--[[
	lua dns resolver library
	See  https://github.com/xjdrew/levent/blob/master/levent/dns.lua for more detail

-- resource record type:
-- TYPE            value and meaning
-- A               1 a host address
-- NS              2 an authoritative name server
-- MD              3 a mail destination (Obsolete - use MX)
-- MF              4 a mail forwarder (Obsolete - use MX)
-- CNAME           5 the canonical name for an alias
-- SOA             6 marks the start of a zone of authority
-- MB              7 a mailbox domain name (EXPERIMENTAL)
-- MG              8 a mail group member (EXPERIMENTAL)
-- MR              9 a mail rename domain name (EXPERIMENTAL)
-- NULL            10 a null RR (EXPERIMENTAL)
-- WKS             11 a well known service description
-- PTR             12 a domain name pointer
-- HINFO           13 host information
-- MINFO           14 mailbox or mail list information
-- MX              15 mail exchange
-- TXT             16 text strings
-- AAAA            28 a ipv6 host address
-- only appear in the question section:
-- AXFR            252 A request for a transfer of an entire zone
-- MAILB           253 A request for mailbox-related records (MB, MG or MR)
-- MAILA           254 A request for mail agent RRs (Obsolete - see MX)
-- *               255 A request for all records
--
-- resource recode class:
-- IN              1 the Internet
-- CS              2 the CSNET class (Obsolete - used only for examples in some obsolete RFCs)
-- CH              3 the CHAOS class
-- HS              4 Hesiod [Dyer 87]
-- only appear in the question section:
-- *               255 any class
-- ]]

--[[
-- struct header {
--  uint16_t tid     # identifier assigned by the program that generates any kind of query.
--  uint16_t flags   # flags
--  uint16_t qdcount # the number of entries in the question section.
--  uint16_t ancount # the number of resource records in the answer section.
--  uint16_t nscount # the number of name server resource records in the authority records section.
--  uint16_t arcount # the number of resource records in the additional records section.
-- }
--
-- request body:
-- struct request {
--  string name
--  uint16_t atype
--  uint16_t class
-- }
--
-- response body:
-- struct response {
--  string name
--  uint16_t atype
--  uint16_t class
--  uint16_t ttl
--  uint16_t rdlength
--  string rdata
-- }
--]]

local skynet = require "skynet"
local socket = require "socket"

local MAX_DOMAIN_LEN = 1024
local MAX_LABEL_LEN = 63
local MAX_PACKET_LEN = 2048
local DNS_HEADER_LEN = 12

local QTYPE = {
	A = 1,
	CNAME = 5,
	AAAA = 28,
}

local QCLASS = {
	IN = 1,
}

local dns = {}

local function verify_domain_name(name)
	if #name > MAX_DOMAIN_LEN then
		return false
	end
	if not name:match("^[%l%d-%.]+$") then
		return false
	end
	for w in name:gmatch("([%w-]+)%.?") do
		if #w > MAX_LABEL_LEN then
			return false
		end
	end
	return true
end

local next_tid = 1
local function gen_tid()
	local tid = next_tid
	next_tid = next_tid + 1
	return tid
end

local function pack_header(t)
	return string.pack(">HHHHHH",
		t.tid, t.flags, t.qdcount, t.ancount or 0, t.nscount or 0, t.arcount or 0)
end

local function pack_question(name, qtype, qclass)
	local labels = {}
	for w in name:gmatch("([%w-]+)%.?") do
		table.insert(labels, string.pack("s1",w))
	end
	table.insert(labels, '\0')
	table.insert(labels, string.pack(">HH", qtype, qclass))
	return table.concat(labels)
end

local function unpack_header(chunk)
	local tid, flags, qdcount, ancount, nscount, arcount, left = string.unpack(">HHHHHH", chunk)
	return {
		tid = tid,
		flags = flags,
		qdcount = qdcount,
		ancount = ancount,
		nscount = nscount,
		arcount = arcount
	}, left
end

-- unpack a resource name
local function unpack_name(chunk, left)
	local t = {}
	local jump_pointer
	local tag, offset, label
	while true do
		tag, left = string.unpack("B", chunk, left)
		if tag & 0xc0 == 0xc0 then
			-- pointer
			offset,left = string.unpack(">H", chunk, left - 1)
			offset = offset & 0x3fff
			if not jump_pointer then
				jump_pointer = left
			end
			-- offset is base 0, need to plus 1
			left = offset + 1
		elseif tag == 0 then
			break
		else
			label, left = string.unpack("s1", chunk, left - 1)
			t[#t+1] = label
		end
	end
	return table.concat(t, "."), jump_pointer or left
end

local function unpack_question(chunk, left)
	local name, left = unpack_name(chunk, left)
	local atype, class, left = string.unpack(">HH", chunk, left)
	return {
		name = name,
		atype = atype,
		class = class
	}, left
end

local function unpack_answer(chunk, left)
	local name, left = unpack_name(chunk, left)
	local atype, class, ttl, rdata, left = string.unpack(">HHI4s2", chunk, left)
	return {
		name = name,
		atype = atype,
		class = class,
		ttl = ttl,
		rdata = rdata
	},left
end

local function unpack_rdata(qtype, chunk)
	if qtype == QTYPE.A then
		local a,b,c,d = string.unpack("BBBB", chunk)
		return string.format("%d.%d.%d.%d", a,b,c,d)
	elseif qtype == QTYPE.AAAA then
		local a,b,c,d,e,f,g,h = string.unpack(">HHHHHHHH", chunk)
		return string.format("%x:%x:%x:%x:%x:%x:%x:%x", a, b, c, d, e, f, g, h)
	else
		error("Error qtype " .. qtype)
	end
end

local dns_server
local request_pool = {}

local function resolve(content)
	if #content < DNS_HEADER_LEN then
		-- drop
		skynet.error("Recv an invalid package when dns query")
		return
	end
	local answer_header,left = unpack_header(content)
	-- verify answer
	assert(answer_header.qdcount == 1, "malformed packet")

	local resp = request_pool[answer_header.tid]
	if not resp then
		skynet.error("Recv an invalid tid when dns query")
		return
	end

	local question,left = unpack_question(content, left)
	if question.name ~= resp.name then
		skynet.error("Recv an invalid name when dns query")
		return
	end

	local ttl
	local answer
	local answers = {}
	for i=1, answer_header.ancount do
		answer, left = unpack_answer(content, left)
		-- only extract qtype address
		if answer.atype == resp.qtype then
			local ip = unpack_rdata(resp.qtype, answer.rdata)
			ttl = ttl and math.min(ttl, answer.ttl) or answer.ttl
			answers[#answers+1] = ip
		end
	end

	if #answers > 0 then
		resp.answers = answers
	end

	skynet.wakeup(resp.co)
end

function dns.server(server, port)
	if not server then
		local f = assert(io.open "/etc/resolv.conf")
		for line in f:lines() do
			server = line:match("%s*nameserver%s+([^%s]+)")
			if server then
				break
			end
		end
		assert(server, "Can't get nameserver")
	end
	dns_server = socket.udp(function(str, from)
		resolve(str)
	end)
	socket.udp_connect(dns_server, server, port or 53)
	return server
end

local function suspend(tid, name, qtype)
	local req = {
		name = name,
		tid = tid,
		qtype = qtype,
		time = skynet.now(),	-- for timeout
		co = coroutine.running(),
	}
	request_pool[tid] = req
	skynet.wait()
	local answers = request_pool[tid].answers
	request_pool[tid] = nil
	assert(answers, "no ip")
	return answers[1], answers
end

function dns.resolve(name, ipv6)
	local qtype = ipv6 and QTYPE.AAAA or QTYPE.A
	local name = name:lower()
	assert(verify_domain_name(name) , "illegal name")
	local question_header = {
		tid = gen_tid(),
		flags = 0x100, -- flags: 00000001 00000000, set RD
		qdcount = 1,
	}
	local req = pack_header(question_header) .. pack_question(name, qtype, QCLASS.IN)
	assert(dns_server, "Call dns.server fist")
	socket.write(dns_server, req)
	return suspend(question_header.tid, name, qtype)
end

return dns