--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/teal-src/prosody/util/smqueue.tl	Thu Mar 23 13:36:52 2023 +0100
@@ -0,0 +1,99 @@
+local queue = require "prosody.util.queue";
+
+local record lib
+	-- T would typically be util.stanza
+	record smqueue<T>
+		_queue : queue.queue<T>
+		_head : integer
+		_tail : integer
+
+		enum ack_errors
+			"tail"
+			"head"
+			"pop"
+		end
+		push : function (smqueue, T)
+		ack : function (smqueue, integer) : { T }, ack_errors
+		resumable : function (smqueue<T>) : boolean
+		resume : function (smqueue<T>)  : queue.queue.iterator, any, integer
+		type consume_iter = function (smqueue<T>) : T
+		consume : function (smqueue<T>) : consume_iter
+
+		table : function (smqueue<T>) : { T }
+	end
+	new : function <T>(integer) : smqueue<T>
+end
+
+local type smqueue = lib.smqueue;
+
+function smqueue:push(v)
+	self._head = self._head + 1;
+	-- Wraps instead of errors
+	assert(self._queue:push(v));
+end
+
+function smqueue:ack(h : integer) : { any }, smqueue.ack_errors
+	if h < self._tail then
+		return nil, "tail";
+	elseif h > self._head then
+		return nil, "head";
+	end
+	-- TODO optimize? cache table fields
+	local acked = {};
+	self._tail = h;
+	local expect = self._head - self._tail;
+	while expect < self._queue:count() do
+		local v = self._queue:pop();
+		if not v then return nil, "pop"; end
+		table.insert(acked, v);
+	end
+	return acked;
+end
+
+function smqueue:count_unacked() : integer
+	return self._head - self._tail;
+end
+
+function smqueue:count_acked() : integer
+	return self._tail;
+end
+
+function smqueue:resumable() : boolean
+	return self._queue:count() >= (self._head - self._tail);
+end
+
+function smqueue:resume() : queue.queue.iterator, any, integer
+	return self._queue:items();
+end
+
+function smqueue:consume() : queue.queue.consume_iter
+	return self._queue:consume()
+end
+
+-- Compatibility layer, plain ol' table
+function smqueue:table() : { any }
+	local t : { any } = {};
+	for i, v in self:resume() do
+		t[i] = v;
+	end
+	return t;
+end
+
+local function freeze(q : smqueue<any>) : { string:integer }
+	return { head = q._head, tail = q._tail }
+end
+
+local queue_mt = {
+	--
+	__name = "smqueue";
+	__index = smqueue;
+	__len = smqueue.count_unacked;
+	__freeze = freeze;
+}
+
+function lib.new<T>(size : integer) : queue.queue<T>
+	assert(size>0);
+	return setmetatable({ _head = 0; _tail = 0; _queue = queue.new(size, true) }, queue_mt);
+end
+
+return lib;