-- (c) William Welch 2004
--
-- This software is provided 'as-is', without any express or implied 
-- warranty. In no event will the authors be held liable for any damages 
-- arising from the use of this software.
--
-- Permission is granted to anyone to use this software for any purpose, 
-- including commercial applications, and to alter it and redistribute it 
-- freely, subject to the following restrictions:
--
-- 1. The origin of this software must not be misrepresented; 
--    you must not claim that you wrote the original software. 
--    If you use this software in a product, an acknowledgment in 
--    the product documentation would be appreciated but is not required.
--
-- 2. Altered source versions must be plainly marked as such, and must 
--    not be misrepresented as being the original software.
--
-- 3. This notice may not be removed or altered from any source distribution.
--
-- This license is commonly known as the zlib/libpng License.
--
-- chksum.jal bvwelch 20 may 2004
-- revised 18 march 2005
-- Internet Checksum in JAL, see RFC 1071.

-- prepare for checksum calculations
procedure chksum_start is
  I32_CLR ( SUM32 )
end procedure

-- compare to "C" example, RFC 1071, section 4.1, page 7.
procedure chksum_calc (byte in ptr, byte in offset, byte in cnt_hi, byte in cnt_lo) is
  var byte hi, lo

  LoadPointer_2 ( ptr , offset )

  while cnt_hi > 0 loop
    for 128 loop
      hi = MemNext2
      lo = MemNext2
      I32_LDL ( CHKTMP , 0 , 0 , hi , lo )
      I32_ADD ( SUM32 , CHKTMP )
    end loop
    cnt_hi = cnt_hi - 1
  end loop

  while cnt_lo > 1 loop
    hi = MemNext2
    lo = MemNext2
    I32_LDL ( CHKTMP , 0 , 0 , hi , lo )
    I32_ADD ( SUM32 , CHKTMP )
    cnt_lo = cnt_lo - 2
  end loop

  if cnt_lo > 0 then
    hi = MemNext2
    I32_LDL ( CHKTMP , 0 , 0 , hi , 0 )
    I32_ADD ( SUM32 , CHKTMP )
  end if
end procedure

procedure chksum_finish (byte out sum_hi, byte out sum_lo) is
  var byte b3 , b2 , b1 , b0

  I32_SVL ( SUM32 , b3 , b2 , b1 , b0 )
  while ( b3 != 0 ) | ( b2 != 0 ) loop
    I32_LDL ( CHKTMP , 0 , 0 , b3 , b2 )
    I32_LDL ( SUM32 , 0 , 0 , b1 , b0 )
    I32_ADD ( SUM32 , CHKTMP )
    I32_SVL ( SUM32 , b3 , b2 , b1 , b0 )
  end loop
  sum_hi = b1
  sum_lo = b0
end procedure

-- regression test from RFC 1071
-- answer should be 0xDDF2
procedure chksum_test is
  var byte sum_hi, sum_lo

  LoadPointer_0 ( PKT )
  MemNext = 0x00
  MemNext = 0x01
  MemNext = 0xf2
  MemNext = 0x03
  MemNext = 0xf4
  MemNext = 0xf5
  MemNext = 0xf6
  MemNext = 0xf7

  chksum_start
  chksum_calc(PKT, 0, 0, 8)
  chksum_finish(sum_hi, sum_lo)

  putc = "C"
  putc = "H"
  putc = "K"
  putc = "S"
  putc = "U"
  putc = "M"
  putc = " "
  if (sum_hi == 0xDD) & (sum_lo == 0xF2) then
    putc = "O"
    putc = "K"
  else
    putc = "B"
    putc = "A"
    putc = "D"
  end if
  putc = 13
  putc = 10
end procedure

-- compute and store the checksum
procedure chksum_write(byte in ptr , byte in start_offset, byte in write_offset, byte in cnt_hi , byte in cnt_lo) is
  var byte sum_hi, sum_lo

  -- first, zero out where the checksum goes.
  MemSetEx ( ptr , write_offset , 0 , 2  )

  chksum_start
  chksum_calc(ptr, start_offset, cnt_hi , cnt_lo)
  chksum_finish(sum_hi, sum_lo)

  -- store the ones-complement of the checksum.
  MemWrEx (ptr , write_offset, (sum_hi ^ 0xff) )
  MemWrEx( ptr , (write_offset + 1), (sum_lo ^ 0xff) )
end procedure

procedure chksum_udp is
  var byte cnt_hi, cnt_lo, sum_hi, sum_lo

  -- build pseudo header
  MemCpyEx ( UDP_PSEUDO , 0 , ip_off , 12 , 8 )
  MemWrEx( UDP_PSEUDO , 8 , 0)
  MemCpyEx( UDP_PSEUDO , 9 , ip_off , 9 , 1 )
  MemCpyEx( UDP_PSEUDO , 10, udp_off , 4 , 2 )

  -- now calculate the checksum of the pseudo header.
  chksum_start
  chksum_calc(UDP_PSEUDO, 0, 0 , 12)

  -- now calculate the checksum of the udp packet itself
  MemSetEx( udp_off , 6, 0 , 2 )
  -- note byte swap
  I16_SVL ( UDP_LEN , cnt_lo , cnt_hi)
  chksum_calc( udp_off, 0 , cnt_hi , cnt_lo)
  chksum_finish(sum_hi, sum_lo)

  -- store the ones-complement of the checksum.
  MemWrEx ( udp_off , 6, (sum_hi ^ 0xff) )
  MemWrEx ( udp_off , 7, (sum_lo ^ 0xff) )
end procedure

procedure chksum_tcp(byte in tcplen_hi , byte in tcplen_lo) is
  var byte sum_hi, sum_lo

  -- build pseudo header
  MemCpyEx ( TCP_PSEUDO , 0 , IP_OFF , 12 , 8 )
  MemWrEx ( tcp_pseudo , 8, 0)
  MemCpyEx( tcp_pseudo , 9, ip_off , 9 , 1 )
  MemWrEx ( tcp_pseudo , 10, tcplen_hi)
  MemWrEx ( tcp_pseudo , 11, tcplen_lo)

  -- now calculate the checksum of the pseudo header.
  chksum_start
  chksum_calc(tcp_pseudo, 0, 0 , 12)

  -- now calculate the checksum of the tcp packet itself
  MemSetEx ( tcp_off , 16, 0, 2)
  chksum_calc(tcp_off, 0, tcplen_hi , tcplen_lo)
  chksum_finish(sum_hi, sum_lo)

  -- store the ones-complement of the checksum.
  MemWrEx( tcp_off , 16, (sum_hi ^ 0xff) )
  MemWrEx( tcp_off , 17, (sum_lo ^ 0xff) )
end procedure