protocol.lua

-- protocol.lua
-- (C) 2009, Michael Meier

local rpcidlength = 8

-- packet table -> binary string
function encodepacket(info, headervers)

   local chunks = {}
   local hv

   if headervers == nil then
      hv = 1
   else
      if headervers == 1 then
	 hv = 1
      else
	 error("unsupported header version")
      end
   end


   table.insert(chunks, string.char(hv))

   if hv == 1 then
      local rpcid = info.rpcid
      local fromid = info.fromid
      local call = info.call

      if #rpcid ~= rpcidlength then error("rpcid has to be 8 bytes long") end
      if #fromid ~= 20 then error("fromid has to be 20 bytes long") end
      if call > 255 or call < 0 then error("call has to be a character number") end

      table.insert(chunks, fromid)
      table.insert(chunks, rpcid)
      table.insert(chunks, string.char(call))

      if info.payload and (call == 1 or call==129) then
	 local encoded = bencode(info.payload)
	 local len = #encoded

	 if len > 0xffff then error("payload too long") end

	 -- generate single bytes for encoding in network byte order
	 local msb = math.floor(len / 256)
	 local lsb = len - (msb * 256)

	 table.insert(chunks, string.char(msb))
	 table.insert(chunks, string.char(lsb))
	 table.insert(chunks, encoded)
      end
   else

   end

   return table.concat(chunks)

end


-- raw packet table -> packet table
function decodepacket(packet)

   packet.decoded = false
   local raw = packet.raw

   local hv = string.byte(raw:sub(1, 1))

   if hv == 1 then
      local fromid = raw:sub(2, 21)
      local rpcid = raw:sub(22, 29)
      local call = raw:sub(30, 30)
      --print("decoding header type 1")
      --print("fromid " .. fromid)
      --print("rpcid " .. rpcid)
      --print("call " .. call)
      if call == "" then return end

      call = string.byte(call)
      
      packet.from.id = fromid
      packet.from.unique = packet.from.addr .. "|" .. packet.from.port .. "|" .. fromid
      packet.rpcid = rpcid
      packet.call = call

      -- error packets don't have payload
      if call == 130 or call == 131 then
	 packet.decoded = true
	 return
      end

      if packet.call == 1 or packet.call == 129 then
	 local msb = string.byte(raw:sub(31, 31))
	 if msb == nil then
	    packet.payload = {}
	    packet.decoded = true
	    return
	 end
	 local lsb = string.byte(raw:sub(32, 32))
	 if lsb == nil then return end

	 local len = (msb * 256) + lsb
	 local encoded = raw:sub(33, 33 + len - 1)
	 if #encoded ~= len then return end
	 local ok, payload = pcall(debencode, encoded)
	 if ok then
	    packet.payload = payload
	    packet.decoded = true
	 else
	    return
	 end
      end

      return
   else
      return
   end


end