local bit=require'bit'

function crc(a) 

  local tab = { 
      0x0000, 0x365E, 0x6CBC, 0x5AE2, 0xD978, 0xEF26, 0xB5C4, 0x839A, 
      0xFF89, 0xC9D7, 0x9335, 0xA56B, 0x26F1, 0x10AF, 0x4A4D, 0x7C13, 
      0xB26B, 0x8435, 0xDED7, 0xE889, 0x6B13, 0x5D4D, 0x07AF, 0x31F1, 
      0x4DE2, 0x7BBC, 0x215E, 0x1700, 0x949A, 0xA2C4, 0xF826, 0xCE78, 
      0x29AF, 0x1FF1, 0x4513, 0x734D, 0xF0D7, 0xC689, 0x9C6B, 0xAA35, 
      0xD626, 0xE078, 0xBA9A, 0x8CC4, 0x0F5E, 0x3900, 0x63E2, 0x55BC, 
      0x9BC4, 0xAD9A, 0xF778, 0xC126, 0x42BC, 0x74E2, 0x2E00, 0x185E, 
      0x644D, 0x5213, 0x08F1, 0x3EAF, 0xBD35, 0x8B6B, 0xD189, 0xE7D7, 
      0x535E, 0x6500, 0x3FE2, 0x09BC, 0x8A26, 0xBC78, 0xE69A, 0xD0C4, 
      0xACD7, 0x9A89, 0xC06B, 0xF635, 0x75AF, 0x43F1, 0x1913, 0x2F4D, 
      0xE135, 0xD76B, 0x8D89, 0xBBD7, 0x384D, 0x0E13, 0x54F1, 0x62AF, 
      0x1EBC, 0x28E2, 0x7200, 0x445E, 0xC7C4, 0xF19A, 0xAB78, 0x9D26, 
      0x7AF1, 0x4CAF, 0x164D, 0x2013, 0xA389, 0x95D7, 0xCF35, 0xF96B, 
      0x8578, 0xB326, 0xE9C4, 0xDF9A, 0x5C00, 0x6A5E, 0x30BC, 0x06E2, 
      0xC89A, 0xFEC4, 0xA426, 0x9278, 0x11E2, 0x27BC, 0x7D5E, 0x4B00, 
      0x3713, 0x014D, 0x5BAF, 0x6DF1, 0xEE6B, 0xD835, 0x82D7, 0xB489, 
      0xA6BC, 0x90E2, 0xCA00, 0xFC5E, 0x7FC4, 0x499A, 0x1378, 0x2526, 
      0x5935, 0x6F6B, 0x3589, 0x03D7, 0x804D, 0xB613, 0xECF1, 0xDAAF, 
      0x14D7, 0x2289, 0x786B, 0x4E35, 0xCDAF, 0xFBF1, 0xA113, 0x974D, 
      0xEB5E, 0xDD00, 0x87E2, 0xB1BC, 0x3226, 0x0478, 0x5E9A, 0x68C4, 
      0x8F13, 0xB94D, 0xE3AF, 0xD5F1, 0x566B, 0x6035, 0x3AD7, 0x0C89, 
      0x709A, 0x46C4, 0x1C26, 0x2A78, 0xA9E2, 0x9FBC, 0xC55E, 0xF300, 
      0x3D78, 0x0B26, 0x51C4, 0x679A, 0xE400, 0xD25E, 0x88BC, 0xBEE2, 
      0xC2F1, 0xF4AF, 0xAE4D, 0x9813, 0x1B89, 0x2DD7, 0x7735, 0x416B, 
      0xF5E2, 0xC3BC, 0x995E, 0xAF00, 0x2C9A, 0x1AC4, 0x4026, 0x7678, 
      0x0A6B, 0x3C35, 0x66D7, 0x5089, 0xD313, 0xE54D, 0xBFAF, 0x89F1, 
      0x4789, 0x71D7, 0x2B35, 0x1D6B, 0x9EF1, 0xA8AF, 0xF24D, 0xC413, 
      0xB800, 0x8E5E, 0xD4BC, 0xE2E2, 0x6178, 0x5726, 0x0DC4, 0x3B9A, 
      0xDC4D, 0xEA13, 0xB0F1, 0x86AF, 0x0535, 0x336B, 0x6989, 0x5FD7, 
      0x23C4, 0x159A, 0x4F78, 0x7926, 0xFABC, 0xCCE2, 0x9600, 0xA05E, 
      0x6E26, 0x5878, 0x029A, 0x34C4, 0xB75E, 0x8100, 0xDBE2, 0xEDBC, 
      0x91AF, 0xA7F1, 0xFD13, 0xCB4D, 0x48D7, 0x7E89, 0x246B, 0x1235 
    } 

  
  local sum = 0 
  for i = 1, #a do 
    local index = bit.band(bit.bxor(sum, a:byte(i)), 0xff) + 1 
    sum =  bit.bxor(bit.rshift(sum, 8), tab[index]) 
  end 

  return bit.band(bit.bnot(sum), 0xffff) 
end 


--
-- reassembly_handler.lua skeleton
--
-- TYPE:        FRONTEND SCRIPT
-- PURPOSE:     Hook into Trisul's TCP reassembly engine 
-- DESCRIPTION: DNP3 over TCP 20000 
-- 
local SweepBuf=require'sweepbuf' 

TrisulPlugin = { 


  -- the ID block, you can skip the fields marked 'optional '
  -- 
  id =  {
    name = "DNP3",
    description = "On to TCP Reassembly at 20000"
  },



  -- reassembly_handler block
  -- 
  reassembly_handler   = {

    -- WHEN CALLED: a new flow is detected (eg from a SYN packet) 
    --  look at flow tuples and decide if you want to reassemble 
    --  return true : to enable reassembly , false to disable
    --  skip this function if you always want to enable 
    filter = function(engine, timestamp, flowkey) 
      return (flowkey:porta_readable()=="20000" or flowkey:portz_readable()=="20000" )
    end,


    -- WHEN CALLED: when a chunk of reassembled payload is available 
    -- 
    -- handle reassembled byte stream here , 
    -- 
    onpayload = function(engine, timestamp, flowkey, direction, seekpos, buffer) 

	print( buffer:hexdump())
	local payload=SweepBuf.new(buffer:tostring())

	print( "magic ="..payload:next_u16_le())
	print( "length ="..payload:next_u8_le())
	print( "control ="..payload:next_u8_le())
	print( "destination ="..payload:next_u16_le())
	print( "source ="..payload:next_u16_le())
	print( "crc ="..payload:next_u16_le())

	local crc_calculated=crc(buffer:tostring(0,8))
	print( "calccrc ="..crc_calculated);

    end,

  },

}
