module:set_global();

local async = require "util.async";
local datetime = require "util.datetime";

local record map_store<K,V>
	-- TODO move to somewhere sensible
	get : function (map_store<K,V>, string, K) : V
	set : function (map_store<K,V>, string, K, V)
end

local enum frequency
	"hourly"
	"daily"
	"weekly"
end

local record task_spec
	id : string -- unique id
	name : string -- name or short description
	when : frequency
	last : integer
	run : function (task_spec, integer)
	save : function (task_spec, integer)
end

local record task_event
	source : module
	item : task_spec
end

local periods : { frequency : integer } = { hourly = 3600, daily = 86400, weekly = 7*86400 }

local active_hosts : { string : boolean } = {  }

function module.add_host(host_module : moduleapi)

	local last_run_times = host_module:open_store("cron", "map") as map_store<string,integer>;
	active_hosts[host_module.host] = true;

	local function save_task(task : task_spec, started_at : integer)
		last_run_times:set(nil, task.id, started_at);
	end

	local function task_added(event : task_event) : boolean
		local task = event.item;
		if task.name == nil then
			task.name = task.when;
		end
		if task.id == nil then
			task.id = event.source.name .. "/" .. task.name:gsub("%W", "_"):lower();
		end
		if task.last == nil then
			task.last = last_run_times:get(nil, task.id);
		end
		task.save = save_task;
		module:log("debug", "%s task %s added, last run %s", task.when, task.id,
			task.last and datetime.datetime(task.last) or "never");
		if task.last == nil then
			-- initialize new tasks so e.g. daily tasks run at ~midnight UTC for now
			local now = os.time();
			task.last = now - now % periods[task.when];
		end
		return true;
	end

	local function task_removed(event : task_event) : boolean
		local task = event.item;
		host_module:log("debug", "Task %s removed", task.id);
		return true;
	end

	host_module:handle_items("task", task_added, task_removed, true);

	function host_module.unload()
		active_hosts[host_module.host]=nil;
	end
end

local function should_run(when : frequency, last : integer) : boolean
	return not last or last + periods[when]*0.995 <= os.time();
end

local function run_task(task : task_spec)
	local started_at = os.time();
	task:run(started_at);
	task.last = started_at;
	task:save(started_at);
end

local task_runner = async.runner(run_task);
module:add_timer(1, function() : integer
	module:log("info", "Running periodic tasks");
	local delay = 3600;
	for host in pairs(active_hosts) do
		module:log("debug", "Running periodic tasks for host %s", host);
		for _, task in ipairs(module:context(host):get_host_items("task") as { task_spec } ) do
			module:log("debug", "Considering %s task %s (%s)", task.when, task.id, task.run);
			if should_run(task.when, task.last) then task_runner:run(task); end
		end
	end
	module:log("debug", "Wait %ds", delay);
	return delay;
end);

-- TODO measure load, pick a good time to do stuff