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<>0 then ShowMessage(IntToHex(crc,6));
  end;
  OpenDialog.Free;
end;

end.

You can compare results of the function with CRC check in Total Commander. Results should be same.