ircd:Ⓜ️:room::message: Add interface to extract rich reply fallbacks.

include/ircd/m/room/message.h

@@ -37,5 +37,23 @@ struct ircd::m::room::message
 	json::property<name::formatted_body, json::string>
 >
 {
+	/// The event ID of the message being replied to; empty if malformed or
+	/// not a reply. If malformed, the message is not considered a reply.
+	id::event reply_to_event() const noexcept;
+
+	/// The user who sent the message being replied to; empty if not a reply
+	/// or the username was missing or malformed.
+	id::user reply_to_user() const noexcept;
+
+	/// The message being replied to, cut to quoted content, which may include
+	/// multiple pseudo-newlines and leading '>' interrupting the text; caller
+	/// must clean that up if required. Empty if not a reply or malformed.
+	string_view reply_to_body() const noexcept;
+
+	/// C2S v1.3 11.3.1 message body stripped of any reply fallback. This is
+	/// the proper way to read the message rather than reading "body" direct;
+	/// returns "body" if not reply.
+	string_view body() const noexcept;
+
 	using super_type::tuple;
 };

matrix/Makefile.am

@@ -126,6 +126,7 @@ libircd_matrix_la_SOURCES += room_members.cc
 libircd_matrix_la_SOURCES += room_origins.cc
 libircd_matrix_la_SOURCES += room_type.cc
 libircd_matrix_la_SOURCES += room_content.cc
+libircd_matrix_la_SOURCES += room_message.cc
 libircd_matrix_la_SOURCES += room_power.cc
 libircd_matrix_la_SOURCES += room_state.cc
 libircd_matrix_la_SOURCES += room_state_history.cc

matrix/room_message.cc

@@ -0,0 +1,149 @@
+// The Construct
+//
+// Copyright (C) The Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2022 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+ircd::string_view
+ircd::m::room::message::body()
+const noexcept
+{
+	const auto reply_to_body
+	{
+		this->reply_to_body()
+	};
+
+	string_view body
+	{
+		json::get<"body"_>(*this)
+	};
+
+	if(reply_to_body)
+	{
+		body = string_view
+		{
+			reply_to_body.end(), body.end()
+		};
+
+		body = lstrip(body, "\\n", 2);
+	}
+
+	return body;
+}
+
+ircd::string_view
+ircd::m::room::message::reply_to_body()
+const noexcept
+{
+	const auto reply_to_user
+	{
+		this->reply_to_user()
+	};
+
+	if(likely(!reply_to_user))
+		return {};
+
+	string_view body
+	{
+		json::get<"body"_>(*this)
+	};
+
+	body = string_view
+	{
+		reply_to_user.end(), body.end()
+	};
+
+	body = lstrip(body, '>', 1);
+	body = lstrip(body, ' ', 1);
+	body = string_view
+	{
+		body.begin(), token(body, "\\n", 0).end()
+	};
+
+	tokens(body, "\\n", [&body]
+	(const string_view &line) noexcept
+	{
+		if(!startswith(line, '>'))
+			return false;
+
+		body = string_view
+		{
+			body.begin(), line.end()
+		};
+
+		return true;
+	});
+
+	return body;
+}
+
+ircd::m::user::id
+ircd::m::room::message::reply_to_user()
+const noexcept
+{
+	string_view body
+	{
+		json::get<"body"_>(*this)
+	};
+
+	if(!startswith(body, '>'))
+		return {};
+
+	body = lstrip(body, '>', 1);
+	body = lstrip(body, ' ', 1);
+	body = lstrip(body, '*', 1);
+	body = lstrip(body, ' ', 1);
+	const string_view ret
+	{
+		between(body, '<', '>')
+	};
+
+	if(!valid(m::id::USER, ret))
+		return {};
+
+	return ret;
+}
+
+ircd::m::event::id
+ircd::m::room::message::reply_to_event()
+const noexcept try
+{
+	const json::object &m_relates_to
+	{
+		json::get<"m.relates_to"_>(*this)
+	};
+
+	if(!m_relates_to || !json::type(m_relates_to, json::OBJECT))
+		return {};
+
+	const json::object &m_in_reply_to
+	{
+		m_relates_to["m.in_reply_to"]
+	};
+
+	const auto &event_id
+	{
+		json::type(m_in_reply_to, json::OBJECT)?
+			m_in_reply_to.get<json::string>("event_id"):
+			m_relates_to.get<json::string>("event_id")
+	};
+
+	if(!event_id || !valid(m::id::EVENT, event_id))
+		return {};
+
+	return event_id;
+}
+catch(const std::exception &e)
+{
+	log::error
+	{
+		log, "Failed to extract m.relates_to m.in_reply_to event_id :%s",
+		e.what(),
+	};
+
+	return {};
+}