ircd:Ⓜ️:room::events: Add viewport metric concept; conf item.

include/ircd/m/room/events.h

@@ -11,6 +11,14 @@
 #pragma once
 #define HAVE_IRCD_M_ROOM_EVENTS_H
 
+// The "viewport" is comprised of events starting from the tophead (most recent
+// in room timeline) and covering about ~20 events leading up to that. Note
+// that this is a completely ad hoc and configurable server value. Events in
+// the viewport must be eval'ed and synced to clients in the order they will
+// be displayed. Events not in the viewport are not /synced to clients and any
+// client request provides event ordering: thus older events (backfills, etc)
+// can be eval'ed without this constraint.
+//
 // The "sounding" is the depth of the first gap. In any attempt to trace
 // the room timeline from the tophead to the m.room.create event: the sounding
 // is the [highest number] depth preventing that.
@@ -24,6 +32,7 @@
 
 namespace ircd::m
 {
+	std::pair<int64_t, event::idx> viewport(const room &);
 	std::pair<int64_t, event::idx> sounding(const room &); // Last missing (one)
 	std::pair<int64_t, event::idx> twain(const room &);
 	std::pair<int64_t, event::idx> hazard(const room &); // First missing (one)
@@ -43,6 +52,8 @@ struct ircd::m::room::events
 	struct horizon;
 	struct missing;
 
+	static conf::item<size_t> viewport_size;
+
 	m::room room;
 	db::domain::const_iterator it;
 	event::fetch _event;

modules/m_room_events.cc

@@ -14,9 +14,41 @@ IRCD_MODULE
 	"Matrix room library"
 };
 
-//
-// tools
-//
+IRCD_MODULE_EXPORT_DATA
+decltype(ircd::m::room::events::viewport_size)
+ircd::m::room::events::viewport_size
+{
+	{ "name",     "ircd.m.room.events.viewport.size" },
+	{ "default",  24L                                },
+};
+
+std::pair<int64_t, ircd::m::event::idx>
+IRCD_MODULE_EXPORT
+ircd::m::viewport(const room &room)
+{
+	std::pair<int64_t, m::event::idx> ret
+	{
+		-1, 0
+	};
+
+	m::room::events it
+	{
+		room
+	};
+
+	const size_t &max
+	{
+		room::events::viewport_size
+	};
+
+	for(size_t i(0); it && i < max; --it, ++i)
+	{
+		ret.first = it.depth();
+		ret.second = it.event_idx();
+	}
+
+	return ret;
+}
 
 std::pair<int64_t, ircd::m::event::idx>
 IRCD_MODULE_EXPORT