module Packets.Checksum
  (
    ipChecksum
  ) where

import           Data.Bits       (shiftR, (.&.))
import qualified Data.ByteString as B
import           Data.Word       (Word16)
import           Prelude         hiding (words)

ipChecksum :: B.ByteString -> Word16
ipChecksum bytes = 0xffff - (fromIntegral . adjustedSum . sum . words) bytes
  where
    adjustedSum x = if x > 0xffff
      then adjustedSum $ x .&. 0xffff + x `shiftR` 16
      else x

    words :: B.ByteString -> [Int]
    words bs =
      let (h, t) = B.splitAt 2 bs in
        case (B.unpack . B.take 2) h of
          [x1, x2] -> (fromIntegral x1 * 0x100 + fromIntegral x2) : words t
          [x1]     -> (fromIntegral x1 * 0x100) : words t
          _        -> []