From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Wyatt Childers <wchilders@nearce.com>
Date: Sat, 4 Jul 2020 23:07:43 -0400
Subject: [PATCH] Optimize the advancement data player iteration to be O(N)
 rather than O(N^2)


diff --git a/src/main/java/net/minecraft/server/PlayerAdvancements.java b/src/main/java/net/minecraft/server/PlayerAdvancements.java
index ecd62d2625bc5def3ac68f642f2c6f156060f129..f8f0be77126d2f3a1c53f7405ce4de4f0dca42be 100644
--- a/src/main/java/net/minecraft/server/PlayerAdvancements.java
+++ b/src/main/java/net/minecraft/server/PlayerAdvancements.java
@@ -437,6 +437,16 @@ public class PlayerAdvancements {
     }
 
     private void ensureVisibility(Advancement advancement) {
+        // Paper start
+        ensureVisibility(advancement, IterationEntryPoint.ROOT);
+    }
+    private enum IterationEntryPoint {
+        ROOT,
+        ITERATOR,
+        PARENT_OF_ITERATOR
+    }
+    private void ensureVisibility(Advancement advancement, IterationEntryPoint entryPoint) {
+        // Paper end
         boolean flag = this.shouldBeVisible(advancement);
         boolean flag1 = this.visible.contains(advancement);
 
@@ -452,15 +462,23 @@ public class PlayerAdvancements {
         }
 
         if (flag != flag1 && advancement.getParent() != null) {
-            this.ensureVisibility(advancement.getParent());
+            // Paper start - If we're not coming from an iterator consider this to be a root entry, otherwise
+            // market that we're entering from the parent of an iterator.
+            this.ensureVisibility(advancement.getParent(), entryPoint == IterationEntryPoint.ITERATOR ? IterationEntryPoint.PARENT_OF_ITERATOR : IterationEntryPoint.ROOT);
         }
 
+        // If this is true, we've went through a child iteration, entered the parent, processed the parent
+        // and are about to reprocess the children. Stop processing here to prevent O(N^2) processing.
+        if (entryPoint == IterationEntryPoint.PARENT_OF_ITERATOR) {
+            return;
+        } // Paper end
+
         Iterator iterator = advancement.getChildren().iterator();
 
         while (iterator.hasNext()) {
             Advancement advancement1 = (Advancement) iterator.next();
 
-            this.ensureVisibility(advancement1);
+            this.ensureVisibility(advancement1, IterationEntryPoint.ITERATOR); // Paper - Mark this call as being from iteration
         }
 
     }