From bfe4057567647862f7f8c377acc4bff3788e42d5 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Fri, 28 Sep 2018 20:46:29 -0400
Subject: [PATCH] Optimize Light Recalculations

The server triggers light recalculations even if the new block
is the same as the old block. At this time, BlockData Properties
do not impact light calculations.

So the only way light should change, is if the block itself
changes from 1 block to another.

Additionally, as of 1.13, water now only blocks 1 light level
instead of 3, meaning that light level decreasing is the same as air.

This means, transitions between water and air also do not need
to recalculate light, which saves a TON of updates caused by
fluids flowing.

diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index 53aab97866..dad294ecbe 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -103,6 +103,9 @@ public class Chunk implements IChunkAccess {
     // Keep this synced with entitySlices.add() and entitySlices.remove()
     private final int[] itemCounts = new int[16];
     private final int[] inventoryEntityCounts = new int[16];
+    public static boolean shouldRecheckLight(Block block, Block block1) {
+        return !(block == block1 || (block1 == Blocks.WATER && block == Blocks.AIR) || (block == Blocks.WATER && block1 == Blocks.AIR));
+    }
     // Paper end
     public boolean areNeighborsLoaded(final int radius) {
         switch (radius) {
@@ -578,7 +581,7 @@ public class Chunk implements IChunkAccess {
             } else {
                 if (flag1) {
                     this.initLighting();
-                } else {
+                } else if (shouldRecheckLight(block, block1)) { // Paper - Optimize light recalculations
                     this.runOrQueueLightUpdate(() -> { // Paper - Queue light update
                     int i1 = iblockdata.b(this.world, blockposition);
                     int j1 = iblockdata1.b(this.world, blockposition);
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index 13f69f1b82..cd25bf3d9e 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -444,7 +444,7 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
             } else {
                 IBlockData iblockdata2 = this.getType(blockposition);
 
-                if (iblockdata2.b(this, blockposition) != iblockdata1.b(this, blockposition) || iblockdata2.e() != iblockdata1.e()) {
+                if (Chunk.shouldRecheckLight(iblockdata.getBlock(), iblockdata2.getBlock()) && iblockdata2.b(this, blockposition) != iblockdata1.b(this, blockposition) || iblockdata2.e() != iblockdata1.e()) { // Paper - optimize light recalculations
                     this.methodProfiler.a("checkLight");
                     chunk.runOrQueueLightUpdate(() -> this.r(blockposition)); // Paper - Queue light update
                     this.methodProfiler.e();
-- 
2.19.0