Converting C to Pb

[Peter Lameijn]

I am trying to port this code to PB. The code is used to generate 16 bit CRC code. (CRC-16, CCITT)
I am a bit puzzled by the part in the do...while loop. (Not really easy to read)
This is the c code:

#define POLY 0x8408
 
unsigned short crc16(data_p, length)
char *data_p;
unsigned short length;
{
       unsigned char i;
       unsigned int data;
       unsigned int crc;
       
       crc = 0xffff;
       
       if (length == 0)
              return (~crc);
       
       do {
              for (i = 0 data = (unsigned int)0xff & *data_p++;
                  i < 8;
                  i++, data >>= 1) {
                    if ((crc & 0x0001) ^ (data & 0x0001))
                           crc = (crc >> 1) ^ POLY;
                    else
                           crc >>= 1;
              }
       } while (--length);
       
       crc = ~crc;
        
       data = crc;
       crc = (crc << 8) | (data >> 8 & 0xFF);
        
       return (crc);
}

Any ideas are welcome...


------------------
Regards,
Peter

[Charles Dietz]

Peter, maybe something like this...

%POLY = &H08408
FUNCTION crc16(BYVAL data_p AS BYTE PTR, length AS WORD) AS WORD
   LOCAL i AS BYTE, dData, crc, n AS DWORD
   crc = &H0ffff
   IF length = 0 THEN
      FOR i = 0 TO 31 'crc = ~crc
         BIT TOGGLE crc, i
      NEXT i
   ELSE
      DO 
         dData = CDWD(&H0ff AND data_p): INCR data_p
         FOR i = 0 TO 7       
            IF (crc AND &H0001) XOR (dData AND &H0001) THEN
               SHIFT RIGHT crc, 1
               crc = crc XOR %POLY
            ELSE
               SHIFT RIGHT crc, 1
            END IF
            SHIFT RIGHT dData, 1
         NEXT i
         DECR length 'added this very necessary stmt
      LOOP WHILE length 
      FOR i = 0 TO 31  'crc = ~crc
         BIT TOGGLE crc, i
      NEXT i
      dData = crc
      SHIFT LEFT crc, 8: SHIFT RIGHT dData, 8
      crc = crc OR (dData AND &H0FF)
   END IF
   FUNCTION = crc
END FUNCTION

Added
Since short was explicitly used for the length variable, I assumed that integers were 4 bytes.
which seems like this algorithm would be for crc32 rather than for crc16. I'm confused.
------------------




[This message has been edited by Charles Dietz (edited August 16, 2007).]

[Aleksandr Dobrev]

One more version

#COMPILE EXE
#DIM ALL
'
#INCLUDE "win32api.inc"
'
%POLY = &H8408??
'
'unsigned short crc16(data_p, length)
FUNCTION crc16 (BYVAL data_p AS BYTE PTR, BYVAL length AS WORD) AS WORD
       LOCAL i AS BYTE     'unsigned char i;
       LOCAL wData AS WORD 'unsigned int data;
       LOCAL crc AS WORD   'unsigned int crc;
       '
       crc = &hFFFF?? 'crc = 0xffff;
       '
       IF length = 0 THEN 'if (length == 0)
             FUNCTION = NOT crc : EXIT FUNCTION ' return (~crc);
       END IF
       '
       DO
              'for (i = 0 wData = (unsigned int)0xff & *data_p++; i < 8; i++, wData >>= 1)
              i = 0 : wData = &h00FF?? AND @Data_p
              WHILE i < 8
                       IF ((crc AND &h0001??) XOR (wData AND &h0001??)) THEN
                            SHIFT RIGHT crc,1   '<- crc = (crc >> 1) ^ %POLY;
                            crc = crc XOR %POLY '<-
                       ELSE
                            SHIFT RIGHT crc,1  'crc >>= 1;
                       END IF
                 INCR i
                 SHIFT RIGHT wData, 1
              WEND
              '
              INCR Data_p
              DECR length
       LOOP WHILE length '(--length);
       '
       crc = NOT crc ' crc = ~crc;
       '
       wDATA = crc ';
       '
       SHIFT RIGHT wData, 8 : wData = wData AND &h00FF?? '<- crc = (crc << 8) | (wDATA >> 8 & 0xFF);
       SHIFT LEFT crc, 8                                 '<-
       crc = crc OR wData                                '<-
       '
       FUNCTION = crc ' RETURN (crc);
END FUNCTION
'
'===============================
'
FUNCTION PBMAIN () AS LONG
   LOCAL cData AS ASCIIZ * 32
   DIM bData(10) AS BYTE
   '
   cData = "abcdefgdddddddD"
   ? "0x" + HEX$(crc16(BYVAL VARPTR(cData) , LEN(cData)), 4) ,, "CRC16 of string ="
   '
   ARRAY ASSIGN bData() = 10,20,30,40,50,60,70,80,90,100,200 ' fill byte array with some data
   ? "0x" + HEX$(crc16(BYVAL VARPTR(bData(0)), UBOUND(bData)-LBOUND(bData)), 4) ,, "CRC16 of byte array ="
   '
END FUNCTION
'

------------------
-=Alex=-

[Peter Lameijn]

Thanks,

I'll try them both out. I've already ported/tried several other code samples, but all give different (and incorrect) answers...
There is a predefined 'test' string for CCITT CRC-16 test:

When the input string is "123456789", the calculated CRC should be: E5CC Hex



------------------
Regards,
Peter

[Charles Dietz]

Looks like I left off the DECR length statement which I just edited in my code above.
Also, Alex's use of bitwise NOT is of course the way to go.

------------------

[Aleksandr Dobrev]

One more crc16 algorithm which is concur with test string of CCITT

#COMPILE EXE
#DIM ALL
'
#INCLUDE "win32api.inc"
'
%POLY  = &H8408??
%Magic = &H16DB??
'
FUNCTION crc16f1(BYVAL wData AS BYTE PTR, BYVAL lenght AS LONG, BYVAL seed AS WORD) AS WORD
  LOCAL b1 , b2 AS BYTE
  LOCAL crc AS WORD
  '
  crc = seed
  '
  IF lenght THEN
      WHILE lenght
          b1 = @wData AND &hff? : INCR wData
          b2 = (crc XOR b1) AND &h0f?
          SHIFT RIGHT crc, 4
          crc = crc XOR (b2 * %Magic )
          SHIFT RIGHT b1, 4
          b2 = (crc XOR b1) AND &h0f?
          SHIFT RIGHT crc, 4
          crc = crc XOR (b2 * %Magic)
          '
          DECR lenght
      WEND
  ELSE
      crc = 0
  END IF
 
  FUNCTION = crc
END FUNCTION
'===============================
'
FUNCTION PBMAIN () AS LONG
   LOCAL cData AS ASCIIZ * 32
   LOCAL sRes AS STRING
   '
'   There is a predefined 'test' string for CCITT CRC-16 test:
'   When the input string is "123456789", the calculated CRC should be: E5CC Hex
   cData = "123456789"
   '
   ? "CRC16 of string ["+cData+"] = 0x" + HEX$(crc16f1(BYVAL VARPTR(cData) , LEN(cData), %POLY), 4) ,, "Result"
   '
END FUNCTION

P.S.
BTW,
very helpful links: http://www.joegeluso.com/software/articles/ccitt.htm http://www.zorc.breitbandkatze.de/crc.html
------------------
-=Alex=-

[This message has been edited by Aleksandr Dobrev (edited August 16, 2007).]

[Peter Lameijn]

I have made some code by reading the specs of CRC-16 a few time. It's not very quick, but the results are OK.
(I've added 2 more reference values)
This is the straigth approach. It converts the incoming datastring into a binary string ("A" becomes "01000001")
and compares bit by bit through complete string.

#Compile Exe
#Dim All
'
#Include "win32api.inc"
'
%POLY  = &H11021
 
Function Crc_16(dStr As String) As Word
  Local CntByte As Long, CntBit As Long, NewData As Byte, OldData As Byte, Buff As String, Crc As Word
  Local dwTemp As Dword
  Buff = String$(16, "1")
  For CntByte = 1 To Len(dStr)
    NewData = Asc(dStr,CntByte)
    For CntBit = 7 To 0 Step -1
      If Bit(NewData, CntBit) Then Buff = Buff & "1" Else Buff = Buff & "0"
    Next
  Next
  Buff = Buff & String$(16, "0")
  dwTemp = Val ("&b" & Mid$(Buff, 1, 16))
  For CntBit = 17 To Len(Buff)
    Shift Left dwTemp, 1
    If Mid$(Buff, CntBit, 1) = "1" Then Bit Set dwTemp, 0 Else Bit Reset dwTemp, 0
    If Bit (dwTemp, 16) Then dwTemp = dwTemp Xor %POLY
  Next
  Function = dwTemp
End Function
 
'===============================
Function PBMain () As Long
  MsgBox "CRC-16 of string " & $Dq & "123456789" & $Dq & " is:   0x" & Hex$(Crc_16("123456789")),,"Result: (expected: 0xE5CC)"
  MsgBox "CRC-16 of empty string" & $Dq & "" & $Dq & " is: is:   0x" & Hex$(Crc_16("")),,"Result: (expected: 0x1D0F)"
  MsgBox "CRC-16 of string " & $Dq & "A" & $Dq & " is:   0x" & Hex$(Crc_16("A")),,"Result: (expected: 0x9479)"
End Function

------------------
Regards,
Peter