button to check/enable.
+**
+**  @return        True if enabled.
+*/
+bool ButtonCheckUnitsNot(const CUnit &unit, const ButtonAction &button)
+{
+    return !ButtonCheckUnitsAnd(unit, button);
+}
+
 /**
 **  Check if network play is enabled.
 **

src/ui/script_ui.cpp

@@ -968,6 +968,8 @@ static int CclDefineButton(lua_State *l)
 				ba.Allowed = ButtonCheckUnitsOr;
 			} else if (!strcmp(value, "check-units-and")) {
 				ba.Allowed = ButtonCheckUnitsAnd;
+			} else if (!strcmp(value, "check-units-not")) {
+				ba.Allowed = ButtonCheckUnitsNot;
 			} else if (!strcmp(value, "check-network")) {
 				ba.Allowed = ButtonCheckNetwork;
 			} else if (!strcmp(value, "check-no-network")) {