modules/m_room: Add interface to count events in a room between two arguments.

include/ircd/m/room.h

@@ -51,6 +51,12 @@ namespace ircd::m
 	int64_t depth(std::nothrow_t, const id::room &);
 	int64_t depth(const id::room &);
 
+	// [GET] Count the events in the room between two (note always >=1 if a!=b)
+	size_t count_since(const room &, const m::event::idx &, const m::event::idx &);
+	size_t count_since(const room &, const m::event::id &, const m::event::id &);
+	size_t count_since(const m::event::idx &, const m::event::idx &);
+	size_t count_since(const m::event::id &, const m::event::id &);
+
 	// [SET] Lowest-level
 	event::id::buf commit(const room &, json::iov &event, const json::iov &content);
 

ircd/m/m.cc

@@ -2702,6 +2702,52 @@ ircd::m::commit(const room &room,
 	return eval.event_id;
 }
 
+size_t
+ircd::m::count_since(const m::event::id &a,
+                     const m::event::id &b)
+{
+	return count_since(index(a), index(b));
+}
+
+size_t
+ircd::m::count_since(const m::event::idx &a,
+                     const m::event::idx &b)
+{
+	// Get the room_id from b here; a might not be in the same room but downstream
+	// the counter seeks to a in the given room and will properly fail there.
+	room::id::buf room_id
+	{
+		m::get(std::max(a, b), "room_id", room_id)
+	};
+
+	return count_since(room_id, a, b);
+}
+
+size_t
+ircd::m::count_since(const room &room,
+                     const m::event::id &a,
+                     const m::event::id &b)
+{
+	return count_since(room, index(a), index(b));
+}
+
+size_t
+ircd::m::count_since(const room &r,
+                     const event::idx &a,
+                     const event::idx &b)
+{
+	using prototype = bool (const room &,
+	                        const event::idx &,
+	                        const event::idx &);
+
+	static m::import<prototype> _count_since
+	{
+		"m_room", "count_since"
+	};
+
+	return _count_since(r, std::min(a, b), std::max(a, b));
+}
+
 ircd::m::id::room::buf
 ircd::m::room_id(const id::room_alias &room_alias)
 {

modules/m_room.cc

@@ -16,6 +16,37 @@ IRCD_MODULE
 	"Matrix state library; modular components."
 };
 
+extern "C" size_t
+count_since(const m::room &room,
+            const m::event::idx &a,
+            const m::event::idx &b)
+{
+	assert(a <= b);
+	m::room::messages it{room};
+	it.seek_idx(a);
+
+	if(!it && !m::exists(room))
+		throw m::NOT_FOUND
+		{
+			"Cannot find room '%s' to count events in",
+			string_view{room.room_id}
+		};
+	else if(!it)
+		throw m::NOT_FOUND
+		{
+			"Event @ idx:%lu or idx:%lu not found in room '%s' or at all",
+			a,
+			b,
+			string_view{room.room_id}
+		};
+
+	size_t ret{0};
+	// Hit the iterator once first otherwise the count will always increment
+	// to `1` erroneously when it ought to show `0`.
+	for(++it; it && it.event_idx() < b; ++it, ++ret);
+	return ret;
+}
+
 extern "C" bool
 random_origin(const m::room &room,
               const m::room::origins::closure &view,