'===========================================================================
' Subject: GENERATES 32-BIT CRC              Date: 03-19-99 (23:35)       
' Author:  Greg Turgeon                      Code: PBCC                   
' Origin:  gturgeon@compuserve.com         Packet: PBCC.ABC
'===========================================================================
$IF 0
+--------------------------------------------------------------------------+
| WESTFORD CUSTOM PC/FX         -*-   CRC32_CC.BAS  -*-            3/19/99 |
| gturgeon@compuserve.com |------------------------------------------------+
| 978-692-8163 |----------+
+--------------+
   
   This file contains the following routines:

         SUB CRC32Init()   
         SUB BuildCRC32TABLE()
         SUB CRC32(BYVAL pBuffer???, BYVAL BuffLen&)
         FUNCTION CRC32Retrieve???()
   
   The CRC function employs the CCITT-32 polynomial 0EDB88320h and 
   therefore generates the same CRC values as do PKZIP and ARJ.

   The code requires PBCC.EXE to compile. This file can be simply 
   $INCLUDEd at the top of the main module file for any PBCC project.

   To use:

   The first call to CRC32Init() initializes the required table of values.
   It also initializes CRC_CURRENT???, which should not be manipulated 
   directly by the application. Subsequent calls to CRC32Init() 
   reinitialize CRC_CURRENT???.

   The primary CRC process is executed in the subroutine 
   CRC32(BYVAL pBuffer???, BYVAL BuffLen&), which accepts a pointer to 
   the target data and long integer specifying data length.

   The function CRC32Retrieve???() returns the current CRC value 
   (properly normalized) but does not clear or reinitialize the current 
   value. Reinitializing is accomplished only through CRC32Init().

$ENDIF


DEFLNG A-Z
'============
$IF NOT %DEF(%NULL)
   %NULL = 0
$ENDIF

GLOBAL CRC32TABLE???(), pTABLE???, CRC_CURRENT???

DECLARE SUB CRC32Init()
DECLARE SUB BuildCRC32TABLE()
DECLARE SUB CRC32(BYVAL pBuffer???, BYVAL BuffLen&)
DECLARE FUNCTION CRC32Retrieve???

'====================
SUB CRC32Init()
if pTABLE??? = %NULL then
   redim CRC32TABLE???(255)
   call BuildCRC32TABLE
   pTABLE??? = varptr(CRC32TABLE???(0))
end if
CRC_CURRENT??? = &h0ffffffff
END SUB

'====================
FUNCTION CRC32Retrieve???
function = NOT CRC_CURRENT???
END FUNCTION


'=========================
SUB CRC32(BYVAL pBuffer???, BYVAL BuffLen&)

!  push     ebx
!  push     esi
!  push     edi

!  mov      edx, CRC_CURRENT???     ;edx holds current CRC throughout
!  mov      ecx, BuffLen&
!  test     ecx, ecx
!  jz       short CRCDone
!  mov      edi, pTABLE???
!  mov      esi, pBuffer???
!  inc      ecx                  ;add 1
!  shr      ecx, 1               ; \2
!  jc       short CRCTop         ;if originally even number of bytes to process

!  mov      al, byte ptr [esi]   ;do odd byte here
!  inc      esi
!  xor      ebx, ebx
!  mov      bl, dl
!  xor      bl, al
!  shr      edx, 8
!  xor      edx, [edi + 4 * ebx]
!  dec      ecx
!  jz       short CRCDone
CRCTop:
!  mov      ax, [esi]            ;al = [byte ptr esi]
!  xor      ebx, ebx
!  mov      bl, dl
!  xor      bl, al
!  shr      edx, 8
!  xor      edx, [edi + 4 * ebx]

!  mov      al, ah               ;al = [byte ptr esi+1]
!  xor      ebx, ebx
!  mov      bl, dl
!  xor      bl, al
!  shr      edx, 8
!  xor      edx, [edi + 4 * ebx]

!  inc      esi
!  inc      esi
!  dec      ecx
!  jz       short CRCDone
!  jmp      short CRCTop
CRCDone:
!  mov      CRC_CURRENT???, edx
!  pop      edi
!  pop      esi
!  pop      ebx
END SUB


'=========================
SUB BuildCRC32TABLE()
'-- Builds necessary table of dword values
'-- The table itself is never altered and thus
'--  must be constructed only once
REGISTER b&, value???
LOCAL    CRC32Polynomial???, a&
LOCAL ppoly AS DWORD PTR, parray AS DWORD PTR

CRC32Polynomial??? = &hEDB88320
ppoly = varptr(CRC32Polynomial???)

parray = varptr(CRC32TABLE???(0))

for a& = 0 to 255
   value??? = a&
   for b& = 8 to 1 step -1
      if istrue(value??? AND 1) then
         value??? = (value??? \ 2) XOR CRC32Polynomial???
      else
         shift right value???, 1
      end if
   next b&
   @parray[a&] = value???     '-- CRC32TABLE???(a&) = value???
next a&
END SUB

'-- END CRC32_CC.BAS