Author: Lou Adler

How to rewrite the last line of text in a text file 

Answer:
1   
2   procedure RewriteLastTextLine(AFileName: string; ANewTextLine: string);
3   const
4     BUFFER_SIZE = 1024; {change this number for different sized buffer}
5     CRLF = #13#10;
6   var
7     fs: TFileStream;
8     buf: PChar;
9     iStartWritePos: Int64;
10  
11    function AssignPos: Boolean;
12    var
13      i: Integer;
14    begin
15      for i := BUFFER_SIZE - 1 downto 0 do
16        if (buf[i] = #13) then
17        begin
18          iStartWritePos := (iStartWritePos - (BUFFER_SIZE - i));
19          Result := True;
20          Exit;
21        end;
22      Result := False;
23    end;
24  
25    procedure ReadABuffer;
26    begin
27      fs.Position := fs.Position - BUFFER_SIZE;
28      fs.read(buf^, BUFFER_SIZE);
29      fs.Position := fs.Position - BUFFER_SIZE;
30    end;
31  
32  begin
33    fs := TFileStream.Create(AFileName, fmOpenReadWrite or fmShareDenyWrite);
34    try
35      GetMem(buf, BUFFER_SIZE);
36      FillMemory(buf, BUFFER_SIZE, 0);
37      fs.Position := fs.Size;
38      iStartWritePos := fs.Position;
39      repeat
40        ReadABuffer
41      until
42        AssignPos;
43      fs.Position := iStartWritePos;
44      fs.write(CRLF, Length(CRLF));
45      fs.write(ANewTextLine[1], Length(ANewTextLine));
46    finally
47      FreeMem(buf, BUFFER_SIZE);
48      fs.Free;
49    end;
50  end;