ircd:Ⓜ️:room: Start a spec faithful display_name(room) diagnostic.

include/ircd/m/room/room.h

@@ -44,6 +44,7 @@ namespace ircd::m
 	string_view version(const mutable_buffer &, const room &);
 	string_view join_rule(const mutable_buffer &out, const room &);
 	string_view membership(const mutable_buffer &out, const room &, const m::id::user &);
+	string_view display_name(const mutable_buffer &out, const room &);
 	id::user::buf any_user(const room &, const string_view &host, const string_view &memshp = "join");
 
 	id::room room_id(const mutable_buffer &, const id::room_alias &);

modules/console.cc

@@ -7842,7 +7842,7 @@ console_cmd__room__top(opt &out, const string_view &line)
 
 	out << std::endl;
 
-	char version_buf[32];
+	char version_buf[32], display_buf[256];
 	out << "index:             " << m::room::index(room_id) << std::endl;
 	out << "version:           " << m::version(version_buf, room_id) << std::endl;
 	out << "federated:         " << std::boolalpha << m::federated(room_id) << std::endl;
@@ -7858,6 +7858,7 @@ console_cmd__room__top(opt &out, const string_view &line)
 	out << "top depth:         " << std::get<int64_t>(top) << std::endl;
 	out << "top index:         " << std::get<m::event::idx>(top) << std::endl;
 	out << "top event:         " << std::get<m::event::id::buf>(top) << std::endl;
+	out << "display name:      " << m::display_name(display_buf, room_id) << std::endl;
 	return true;
 }
 

modules/m_room.cc

@@ -18,3 +18,95 @@ IRCD_MODULE
 {
 	"Matrix room library"
 };
+
+ircd::string_view
+IRCD_MODULE_EXPORT
+ircd::m::display_name(const mutable_buffer &out,
+                      const room &room)
+{
+	const room::state state
+	{
+		room
+	};
+
+	// 1. If the room has an m.room.name state event with a non-empty name
+	// field, use the name given by that field.
+	string_view ret;
+	{
+		const event::idx event_idx
+		{
+			state.get(std::nothrow, "m.room.name", "")
+		};
+
+		m::get(std::nothrow, event_idx, "content", [&out, &ret]
+		(const json::object &content)
+		{
+			const json::string &name
+			{
+				content.get("name")
+			};
+
+			ret = strlcpy
+			{
+				out, name
+			};
+		});
+	}
+
+	// 2. If the room has an m.room.canonical_alias state event with a
+	// non-empty alias field, use the alias given by that field as the name.
+	if(!ret)
+	{
+		const event::idx event_idx
+		{
+			state.get(std::nothrow, "m.room.canonical_alias", "")
+		};
+
+		m::get(std::nothrow, event_idx, "content", [&out, &ret]
+		(const json::object &content)
+		{
+			const json::string &alias
+			{
+				content.get("alias")
+			};
+
+			ret = strlcpy
+			{
+				out, alias
+			};
+		});
+	}
+
+	// 3. If neither of the above conditions are met, the client can optionally
+	// guess an alias from the m.room.alias events in the room. This is a
+	// temporary measure while clients do not promote canonical aliases as
+	// prominently. This step may be removed in a future version of the
+	// specification.
+
+	// 4. If none of the above conditions are met, a name should be composed
+	// based on the members of the room. Clients should consider m.room.member
+	// events for users other than the logged-in user, as defined below.
+
+	// i. If the number of m.heroes for the room are greater or equal to
+	// m.joined_member_count + m.invited_member_count - 1, then use the
+	// membership events for the heroes to calculate display names for the
+	// users (disambiguating them if required) and concatenating them. For
+	// example, the client may choose to show "Alice, Bob, and Charlie
+	// (@charlie:example.org)" as the room name. The client may optionally
+	// limit the number of users it uses to generate a room name.
+
+	// ii. If there are fewer heroes than m.joined_member_count +
+	// m.invited_member_count - 1, and m.joined_member_count +
+	// m.invited_member_count is greater than 1, the client should use the
+	// heroes to calculate display names for the users (disambiguating them if
+	// required) and concatenating them alongside a count of the remaining
+	// users. For example, "Alice, Bob, and 1234 others".
+
+	// iii. If m.joined_member_count + m.invited_member_count is less than or
+	// equal to 1 (indicating the member is alone), the client should use the
+	// rules above to indicate that the room was empty. For example, "Empty Room
+	// (was Alice)", "Empty Room (was Alice and 1234 others)", or "Empty Room"
+	// if there are no heroes.
+
+	return ret;
+}