Add support for permissions in the Things API.

This commit introduces the PermissionThing and associated parser. The parser determines the value (grant/revoke) of the permission by looking at the first character of the input string - if it is a minus (-) or caret (^), the value is false (revoke), otherwise it is true (grant). To distinguish permissions from other things, the parser requires a prefix of "perm:".

src/main/java/com/garbagemule/MobArena/things/PermissionThing.java

@@ -0,0 +1,63 @@
+package com.garbagemule.MobArena.things;
+
+import com.garbagemule.MobArena.MobArena;
+import org.bukkit.entity.Player;
+import org.bukkit.permissions.PermissionAttachment;
+import org.bukkit.permissions.PermissionAttachmentInfo;
+
+public class PermissionThing implements Thing {
+    private final String perm;
+    private final boolean value;
+    private final MobArena plugin;
+
+    public PermissionThing(String perm, boolean value, MobArena plugin) {
+        this.perm = perm;
+        this.value = value;
+        this.plugin = plugin;
+    }
+
+    @Override
+    public boolean giveTo(Player player) {
+        removeExistingAttachment(player);
+        PermissionAttachment attachment = player.addAttachment(plugin);
+        attachment.setPermission(perm, value);
+        return true;
+    }
+
+    @Override
+    public boolean takeFrom(Player player) {
+        removeExistingAttachment(player);
+        return true;
+    }
+
+    private void removeExistingAttachment(Player player) {
+        player.getEffectivePermissions().stream()
+            .filter(info -> info.getAttachment() != null)
+            .filter(info -> info.getAttachment().getPlugin().equals(plugin))
+            .filter(info -> info.getPermission().equals(perm))
+            .map(PermissionAttachmentInfo::getAttachment)
+            .findAny()
+            .ifPresent(PermissionAttachment::remove);
+    }
+
+    String getPermission() {
+        return perm;
+    }
+
+    boolean getValue() {
+        return value;
+    }
+
+    @Override
+    public boolean heldBy(Player player) {
+        return player.hasPermission(perm);
+    }
+
+    @Override
+    public String toString() {
+        if (value) {
+            return perm;
+        }
+        return "-" + perm;
+    }
+}

src/main/java/com/garbagemule/MobArena/things/PermissionThingParser.java

@@ -0,0 +1,33 @@
+package com.garbagemule.MobArena.things;
+
+import com.garbagemule.MobArena.MobArena;
+
+class PermissionThingParser implements ThingParser {
+    private static final String PREFIX = "perm:";
+
+    private MobArena plugin;
+
+    PermissionThingParser(MobArena plugin) {
+        this.plugin = plugin;
+    }
+
+    @Override
+    public PermissionThing parse(String s) {
+        String value = trimPrefix(s);
+        if (value == null) {
+            return null;
+        }
+
+        if (value.startsWith("-") || value.startsWith("^")) {
+            return new PermissionThing(value.substring(1), false, plugin);
+        }
+        return new PermissionThing(value, true, plugin);
+    }
+
+    private String trimPrefix(String s) {
+        if (s.startsWith(PREFIX)) {
+            return s.substring(PREFIX.length());
+        }
+        return null;
+    }
+}

src/main/java/com/garbagemule/MobArena/things/ThingManager.java

@@ -13,6 +13,7 @@ public class ThingManager implements ThingParser {
         parsers = new ArrayList<>();
         parsers.add(new CommandThingParser());
         parsers.add(new MoneyThingParser(plugin));
+        parsers.add(new PermissionThingParser(plugin));
         parsers.add(new PotionEffectThingParser());
         items = parser;
     }