diff --git a/src/editor/diff.js b/src/editor/diff.js
index 2c82e22793..3b7b2aa79d 100644
--- a/src/editor/diff.js
+++ b/src/editor/diff.js
@@ -25,16 +25,6 @@ function firstDiff(a, b) {
     return compareLen;
 }
 
-function lastDiff(a, b) {
-    const compareLen = Math.min(a.length, b.length);
-    for (let i = 0; i < compareLen; ++i) {
-        if (a[a.length - i] !== b[b.length - i]) {
-            return i;
-        }
-    }
-    return compareLen;
-}
-
 function diffStringsAtEnd(oldStr, newStr) {
     const len = Math.min(oldStr.length, newStr.length);
     const startInCommon = oldStr.substr(0, len) === newStr.substr(0, len);
@@ -52,22 +42,14 @@ function diffStringsAtEnd(oldStr, newStr) {
     }
 }
 
+// assumes only characters have been deleted at one location in the string, and none added
 export function diffDeletion(oldStr, newStr) {
     if (oldStr === newStr) {
         return {};
     }
     const firstDiffIdx = firstDiff(oldStr, newStr);
-    const lastDiffIdx = oldStr.length - lastDiff(oldStr, newStr) + 1;
-    return {at: firstDiffIdx, removed: oldStr.substring(firstDiffIdx, lastDiffIdx)};
-}
-
-export function diffInsertion(oldStr, newStr) {
-    const diff = diffDeletion(newStr, oldStr);
-    if (diff.removed) {
-        return {at: diff.at, added: diff.removed};
-    } else {
-        return diff;
-    }
+    const amount = oldStr.length - newStr.length;
+    return {at: firstDiffIdx, removed: oldStr.substr(firstDiffIdx, amount)};
 }
 
 export function diffAtCaret(oldValue, newValue, caretPosition) {
diff --git a/test/editor/diff-test.js b/test/editor/diff-test.js
new file mode 100644
index 0000000000..257c35c22f
--- /dev/null
+++ b/test/editor/diff-test.js
@@ -0,0 +1,48 @@
+/*
+Copyright 2019 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import expect from 'expect';
+import {diffDeletion} from "../../src/editor/diff";
+
+describe('editor/diff', function() {
+    describe('diffDeletion', function() {
+        it('at start of string', function() {
+            const diff = diffDeletion("hello", "ello");
+            expect(diff.at).toBe(0);
+            expect(diff.removed).toBe("h");
+        });
+        it('removing whole string', function() {
+            const diff = diffDeletion("hello", "");
+            expect(diff.at).toBe(0);
+            expect(diff.removed).toBe("hello");
+        });
+        it('in middle of string', function() {
+            const diff = diffDeletion("hello", "hllo");
+            expect(diff.at).toBe(1);
+            expect(diff.removed).toBe("e");
+        });
+        it('in middle of string with duplicate character', function() {
+            const diff = diffDeletion("hello", "helo");
+            expect(diff.at).toBe(3);
+            expect(diff.removed).toBe("l");
+        });
+        it('at end of string', function() {
+            const diff = diffDeletion("hello", "hell");
+            expect(diff.at).toBe(4);
+            expect(diff.removed).toBe("o");
+        });
+    });
+});