From b94402025bca224a1bea86a9d368887cc8e3e1e8 Mon Sep 17 00:00:00 2001
From: Nick Mills-Barrett <nick@beeper.com>
Date: Thu, 29 Oct 2026 16:30:02 +0000
Subject: [PATCH] PLAT-39405 connector: turn Reddit @username text into Matrix
 mentions

Reddit sends mentions as plain "@username" text with no m.mentions or
formatted body, so they arrived in Beeper as plain text.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 pkg/connector/mentions.go      | 66 ++++++++++++++++++++++++++++++++++
 pkg/connector/mentions_test.go | 34 ++++++++++++++++++
 pkg/connector/tomatrix.go      |  3 ++
 3 files changed, 103 insertions(+)
 create mode 100644 pkg/connector/mentions.go
 create mode 100644 pkg/connector/mentions_test.go

diff --git a/pkg/connector/mentions.go b/pkg/connector/mentions.go
new file mode 100644
index 0000000..ca4681e
--- /dev/null
+++ b/pkg/connector/mentions.go
@@ -0,0 +1,66 @@
+package connector
+
+import (
+	"fmt"
+	"html"
+	"regexp"
+	"strings"
+
+	"maunium.net/go/mautrix/bridgev2"
+	"maunium.net/go/mautrix/event"
+	"maunium.net/go/mautrix/id"
+)
+
+var mentionPattern = regexp.MustCompile(`(?:^|[^\w@/])(@[\w-]{3,20})`)
+
+// Reddit sends mentions as plain "@username" text with no Matrix mention data.
+func (r *RedditClient) convertMentions(portal *bridgev2.Portal, content *event.MessageEventContent) {
+	meta, _ := portal.Metadata.(*PortalMetadata)
+	if content.Format != "" || meta == nil || meta.State == nil {
+		return
+	}
+	formatted, mentioned := linkMentions(content.Body, meta.State.Members, r.beeperMXID)
+	if len(mentioned) == 0 {
+		return
+	}
+	content.Format = event.FormatHTML
+	content.FormattedBody = formatted
+	content.Mentions = &event.Mentions{UserIDs: mentioned}
+}
+
+func (r *RedditClient) beeperMXID(redditMXID id.UserID) id.UserID {
+	if userID := makeUserID(redditMXID); userID != r.userID {
+		return r.main.Bridge.Matrix.FormatGhostMXID(userID)
+	}
+	return r.userLogin.UserMXID
+}
+
+func linkMentions(body string, members map[id.UserID]RoomMember, beeperMXID func(id.UserID) id.UserID) (string, []id.UserID) {
+	usernames := make(map[string]id.UserID, len(members))
+	for mxid, member := range members {
+		if member.Name != "" {
+			usernames[strings.ToLower(stripUserPrefix(member.Name))] = mxid
+		}
+	}
+	var formatted strings.Builder
+	var mentioned []id.UserID
+	last := 0
+	for _, match := range mentionPattern.FindAllStringSubmatchIndex(body, -1) {
+		start, end := match[2], match[3]
+		redditMXID, ok := usernames[strings.ToLower(body[start+1:end])]
+		if !ok {
+			continue
+		}
+		target := beeperMXID(redditMXID)
+		formatted.WriteString(plainToHTML(body[last:start]))
+		_, _ = fmt.Fprintf(&formatted, `<a href="%s">%s</a>`, target.URI().MatrixToURL(), html.EscapeString(body[start:end]))
+		mentioned = append(mentioned, target)
+		last = end
+	}
+	formatted.WriteString(plainToHTML(body[last:]))
+	return formatted.String(), mentioned
+}
+
+func plainToHTML(text string) string {
+	return strings.ReplaceAll(html.EscapeString(text), "\n", "<br>")
+}
diff --git a/pkg/connector/mentions_test.go b/pkg/connector/mentions_test.go
new file mode 100644
index 0000000..29fd19b
--- /dev/null
+++ b/pkg/connector/mentions_test.go
@@ -0,0 +1,34 @@
+package connector
+
+import (
+	"slices"
+	"testing"
+
+	"maunium.net/go/mautrix/id"
+)
+
+func TestLinkMentions(t *testing.T) {
+	members := map[id.UserID]RoomMember{
+		"@t2_a:reddit.com": {Name: "Some_User-1"},
+		"@t2_b:reddit.com": {Name: "other"},
+		"@t2_c:reddit.com": {},
+	}
+	beeperMXID := func(mxid id.UserID) id.UserID { return "@ghost_" + id.UserID(makeUserID(mxid)) + ":beeper.local" }
+	for _, tc := range []struct {
+		name, body, want string
+		mentioned        []id.UserID
+	}{
+		{"start", "@some_user-1 hi", `<a href="https://matrix.to/#/@ghost_t2_a:beeper.local">@some_user-1</a> hi`, []id.UserID{"@ghost_t2_a:beeper.local"}},
+		{"two", "hey @other & @Some_User-1!", `hey <a href="https://matrix.to/#/@ghost_t2_b:beeper.local">@other</a> &amp; <a href="https://matrix.to/#/@ghost_t2_a:beeper.local">@Some_User-1</a>!`, []id.UserID{"@ghost_t2_b:beeper.local", "@ghost_t2_a:beeper.local"}},
+		{"not a member", "@stranger <b>", "@stranger &lt;b&gt;", nil},
+		{"email and url", "a@other.com reddit.com/@other", "a@other.com reddit.com/@other", nil},
+		{"newline", "line\n@other", `line<br><a href="https://matrix.to/#/@ghost_t2_b:beeper.local">@other</a>`, []id.UserID{"@ghost_t2_b:beeper.local"}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			formatted, mentioned := linkMentions(tc.body, members, beeperMXID)
+			if formatted != tc.want || !slices.Equal(mentioned, tc.mentioned) {
+				t.Fatalf("linkMentions(%q) = %q, %v", tc.body, formatted, mentioned)
+			}
+		})
+	}
+}
diff --git a/pkg/connector/tomatrix.go b/pkg/connector/tomatrix.go
index 22abddf..f8f7bcc 100644
--- a/pkg/connector/tomatrix.go
+++ b/pkg/connector/tomatrix.go
@@ -47,6 +47,9 @@ func (r *RedditClient) convertMessage(ctx context.Context, portal *bridgev2.Port
 			return nil, err
 		}
 	case event.MsgText, event.MsgNotice, event.MsgEmote:
+		if portal != nil {
+			r.convertMentions(portal, &content)
+		}
 	default:
 		return nil, bridgev2.ErrUnsupportedMessageType
 	}