local log = require "util.logger".init("util.async");

local new_id = require "util.id".short;

local function checkthread()

	local thread, main = coroutine.running();

	if not thread or main then

		error("Not running in an async context, see https://prosody.im/doc/developers/util/async");

	end

	return thread;

end

local function runner_continue(thread)

	-- ASSUMPTION: runner is in 'waiting' state (but we don't have the runner to know for sure)

	if coroutine.status(thread) ~= "suspended" then -- This should suffice

		log("warn", "unexpected async state: thread not suspended");

		return false;

	end

	local ok, state, runner = coroutine.resume(thread);

	if not ok then

		local err = state;

		-- Running the coroutine failed, which means we have to find the runner manually,

		-- in order to inform the error handler

		local level = 0;

		-- Find the 'level' of the top-most function (0 == current level, 1 == caller, ...)

		while debug.getinfo(thread, level, "") do level = level + 1; end

		ok, runner = debug.getlocal(thread, level-1, 1);

		if ok ~= "self" or runner.thread ~= thread then

			log("warn", "unexpected async state: unable to locate runner during error handling, got %s", ok);

			return false;

		end

		local error_handler = runner.watchers.error;

		if error_handler then error_handler(runner, debug.traceback(thread, err)); end

		local ready_handler = runner.watchers.ready;

		runner.state, runner.thread = "ready", nil;

		if ready_handler then ready_handler(runner); end

		runner.notified_state = "ready";

	elseif state == "ready" then

		-- If state is 'ready', it is our responsibility to update runner.state from 'waiting'.

		-- We also have to :run(), because the queue might have further items that will not be

		-- processed otherwise. FIXME: It's probably best to do this in a nexttick (0 timer).

		runner.state = "ready";

		runner:run();

	end

	return true;

end

local function waiter(num)

	local thread = checkthread();

	num = num or 1;

	local waiting;

	return function ()

		if num == 0 then return; end -- already done

		waiting = true;

		coroutine.yield("wait");

	end, function ()

		num = num - 1;

		if num == 0 and waiting then

			runner_continue(thread);

		elseif num < 0 then

			error("done() called too many times");

		end

	end;

end

local function guarder()

	local guards = {};

	return function (id, func)

		local thread = checkthread();

		local guard = guards[id];

		if not guard then

			guard = {};

			guards[id] = guard;

			log("debug", "New guard!");

		else

			table.insert(guard, thread);

			log("debug", "Guarded. %d threads waiting.", #guard)

			coroutine.yield("wait");

		end

		local function exit()

			local next_waiting = table.remove(guard, 1);

			if next_waiting then

				log("debug", "guard: Executing next waiting thread (%d left)", #guard)

				runner_continue(next_waiting);

			else

				log("debug", "Guard off duty.")

				guards[id] = nil;

			end

		end

		if func then

			func();

			exit();

			return;

		end

		return exit;

	end;

end

local runner_mt = {};

runner_mt.__index = runner_mt;

local function runner_create_thread(func, self)

	local thread = coroutine.create(function (self) -- luacheck: ignore 432/self

		while true do

			func(coroutine.yield("ready", self));

		end

	end);

	assert(coroutine.resume(thread, self)); -- Start it up, it will return instantly to wait for the first input

	return thread;

end

local empty_watchers = {};

local function runner(func, watchers, data)

	return setmetatable({ func = func, thread = false, state = "ready", notified_state = "ready",

		queue = {}, watchers = watchers or empty_watchers, data = data, id = new_id() }

	, runner_mt);

end

-- Add a task item for the runner to process

function runner_mt:run(input)

	if input ~= nil then

		table.insert(self.queue, input);

		self:log("debug", "queued new work item, %d items queued", #self.queue);

	end

	if self.state ~= "ready" then

		-- The runner is busy. Indicate that the task item has been

		-- queued, and return information about the current runner state

		return true, self.state, #self.queue;

	end

	local q, thread = self.queue, self.thread;

	if not thread or coroutine.status(thread) == "dead" then

		self:log("debug", "creating new coroutine");

		-- Create a new coroutine for this runner

		thread = runner_create_thread(self.func, self);

		self.thread = thread;

	end

	-- Process task item(s) while the queue is not empty, and we're not blocked

	local n, state, err = #q, self.state, nil;

	self.state = "running";

	self:log("debug", "running main loop");

	while n > 0 and state == "ready" and not err do

		local consumed;

		-- Loop through queue items, and attempt to run them

		for i = 1,n do

			local queued_input = q[i];

			local ok, new_state = coroutine.resume(thread, queued_input);

			if not ok then

				-- There was an error running the coroutine, save the error, mark runner as ready to begin again

				consumed, state, err = i, "ready", debug.traceback(thread, new_state);

				self.thread = nil;

				break;

			elseif new_state == "wait" then

				 -- Runner is blocked on waiting for a task item to complete

				consumed, state = i, "waiting";

				break;

			end

		end

		-- Loop ended - either queue empty because all tasks passed without blocking (consumed == nil)

		-- or runner is blocked/errored, and consumed will contain the number of tasks processed so far

		if not consumed then consumed = n; end

		-- Remove consumed items from the queue array

		if q[n+1] ~= nil then

			n = #q;

		end

		for i = 1, n do

			q[i] = q[consumed+i];

		end

		n = #q;

	end

	-- Runner processed all items it can, so save current runner state

	self.state = state;

	if err or state ~= self.notified_state then

		self:log("debug", "changed state from %s to %s", self.notified_state, err and ("error ("..state..")") or state);

		if err then

			state = "error"

		else

			self.notified_state = state;

		end

		local handler = self.watchers[state];

		if handler then handler(self, err); end

	end

	return true, state, n;

end

-- Add a task item to the queue without invoking the runner, even if it is idle

function runner_mt:enqueue(input)

	table.insert(self.queue, input);

	self:log("debug", "queued new work item, %d items queued", #self.queue);

end

function runner_mt:log(level, fmt, ...)

	return log(level, "[runner %s] "..fmt, self.id, ...);

end

return { waiter = waiter, guarder = guarder, runner = runner };