Implement a cache for top tens

src/main/java/world/bentobox/level/LevelsManager.java

@@ -2,6 +2,7 @@ package world.bentobox.level;
 
 import java.math.BigInteger;
 import java.text.DecimalFormat;
+import java.time.Instant;
 import java.util.AbstractMap;
 import java.util.Collections;
 import java.util.HashMap;
@@ -31,6 +32,7 @@ import world.bentobox.level.events.IslandPreLevelEvent;
 import world.bentobox.level.objects.IslandLevels;
 import world.bentobox.level.objects.LevelsData;
 import world.bentobox.level.objects.TopTenData;
+import world.bentobox.level.util.CachedData;
 
 public class LevelsManager {
     private static final String INTOPTEN = "intopten";
@@ -52,6 +54,8 @@ public class LevelsManager {
     private final Map<String, IslandLevels> levelsCache;
     // Top ten lists
     private final Map<World, TopTenData> topTenLists;
+    // Cache for top tens
+    private Map<World, CachedData> cache = new HashMap<>();
 
     public LevelsManager(Level addon) {
         this.addon = addon;
@@ -339,7 +343,19 @@ public class LevelsManager {
     @NonNull
     public Map<String, Long> getTopTen(@NonNull World world, int size) {
         createAndCleanRankings(world);
-	// Return the sorted map
+        CachedData cachedData = cache.get(world);
+        Instant now = Instant.now();
+
+        if (cachedData != null && cachedData.getLastUpdated().plusSeconds(1).isAfter(now)) {
+            return cachedData.getCachedMap();
+        } else {
+            Map<String, Long> newTopTen = calculateTopTen(world, size);
+            cache.put(world, new CachedData(newTopTen, now));
+            return newTopTen;
+        }
+    }
+
+    private Map<String, Long> calculateTopTen(@NonNull World world, int size) {
         return Collections.unmodifiableMap(topTenLists.get(world).getTopTen().entrySet().stream()
                 .filter(l -> l.getValue() > 0).sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))
                 .limit(size)
@@ -416,6 +432,8 @@ public class LevelsManager {
     public void removeEntry(World world, String uuid) {
         if (topTenLists.containsKey(world)) {
             topTenLists.get(world).getTopTen().remove(uuid);
+            // Invalidate the cache because of this deletion
+            cache.remove(world);
         }
 
     }

src/main/java/world/bentobox/level/util/CachedData.java

@@ -0,0 +1,30 @@
+package world.bentobox.level.util;
+
+import java.time.Instant;
+import java.util.Map;
+
+/**
+ * Cache for top tens
+ */
+public class CachedData {
+    private Map<String, Long> cachedMap;
+    private Instant lastUpdated;
+
+    public CachedData(Map<String, Long> cachedMap, Instant lastUpdated) {
+        this.cachedMap = cachedMap;
+        this.lastUpdated = lastUpdated;
+    }
+
+    public Map<String, Long> getCachedMap() {
+        return cachedMap;
+    }
+
+    public Instant getLastUpdated() {
+        return lastUpdated;
+    }
+
+    public void updateCache(Map<String, Long> newMap, Instant newUpdateTime) {
+        this.cachedMap = newMap;
+        this.lastUpdated = newUpdateTime;
+    }
+}