Page 1 of 1

Configurable CRC procedure (1 - 32 bits)

Posted: Sat Aug 24, 2019 3:34 pm
by wilbert
After workin on a CRC module ( viewtopic.php?f=12&t=73462 ),
I realized that I could do most of it in a single procedure without the use of asm.
It's not as fast and doesn't support CRC with a width above 32 bits but seems to work fine.
In it's current configuration, the result is the same as the PB builtin CRC32 algorithm.

Some other often used configurations ...
CRC-16/CCITT: Width=16, Poly=$1021, Init=$ffff, RefIn=#False, RefOut=#False, XOrOut=0
CRC-16/MODBUS: Width=16, Poly=$8005, Init=$ffff, RefIn=#True, RefOut=#True, XOrOut=0

For more of them, see http://reveng.sourceforge.net/crc-catalogue/

Code: Select all

Procedure.l CRC(*Buffer, Size)
  
  Static.l Width  = 32
  Static.l Poly   = $04c11db7
  Static.l Init   = -1
  Static.l RefIn  = #True
  Static.l RefOut = #True
  Static.l XOrOut = -1
  
  Static.l M
  Static Dim T.l(1023)
  Protected.l i,j,C,R,*B1.Ascii,*B4.Long
  
  ; Calculate CRC lookup table
  If Not M
    M=~((-2)<<(Width-1))
    If RefIn
      R=0: For i=1 To Width: R|((Poly>>(i-1))&1)<<(Width-i): Next
    Else
      R=Poly<<(32-Width)
    EndIf
    For i=0 To 255
      If RefIn
        C=i: For j=0 To 7: C=((C>>1)&$7fffffff)!((-(C&1))&R): Next
      Else
        C=i<<24: For j=0 To 7: C=(C<<1)!((C>>31)&R): Next
        C=(C<<24)|(((C>>8)&255)<<16)|(((C>>16)&255)<<8)|((C>>24)&255)
      EndIf
      T(i)=C
    Next
    For i=0 To 255
      C=T(i): For j=1 To 3: C=((C>>8)&$ffffff)!T(C&255): T((j<<8)|i)=C: Next
    Next    
  EndIf
  
  ; 4 byte loop
  C=Init&M: *B4=*Buffer
  While Size >= 4
    C!*B4\l: *B4+4: Size-4
    C=T(C&255+768)!T(((C>>8)&255)+512)!T(((C>>16)&255)+256)!T(((C>>24)&255))
  Wend
  
  ; 1 byte loop
  *B1=*B4
  While Size
    C=((C>>8)&$ffffff)!T((C!*B1\a)&255)
    *B1+1: Size-1
  Wend
  
  ; Finalize and output CRC
  If Not RefIn
    C=((C<<24)|(((C>>8)&255)<<16)|(((C>>16)&255)<<8)|((C>>24)&255))>>(32-Width)
  EndIf  
  If RefOut<>RefIn
    R=0: For i=1 To Width: R|((C>>(i-1))&1)<<(Width-i): Next: C=R
  EndIf
  ProcedureReturn (C!XOrOut)&M  
    
EndProcedure

Re: Configurable CRC procedure (1 - 32 bits)

Posted: Sat Aug 24, 2019 7:16 pm
by Kwai chang caine
That works to on the same machine :wink:
I obtain "378246432" all the time

Code: Select all

*Mem = AllocateMemory(1234567, #PB_Memory_NoClear)
RandomSeed(0)
RandomData(*Mem, 1234567)
Debug CRC(*Mem, 1234567)
FreeMemory(*Mem)