Add condition argument to Rule annotation (#998)

src/main/java/carpet/settings/Condition.java

@@ -0,0 +1,5 @@
+package carpet.settings;
+
+public interface Condition {
+    boolean isTrue();
+}

src/main/java/carpet/settings/Rule.java

@@ -64,4 +64,9 @@ public @interface Rule
      * The class of the validator checked when the rule is changed.
      */
     Class<? extends Validator>[] validate() default {};
+
+    /**
+     * The class of the condition checked when the rule is parsed.
+     */
+    Class<? extends Condition>[] condition() default {};
 }

src/main/java/carpet/settings/SettingsManager.java

@@ -30,6 +30,7 @@ import java.io.BufferedReader;
 import java.io.BufferedWriter;
 import java.io.IOException;
 import java.io.PrintStream;
+import java.lang.reflect.Constructor;
 import java.lang.reflect.Field;
 import java.nio.file.Files;
 import java.nio.file.NoSuchFileException;
@@ -143,7 +144,23 @@ public class SettingsManager
             Rule rule = f.getAnnotation(Rule.class);
             if (rule == null) continue;
             ParsedRule parsed = new ParsedRule(f, rule, this);
-            rules.put(parsed.name, parsed);
+
+            List<Boolean> conditions = new ArrayList<>();
+            for (Class<? extends Condition> condition : rule.condition()) {
+                try
+                {
+                    Constructor<?> constr = condition.getDeclaredConstructor();
+                    constr.setAccessible(true);
+                    conditions.add(((Condition) constr.newInstance()).isTrue());
+                }
+                catch (ReflectiveOperationException e)
+                {
+                    throw new RuntimeException(e);
+                }
+            }
+
+            if (rule.condition().length == 0 || conditions.stream().allMatch(Boolean::valueOf))
+                rules.put(parsed.name, parsed);
         }
     }