From dfe26262ec9197a722aaf1359efd8034e91c030f Mon Sep 17 00:00:00 2001
From: James Allen <james@sharelatex.com>
Date: Fri, 16 Sep 2016 11:50:44 +0100
Subject: [PATCH] Return a No-op if diff returns nothing

---
 .../app/coffee/UpdateCompressor.coffee        | 22 ++++++++++++++-----
 .../UpdateCompressorTests.coffee              | 15 +++++--------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/services/track-changes/app/coffee/UpdateCompressor.coffee b/services/track-changes/app/coffee/UpdateCompressor.coffee
index f55d7462ea..3ae5adf8fb 100644
--- a/services/track-changes/app/coffee/UpdateCompressor.coffee
+++ b/services/track-changes/app/coffee/UpdateCompressor.coffee
@@ -162,16 +162,28 @@ module.exports = UpdateCompressor =
 		else if firstOp.d? and secondOp.i? and firstOp.p == secondOp.p
 			offset = firstOp.p
 			diff_ops = @diffAsShareJsOps(firstOp.d, secondOp.i)
-			return diff_ops.map (op) ->
-				op.p += offset
-				return {
+			if diff_ops.length == 0
+				return [{ # Noop
 					meta:
 						start_ts: firstUpdate.meta.start_ts
 						end_ts:   secondUpdate.meta.end_ts
 						user_id:  firstUpdate.meta.user_id
-					op: op
+					op:
+						p: firstOp.p
+						i: ""
 					v: secondUpdate.v
-				}
+				}]
+			else
+				return diff_ops.map (op) ->
+					op.p += offset
+					return {
+						meta:
+							start_ts: firstUpdate.meta.start_ts
+							end_ts:   secondUpdate.meta.end_ts
+							user_id:  firstUpdate.meta.user_id
+						op: op
+						v: secondUpdate.v
+					}
 
 		else
 			return [firstUpdate, secondUpdate]
diff --git a/services/track-changes/test/unit/coffee/UpdateCompressor/UpdateCompressorTests.coffee b/services/track-changes/test/unit/coffee/UpdateCompressor/UpdateCompressorTests.coffee
index d2e449b646..b4858b79b0 100644
--- a/services/track-changes/test/unit/coffee/UpdateCompressor/UpdateCompressorTests.coffee
+++ b/services/track-changes/test/unit/coffee/UpdateCompressor/UpdateCompressorTests.coffee
@@ -7,7 +7,8 @@ SandboxedModule = require('sandboxed-module')
 
 describe "UpdateCompressor", ->
 	beforeEach ->
-		@UpdateCompressor = SandboxedModule.require modulePath
+		@UpdateCompressor = SandboxedModule.require modulePath, requires:
+			"../lib/diff_match_patch": require("../../../../app/lib/diff_match_patch")
 		@user_id = "user-id-1"
 		@other_user_id = "user-id-2"
 		@bigstring = ("a" for [0 .. 2*1024*1024]).join("")
@@ -366,23 +367,19 @@ describe "UpdateCompressor", ->
 					meta: start_ts: @ts1, end_ts: @ts2, user_id: @user_id
 					v: 43
 				}]
-
-			it "should do a diff of the content", ->
+			
+			it "should return a no-op if the delete and insert are the same", ->
 				expect(@UpdateCompressor.compressUpdates [{
 					op: { p: 3, d: "one two three four five six seven eight" }
 					meta: ts: @ts1, user_id: @user_id
 					v: 42
 				}, {
-					op: { p: 3, i: "one 2 three four five six seven eight" }
+					op: { p: 3, i: "one two three four five six seven eight" }
 					meta: ts: @ts2, user_id: @user_id
 					v: 43
 				}])
 				.to.deep.equal [{
-					op: { p: 7, d: "two" }
-					meta: start_ts: @ts1, end_ts: @ts2, user_id: @user_id
-					v: 43
-				}, {
-					op: { p: 7, i: "2" }
+					op: { p: 3, i: "" }
 					meta: start_ts: @ts1, end_ts: @ts2, user_id: @user_id
 					v: 43
 				}]