markdown: Correctly retrieve indices for repeated matches.

The same pattern being matched multiple times in a topic cannot be
properly ordered using topic_name.find(match_text) and etc. when there
are multiple matches of the same pattern in the topic.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

frontend_tests/node_tests/markdown_parse.js

@@ -230,3 +230,44 @@ run_test("topic links", () => {
         },
     ]);
 });
+
+run_test("topic links repeated", () => {
+    // Links generated from repeated patterns should preserve the order.
+    const topic =
+        "#foo101 https://google.com #foo102 #foo103 https://google.com #foo101 #foo102 #foo103";
+    const topic_links = markdown.get_topic_links({topic, get_linkifier_map});
+    assert.deepEqual(topic_links, [
+        {
+            text: "#foo101",
+            url: "http://foo.com/101",
+        },
+        {
+            text: "https://google.com",
+            url: "https://google.com",
+        },
+        {
+            text: "#foo102",
+            url: "http://foo.com/102",
+        },
+        {
+            text: "#foo103",
+            url: "http://foo.com/103",
+        },
+        {
+            text: "https://google.com",
+            url: "https://google.com",
+        },
+        {
+            text: "#foo101",
+            url: "http://foo.com/101",
+        },
+        {
+            text: "#foo102",
+            url: "http://foo.com/102",
+        },
+        {
+            text: "#foo103",
+            url: "http://foo.com/103",
+        },
+    ]);
+});

static/js/markdown.js

@@ -296,17 +296,15 @@ export function get_topic_links({topic, get_linkifier_map}) {
             }
             // We store the starting index as well, to sort the order of occurrence of the links
             // in the topic, similar to the logic implemented in zerver/lib/markdown/__init__.py
-            links.push({url: link_url, text: match[0], index: topic.indexOf(match[0])});
+            links.push({url: link_url, text: match[0], index: match.index});
         }
     }
 
     // Also make raw URLs navigable
     const url_re = /\b(https?:\/\/[^\s<]+[^\s"'),.:;<\]])/g; // Slightly modified from third/marked.js
-    const matches = topic.match(url_re);
-    if (matches) {
-        for (const match of matches) {
-            links.push({url: match, text: match, index: topic.indexOf(match)});
-        }
+    let match;
+    while ((match = url_re.exec(topic)) !== null) {
+        links.push({url: match[0], text: match[0], index: match.index});
     }
     links.sort((a, b) => a.index - b.index);
     for (const match of links) {

zerver/lib/markdown/__init__.py

@@ -2414,13 +2414,20 @@ def topic_links(linkifiers_key: int, topic_name: str) -> List[Dict[str, str]]:
                 dict(
                     url=url_format_string % match_details,
                     text=match_text,
-                    index=topic_name.find(match_text),
+                    index=m.start(),
                 )
             ]
 
+    pos = 0
     # Also make raw URLs navigable.
-    for sub_string in basic_link_splitter.split(topic_name):
-        link_match = re.match(get_web_link_regex(), sub_string)
+    while pos < len(topic_name):
+        # Assuming that basic_link_splitter matches 1 character,
+        # we match segments of the string for URL divided by the matched character.
+        next_split = basic_link_splitter.search(topic_name, pos)
+        end = next_split.start() if next_split is not None else len(topic_name)
+        # We have to match the substring because LINK_REGEX
+        # matches the start of the entire string with "^"
+        link_match = re.match(get_web_link_regex(), topic_name[pos:end])
         if link_match:
             actual_match_url = link_match.group("url")
             result = urlsplit(actual_match_url)
@@ -2431,9 +2438,9 @@ def topic_links(linkifiers_key: int, topic_name: str) -> List[Dict[str, str]]:
                 url = result._replace(scheme="https").geturl()
             else:
                 url = actual_match_url
-            matches.append(
-                dict(url=url, text=actual_match_url, index=topic_name.find(actual_match_url))
-            )
+            matches.append(dict(url=url, text=actual_match_url, index=pos))
+        # Move pass the next split point, and start matching the URL from there
+        pos = end + 1
 
     # In order to preserve the order in which the links occur, we sort the matched text
     # based on its starting index in the topic. We pop the index field before returning.

zerver/tests/test_markdown.py

@@ -1340,6 +1340,21 @@ class MarkdownTest(ZulipTestCase):
             ],
         )
 
+        msg.set_topic_name("#111 https://google.com #111 #222 #111 https://google.com #222")
+        converted_topic = topic_links(realm.id, msg.topic_name())
+        self.assertEqual(
+            converted_topic,
+            [
+                {"url": "https://trac.example.com/ticket/111", "text": "#111"},
+                {"url": "https://google.com", "text": "https://google.com"},
+                {"url": "https://trac.example.com/ticket/111", "text": "#111"},
+                {"url": "https://trac.example.com/ticket/222", "text": "#222"},
+                {"url": "https://trac.example.com/ticket/111", "text": "#111"},
+                {"url": "https://google.com", "text": "https://google.com"},
+                {"url": "https://trac.example.com/ticket/222", "text": "#222"},
+            ],
+        )
+
         msg.set_topic_name("#444 #555 #666")
         converted_topic = topic_links(realm.id, msg.topic_name())
         self.assertEqual(