Calc CRC-CCITT (XModem)

[A du Toit]

Dear all,

Maybe someone might find this useful:

' ************************************
' * Original code in C, by Mr L.Bies *
' * Ported to PB9 by Mr J.A.du Toit  *
' ****************************************************************************************
' * + Usefull Links:                                                                     *
' *~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*
' *   http://www.lammertbies.nl/comm/software/index.html                                 *
' *   http://www.lammertbies.nl/comm/info/crc-calculation.html?crc=000b001600&method=hex *
' ****************************************************************************************
#COMPILE EXE
#DIM ALL

%TRUE = -1

GLOBAL crc_tabccitt() AS WORD
GLOBAL crc_tabccitt_init AS LONG

SUB init_crcccitt_tab()
 LOCAL i AS WORD
 LOCAL j AS WORD
 LOCAL crc AS WORD
 LOCAL c AS WORD
 LOCAL P_CCITT AS WORD
  P_CCITT = &h1021
  REDIM crc_tabccitt(0 TO 255)
  
  FOR i = 0 TO 255
    crc = 0
    c = i : SHIFT LEFT c, 8
    FOR j = 0 TO 7
      IF ISTRUE((crc XOR c) AND &h8000) THEN
        SHIFT LEFT crc, 1
        crc = crc XOR P_CCITT
      ELSE
        SHIFT LEFT crc, 1
      END IF

      SHIFT LEFT c, 1
    NEXT
    crc_tabccitt(i) = crc
  NEXT
  crc_tabccitt_init = %TRUE
END SUB

FUNCTION update_crc_ccitt(BYVAL crc AS WORD, BYVAL c AS BYTE) AS WORD
 LOCAL tmp AS WORD
 LOCAL short_c AS WORD
  short_c = &h00FF AND c
  IF ISFALSE(crc_tabccitt_init) THEN init_crcccitt_tab
  tmp = crc: SHIFT RIGHT tmp, 8
  tmp = tmp XOR short_c
  SHIFT LEFT crc, 8
  crc = crc XOR crc_tabccitt(tmp)
  FUNCTION = crc
END FUNCTION
  
FUNCTION PBMAIN () AS LONG
 DIM A() AS BYTE
 DIM B() AS BYTE
 LOCAL i AS LONG
 LOCAL crc_ccitt_0000 AS WORD

  REDIM A(0 TO 4)
  ARRAY ASSIGN A() = &h00, &h0b, &h00, &h16, &h00

  crc_ccitt_0000 = 0
  FOR i = 0 TO 4
    crc_ccitt_0000 = update_crc_ccitt(crc_ccitt_0000, A(i))
  NEXT
  'crc_ccitt_0000 = MAK(WORD, HI(BYTE, crc_ccitt_0000), LO(BYTE, crc_ccitt_0000))
  MSGBOX HEX$(crc_ccitt_0000, 4) + "  expect B7 CA"


  REDIM B(0 TO 8)
  ARRAY ASSIGN B() = &h31, &h32, &h33, &h34, &h35, &h36, &h37, &h38, &h39 ' "123456789"

  crc_ccitt_0000 = 0
  FOR i = 0 TO 8
    crc_ccitt_0000 = update_crc_ccitt(crc_ccitt_0000, B(i))
  NEXT
  MSGBOX HEX$(crc_ccitt_0000, 4) + "  expect 31 C3"
END FUNCTION