Avoid checking for loaded chunks for lighting and entity ticking.

When a chunk is loaded the server tries to ensure it has its initial light
calculations done before sending it to the player. When ticking entities
the server tries to ensure the entity does not walk into an unloaded chunk.
To accomplish these the server checks a one chunk radius around the chunk
to be lit or a two chunk radius around the chunk the entity is in. These
lookups happen every tick even though their result is unlikely to change
that often. To reduce the cost of these checks we replace them with a
system to keep track of what neighbor chunks a chunk has loaded and update
it when chunks load or unload which is a much less frequent action. On a
server with ten players this change removes about 100,000 calls a tick to
LongObjectHashMap's containsKey method.

src/main/java/net/minecraft/server/Chunk.java

@@ -41,6 +41,34 @@ public class Chunk {
     public long s;
     private int x;
 
+    // CraftBukkit start - Neighbor loaded cache for chunk lighting and entity ticking
+    private int neighbors = 0x1 << 12;
+
+    public boolean areNeighborsLoaded(final int radius) {
+        switch(radius) {
+            case 2:
+                return this.neighbors == Integer.MAX_VALUE >> 6;
+            case 1:
+                final int mask =
+                    //        x        z   offset            x        z   offset            x         z   offset
+                    ( 0x1 << (1 * 5 +  1 + 12) ) | ( 0x1 << (0 * 5 +  1 + 12) ) | ( 0x1 << (-1 * 5 +  1 + 12) ) |
+                    ( 0x1 << (1 * 5 +  0 + 12) ) | ( 0x1 << (0 * 5 +  0 + 12) ) | ( 0x1 << (-1 * 5 +  0 + 12) ) |
+                    ( 0x1 << (1 * 5 + -1 + 12) ) | ( 0x1 << (0 * 5 + -1 + 12) ) | ( 0x1 << (-1 * 5 + -1 + 12) );
+                return (this.neighbors & mask) == mask;
+            default:
+                throw new UnsupportedOperationException(String.valueOf(radius));
+        }
+    }
+
+    public void setNeighborLoaded(final int x, final int z) {
+        this.neighbors |= 0x1 << (x * 5 + 12 + z);
+    }
+
+    public void setNeighborUnloaded(final int x, final int z) {
+        this.neighbors &= ~(0x1 << (x * 5 + 12 + z));
+    }
+    // CraftBukkit end
+
     public Chunk(World world, int i, int j) {
         this.sections = new ChunkSection[16];
         this.v = new byte[256];

src/main/java/net/minecraft/server/ChunkProviderServer.java

@@ -94,6 +94,10 @@ public class ChunkProviderServer implements IChunkProvider {
     }
 
     // CraftBukkit start - Add async variant, provide compatibility
+    public Chunk getChunkIfLoaded(int x, int z) {
+        return this.chunks.get(LongHash.toLong(x, z));
+    }
+
     public Chunk getChunkAt(int i, int j) {
         return getChunkAt(i, j, null);
     }
@@ -166,6 +170,21 @@ public class ChunkProviderServer implements IChunkProvider {
                  */
                 server.getPluginManager().callEvent(new org.bukkit.event.world.ChunkLoadEvent(chunk.bukkitChunk, newChunk));
             }
+
+            // Update neighbor counts
+            for (int x = -2; x < 3; x++) {
+                for (int z = -2; z < 3; z++) {
+                    if (x == 0 && z == 0) {
+                        continue;
+                    }
+
+                    Chunk neighbor = this.getChunkIfLoaded(chunk.locX + x, chunk.locZ + z);
+                    if (neighbor != null) {
+                        neighbor.setNeighborLoaded(-x, -z);
+                        chunk.setNeighborLoaded(x, z);
+                    }
+                }
+            }
             // CraftBukkit end
             chunk.a(this, this, i, j);
         }
@@ -327,6 +346,21 @@ public class ChunkProviderServer implements IChunkProvider {
 
                     // this.unloadQueue.remove(olong);
                     // this.chunks.remove(olong.longValue());
+
+                    // Update neighbor counts
+                    for (int x = -2; x < 3; x++) {
+                        for (int z = -2; z < 3; z++) {
+                            if (x == 0 && z == 0) {
+                                continue;
+                            }
+
+                            Chunk neighbor = this.getChunkIfLoaded(chunk.locX + x, chunk.locZ + z);
+                            if (neighbor != null) {
+                                neighbor.setNeighborUnloaded(-x, -z);
+                                chunk.setNeighborUnloaded(x, z);
+                            }
+                        }
+                    }
                 }
             }
             // CraftBukkit end
@@ -357,8 +391,8 @@ public class ChunkProviderServer implements IChunkProvider {
     }
 
     public int getLoadedChunks() {
-        // CraftBukkit - this.chunks.count() -> .values().size()
-        return this.chunks.values().size();
+        // CraftBukkit - this.chunks.count() -> this.chunks.size()
+        return this.chunks.size();
     }
 
     public void recreateStructures(int i, int j) {}

src/main/java/net/minecraft/server/World.java

@@ -115,6 +115,10 @@ public abstract class World implements IBlockAccess {
         return (CraftServer) Bukkit.getServer();
     }
 
+    public Chunk getChunkIfLoaded(int x, int z) {
+        return ((ChunkProviderServer) this.chunkProvider).getChunkIfLoaded(x, z);
+    }
+
     // Changed signature - added gen and env
     public World(IDataManager idatamanager, String s, WorldSettings worldsettings, WorldProvider worldprovider, MethodProfiler methodprofiler, ChunkGenerator gen, org.bukkit.World.Environment env) {
         this.generator = gen;
@@ -1396,7 +1400,10 @@ public abstract class World implements IBlockAccess {
         int j = MathHelper.floor(entity.locZ);
         byte b0 = 32;
 
-        if (!flag || this.b(i - b0, 0, j - b0, i + b0, 0, j + b0)) {
+        // CraftBukkit start - Use neighbor cache instead of looking up
+        Chunk startingChunk = this.getChunkIfLoaded(i >> 4, j >> 4);
+        if (!flag || (startingChunk != null && startingChunk.areNeighborsLoaded(2)) /* this.b(i - b0, 0, j - b0, i + b0, 0, j + b0) */) {
+            // CraftBukkit end
             entity.S = entity.locX;
             entity.T = entity.locY;
             entity.U = entity.locZ;
@@ -2165,7 +2172,10 @@ public abstract class World implements IBlockAccess {
     }
 
     public boolean c(EnumSkyBlock enumskyblock, int i, int j, int k) {
-        if (!this.areChunksLoaded(i, j, k, 17)) {
+        // CraftBukkit start - Use neighbor cache instead of looking up
+        Chunk chunk = this.getChunkIfLoaded(i >> 4, k >> 4);
+        if (chunk == null || !chunk.areNeighborsLoaded(1) /* !this.areChunksLoaded(i, j, k, 17)*/) {
+            // CraftBukkit end
             return false;
         } else {
             int l = 0;

src/main/java/org/bukkit/craftbukkit/chunkio/ChunkIOProvider.java

@@ -48,6 +48,21 @@ class ChunkIOProvider implements AsynchronousExecutor.CallBackProvider<QueuedChu
             server.getPluginManager().callEvent(new org.bukkit.event.world.ChunkLoadEvent(chunk.bukkitChunk, false));
         }
 
+        // Update neighbor counts
+        for (int x = -2; x < 3; x++) {
+            for (int z = -2; z < 3; z++) {
+                if (x == 0 && z == 0) {
+                    continue;
+                }
+
+                Chunk neighbor = queuedChunk.provider.getChunkIfLoaded(chunk.locX + x, chunk.locZ + z);
+                if (neighbor != null) {
+                    neighbor.setNeighborLoaded(-x, -z);
+                    chunk.setNeighborLoaded(x, z);
+                }
+            }
+        }
+
         chunk.a(queuedChunk.provider, queuedChunk.provider, queuedChunk.x, queuedChunk.z);
     }