local _M = { }

_M.source , _M.resolution , _M.gettime = (function()
	local has_syscall , syscall = pcall ( require , "syscall" )
	if has_syscall and syscall.clock_gettime and syscall.c.CLOCK then
		local clock_id = syscall.c.CLOCK.REALTIME
		local function timespec_to_number ( timespec )
			return tonumber ( timespec.tv_sec ) + tonumber ( timespec.tv_nsec ) * 1e-9
		end
		return "syscall.clock_gettime(CLOCK_REALTIME)" ,
			syscall.clock_getres and timespec_to_number ( syscall.clock_getres ( clock_id ) ) or 1e-9 ,
			function ( )
				return timespec_to_number ( syscall.clock_gettime ( clock_id ) )
			end
	end

	local has_unix , unix = pcall ( require , "unix" )
	-- On Apple devices lunix only uses gettimeofday()
	if has_unix and unix.clock_gettime and unix.uname and unix.uname().sysname ~= "Darwin" then
		return "unix.clock_gettime(CLOCK_REALTIME)" , 1e-9 , function ( )
			return unix.clock_gettime ( )
		end
	end

	if has_syscall and syscall.gettimeofday then
		local function timeval_to_number ( timeval )
			return tonumber ( timeval.tv_sec ) + tonumber ( timeval.tv_nsec ) * 1e-6
		end
		return "syscall.gettimeofday()" , 1e-6 ,
			function ( )
				return timeval_to_number ( syscall.gettimeofday ( ) )
			end
	end

	if has_unix and unix.gettimeofday then
		return "unix.gettimeofday()", 1e-6, unix.gettimeofday
	end

	local has_socket , socket = pcall ( require , "socket" )
	if has_socket and socket.gettime then
		-- on windows, this uses GetSystemTimeAsFileTime, which has resolution of 1e-7
		-- on linux, this uses gettimeofday, which has resolution of 1e-6
		return "socket.gettime()" , 1e-6 , socket.gettime
	end

	if ngx and ngx.now then -- luacheck: ignore 113
		return "ngx.now()" , 1e-3 , ngx.now -- luacheck: ignore 113
	end

	return "os.time()" , 1 , os.time
end)()

return _M