f.hs

bytestring :: BS.ByteString -> Parser e ()
bytestring (BS.PS (ForeignPtr fp con) _ len) = go64 fp 0#
  where
    go64 :: Addr# -> Int# -> Parser e ()
    go64 addr n | tagToEnum# (n <# w64s)
                = do scan64# (W# (indexWord64OffAddr# addr 0#))
                     go64 (addr `plusAddr#` 8#) (n -# 1#)
                | otherwise = go32 addr w32s

    go32 addr 1# = scan32# (W32# (indexWord32OffAddr# addr 0#)) >> go16 (addr `plusAddr#` 4#) r32
    go32 addr 0# = go16 addr w16s
    go32 _    _ = empty


    go16 addr 1# = scan16# (W16# (indexWord16OffAddr# addr 0#)) >> go8 (addr `plusAddr#` 2#) w8s
    go16 addr 0# = go8 addr w8s
    go16 _    _ = empty

    go8 addr 1# = scan8# (W8# (indexWord8OffAddr# addr 0#))
    go8 addr 0# = pure ()
    go8 _    _  = empty

    !(I# w64s,I# r64) = len `quotRem` 8
    !(I# w32s,I# r32) = I# r64 `quotRem` 4
    !(I# w16s,I# w8s) = I# r32 `quotRem` 2