module Network.EBus.Layer2(
  ebusParserLayer2,
  EbusPacket,
  ebusPacketType,
  ebusPacketSource,
  ebusPacketDestination,
  ebusPacketPrimaryCommand,
  ebusPacketSecondaryCommand,
  ebusPacketPayload,
  ebusPacketPayloadSlave) where

import Control.Applicative ((<|>))
import Data.Attoparsec (anyWord8, count, skipMany, try, Parser, word8)
import Data.Bits (xor)
import Data.Word (Word8)

-- | Ebus Layer2 Constants
ebusConstant :: String -> Word8
ebusConstant "SYN" = 0xaa
ebusConstant "ACK" = 0x00
ebusConstant "BROADCAST" = 0xff
ebusConstant "ESCAPE" = 0xa9
ebusConstant "ESCAPE_ESCAPE" = 0x00
ebusConstant "ESCAPE_SYN" = 0x01
ebusConstant _ = 0x00

-- | Ebus Packet Types
data EbusType = EbusBroadcast 
              | EbusMasterMaster 
              | EbusMasterSlave
              deriving (Show)

-- | Ebus Packet
data EbusPacket = EbusPacket { 
  ebusPacketType :: EbusType,
  ebusPacketSource :: Word8,
  ebusPacketDestination :: Word8,
  ebusPacketPrimaryCommand :: Word8,
  ebusPacketSecondaryCommand :: Word8,
  ebusPacketPayload :: [Word8],
  ebusPacketPayloadSlave :: Maybe [Word8]
  } deriving (Show)

-- | Ebus Binary Escape Sequences for payload data
parserPayload :: Parser Word8
parserPayload = do{ _ <- word8 $ ebusConstant "ESCAPE"
                  ; _ <- word8 $ ebusConstant "ESCAPE_ESCAPE"
                  ; return $ ebusConstant "ESCAPE"}
                <|>
                do{ _ <- word8 $ ebusConstant "ESCAPE"
                  ; _ <- word8 $ ebusConstant "ESCAPE_SYN"
                  ; return $ ebusConstant "SYN"}
                <|>
                anyWord8;

-- | Parse one Ebus Packet, skips leading SYNs
ebusParserLayer2 :: Parser EbusPacket
ebusParserLayer2 = do{ 
  skipMany $ word8 $ ebusConstant "SYN"
  ; source <- anyWord8
  ; destination <- anyWord8
  ; primaryCommand <- anyWord8
  ; secondaryCommand <- anyWord8
  ; payloadLength <- anyWord8
  ; payload <- count (fromIntegral payloadLength) parserPayload
  ; followup <-
    if destination == 0xfe then
      -- Broadcast
      do{ crc <- anyWord8
        ; _ <- {- syn -} word8 $ ebusConstant "SYN"
        ; if ebusCrc ([source, destination, 
                       primaryCommand, secondaryCommand, payloadLength] 
                      ++ payload) == crc then
            return $ EbusPacket EbusBroadcast source destination primaryCommand secondaryCommand payload Nothing
          else
            fail "Broadcast: CRC Check failed"}
      <|>
      fail "Failed to parse Broadcast Paket"
    else
      -- Master-Master
      try( 
        do{ crc <- anyWord8
          ; _ <- {- ack -} word8 $ ebusConstant "ACK"
          ; _ <- {- syn -} word8 $ ebusConstant "SYN"
          ; if ebusCrc ([source, destination, 
                         primaryCommand, secondaryCommand, payloadLength] 
                        ++ payload) == crc then
              return $ EbusPacket EbusMasterMaster source destination primaryCommand secondaryCommand payload Nothing
            else
              fail "Master-Master: CRC Check failed"})
      <|> 
      -- Master Slave
      try(
        do{ crc <- anyWord8
          ; _ <- {- ack -} word8 $ ebusConstant "ACK"
          ; payloadSlaveLength <- anyWord8
          ; payloadSlave <- count (fromIntegral payloadSlaveLength) parserPayload
          ; crcSlave <- anyWord8
          ; _ <- {- ackSlave -} word8 $ ebusConstant "ACK"
          ; _ <- {- synSlave -} word8 $ ebusConstant "SYN"
          ; if ebusCrc ([source, destination, 
                         primaryCommand, secondaryCommand, payloadLength] 
                        ++ payload) == crc &&
               ebusCrc ([payloadSlaveLength] ++ payloadSlave) == crcSlave then
              return $ EbusPacket EbusMasterSlave source destination primaryCommand secondaryCommand payload (Just payloadSlave)
            else
              fail "Master-Slave: CRC Check failed"})

      <|> 
      fail "Failed to parse Master-Master/Master-Slave Packet"
  ; return followup
    <|> fail "Failed to parse packet"
  }

-- | Ebus CRC Calculation using a Table of precalculated values
ebusCrc :: [Word8] -> Word8
ebusCrc input = foldl step 0 input
  where step acc x = (crctab (fromIntegral acc)) `xor` x

-- | Table for CRC Calculation
crctab :: Int -> Word8
crctab pos = [
    0, 155, 173, 54, 193, 90, 108, 247, 25, 130,   --0 -9
    180, 47, 216, 67, 117, 238, 50, 169, 159, 4,   --10-19
    243, 104, 94, 197, 43, 176, 134, 29, 234, 113, --20
    71, 220, 100, 255, 201, 82, 165, 62, 8, 147,   --30
    125, 230, 208, 75, 188, 39, 17, 138, 86, 205,
    251, 96, 151, 12, 58, 161, 79, 212, 226, 121,
    142, 21, 35, 184, 200, 83, 101, 254, 9, 146,
    164, 63, 209, 74, 124, 231, 16, 139, 189, 38,
    250, 97, 87, 204, 59, 160, 150, 13, 227, 120,
    78, 213, 34, 185, 143, 20, 172, 55, 1, 154,
    109, 246, 192, 91, 181, 46, 24, 131, 116, 239, --100
    217, 66, 158, 5, 51, 168, 95, 196, 242, 105,
    135, 28, 42, 177, 70, 221, 235, 112, 11, 144,
    166, 61, 202, 81, 103, 252, 18, 137, 191, 36,
    211, 72, 126, 229, 57, 162, 148, 15, 248, 99,
    85, 206, 32, 187, 141, 22, 225, 122, 76, 215,
    111, 244, 194, 89, 174, 53, 3, 152, 118, 237,
    219, 64, 183, 44, 26, 129, 93, 198, 240, 107,
    156, 7, 49, 170, 68, 223, 233, 114, 133, 30,
    40, 179, 195, 88, 110, 245, 2, 153, 175, 52,
    218, 65, 119, 236, 27, 128, 182, 45, 241, 106, --200
    92, 199, 48, 171, 157, 6, 232, 115, 69, 222,
    41, 178, 132, 31, 167, 60, 10, 145, 102, 253,
    203, 80, 190, 37, 19, 136, 127, 228, 210, 73,
    149, 14, 56, 163, 84, 207, 249, 98, 140, 23,
    33, 186, 77, 214, 224, 123] !! pos