Delphi XE2 TZipFile: replace a file in zip archive - delphi

Question

I'd like to replace a file (= delete old and add new) in a zip archive with the Delphi XE2/XE3 standard System.Zip unit. But there are no replace/delete methods. Does anybody have an idea how it could be achieved without needing to extract all files and add them to a new archive?
I have this code, but it adds the "document.txt" once more if it's already present:
var
ZipFile: TZipFile;
SS: TStringStream;
const
ZipDocument = 'E:\document.zip';
begin
ZipFile := TZipFile.Create; //Zipfile: TZipFile
SS := TStringStream.Create('hello');
try
if FileExists(ZipDocument) then
ZipFile.Open(ZipDocument, zmReadWrite)
else
ZipFile.Open(ZipDocument, zmWrite);
ZipFile.Add(SS, 'document.txt');
ZipFile.Close;
finally
SS.Free;
ZipFile.Free;
end;
end;
Note: I used TPAbbrevia before (that did the job), but I'd like to use Delphi's Zip unit now. So please do not answer something like "use another library". Thank you.

Answer 1

I'd recommend Abbrevia because I'm biased :), you already know it, and it doesn't require any hacks. Barring that, here's your hack:
type
TZipFileHelper = class helper for TZipFile
procedure Delete(FileName: string);
end;
{ TZipFileHelper }
procedure TZipFileHelper.Delete(FileName: string);
var
i, j: Integer;
StartOffset, EndOffset, Size: UInt32;
Header: TZipHeader;
Buf: TBytes;
begin
i := IndexOf(FileName);
if i <> -1 then begin
// Find extents for existing file in the file stream
StartOffset := Self.FFiles[i].LocalHeaderOffset;
EndOffset := Self.FEndFileData;
for j := 0 to Self.FFiles.Count - 1 do begin
if (Self.FFiles[j].LocalHeaderOffset > StartOffset) and
(Self.FFiles[j].LocalHeaderOffset <= EndOffset) then
EndOffset := Self.FFiles[j].LocalHeaderOffset;
end;
Size := EndOffset - StartOffset;
// Update central directory header data
Self.FFiles.Delete(i);
for j := 0 to Self.FFiles.Count - 1 do begin
Header := Self.FFiles[j];
if Header.LocalHeaderOffset > StartOffset then begin
Header.LocalHeaderOffset := Header.LocalHeaderOffset - Size;
Self.FFiles[j] := Header;
end;
end;
// Remove existing file stream
SetLength(Buf, Self.FEndFileData - EndOffset);
Self.FStream.Position := EndOffset;
if Length(Buf) > 0 then
Self.FStream.Read(Buf[0], Length(Buf));
Self.FStream.Size := StartOffset;
if Length(Buf) > 0 then
Self.FStream.Write(Buf[0], Length(Buf));
Self.FEndFileData := Self.FStream.Position;
end;
end;
Usage:
ZipFile.Delete('document.txt');
ZipFile.Add(SS, 'document.txt');