diff --git a/include/ircd/m/room/events.h b/include/ircd/m/room/events.h
index cf6a306a8..6464ae23d 100644
--- a/include/ircd/m/room/events.h
+++ b/include/ircd/m/room/events.h
@@ -11,20 +11,22 @@
 #pragma once
 #define HAVE_IRCD_M_ROOM_EVENTS_H
 
+// 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.
+//
+// The "twain" marks the depth at the end of the first gap; the server is in
+// possession of one or more events again at the twain.
+//
+// The "hazard" is the depth of the first gap starting from the m.room.create
+// event toward the tophead. In any attempt to trace the room timeline with
+// an increasing depth, the hazard is the next gap to frontfill.
+
 namespace ircd::m
 {
-	// [GET] Find gaps in the room's event graph. A gap is where no events
-	// have been obtained at that depth. Each gap is reported to the closure
-	// with a separate invocation. The range is [inclusive, exclusive].
-	using depth_range = std::pair<int64_t, int64_t>;
-	using depth_range_closure = std::function<bool (const depth_range &, const event::idx &)>;
-	bool for_each_depth_gap(const room &, const depth_range_closure &);
-	bool rfor_each_depth_gap(const room &, const depth_range_closure &);
-
-	bool sounding(const room &, const depth_range_closure &); // Last missing (all)
-	std::pair<int64_t, event::idx> hazard(const room &); // First missing (one)
 	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)
 }
 
 /// Interface to room events
@@ -37,6 +39,9 @@ namespace ircd::m
 ///
 struct ircd::m::room::events
 {
+	struct missing;
+	struct sounding;
+
 	m::room room;
 	db::domain::const_iterator it;
 	event::fetch _event;
@@ -88,3 +93,48 @@ struct ircd::m::room::events
 	static size_t count(const event::idx &, const event::idx &);
 	static size_t count(const event::id &, const event::id &);
 };
+
+/// Find missing room events. This is an interface to the event-horizon for
+/// this room, organized as a breadth-first iteration of missing references.
+///
+/// The closure is invoked with the first argument being the event_id unknown
+/// to the server, followed by the depth and event::idx of the event making the
+/// reference.
+///
+struct ircd::m::room::events::missing
+{
+	using closure = std::function<bool (const event::id &, const uint64_t &, const event::idx &)>;
+
+	m::room room;
+
+  public:
+	bool for_each(const closure &) const;
+
+	missing() = default;
+	missing(const m::room &room)
+	:room{room}
+	{}
+};
+
+/// Find gaps in the room's events. A gap is where this server has no events
+/// at a certain depth. This is a path-finding diagnostic interface, useful to
+/// understand what areas of the timeline have not been acquired by the server
+/// to calculate backfill requests, etc. This interface is depth-first oriented,
+/// rather than the breadth-first room::events::missing interface.
+///
+struct ircd::m::room::events::sounding
+{
+	using range = std::pair<int64_t, int64_t>;
+	using closure = std::function<bool (const range &, const event::idx &)>;
+
+	m::room room;
+
+  public:
+	bool for_each(const closure &) const;
+	bool rfor_each(const closure &) const;
+
+	sounding() = default;
+	sounding(const m::room &room)
+	:room{room}
+	{}
+};
diff --git a/modules/console.cc b/modules/console.cc
index 8b1a8b2bf..38ed1e858 100644
--- a/modules/console.cc
+++ b/modules/console.cc
@@ -7807,7 +7807,12 @@ console_cmd__room__top(opt &out, const string_view &line)
 	out << "recent gaps: " << std::endl;
 
 	size_t gap_count(4);
-	m::rfor_each_depth_gap(room, [&out, &gap_count]
+	const m::room::events::sounding gaps
+	{
+		room
+	};
+
+	gaps.for_each([&out, &gap_count]
 	(const auto &range, const auto &event_idx)
 	{
 		out << std::right << std::setw(8) << range.first
@@ -8101,19 +8106,18 @@ console_cmd__room__sounding(opt &out, const string_view &line)
 		m::room::index(room)
 	};
 
+	const auto sounding
+	{
+		m::sounding(room)
+	};
+
 	out << "head:      " << std::setw(8) << m::depth(room)
 	    << "   " << m::event_id(head) << " (" << head << ")"
 	    << std::endl;
 
-	m::sounding(room, [&out]
-	(const auto &range, const auto &event_idx)
-	{
-		out << "sounding:  " << std::setw(8) << range.second
-		    << "   " << m::event_id(event_idx) << " (" << event_idx << ")"
-		    << std::endl;
-
-		return true;
-	});
+	out << "sounding:  " << std::setw(8) << sounding.first
+	    << "   " << m::event_id(sounding.second) << " (" << sounding.second << ")"
+	    << std::endl;
 
 	out << "twain:     " << std::setw(8) << twain.first
 	    << std::endl;
@@ -8179,10 +8183,15 @@ console_cmd__room__depth__gaps(opt &out, const string_view &line)
 		return true;
 	}};
 
+	const m::room::events::sounding gaps
+	{
+		room
+	};
+
 	if(param["reverse"] == "reverse")
-		m::rfor_each_depth_gap(room, closure);
+		gaps.rfor_each(closure);
 	else
-		m::for_each_depth_gap(room, closure);
+		gaps.for_each(closure);
 
 	return true;
 }
diff --git a/modules/m_room_events.cc b/modules/m_room_events.cc
index 0f78e63f0..de55373ce 100644
--- a/modules/m_room_events.cc
+++ b/modules/m_room_events.cc
@@ -14,6 +14,10 @@ IRCD_MODULE
 	"Matrix room library"
 };
 
+//
+// tools
+//
+
 std::pair<int64_t, ircd::m::event::idx>
 IRCD_MODULE_EXPORT
 ircd::m::twain(const room &room)
@@ -23,7 +27,12 @@ ircd::m::twain(const room &room)
 		-1, 0
 	};
 
-	rfor_each_depth_gap(room, [&ret]
+	const room::events::sounding s
+	{
+		room
+	};
+
+	s.rfor_each([&ret]
 	(const auto &range, const auto &event_idx)
 	{
 		ret.first = range.first - 1;
@@ -43,7 +52,12 @@ ircd::m::sounding(const room &room)
 		-1, 0
 	};
 
-	rfor_each_depth_gap(room, [&ret]
+	const room::events::sounding s
+	{
+		room
+	};
+
+	s.rfor_each([&ret]
 	(const auto &range, const auto &event_idx)
 	{
 		ret.first = range.second;
@@ -58,8 +72,17 @@ std::pair<int64_t, ircd::m::event::idx>
 IRCD_MODULE_EXPORT
 ircd::m::hazard(const room &room)
 {
-	std::pair<int64_t, m::event::idx> ret {0, 0};
-	for_each_depth_gap(room, [&ret]
+	std::pair<int64_t, m::event::idx> ret
+	{
+		0, 0
+	};
+
+	const room::events::sounding s
+	{
+		room
+	};
+
+	s.for_each([&ret]
 	(const auto &range, const auto &event_idx)
 	{
 		ret.first = range.first;
@@ -70,96 +93,9 @@ ircd::m::hazard(const room &room)
 	return ret;
 }
 
-bool
-IRCD_MODULE_EXPORT
-ircd::m::sounding(const room &room,
-                  const depth_range_closure &closure)
-{
-	bool ret(true);
-	int64_t depth(-1);
-	rfor_each_depth_gap(room, [&depth, &ret, &closure]
-	(const auto &range, const auto &event_idx)
-	{
-		if(depth != -1 && depth != range.second)
-			return false;
-
-		depth = range.second;
-		if(!closure(range, event_idx))
-			return false;
-
-		return true;
-	});
-
-	return ret;
-}
-
-bool
-IRCD_MODULE_EXPORT
-ircd::m::rfor_each_depth_gap(const room &room,
-                             const depth_range_closure &closure)
-{
-	room::events it
-	{
-		room
-	};
-
-	if(!it)
-		return true;
-
-	event::idx idx{0};
-	for(depth_range range{0L, it.depth()}; it; --it)
-	{
-		range.first = it.depth();
-		if(range.first == range.second)
-		{
-			idx = it.event_idx();
-			continue;
-		}
-
-		--range.second;
-		if(range.first == range.second)
-		{
-			idx = it.event_idx();
-			continue;
-		}
-
-		if(!closure({range.first+1, range.second+1}, idx))
-			return false;
-
-		range.second = range.first;
-	}
-
-	return true;
-}
-
-bool
-IRCD_MODULE_EXPORT
-ircd::m::for_each_depth_gap(const room &room,
-                            const depth_range_closure &closure)
-{
-	room::events it
-	{
-		room, int64_t(0L)
-	};
-
-	for(depth_range range{0L, 0L}; it; ++it)
-	{
-		range.second = it.depth();
-		if(range.first == range.second)
-			continue;
-
-		++range.first;
-		if(range.first == range.second)
-			continue;
-
-		if(!closure(range, it.event_idx()))
-			return false;
-
-		range.first = range.second;
-	}
-
-	return true;
-}
+//
+// room::events
+//
 
 size_t
 IRCD_MODULE_EXPORT
@@ -228,3 +164,92 @@ ircd::m::room::events::count(const m::room &room,
 	for(++it; it && it.event_idx() < b; ++it, ++ret);
 	return ret;
 }
+
+//
+// room::events::events
+//
+// (see: ircd/m_room.cc for now)
+
+//
+// room::events::missing
+//
+
+bool
+IRCD_MODULE_EXPORT
+ircd::m::room::events::missing::for_each(const closure &closure)
+const
+{
+	return true;
+}
+
+//
+// room::events::sounding
+//
+
+bool
+IRCD_MODULE_EXPORT
+ircd::m::room::events::sounding::rfor_each(const closure &closure)
+const
+{
+	room::events it
+	{
+		room
+	};
+
+	if(!it)
+		return true;
+
+	event::idx idx{0};
+	for(sounding::range range{0L, it.depth()}; it; --it)
+	{
+		range.first = it.depth();
+		if(range.first == range.second)
+		{
+			idx = it.event_idx();
+			continue;
+		}
+
+		--range.second;
+		if(range.first == range.second)
+		{
+			idx = it.event_idx();
+			continue;
+		}
+
+		if(!closure({range.first+1, range.second+1}, idx))
+			return false;
+
+		range.second = range.first;
+	}
+
+	return true;
+}
+
+bool
+IRCD_MODULE_EXPORT
+ircd::m::room::events::sounding::for_each(const closure &closure)
+const
+{
+	room::events it
+	{
+		room, int64_t(0L)
+	};
+
+	for(sounding::range range{0L, 0L}; it; ++it)
+	{
+		range.second = it.depth();
+		if(range.first == range.second)
+			continue;
+
+		++range.first;
+		if(range.first == range.second)
+			continue;
+
+		if(!closure(range, it.event_idx()))
+			return false;
+
+		range.first = range.second;
+	}
+
+	return true;
+}