welcome message: link @zio with a mention entity

internal/domain/welcome_message.go

@@ -10,6 +10,14 @@ import (
 	"time"
 )
 
+// officialUpdatesChannelMention is the public @username of the updates channel
+// linked from the welcome message when the resolved body includes it. It is
+// carried in a MessageEntityMention wherever it appears in body so clients
+// render it as a tappable link; the mention itself lives in the default
+// templates in login_welcome_template.go (or an admin-panel override), not
+// hardcoded here.
+const officialUpdatesChannelMention = "@zio"
+
 // OfficialWelcomeMessage builds the account-visible incoming message sent
 // from the official system account on every completed sign-in (SignUp and
 // every subsequent SignIn/SignInWithEmail), regardless of delivery channel.
@@ -28,13 +36,42 @@ func OfficialWelcomeMessage(userID int64, body string, date int) (Message, error
 	if userID <= 0 || IsSystemUserID(userID) || body == "" || date < 0 || date > math.MaxInt32 {
 		return Message{}, fmt.Errorf("%w: user=%d date=%d", ErrLoginCodeDeliveryInvalid, userID, date)
 	}
-	return Message{
+	msg := Message{
 		OwnerUserID: userID,
 		Peer:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
 		From:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
 		Date:        date,
 		Body:        body,
-	}, nil
+	}
+	if ent, ok := usernameMentionEntity(body, officialUpdatesChannelMention); ok {
+		msg.Entities = []MessageEntity{ent}
+	}
+	return msg, nil
+}
+
+// usernameMentionEntity locates mention ("@name") in text and returns a
+// MessageEntityMention spanning it, with the UTF-16 offset/length clients expect.
+func usernameMentionEntity(text, mention string) (MessageEntity, bool) {
+	before, _, found := strings.Cut(text, mention)
+	if !found {
+		return MessageEntity{}, false
+	}
+	return MessageEntity{
+		Type:   MessageEntityMention,
+		Offset: utf16Len(before),
+		Length: utf16Len(mention),
+	}, true
+}
+
+func utf16Len(s string) int {
+	n := 0
+	for _, r := range s {
+		n++
+		if r > 0xffff {
+			n++
+		}
+	}
+	return n
 }
 
 // SignInMethodLabel returns the human-readable method name embedded in

internal/domain/welcome_message_test.go

@@ -3,6 +3,7 @@ package domain
 import (
 	"errors"
 	"testing"
+	"unicode/utf16"
 )
 
 func TestWelcomeMessageContentAndFingerprint(t *testing.T) {
@@ -52,3 +53,25 @@ func TestNextWelcomeRevision(t *testing.T) {
 		t.Fatalf("zero revision err = %v", err)
 	}
 }
+
+func TestOfficialWelcomeMessageLinksUpdatesChannel(t *testing.T) {
+	body := "Welcome! Join " + officialUpdatesChannelMention + " for updates."
+	msg, err := OfficialWelcomeMessage(1780243200, body, 1_700_000_000)
+	if err != nil {
+		t.Fatalf("body %q: %v", body, err)
+	}
+	if len(msg.Entities) != 1 {
+		t.Fatalf("body %q: entities = %+v, want one mention", body, msg.Entities)
+	}
+	ent := msg.Entities[0]
+	if ent.Type != MessageEntityMention {
+		t.Fatalf("entity type = %q, want mention", ent.Type)
+	}
+	units := utf16.Encode([]rune(msg.Body))
+	if ent.Offset < 0 || ent.Length <= 0 || ent.Offset+ent.Length > len(units) {
+		t.Fatalf("entity %+v out of bounds for body of %d utf16 units", ent, len(units))
+	}
+	if got := string(utf16.Decode(units[ent.Offset : ent.Offset+ent.Length])); got != officialUpdatesChannelMention {
+		t.Fatalf("entity spans %q, want %q", got, officialUpdatesChannelMention)
+	}
+}