local tz_info_mt = require "luatz.tzinfo".tz_info_mt
local tt_info_mt = require "luatz.tzinfo".tt_info_mt


local function read_int32be ( fd )
	local data , err = fd:read ( 4 )
	if data == nil then return nil , err end
	local o1 , o2 , o3 , o4 = data:byte ( 1 , 4 )

	local unsigned = o4 + o3*2^8 + o2*2^16 + o1*2^24
	if unsigned >= 2^31 then
		return unsigned - 2^32
	else
		return unsigned
	end
end

local function read_int64be ( fd )
	local data , err = fd:read ( 8 )
	if data == nil then return nil , err end
	local o1 , o2 , o3 , o4 , o5 , o6 , o7 , o8 = data:byte ( 1 , 8 )

	local unsigned = o8 + o7*2^8 + o6*2^16 + o5*2^24 + o4*2^32 + o3*2^40 + o2*2^48 + o1*2^56
	if unsigned >= 2^63 then
		return unsigned - 2^64
	else
		return unsigned
	end
end

local function read_flags ( fd , n )
	local data , err = fd:read ( n )
	if data == nil then return nil , err end

	local res = { }
	for i=1, n do
		res[i] = data:byte(i,i) ~= 0
	end
	return res
end

local fifteen_nulls = ("\0"):rep(15)
local function read_tz ( fd )
	assert ( fd:read(4) == "TZif" , "Invalid TZ file" )
	local version = assert ( fd:read(1) )
	if version == "\0" or version == "2" then
		local MIN_TIME = -2^32+1

		assert ( assert ( fd:read(15) ) == fifteen_nulls , "Expected 15 nulls" )

		-- The number of UTC/local indicators stored in the file.
		local tzh_ttisgmtcnt = assert ( read_int32be ( fd ) )

		-- The number of standard/wall indicators stored in the file.
		local tzh_ttisstdcnt = assert ( read_int32be ( fd ) )

		-- The number of leap seconds for which data is stored in the file.
		local tzh_leapcnt = assert ( read_int32be ( fd ) )

		-- The number of "transition times" for which data is stored in the file.
		local tzh_timecnt = assert ( read_int32be ( fd ) )

		-- The number of "local time types" for which data is stored in the file (must not be zero).
		local tzh_typecnt = assert ( read_int32be ( fd ) )

		-- The number of characters of "timezone abbreviation strings" stored in the file.
		local tzh_charcnt = assert ( read_int32be ( fd ) )

		local transition_times = { }
		for i=1, tzh_timecnt do
			transition_times [ i ] = assert ( read_int32be ( fd ) )
		end
		local transition_time_ind = { assert ( fd:read ( tzh_timecnt ) ):byte ( 1 , -1 ) }

		local ttinfos = { }
		for i=1, tzh_typecnt do
			ttinfos [ i ] = {
				gmtoff = assert ( read_int32be ( fd ) ) ;
				isdst  = assert ( fd:read ( 1 ) ) ~= "\0" ;
				abbrind = assert ( fd:read ( 1 ) ):byte ( ) ;
			}
		end

		local abbreviations = assert ( fd:read ( tzh_charcnt ) )

		local leap_seconds = { }
		for i=1, tzh_leapcnt do
			leap_seconds [ i ] = {
				offset = assert ( read_int32be ( fd ) ) ;
				n = assert ( read_int32be ( fd ) ) ;
			}
		end

		local isstd = assert ( read_flags ( fd , tzh_ttisstdcnt ) )

		local isgmt = assert ( read_flags ( fd , tzh_ttisgmtcnt ) )

		local TZ

		if version == "2" then
			--[[
			For version-2-format timezone files, the above header and data is followed by a second header and data,
			identical in format except that eight bytes are used for each transition time or leap-second time.
			]]
			assert ( fd:read(5) == "TZif2" )
			assert ( assert ( fd:read(15) ) == fifteen_nulls , "Expected 15 nulls" )

			MIN_TIME = -2^64+1

			-- The number of UTC/local indicators stored in the file.
			tzh_ttisgmtcnt = assert ( read_int32be ( fd ) )

			-- The number of standard/wall indicators stored in the file.
			tzh_ttisstdcnt = assert ( read_int32be ( fd ) )

			-- The number of leap seconds for which data is stored in the file.
			tzh_leapcnt = assert ( read_int32be ( fd ) )

			-- The number of "transition times" for which data is stored in the file.
			tzh_timecnt = assert ( read_int32be ( fd ) )

			-- The number of "local time types" for which data is stored in the file (must not be zero).
			tzh_typecnt = assert ( read_int32be ( fd ) )

			-- The number of characters of "timezone abbreviation strings" stored in the file.
			tzh_charcnt = assert ( read_int32be ( fd ) )

			transition_times = { }
			for i=1, tzh_timecnt do
				transition_times [ i ] = assert ( read_int64be ( fd ) )
			end
			transition_time_ind = { assert ( fd:read ( tzh_timecnt ) ):byte ( 1 , -1 ) }

			ttinfos = { }
			for i=1, tzh_typecnt do
				ttinfos [ i ] = {
					gmtoff = assert ( read_int32be ( fd ) ) ;
					isdst  = assert ( fd:read ( 1 ) ) ~= "\0" ;
					abbrind = assert ( fd:read ( 1 ) ):byte ( ) ;
				}
			end

			abbreviations = assert ( fd:read ( tzh_charcnt ) )

			leap_seconds = { }
			for i=1, tzh_leapcnt do
				leap_seconds [ i ] = {
					offset = assert ( read_int64be ( fd ) ) ;
					n = assert ( read_int32be ( fd ) ) ;
				}
			end

			isstd = assert ( read_flags ( fd , tzh_ttisstdcnt ) )

			isgmt = assert ( read_flags ( fd , tzh_ttisgmtcnt ) )

			--[[
			After the second header and data comes a newline-enclosed, POSIX-TZ-environment-variable-style string
			for use in handling instants after the last transition time stored in the file
			(with nothing between the newlines if there is no POSIX representation for such instants).
			]]
			assert ( assert ( fd:read ( 1 ) ) == "\n" , "Expected newline at end of version 2 header" )

			TZ = assert ( fd:read ( "*l" ) )
			if #TZ == 0 then
				TZ = nil
			end
		end

		for i=1, tzh_typecnt do
			local v = ttinfos [ i ]
			v.abbr = abbreviations:sub ( v.abbrind+1 , v.abbrind+3 )
			v.isstd = isstd [ i ] or false
			v.isgmt = isgmt [ i ] or false
			setmetatable ( v , tt_info_mt )
		end

		--[[
		Use the first standard-time ttinfo structure in the file
		(or simply the first ttinfo structure in the absence of a standard-time structure)
		if either tzh_timecnt is zero or the time argument is less than the first transition time recorded in the file.
		]]
		local first = 1
		do
			for i=1, tzh_ttisstdcnt do
				if isstd[i] then
					first = i
					break
				end
			end
		end

		local res = {
			[0] = {
				transition_time = MIN_TIME ;
				info = ttinfos [ first ] ;
			}
		}
		for i=1, tzh_timecnt do
			res [ i ] = {
				transition_time = transition_times [ i ] ;
				info = ttinfos [ transition_time_ind [ i ]+1 ] ;
			}
		end
		return setmetatable ( res , tz_info_mt )
	else
		error ( "Unsupported version" )
	end
end

local function read_tzfile ( path )
	local fd = assert ( io.open ( path , "rb" ) )
	local tzinfo = read_tz ( fd )
	fd:close ( )
	return tzinfo
end

return {
	read_tz = read_tz ;
	read_tzfile = read_tzfile ;
}