ircd:Ⓜ️:node: Start preliminary mitsein interface.

include/ircd/m/node.h

@@ -38,6 +38,7 @@ struct ircd::m::node
 {
 	struct room;
 	struct keys;
+	struct mitsein;
 
 	string_view node_id;
 
@@ -77,6 +78,29 @@ struct ircd::m::node::keys
 	{}
 };
 
+/// Interface to the other nodes visible to a node from common rooms.
+struct ircd::m::node::mitsein
+{
+	using closure = std::function<bool (const string_view &, const m::room &)>;
+
+	m::node node;
+
+  public:
+	// All common rooms with node
+	bool for_each(const m::node &, const string_view &membership, const closure &) const;
+	bool for_each(const m::node &, const closure &) const;
+
+	// Counting convenience
+	size_t count(const m::node &, const string_view &membership = {}) const;
+
+	// Existential convenience (does `node` and `other` share any common room).
+	bool has(const m::node &other, const string_view &membership = {}) const;
+
+	mitsein(const m::node &node)
+	:node{node}
+	{}
+};
+
 inline
 ircd::m::node::node(const string_view &node_id)
 :node_id

matrix/node.cc

@@ -146,3 +146,53 @@ const
 		closure(key);
 	});
 }
+
+//
+// node::mitsein
+//
+
+bool
+ircd::m::node::mitsein::has(const m::node &other,
+                            const string_view &membership)
+const
+{
+	return !for_each(other, membership, []
+	(const auto &, const auto &)
+	{
+		return false;
+	});
+}
+
+size_t
+ircd::m::node::mitsein::count(const m::node &other,
+                              const string_view &membership)
+const
+{
+	size_t ret(0);
+	for_each(other, membership, [&ret]
+	(const auto &, const auto &)
+	{
+		++ret;
+		return true;
+	});
+
+	return ret;
+}
+
+bool
+ircd::m::node::mitsein::for_each(const m::node &other,
+                                 const closure &closure)
+const
+{
+	return for_each(other, string_view{}, closure);
+}
+
+bool
+ircd::m::node::mitsein::for_each(const m::node &other,
+                                 const string_view &membership,
+                                 const closure &closure)
+const
+{
+	always_assert(0);
+	return true;
+}