1
Я нашел this Delphi экзамен. Предполагается встроить CRC и проверить текущий CRC. Оба должны совпадать, но я получаю разные результаты. Как это исправить? И как его ускорить?Почему встроенный CRC и текущий CRC отличаются?
CRC32Calc.pas
unit CRC32Calc;
interface
uses Classes, SysUtils, windows, messages;
type
Long = record
LoWord: Word;
HiWord: Word;
end;
const
CRCPOLY = $EDB88320;
procedure BuildCRCTable;
function RecountCRC(b: byte; CrcOld: LongWord): LongWord;
function GetCRC32(FileName: string; Full: boolean): string;
function SetEmbeddedCRC(FileName: string): string;
function GetEmbeddedCRC(FileName: string): string;
function BytesToHexStr(pB: PByte; BufSize: LongWord): String;
function HexStrToBytes(Str: String): String;
implementation
var
CRCTable: array [0 .. 512] Of LongWord;
// A helper routine that creates and initializes
// the lookup table that is used when calculating a CRC polynomial
procedure BuildCRCTable;
var
i, j: Word;
r: LongWord;
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;
// A helper routine that recalculates polynomial relative to the specified byte
function RecountCRC(b: byte; CrcOld: LongWord): LongWord;
begin
RecountCRC := CRCTable[byte(CrcOld xor LongWord(b))
] xor ((CrcOld shr 8) and $00FFFFFF)
end;
// A helper routine that converts Word into String
function HextW(w: Word): string;
const
h: array [0 .. 15] Of char = 'ABCDEF';
begin
HextW := '';
HextW := h[Hi(w) shr 4] + h[Hi(w) and $F] + h[Lo(w) shr 4] + h[Lo(w) and $F];
end;
// A helper routine that converts LongWord into String
function HextL(l: LongWord): string;
begin
with Long(l) do
HextL := HextW(HiWord) + HextW(LoWord);
end;
// Calculate CRC32 checksum for the specified file
function GetCRC32(FileName: string; Full: boolean): string;
var
f: TFileStream;
i, CRC: LongWord;
aBt: byte;
begin
// Build a CRC table
BuildCRCTable;
CRC := $FFFFFFFF;
// Open the file
f := TFileStream.Create(FileName, (fmOpenRead or fmShareDenyNone));
// To calculate CRC for the whole file use this loop boundaries
if Full then
for i := 0 to f.Size - 1 do
begin
f.Read(aBt, 1);
CRC := RecountCRC(aBt, CRC);
end
else
// To calculate CRC for the file excluding the last 4 bytes
// use these loop boundaries
for i := 0 to f.Size - 5 do
begin
f.Read(aBt, 1);
CRC := RecountCRC(aBt, CRC);
end;
f.Destroy;
CRC := Not CRC;
Result := HextL(CRC);
end;
// Calculate CRC and writes it to the end of file
function SetEmbeddedCRC(FileName: string): string;
var
f: TFileStream;
CRCOffset: LongWord;
CRC: string;
begin
f := TFileStream.Create(FileName, (fmOpenReadWrite or fmShareDenyNone));
CRCOffset := f.Size;
// Append a placeholder for actual CRC to the file
f.Seek(CRCOffset, TSeekOrigin.soBeginning);
f.Write(PByte(HexStrToBytes('FFFFFFFF'))^, 4);
// Obtain CRC
CRC := GetCRC32(FileName, True);
// Write CRC to the end of file
f.Seek(CRCOffset, TSeekOrigin.soBeginning);
f.Write(PByte(HexStrToBytes(CRC))^, 4);
f.Destroy;
Result := CRC;
end;
// Extract the CRC that was stored at last 4 bytes of a file
function GetEmbeddedCRC(FileName: string): string;
var
f: TFileStream;
CRCOffset: LongWord;
pB: PByte;
begin
GetMem(pB, 4);
// Open file
f := TFileStream.Create(FileName, (fmOpenRead or fmShareDenyNone));
// Proceed upto the end of file
CRCOffset := f.Size - 4;
f.Seek(CRCOffset, TSeekOrigin.soBeginning);
// Read the last four bytes where the CRC is stored
f.Read(pB^, 4);
f.Destroy;
Result := BytesToHexStr(pB, 4);
end;
// A helper routine that converts byte value to string with hexadecimal integer
function BytesToHexStr(pB: PByte; BufSize: LongWord): String;
var
i, j, b: LongWord;
begin
SetLength(Result, 2 * BufSize);
for i := 1 to BufSize do
begin
for j := 0 to 1 do
begin
if j = 1 then
b := pB^ div 16
else
b := pB^ - (pB^ div 16) * 16;
case b of
0:
Result[2 * i - j] := '0';
1:
Result[2 * i - j] := '1';
2:
Result[2 * i - j] := '2';
3:
Result[2 * i - j] := '3';
4:
Result[2 * i - j] := '4';
5:
Result[2 * i - j] := '5';
6:
Result[2 * i - j] := '6';
7:
Result[2 * i - j] := '7';
8:
Result[2 * i - j] := '8';
9:
Result[2 * i - j] := '9';
10:
Result[2 * i - j] := 'A';
11:
Result[2 * i - j] := 'B';
12:
Result[2 * i - j] := 'C';
13:
Result[2 * i - j] := 'D';
14:
Result[2 * i - j] := 'E';
15:
Result[2 * i - j] := 'F';
end;
end;
Inc(pB);
end;
end;
// A helper routine that converts string with hexadecimal integer to byte value
function HexStrToBytes(Str: String): String;
var
b, b2: byte;
lw, lw2, lw3: LongWord;
begin
lw := Length(Str) div 2;
SetLength(Result, lw);
for lw2 := 1 to lw do
begin
b := 0;
for lw3 := 0 to 1 do
begin
case Str[2 * lw2 - lw3] of
'0':
b2 := 0;
'1':
b2 := 1;
'2':
b2 := 2;
'3':
b2 := 3;
'4':
b2 := 4;
'5':
b2 := 5;
'6':
b2 := 6;
'7':
b2 := 7;
'8':
b2 := 8;
'9':
b2 := 9;
'a':
b2 := 10;
'b':
b2 := 11;
'c':
b2 := 12;
'd':
b2 := 13;
'e':
b2 := 14;
'f':
b2 := 15;
'A':
b2 := 10;
'B':
b2 := 11;
'C':
b2 := 12;
'D':
b2 := 13;
'E':
b2 := 14;
'F':
b2 := 15;
else
b2 := 0;
end;
if lw3 = 0 then
b := b2
else
b := b + 16 * b2;
end;
Result[lw2] := char(b);
end;
end;
end.
AppendCRC
program AppendCRC;
{$APPTYPE CONSOLE}
uses
SysUtils, Classes,
CRC32Calc in '..\CRC32Checker\CRC32Calc.pas';
var
FileName: string;
begin
{ TODO -oUser -cConsole Main : Insert code here }
if ParamCount = 1 then
begin
FileName := ParamStr(1);
// Verify whether a file exists
if not FileExists(FileName) then
begin
WriteLn('The specified file does not exist.');
Exit;
end;
WriteLn('Full checksum (before): ' + GetCRC32(FileName, True));
SetEmbeddedCRC(FileName);
WriteLn('Half checksum: ' + GetCRC32(FileName, False));
WriteLn('Full checksum (after): ' + GetCRC32(FileName, True));
WriteLn('GetEmbeddedCRC: :' + GetEmbeddedCRC(FileName));
WriteLn('The checksum was successfully embedded.')
end
else
begin;
WriteLn('Wrong parameters.');
WriteLn('Parameter1 - Full path to file.');;
end;
end.
Мои результаты:
AppendCRC.exe Hello_Delphi_World.exe
Full checksum (before): 1912DA64
Half checksum: 1912DA64
Full checksum (after): B3F0A43E
GetEmbeddedCRC: :4400A000
The checksum was successfully embedded.
Я использую Delphi xe5.
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