CRC32 (Cyclic Redundency Check)

One of the most effective method to check if the file is ok is to calculate CRC32 checksum and compare it with previously saved value. Using CRC check, you can also compare two files, verify downloaded files etc…
Even if the source code is rather long, the function itself is short and not complicated. It reads file “byte by byte” and calculate checksum using some formula. It is not really necessary to understand this algorithm, don’t bother yourself with “magic” numbers in array. Just use code as it is.

unit Unit1;
 
interface
 
uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs, StdCtrls;
 
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
 TForm1 = class(TForm)
 Button1: TButton;
 procedure Button1Click(Sender: TObject);
 private
 { Private declarations }
 public
 { Public declarations }
 end;
 
var
 Form1: TForm1;
 
implementation
 
{$R *.dfm}
 
procedure CalcCRC32(FileName: String; var CRC32: dword);
var F: file;
 BytesRead: dword;
 Buffer: Array[1..65521] of byte;
 i: Word;
begin
 FileMode := 0;
 CRC32 := $ffffffff;
 {$I-}
 AssignFile(F, FileName);
 Reset(F, 1);
 if IOResult = 0 then
 begin
 repeat
 BlockRead(F, Buffer, SizeOf(Buffer), BytesRead);
 for i := 1 to BytesRead do CRC32 := (CRC32 shr 8) xor Table[Buffer[i] xor (CRC32 and $000000FF)];
 until BytesRead = 0;
 end;
 CloseFile(F);
 {$I+}
 CRC32 := not CRC32;
end;
 
procedure TForm1.Button1Click(Sender: TObject);
var OpenDialog: TOpenDialog;
 CRC: Dword;
begin
 OpenDialog := TOpenDialog.Create(nil);
 if OpenDialog.Execute then
 begin
 CalcCRC32(OpenDialog.FileName, crc);
 if crc&amp;lt;&amp;gt;0 then ShowMessage(IntToHex(crc,6));
 end;
 OpenDialog.Free;
end;
 
end.

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unit Unit1;

interface

uses

Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

Dialogs, StdCtrls;

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

TForm1 = class(TForm)

Button1: TButton;

procedure Button1Click(Sender: TObject);

private

{ Private declarations }

public

{ Public declarations }

end;

var

Form1: TForm1;

implementation

{$R *.dfm}

procedure CalcCRC32(FileName: String; var CRC32: dword);

var F: file;

BytesRead: dword;

Buffer: Array[1..65521] of byte;

i: Word;

begin

FileMode := 0;

CRC32 := $ffffffff;

{$I-}

AssignFile(F, FileName);

Reset(F, 1);

if IOResult = 0 then

begin

repeat

BlockRead(F, Buffer, SizeOf(Buffer), BytesRead);

for i := 1 to BytesRead do CRC32 := (CRC32 shr 8) xor Table[Buffer[i] xor (CRC32 and $000000FF)];

until BytesRead = 0;

end;

CloseFile(F);

{$I+}

CRC32 := not CRC32;

end;

procedure TForm1.Button1Click(Sender: TObject);

var OpenDialog: TOpenDialog;

CRC: Dword;

begin

OpenDialog := TOpenDialog.Create(nil);

if OpenDialog.Execute then

begin

CalcCRC32(OpenDialog.FileName, crc);

if crc&lt;&gt;0 then ShowMessage(IntToHex(crc,6));

end;

OpenDialog.Free;

end;

end.

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CRC32 (Cyclic Redundency Check)