welcome message: link @zio with a mention entity

internal/domain/welcome_message.go

@@ -6,7 +6,12 @@ import (
 	"strings"
 )
 
-const officialWelcomeMessageTemplate = "👋 Welcome to OwpenGram!\n\nYou just signed in via %s.\n\nIf this wasn't you, revoke this session from \"Settings > Privacy and Security > Active sessions\" immediately.\n\nIf you haven't already, feel free to join @zio for all the latest updates!"
+// officialUpdatesChannelMention is the public @username of the updates channel
+// linked from the welcome message. It is carried both in the template text and
+// in a MessageEntityMention so clients render it as a tappable link.
+const officialUpdatesChannelMention = "@zio"
+
+const officialWelcomeMessageTemplate = "👋 Welcome to OwpenGram!\n\nYou just signed in via %s.\n\nIf this wasn't you, revoke this session from \"Settings > Privacy and Security > Active sessions\" immediately.\n\nIf you haven't already, feel free to join " + officialUpdatesChannelMention + " for all the latest updates!"
 
 // OfficialWelcomeMessage builds the account-visible incoming message sent
 // from the official system account on every completed sign-in (SignUp and
@@ -20,13 +25,43 @@ func OfficialWelcomeMessage(userID int64, method string, date int) (Message, err
 	if userID <= 0 || IsSystemUserID(userID) || method == "" || date < 0 || date > math.MaxInt32 {
 		return Message{}, fmt.Errorf("%w: user=%d method=%q date=%d", ErrLoginCodeDeliveryInvalid, userID, method, date)
 	}
-	return Message{
+	body := fmt.Sprintf(officialWelcomeMessageTemplate, method)
+	msg := Message{
 		OwnerUserID: userID,
 		Peer:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
 		From:        Peer{Type: PeerTypeUser, ID: OfficialSystemUserID},
 		Date:        date,
-		Body:        fmt.Sprintf(officialWelcomeMessageTemplate, method),
-	}, nil
+		Body:        body,
+	}
+	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

@@ -0,0 +1,29 @@
+package domain
+
+import (
+	"testing"
+	"unicode/utf16"
+)
+
+func TestOfficialWelcomeMessageLinksUpdatesChannel(t *testing.T) {
+	for _, method := range []string{"phone number", "email"} {
+		msg, err := OfficialWelcomeMessage(1780243200, method, 1_700_000_000)
+		if err != nil {
+			t.Fatalf("method %q: %v", method, err)
+		}
+		if len(msg.Entities) != 1 {
+			t.Fatalf("method %q: entities = %+v, want one mention", method, 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)
+		}
+	}
+}