Feature: World border shrinking after x time passed

src/main/java/me/metallicgoat/tweaksaddon/MBedwarsTweaksAddon.java

@@ -87,6 +87,7 @@ public void registerEvents() {
     manager.registerEvents(new RemoveInvisOnDamage(), plugin);
     manager.registerEvents(new SpecialItemCooldown(), plugin);
     manager.registerEvents(new TrackerDistance(), plugin);
+    manager.registerEvents(new WorldBorderResize(), plugin);
     //manager.registerEvents(new TieBreaker(), plugin);
 
     // Spawners

src/main/java/me/metallicgoat/tweaksaddon/config/MainConfig.java

@@ -455,6 +455,18 @@ public class MainConfig {
   @Config public static int friendly_villagers_range = 20;
   @Config public static boolean friendly_villagers_check_visibility = true;
 
+  @Config(
+      description = {
+          "Resize the world border (scale = % of its original size) after a certain amount of time (in seconds) after the game starts",
+          "After start_time has passed, the border will resize for the given duration time (in seconds)",
+          "This only works if the arena has the type WORLD and if the match world has a vanilla world border (command: /minecraft:worldborder)"
+      }
+  )
+  public static boolean world_border_resize_enabled = false;
+  @Config public static int world_border_resize_start_time = 600;
+  @Config public static int world_border_resize_duration = 60;
+  @Config public static int world_border_resize_scale = 50;
+
   // ===== PLACEHOLDER API
   @SectionTitle(title = "PLACEHOLDER API")
 

src/main/java/me/metallicgoat/tweaksaddon/tweaks/misc/WorldBorderResize.java

@@ -0,0 +1,106 @@
+package me.metallicgoat.tweaksaddon.tweaks.misc;
+
+import de.marcely.bedwars.api.arena.Arena;
+import de.marcely.bedwars.api.arena.ArenaPersistentStorage;
+import de.marcely.bedwars.api.arena.ArenaStatus;
+import de.marcely.bedwars.api.arena.RegenerationType;
+import de.marcely.bedwars.api.event.arena.ArenaStatusChangeEvent;
+import de.marcely.bedwars.api.event.arena.ArenaUnloadEvent;
+import de.marcely.bedwars.api.event.arena.RoundStartEvent;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+import me.metallicgoat.tweaksaddon.MBedwarsTweaksPlugin;
+import me.metallicgoat.tweaksaddon.config.MainConfig;
+import org.bukkit.Bukkit;
+import org.bukkit.WorldBorder;
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.Listener;
+import org.bukkit.scheduler.BukkitTask;
+
+public class WorldBorderResize implements Listener {
+
+  private static final String KEY_ORIGINAL_SIZE = "tweaks:world_border_original_size";
+
+  private Map<Arena, BukkitTask> activeTasks = new HashMap<>();
+
+  @EventHandler
+  public void onRoundStart(RoundStartEvent event) {
+    start(event.getArena());
+  }
+
+  @EventHandler(priority = EventPriority.MONITOR)
+  public void onRoundStop(ArenaStatusChangeEvent event) {
+    if (event.getNewStatus() != ArenaStatus.RUNNING)
+      cancel(event.getArena());
+  }
+
+  @EventHandler
+  public void onArenaUnload(ArenaUnloadEvent event) {
+    cancel(event.getArena());
+  }
+
+  private void start(Arena arena) {
+    if (!MainConfig.world_border_resize_enabled || arena.getRegenerationType() != RegenerationType.WORLD)
+      return;
+    if (arena.getGameWorld().getWorldBorder().getSize() >= 100_000) // probably vanilla border (disabled)
+      return;
+
+    cancel(arena);
+
+    final BukkitTask scheduler = Bukkit.getScheduler().runTaskLater(
+        MBedwarsTweaksPlugin.getInstance(),
+        () -> resize(arena),
+        20 * MainConfig.world_border_resize_start_time);
+
+    this.activeTasks.put(arena, scheduler);
+  }
+
+  private void resize(Arena arena) {
+    final WorldBorder border = arena.getGameWorld().getWorldBorder();
+
+    // store original size
+    {
+      final ArenaPersistentStorage storage = arena.getPersistentStorage();
+
+      storage.setSynchronizedFlag(KEY_ORIGINAL_SIZE, false);
+      storage.set(KEY_ORIGINAL_SIZE, border.getSize());
+      storage.saveAsync();
+    }
+
+    // tell bukkit to resize
+    border.setSize(
+        border.getSize()*(MainConfig.world_border_resize_scale/100D),
+        MainConfig.world_border_resize_duration);
+
+    // clean up
+    this.activeTasks.remove(arena);
+  }
+
+  private void cancel(Arena arena) {
+    if (!MainConfig.world_border_resize_enabled)
+      return;
+
+    // cancel scheduler
+    {
+      final BukkitTask scheduler = this.activeTasks.remove(arena);
+
+      if (scheduler != null)
+        scheduler.cancel();
+    }
+
+    // restore border size
+    {
+      final ArenaPersistentStorage storage = arena.getPersistentStorage();
+      final Optional<Double> originalSize = storage.getDouble(KEY_ORIGINAL_SIZE);
+
+      if (originalSize.isPresent()) {
+        final WorldBorder border = arena.getGameWorld().getWorldBorder();
+
+        border.setSize(originalSize.get(), 1);
+        storage.remove(KEY_ORIGINAL_SIZE);
+      }
+    }
+  }
+}