From babace0c059965b01767797adace4e765a4650d7 Mon Sep 17 00:00:00 2001
From: Andrew Clark <acdlite@fb.com>
Date: Tue, 13 Dec 2016 10:55:55 -0800
Subject: [PATCH] Add fast path for appending updates to the end of the queue

This is the most common case, so we should avoid scanning the entire
list to get to the end.
---
 .../shared/fiber/ReactFiberUpdateQueue.js     | 57 +++++++++++--------
 1 file changed, 33 insertions(+), 24 deletions(-)

diff --git a/src/renderers/shared/fiber/ReactFiberUpdateQueue.js b/src/renderers/shared/fiber/ReactFiberUpdateQueue.js
index 9938db1eed..9c9aadccbd 100644
--- a/src/renderers/shared/fiber/ReactFiberUpdateQueue.js
+++ b/src/renderers/shared/fiber/ReactFiberUpdateQueue.js
@@ -176,26 +176,41 @@ function insertUpdate(fiber : Fiber, update : Update) : void {
 
   const priorityLevel = update.priorityLevel;
 
-  // TODO: Fast path where last item in the queue has greater or equal priority.
-
-  let insertAfter1 = null;
-  let insertBefore1 = queue1.first;
-  while (insertBefore1 && comparePriority(insertBefore1.priorityLevel, priorityLevel) <= 0) {
-    insertAfter1 = insertBefore1;
-    insertBefore1 = insertBefore1.next;
-  }
-  const update1 = update;
-
-  let insertAfter2 = null;
-  let insertBefore2 = null;
-  let update2 = null;
-  if (queue2) {
-    insertBefore2 = queue2.first;
-    while (insertBefore2 && comparePriority(insertBefore2.priorityLevel, priorityLevel) <= 0) {
-      insertAfter2 = insertBefore2;
-      insertBefore2 = insertBefore2.next;
+  let queue = queue1;
+  let insertAfter1;
+  let insertBefore1;
+  let insertAfter2;
+  let insertBefore2;
+  for (let i = 0; queue && i < 2; i++) {
+    let insertAfter = null;
+    let insertBefore = null;
+    if (queue.last && comparePriority(queue.last.priorityLevel, priorityLevel) <= 0) {
+      // Fast path for the common case where the update should be inserted at
+      // the end of the queue.
+      insertAfter = queue.last;
+    } else {
+      insertBefore = queue.first;
+      while (insertBefore && comparePriority(insertBefore.priorityLevel, priorityLevel) <= 0) {
+        insertAfter = insertBefore;
+        insertBefore = insertBefore.next;
+      }
     }
+    if (i === 0) {
+      insertAfter1 = insertAfter;
+      insertBefore1 = insertBefore;
+      queue = queue2;
+    } else {
+      insertAfter2 = insertAfter;
+      insertBefore2 = insertBefore;
+      queue = null;
+    }
+  }
 
+  const update1 = update;
+  insertUpdateIntoQueue(queue1, update1, insertAfter1, insertBefore1);
+
+  if (queue2) {
+    let update2;
     if (insertBefore1 === insertBefore2) {
       // The update is inserted into the same position of both lists. There's no
       // need to clone the update.
@@ -206,11 +221,6 @@ function insertUpdate(fiber : Fiber, update : Update) : void {
       // time we commit.
       update2 = cloneUpdate(update1);
     }
-  }
-
-  // Now we're ready to insert the updates into their queues.
-  insertUpdateIntoQueue(queue1, update1, insertAfter1, insertBefore1);
-  if (queue2 && update2) {
     insertUpdateIntoQueue(queue2, update2, insertAfter2, insertBefore2);
   }
 }
@@ -296,7 +306,6 @@ exports.addForceUpdate = addForceUpdate;
 
 
 function addCallback(fiber : Fiber, callback: Callback, priorityLevel : PriorityLevel) : void {
-  // TODO: Fast path where last item in queue has equal priority.
   const update : Update = {
     priorityLevel,
     partialState: null,