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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
 
MAPIMail
Paradox to MS Access converter
Protected Storage Viewer
Database Information Manager
Excel Reader (dll)
SMDBGrid
ExcelFile Viewer
dBase Viewer
SMImport suite
Viewer for MS Outlook Messages
 
 


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