unit test + fixes + comments + simplication for diffDeletion

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");
+        });
+    });
+});