Truncate original messages more gently when replying

changelog.d/1428.feature

@@ -0,0 +1 @@
+Truncate original messages more gently when replying

spec/unit/Template.spec.js

@@ -2,14 +2,12 @@
 const { renderTemplate } = require("../../lib/util/Template.js");
 
 describe("renderTemplate", function() {
-    it("should replace placeholders with submitted values", (done) => {
+    it("should replace placeholders with submitted values", () => {
         const template = '$FOO bar $BAZ';
         const result = renderTemplate(template, {
             foo: 'one',
             baz: 'two',
         });
         expect(result).toEqual('one bar two');
-
-        done();
     });
 });

spec/unit/TrimString.spec.js

@@ -0,0 +1,28 @@
+"use strict";
+const { trimString } = require("../../lib/util/TrimString.js");
+
+describe("trimString", function() {
+    it("should not cut unicode characters in half", () => {
+        const input = "lol 😅";
+        const result = trimString(input, 5);
+        expect(result).toEqual(input);
+    });
+
+    it("should trim trailing whitespace by itself", () => {
+        const input = "lol 😅";
+        const result = trimString(input, 4);
+        expect(result).toEqual('lol');
+    });
+
+    it('should stop trimming at the word boundary if reasonable', () => {
+        const input = "this sentence is waaaaay too long";
+        const result = trimString(input, 20);
+        expect(result).toEqual('this sentence is');
+    });
+
+    it('should give up looking for a word boundary if result would become too short', () => {
+        const input = "we're in Llanfairpwllgwyngyll";
+        const result = trimString(input, 24);
+        expect(result).toContain("we're in Llan");
+    });
+});

src/bridge/IrcBridge.ts

@@ -198,7 +198,9 @@ export class IrcBridge {
         });
         this.matrixHandler = new MatrixHandler(this, this.config.ircService.matrixHandler || {}, this.membershipQueue);
         this.privacyProtection = new PrivacyProtection(this);
-        this.ircHandler = new IrcHandler(this, this.config.ircService.ircHandler, this.membershipQueue, this.privacyProtection);
+        this.ircHandler = new IrcHandler(
+            this, this.config.ircService.ircHandler, this.membershipQueue, this.privacyProtection
+        );
 
         // By default the bridge will escape mxids, but the irc bridge isn't ready for this yet.
         MatrixUser.ESCAPE_DEFAULT = false;

src/bridge/MatrixHandler.ts

@@ -11,6 +11,7 @@ import { toIrcLowerCase } from "../irc/formatting";
 import { AdminRoomHandler } from "./AdminRoomHandler";
 import { trackChannelAndCreateRoom } from "./RoomCreation";
 import { renderTemplate } from "../util/Template";
+import { trimString } from "../util/TrimString";
 
 async function reqHandler(req: BridgeRequest, promise: PromiseLike<unknown>) {
     try {
@@ -1247,7 +1248,6 @@ export class MatrixHandler {
                 else {
                     rplSource = eventContent.content.body;
                 }
-                rplSource = rplSource.substring(0, REPLY_SOURCE_MAX_LENGTH);
                 cachedEvent = {sender: rplName, body: rplSource, timestamp: eventContent.origin_server_ts};
                 this.cacheEvent(eventId, cachedEvent);
             }
@@ -1268,10 +1268,10 @@ export class MatrixHandler {
         // Get the first non-blank line from the source.
         const lines = rplSource.split('\n').filter((line) => !/^\s*$/.test(line))
         if (lines.length > 0) {
-            // Cut to a maximum length.
-            rplSource = lines[0].substring(0, REPLY_SOURCE_MAX_LENGTH);
+            rplSource = trimString(lines[0], REPLY_SOURCE_MAX_LENGTH);
+
             // Ellipsis if needed.
-            if (lines[0].length > REPLY_SOURCE_MAX_LENGTH) {
+            if (lines.length > 1 || rplSource.length < lines[0].length) {
                 rplSource = rplSource + "...";
             }
         }

src/util/TrimString.ts

@@ -0,0 +1,46 @@
+/*
+Copyright 2021 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.
+*/
+
+// since trimRight()/trimEnd() may break unicode characters
+function trimTrailingWhitespace(input: string): string {
+    return input.replace(/\s*$/u, '');
+}
+
+// a unicode-aware substring(0, $x) that tries to not break words if possible
+export function trimString(input: string, maxLength: number): string {
+    const re = new RegExp(`^([\\s\\S]{0,${maxLength}})(\\p{L}?)`, 'u');
+    const match = input.match(re);
+
+    if (!match) {
+        // fallback to a dumb substring() if the regex failed for any reason
+        return trimTrailingWhitespace(input.substring(0, maxLength));
+    }
+
+    const trimmed = trimTrailingWhitespace(match[1]);
+
+    if (match[2]) {
+        // find as much as you can that is followed by a word boundary,
+        // shorter than what we have now, but at least 75% of the desired length
+        const smallerMatch = trimmed.match(/^([\s\S]*\S)\b[\s\S]/u);
+        const minLength = maxLength * 0.75;
+
+        if (smallerMatch && smallerMatch[1].length >= minLength) {
+            return smallerMatch[1];
+        }
+    }
+
+    return trimmed;
+}