datagram.lua

local binary = require 'shared.binary'

-- Datagram format
-- 
--  uint32 seq            Sequence number of the packet
--  uint32 ack            Last acknowledged sequence number of remote channel
--  uint16 fragment_ofs   Offset of fragment
--  uint16 fragment_len   Length of fragment, zero if single message
--  uint8  cmd            Command (see command.lua)
--- binary data           Arbitrary data


-- Packet settings
local UDP_PACKETSIZE = 1500
local HEADER_SIZE = 13
local BODY_SIZE = UDP_PACKETSIZE - 13

local datagram = {
    BODY_SIZE = BODY_SIZE
}

function datagram.pack(seq, ack, fragment_ofs, fragment_len, cmd, data)
    local buf = {}
    table.insert(buf, binary.encode_uint32(seq))
    table.insert(buf, binary.encode_uint32(ack))
    table.insert(buf, binary.encode_uint16(fragment_ofs))
    table.insert(buf, binary.encode_uint16(fragment_len))
    table.insert(buf, binary.encode_uint8(cmd))
    table.insert(buf, data)
    return table.concat(buf)
end

function datagram.unpack(datagram)
    local seq           = binary.decode_uint32(string.sub(datagram, 1, 4))
    local ack           = binary.decode_uint32(string.sub(datagram, 5, 8))
    local fragment_ofs  = binary.decode_uint16(string.sub(datagram, 9, 10))
    local fragment_len  = binary.decode_uint16(string.sub(datagram, 11, 12))
    local cmd           = binary.decode_uint8(string.sub(datagram, 13, 13))
    local data          = string.sub(datagram, DGRAM_HEADER_SIZE + 1)
end

return datagram