--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mod_muc_markers/mod_muc_markers.lua	Mon Apr 13 15:48:58 2020 +0100
@@ -0,0 +1,53 @@
+-- Track messages received by users of the MUC
+
+-- We rewrite the 'id' attribute of outgoing stanzas to match the stanza (archive) id
+-- This module is therefore incompatible with the muc#stable_id feature
+-- We rewrite the id because XEP-0333 doesn't tell clients explicitly which id to use
+-- in marker reports. However it implies the 'id' attribute through examples, and this
+-- is what some clients implement.
+-- Notably Conversations will ack the origin-id instead. We need to update the XEP to
+-- clarify the correct behaviour.
+
+local xmlns_markers = "urn:xmpp:chat-markers:0";
+
+local muc_marker_map_store = module:open_store("muc_markers", "map");
+
+local function get_stanza_id(stanza, by_jid)
+	for tag in stanza:childtags("stanza-id", "urn:xmpp:sid:0") do
+		if tag.attr.by == by_jid then
+			return tag.attr.id;
+		end
+	end
+	return nil;
+end
+
+module:hook("muc-broadcast-message", function (event)
+	local stanza = event.stanza;
+
+	local archive_id = get_stanza_id(stanza, event.room.jid);
+	-- We are not interested in stanzas that didn't get archived
+	if not archive_id then return; end
+
+	-- Add stanza id as id attribute
+	stanza.attr.id = archive_id;
+	-- Add markable element to request markers from clients
+	stanza:tag("markable", { xmlns = xmlns_markers }):up();
+end, -1);
+
+module:hook("muc-occupant-groupchat", function (event)
+	local marker = event.stanza:get_child("received", xmlns_markers);
+	if not marker then return; end
+
+	-- Store the id that the user has received to
+	module:log("warn", "New marker for %s: %s", event.occupant.bare_jid, marker.attr.id);
+	muc_marker_map_store:set(event.occupant.bare_jid, event.room.jid, marker.attr.id);
+
+	-- Prevent stanza from reaching the room (it's just noise)
+	return true;
+end);
+
+-- Public API
+
+function get_user_read_marker(user_jid, room_jid)
+	return muc_marker_map_store:get(user_jid, room_jid);
+end