Calculating a checksum

27 Aug 2010 2:40 PM

Leo

Hi all can someone please help me convert this C sample into VB6 please
WORD CalcTitleChecksum(
char* pszTitle, // A pointer to the name of the savegame
int cb)          // set to either 39 or 47 for TTO or TTD savegames
{
WORD sum = 0;

for(int i=0; i<cb; i++)
{
    sum += pszTitle[i];
    sum = (sum << 1) + (sum >> 15);
}

return sum ^ 0xAAAA;
}

It comes from http://www.ttdpatch.net/chris_becke_ttdlx.html#hack

I have tried
Dim lCalculatedChecksum As Long
   Dim lChecksum As Long
   Dim Data() As Byte
   Dim tl As New CTwiddleLong

   For i = 0 To 46
      lCalculatedChecksum = lCalculatedChecksum + Data(i) '+ Data(i +
1)
      tl.Value = lCalculatedChecksum
      lCalculatedChecksum = tl.RotateL
   Next

lCalculatedChecksum = (lCalculatedChecksum Xor &HAAAA)

Data() is being filled from a file.
CTwiddleLong comes from Karl's Twiddle sample.

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27 Aug 2010 4:25 PM

Jim Mack

Leo wrote:
Show quoteHide quote

> Hi all can someone please help me convert this C sample into VB6
> please WORD CalcTitleChecksum(
>   char* pszTitle, // A pointer to the name of the savegame
>   int cb)          // set to either 39 or 47 for TTO or TTD
> savegames {
>   WORD sum = 0;
>
>   for(int i=0; i<cb; i++)
>   {
>     sum += pszTitle[i];
>     sum = (sum << 1) + (sum >> 15);
>   }
>
>   return sum ^ 0xAAAA;
> }
>
> It comes from http://www.ttdpatch.net/chris_becke_ttdlx.html#hack
>
> I have tried
> Dim lCalculatedChecksum As Long
>    Dim lChecksum As Long
>    Dim Data() As Byte
>    Dim tl As New CTwiddleLong
>
>    For i = 0 To 46
>       lCalculatedChecksum = lCalculatedChecksum + Data(i) '+ Data(i
> + 1)
>       tl.Value = lCalculatedChecksum
>       lCalculatedChecksum = tl.RotateL
>    Next
>
> lCalculatedChecksum = (lCalculatedChecksum Xor &HAAAA)
>
> Data() is being filled from a file.
> CTwiddleLong comes from Karl's Twiddle sample.

You'll have problems with that because you're rotating a long and not
a short, and because VB doesn't offer an unsigned WORD data type. All
your rotates will leave a zero in the lowest bit, and eventually
you'll wind up with a zero result by shifting all those 0 bits left 47
times.

Using VB alone I can see at least one (naive) solution -- I'm sure
there are cleverer ones:

Dim Sum As Long ' the chinese variable
Dim Idx As Long

For Idx = 0 to 46
   Sum = (Sum + Data(Idx)) * 2
   '
   ' * 2 is like a left shift << 1
   '
   Sum = Sum - CLng(CBool(Sum And 65536)) And 65535
   '
   ' equivalent to:
   ' If Sum And &h10000 Then
   '   Sum = Sum + 1
   ' End If
   ' Sum = Sum And &HFFFF&

Next

Sum = Sum Xor &HAAAA&

--
   Jim Mack
   Twisted tees at http://www.cafepress.com/2050inc
   "We sew confusion"

27 Aug 2010 8:04 PM

DickGrier

Hi Jim,

Actually, I think that's about as good as it gets in VB6 (VB.NET has
bit-shift operators... No, I'm not saying one should go that route because
of that).

Dick

--
Richard Grier, Consultant, Hard & Software 12962 West Louisiana Avenue
Lakewood, CO 80228 303-986-2179 (voice) Homepage: www.hardandsoftware.net
Author of Visual Basic Programmer's Guide to Serial Communications, 4th
Edition ISBN 1-890422-28-2 (391 pages) published July 2004, Revised July
2006.

27 Aug 2010 8:19 PM

Mike Williams

"DickGrier" <dick_grierNOSPAM@msn.com> wrote in message
news:OAzKFMiRLHA.2100@TK2MSFTNGP04.phx.gbl...

> Actually, I think that's about as good as it gets
> in VB6 (VB.NET has bit-shift operators...

So does PowerBasic, and so do lots of other programming languages and,
unlike VB.NOT, PowerBasic is not bloatware.

Mike