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"Cyclical Redundancy Check - Two versions of CRC16", by andy_a

CRC's ensure that data is not corrupted during transmission. Whether loading a file from disk or transferring via modem CRC's ensure data integrity.

Found a QBasic listing (author unknown) here:
www.fys.ruu.nl/
bergmann/crc.bas

Converted to Blitz 2005.1.6

Disclaimer: I program as a hobby. DON'T use this on *ANYTHING* important. User assumes ALL responsibility. Use at your own risk!

Code

;CRC16 & CRC32 functions in Blitz BASIC
;Converted from a QBasic listing (author unknown)
;
;Downloaded from here: www.fys.ruu.nl/
bergmann/crc.bas
;
;Converted to Blitz 2005.1.6

Dim Power(7)                                                  ;For the 8 Powers of 2

For I = 0 To 7                                                  ;Calculate Once To Increase
     Power(I) = 2 ^ I                                        ;Speed Within Fuctions
Next

;simple usage.
Print "CRC16 is:"+CRC16("Google is your friend...")
Print "CRC16 is:"+CRC16("Google is your freind...")

Print " "

Print "CRC32 is:"+CRC16("Google is your friend...")
Print "CRC32 is:"+CRC16("Google is your freind...")

a$ = Input("Press Enter to exit")

End

Function CRC16% (B$)                                        ;Calculates CRC For Each Block
;NB: CRC-16/CITT implementation
     CRC% = 0                                                  ;Reset For Each Text Block
     For I = 1 To Len(B$)                                   ;Calculate For Length of Block
             ByteVal = Asc(Mid$(B$, I, 1))
          For J = 7 To 0 Step -1
                TestBit = ((CRC And 32768) = 32768) Xor ((ByteVal And Power(J)) = Power(J))
                CRC = ((CRC And 32767) * 2)
                If TestBit Then CRC = CRC Xor 4129          ;&H1021     ; <-- This For 16 Bit CRC
             Next
     Next 
     Return CRC                                                  ;Return the Word Value
End Function

;=================== NOTE: =======================
CRC32 (below) is the 'ARC' implementation of CRC16
;=================================================
; For true CRC32 see: http://www.blitzbasic.com/codearcs/codearcs.php?code=790

Function CRC32% (B$)                                        ;Calculates CRC For Each Block
     CRC% = 0                                                  ;Reset For Each Text Block
     For I = 1 To Len(B$)                                   ;Calculate For Length of Block
        ByteVal = Asc(Mid$(B$, I, 1))
        For J = 7 To 0 Step -1
           TestBit = ((CRC And 32768) = 32768) Xor ((ByteVal And Power(J)) = Power(J))
           CRC = ((CRC And 32767) * 2)
           If TestBit Then CRC = CRC Xor 32773          ;&H8005   ; <-- This For 32 Bit CRC
        Next
     Next
     Return CRC                                                  ;Return the Word Value
End Function

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