How to program a FAST CRC32 check

Dear Programmers,

I have a problem to calculate the CRC32 from a file. I have found a routine that can do this job (listing is at the end) but there is a small problem:
I have calculated the CRC32 with this program (compiled) and with other commercial programmes with a 3 mb big file.
Fact is that the commercial programmes had done the job nearly immediately and my program needs some seconds !!
I think this is too much time.
Could you give me a faster routine to get the CRC32 from a file ????

Thank you for your efforts

Michael



' ************************************************************************

'// At top level of a module, always include to be sure that all variables have the right type
Option Compare Text

'// Then declare this array variable Crc32Table
Private Crc32Table(255) As Long

'// Then all we have to do is writing public functions like these...
Public Function InitCrc32(Optional ByVal Seed As Long = &HEDB88320, Optional ByVal Precondition As Long = &HFFFFFFFF) As Long

   '// Declare counter variable iBytes, counter variable iBits, value variables lCrc32 and lTempCrc32
   Dim iBytes As Integer, iBits As Integer, lCrc32 As Long, lTempCrc32 As Long

   '// Turn on error trapping
   On Error Resume Next

   '// Iterate 256 times
   For iBytes = 0 To 255

      '// Initiate lCrc32 to counter variable
      lCrc32 = iBytes

      '// Now iterate through each bit in counter byte
      For iBits = 0 To 7
         '// Right shift unsigned long 1 bit
         lTempCrc32 = lCrc32 And &HFFFFFFFE
         lTempCrc32 = lTempCrc32 \ &H2
         lTempCrc32 = lTempCrc32 And &H7FFFFFFF

         '// Now check if temporary is less than zero and then mix Crc32 checksum with Seed value
         If (lCrc32 And &H1) <> 0 Then
            lCrc32 = lTempCrc32 Xor Seed
         Else
            lCrc32 = lTempCrc32
         End If
      Next

      '// Put Crc32 checksum value in the holding array
      Crc32Table(iBytes) = lCrc32
   Next

   '// After this is done, set function value to the precondition value
   InitCrc32 = Precondition

End Function

'// The function above is the initializing function, now we have to write the computation function
Public Function AddCrc32(ByVal Item As String, ByVal Crc32 As Long) As Long

   '// Declare following variables
   Dim bCharValue As Byte, iCounter As Integer, lIndex As Long
   Dim lAccValue As Long, lTableValue As Long

   '// Turn on error trapping
   On Error Resume Next

   '// Iterate through the string that is to be checksum-computed
   For iCounter = 1 To Len(Item)

      '// Get ASCII value for the current character
      bCharValue = Asc(Mid$(Item, iCounter, 1))

      '// Right shift an Unsigned Long 8 bits
      lAccValue = Crc32 And &HFFFFFF00
      lAccValue = lAccValue \ &H100
      lAccValue = lAccValue And &HFFFFFF

      '// Now select the right adding value from the holding table
      lIndex = Crc32 And &HFF
      lIndex = lIndex Xor bCharValue
      lTableValue = Crc32Table(lIndex)

      '// Then mix new Crc32 value with previous accumulated Crc32 value
      Crc32 = lAccValue Xor lTableValue
   Next

   '// Set function value the the new Crc32 checksum
   AddCrc32 = Crc32

End Function

'// At last, we have to write a function so that we can get the Crc32 checksum value at any time
Public Function GetCrc32(ByVal Crc32 As Long) As Long
   '// Turn on error trapping
   On Error Resume Next

   '// Set function to the current Crc32 value
   GetCrc32 = Crc32 Xor &HFFFFFFFF

End Function




Private Sub Command1_Click()

  Dim lCrc32Value As Long
  On Error Resume Next
   
   
  lCrc32Value = InitCrc32()
   
   
   
   
  Open "c:\autoexec.bat" For Binary As #1
   
  whole& = LOF(1) \ 20000
  part& = LOF(1) Mod 20000
  Original$ = String$(20000, 0)
  Cracked$ = String$(20000, 0)
  start& = 1
  Dateigröße = LOF(1)
       
               
  Do
        x& = x& + 1
       
            If x& <= whole& Then
            Get #1, start&, Original$
            Buffergröße = whole&
                   
        Else
           
            Original$ = String$(part&, 0)
                     
            Get #1, start&, Original$
            Buffergröße = part&
             
        End If
               
               
           ' Call CRC32
           lCrc32Value = AddCrc32(Original$, lCrc32Value)
           a$ = Hex$(GetCrc32(lCrc32Value))
           Debug.Print a$
   
       
               start& = start& + 20000
       
               If start& >= Dateigröße Then Exit Do
  Loop
       
  Close
   
  MsgBox "CRC32: " & a$
 

End Sub


Zocko2000Asked:
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VbmasterCommented:
Working with strings are VERY SLOW compared to bytes. Read the file data into a byte array instead of a string, this way you don't have to use for example the Mid$ and Asc (<- very slow function) functions.

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VbmasterCommented:
When you compile it use the advanced settings (remove integer checks and array bound checks), those settings improved my own crc32 alrogithm a lot.

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chris_aCommented:
You can use the OCX on my site http://www.sirocco.f9.co.uk it does all manner of CRCs in ASM language so it is about 800 times quicker than VB, there is a property page extension there also that includes a simple DLL you can use also.

Chris
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VbmasterCommented:
Just wanted to tell you that that was a very kewl OCX, chris_a. It was 9x faster than my VB code so I guess I'll be using that one instead. Just wanted to thank you for a great product! ;)
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chris_aCommented:
Just 9x faster, if you hide the control (progress bar) things should get better, was that v1.2 or the 1.5b (I am just finalising custom crc's)
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VbmasterCommented:
I tested a 4.5Mb and I did hide the control, but now I see that hiding/showing the control does not have any compact on the performance.

The CRC type is the correct CRC32 (CRC32Fwd).

Your OCX takes 0.17 secs and my code takes 1.59  (9.35x)
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chris_aCommented:
Sorry my mistake a disk based file does limit things a bit, but 26MB/sec is not too shabby.

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VbmasterCommented:
Oh forgot to say that, I read it into a byte array first, to eliminate any uneven disk performance.

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chris_aCommented:
Ouch, not just to calculate the CRC I hope, if so try the v1.5B, it does files on the disk async if required
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Zocko2000Author Commented:
Hello Chris,

Thank you very much for your help. Your solution is much faster as I hoped for. Thank you for your help!

Michael
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