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How to identifying a file through CRC 32

Title: How to identifying a file through CRC-32 // The constants here are for the CRC-32 generator // polynomial, as defined in the Microsoft // Systems Journal, March 1995, pp. 107-108 const Table: array[0..255] of DWORD = ($00000000, $77073096, $EE0E612C, $990951BA, $076DC419, $706AF48F, $E963A535, $9E6495A3, $0EDB8832, $79DCB8A4, $E0D5E91E, $97D2D988, $09B64C2B, $7EB17CBD, $E7B82D07, $90BF1D91, $1DB71064, $6AB020F2, $F3B97148, $84BE41DE, $1ADAD47D, $6DDDE4EB, $F4D4B551, $83D385C7, $136C9856, $646BA8C0, $FD62F97A, $8A65C9EC, $14015C4F, $63066CD9, $FA0F3D63, $8D080DF5, $3B6E20C8, $4C69105E, $D56041E4, $A2677172, $3C03E4D1, $4B04D447, $D20D85FD, $A50AB56B, $35B5A8FA, $42B2986C, $DBBBC9D6, $ACBCF940, $32D86CE3, $45DF5C75, $DCD60DCF, $ABD13D59, $26D930AC, $51DE003A, $C8D75180, $BFD06116, $21B4F4B5, $56B3C423, $CFBA9599, $B8BDA50F, $2802B89E, $5F058808, $C60CD9B2, $B10BE924, $2F6F7C87, $58684C11, $C1611DAB, $B6662D3D, $76DC4190, $01DB7106, $98D220BC, $EFD5102A, $71B18589, $06B6B51F, $9FBFE4A5, $E8B8D433, $7807C9A2, $0F00F934, $9609A88E, $E10E9818, $7F6A0DBB, $086D3D2D, $91646C97, $E6635C01, $6B6B51F4, $1C6C6162, $856530D8, $F262004E, $6C0695ED, $1B01A57B, $8208F4C1, $F50FC457, $65B0D9C6, $12B7E950, $8BBEB8EA, $FCB9887C, $62DD1DDF, $15DA2D49, $8CD37CF3, $FBD44C65, $4DB26158, $3AB551CE, $A3BC0074, $D4BB30E2, $4ADFA541, $3DD895D7, $A4D1C46D, $D3D6F4FB, $4369E96A, $346ED9FC, $AD678846, $DA60B8D0, $44042D73, $33031DE5, $AA0A4C5F, $DD0D7CC9, $5005713C, $270241AA, $BE0B1010, $C90C2086, $5768B525, $206F85B3, $B966D409, $CE61E49F, $5EDEF90E, $29D9C998, $B0D09822, $C7D7A8B4, $59B33D17, $2EB40D81, $B7BD5C3B, $C0BA6CAD, $EDB88320, $9ABFB3B6, $03B6E20C, $74B1D29A, $EAD54739, $9DD277AF, $04DB2615, $73DC1683, $E3630B12, $94643B84, $0D6D6A3E, $7A6A5AA8, $E40ECF0B, $9309FF9D, $0A00AE27, $7D079EB1, $F00F9344, $8708A3D2, $1E01F268, $6906C2FE, $F762575D, $806567CB, $196C3671, $6E6B06E7, $FED41B76, $89D32BE0, $10DA7A5A, $67DD4ACC, $F9B9DF6F, $8EBEEFF9, $17B7BE43, $60B08ED5, $D6D6A3E8, $A1D1937E, $38D8C2C4, $4FDFF252, $D1BB67F1, $A6BC5767, $3FB506DD, $48B2364B, $D80D2BDA, $AF0A1B4C, $36034AF6, $41047A60, $DF60EFC3, $A867DF55, $316E8EEF, $4669BE79, $CB61B38C, $BC66831A, $256FD2A0, $5268E236, $CC0C7795, $BB0B4703, $220216B9, $5505262F, $C5BA3BBE, $B2BD0B28, $2BB45A92, $5CB36A04, $C2D7FFA7, $B5D0CF31, $2CD99E8B, $5BDEAE1D, $9B64C2B0, $EC63F226, $756AA39C, $026D930A, $9C0906A9, $EB0E363F, $72076785, $05005713, $95BF4A82, $E2B87A14, $7BB12BAE, $0CB61B38, $92D28E9B, $E5D5BE0D, $7CDCEFB7, $0BDBDF21, $86D3D2D4, $F1D4E242, $68DDB3F8, $1FDA836E, $81BE16CD, $F6B9265B, $6FB077E1, $18B74777, $88085AE6, $FF0F6A70, $66063BCA, $11010B5C, $8F659EFF, $F862AE69, $616BFFD3, $166CCF45, $A00AE278, $D70DD2EE, $4E048354, $3903B3C2, $A7672661, $D06016F7, $4969474D, $3E6E77DB, $AED16A4A, $D9D65ADC, $40DF0B66, $37D83BF0, $A9BCAE53, $DEBB9EC5, $47B2CF7F, $30B5FFE9, $BDBDF21C, $CABAC28A, $53B39330, $24B4A3A6, $BAD03605, $CDD70693, $54DE5729, $23D967BF, $B3667A2E, $C4614AB8, $5D681B02, $2A6F2B94, $B40BBE37, $C30C8EA1, $5A05DF1B, $2D02EF8D); type //----------------------------------crc32---------------------------------- {$IFDEF VER130} // This is a bit awkward // 8-byte integer TInteger8 = Int64; // Delphi 5 {$ELSE} {$IFDEF VER120} TInteger8 = Int64; // Delphi 4 {$ELSE} TInteger8 = COMP; // Delphi 2 or 3 {$ENDIF} {$ENDIF} //----------------------------------crc32---------------------------------- // Use CalcCRC32 as a procedure so CRCValue can be passed in but // also returned. This allows multiple calls to CalcCRC32 for // the "same" CRC-32 calculation. procedure CalcCRC32(p: Pointer; ByteCount: DWORD; var CRCValue: DWORD); // The following is a little cryptic (but executes very quickly). // The algorithm is as follows: // 1. exclusive-or the input byte with the low-order byte of // the CRC register to get an INDEX // 2. shift the CRC register eight bits to the right // 3. exclusive-or the CRC register with the contents of Table[INDEX] // 4. repeat steps 1 through 3 for all bytes var i: DWORD; q: ^BYTE; begin q := p; for i := 0 to ByteCount - 1 do begin CRCvalue := (CRCvalue shr 8) xor Table[q^ xor (CRCvalue and $000000FF)]; Inc(q) end end {CalcCRC32}; function CalcStringCRC32(s: string; out CRC32: DWORD): Boolean; var CRC32Table: DWORD; begin // Verify the table used to compute the CRCs has not been modified. // Thanks to Gary Williams for this suggestion, Jan. 2003. CRC32Table := $FFFFFFFF; CalcCRC32(Addr(Table[0]), SizeOf(Table), CRC32Table); CRC32Table := not CRC32Table; if CRC32Table $6FCF9E13 then ShowMessage('CRC32 Table CRC32 is ' + IntToHex(Crc32Table, 8) + ', expecting $6FCF9E13') else begin CRC32 := $FFFFFFFF; // To match PKZIP if Length(s) 0 // Avoid access violation in D4 then CalcCRC32(Addr(s[1]), Length(s), CRC32); CRC32 := not CRC32; // To match PKZIP end; end; procedure CalcFileCRC32(FromName: string; var CRCvalue: DWORD; var TotalBytes: TInteger8; var error: Word); var Stream: TMemoryStream; begin error := 0; CRCValue := $FFFFFFFF; Stream := TMemoryStream.Create; try try Stream.LoadFromFile(FromName); if Stream.Size 0 then CalcCRC32(Stream.Memory, Stream.Size, CRCvalue) except on E: EReadError do error := 1 end; CRCvalue := not CRCvalue finally Stream.Free end; end; procedure TForm1.Button1Click(Sender: TObject); var s: string; CRC32: DWORD; begin s := 'Test String'; if CalcStringCRC32(s, CRC32) then ShowMessage(IntToStr(crc32)); end;