local adns = require "net.adns";

local basic = require "net.resolvers.basic";

local inet_pton = require "util.net".pton;

local idna_to_ascii = require "util.encodings".idna.to_ascii;

local methods = {};

local resolver_mt = { __index = methods };

local function new_target_selector(rrset)

	local rr_count = rrset and #rrset;

	if not rr_count or rr_count == 0 then

		rrset = nil;

	else

		table.sort(rrset, function (a, b) return a.srv.priority < b.srv.priority end);

	end

	local rrset_pos = 1;

	local priority_bucket, bucket_total_weight, bucket_len, bucket_used;

	return function ()

		if not rrset then return; end

		if not priority_bucket or bucket_used >= bucket_len then

			if rrset_pos > rr_count then return; end -- Used up all records

			-- Going to start on a new priority now. Gather up all the next

			-- records with the same priority and add them to priority_bucket

			priority_bucket, bucket_total_weight, bucket_len, bucket_used = {}, 0, 0, 0;

			local current_priority;

			repeat

				local curr_record = rrset[rrset_pos].srv;

				if not current_priority then

					current_priority = curr_record.priority;

				elseif current_priority ~= curr_record.priority then

					break;

				end

				table.insert(priority_bucket, curr_record);

				bucket_total_weight = bucket_total_weight + curr_record.weight;

				bucket_len = bucket_len + 1;

				rrset_pos = rrset_pos + 1;

			until rrset_pos > rr_count;

		end

		bucket_used = bucket_used + 1;

		local n, running_total = math.random(0, bucket_total_weight), 0;

		local target_record;

		for i = 1, bucket_len do

			local candidate = priority_bucket[i];

			if candidate then

				running_total = running_total + candidate.weight;

				if running_total >= n then

					target_record = candidate;

					bucket_total_weight = bucket_total_weight - candidate.weight;

					priority_bucket[i] = nil;

					break;

				end

			end

		end

		return target_record;

	end;

end

-- Find the next target to connect to, and

-- pass it to cb()

function methods:next(cb)

	if self.resolver or self._get_next_target then

		if not self.resolver then -- Do we have a basic resolver currently?

			-- We don't, so fetch a new SRV target, create a new basic resolver for it

			local next_srv_target = self._get_next_target and self._get_next_target();

			if not next_srv_target then

				-- No more SRV targets left

				cb(nil);

				return;

			end

			-- Create a new basic resolver for this SRV target

			self.resolver = basic.new(next_srv_target.target, next_srv_target.port, self.conn_type, self.extra);

		end

		-- Look up the next (basic) target from the current target's resolver

		self.resolver:next(function (...)

			if self.resolver then

				self.last_error = self.resolver.last_error;

			end

			if ... == nil then

				self.resolver = nil;

				self:next(cb);

			else

				cb(...);

			end

		end);

		return;

	elseif self.in_progress then

		cb(nil);

		return;

	end

	if not self.hostname then

		self.last_error = "hostname failed IDNA";

		cb(nil);

		return;

	end

	self.in_progress = true;

	local function ready()

		self:next(cb);

	end

	-- Resolve DNS to target list

	local dns_resolver = adns.resolver();

	dns_resolver:lookup(function (answer, err)

		if not answer and not err then

			-- net.adns returns nil if there are zero records or nxdomain

			answer = {};

		end

		if answer then

				self.last_error = "Validation error in SRV lookup";

				ready();

				return;

			end

			if #answer == 0 then

				if self.extra and self.extra.default_port then

					self.resolver = basic.new(self.hostname, self.extra.default_port, self.conn_type, self.extra);

				else

					self.last_error = "zero SRV records found";

				end

				ready();

				return;

			end

			if #answer == 1 and answer[1].srv.target == "." then -- No service here

				self.last_error = "service explicitly unavailable";

				ready();

				return;

			end

			self._get_next_target = new_target_selector(answer);

		else

			self.last_error = err;

		end

		ready();

	end, "_" .. self.service .. "._" .. self.conn_type .. "." .. self.hostname, "SRV", "IN");

end

local function new(hostname, service, conn_type, extra)

	local is_ip = inet_pton(hostname);

	if not is_ip and hostname:sub(1,1) == '[' then

		is_ip = inet_pton(hostname:sub(2,-2));

	end

	if is_ip and extra and extra.default_port then

		return basic.new(hostname, extra.default_port, conn_type, extra);

	end

	return setmetatable({

		hostname = idna_to_ascii(hostname);

		service = service;

		conn_type = conn_type or "tcp";

		extra = extra;

	}, resolver_mt);

end

return {

	new = new;

};