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#48: Algorithm of CRC32 calculation for file

If you need calculate a CRC32 for some file or some string, then you must do:
1. build a CRC table
2. calculate a CRC for each line of your file
3. calculate a total CRC

1. CRC table creation:

type
  Long = record
    LoWord: Word;
    HiWord: Word;
  end;

const
  CRCPOLY = $EDB88320;

var
  CRCTable: array[0..512] Of Longint;

procedure BuildCRCTable;
var
  i, j: Word;
  r: Longint;
begin
  FillChar(CRCTable, SizeOf(CRCTable), 0);
  for i := 0 to 255 do
  begin
    r := i shl 1;
    for j := 8 downto 0 do
      if (r and 1) <> 0 then
        r := (r Shr 1) xor CRCPOLY
      else
        r := r shr 1;
    CRCTable[i] := r;
   end;
end;

2. CRC calculation for file:

function RecountCRC(b: byte; CrcOld: Longint): Longint;
begin
  RecountCRC := CRCTable[byte(CrcOld xor Longint(b))] xor ((CrcOld shr 8) and $00FFFFFF)
end;

function HextW(w: Word): string;
const
  h: array[0..15] Of char = '0123456789ABCDEF';
begin
  HextW := '';
  HextW := h[Hi(w) shr 4] + h[Hi(w) and $F] + h[Lo(w) shr 4]+h[Lo(w) and $F];
end;

function HextL(l: Longint): string;
begin
  with Long(l) do
    HextL := HextW(HiWord) + HextW(LoWord);
end;

function GetCRC32(FileName: string): string;
var
  Buffer: PChar;
  f: File of Byte;
  b: array[0..255] of Byte;
  CRC: Longint;
  e, i: Integer;
begin
  BuildCRCTable;
  CRC := $FFFFFFFF;
  AssignFile(F, FileName);
  FileMode := 0;
  Reset(F);
  GetMem(Buffer, SizeOf(B));
  repeat
    FillChar(b, SizeOf(b), 0);
    BlockRead(F, b, SizeOf(b), e);
    for i := 0 to (e-1) do
     CRC := RecountCRC(b[i], CRC);
  until (e < 255) or (IOresult <> 0);
  FreeMem(Buffer, SizeOf(B));
  CloseFile(F);
  CRC := Not CRC;
  Result := '$' + HextL(CRC);
end;

Published: November 30, 1999

See also

	Paradox Viewer
	Database Information Manager
	SMDBGrid
	DBISAM Password Recovery
	dBase Viewer
	SMMsg suite
	Paradox Password Recovery
	MAPIMail
	SMImport suite
	Viewer for MS Outlook Messages

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