Is it possible to delete bytes from the beginning of a file?

Question

I know that I can efficiently truncate a file and remove bytes from the end of the file.

Is there a corresponding efficient way to truncate files by deleting content fr

Answer 1

A very simple solution would be to shift (move) blocks of data from the "target position offset" towards BOF, and then trim (truncate) the leftovers:

--------------------------
|******|xxxxxx|yyyyyy|zzz|
--------------------------
BOF  <-^ (target position offset)


--------------------------
|xxxxxx|yyyyyy|zzz|******|
--------------------------
                  ^ EOF

---

Since @David posted a code based on TStream, here is some code based on "low level" I/O pascal style:

function FileDeleteFromBOF(const FileName: string; const Offset: Cardinal): Boolean;
var
  Buf: Pointer;
  BufSize, FSize,
  NumRead, NumWrite,
  OffsetFrom, OffsetTo: Cardinal;
  F: file;
begin
  {$IOCHECKS OFF}
  Result := False;
  AssignFile(F, FileName);
  try
    FileMode := 2; // Read/Write
    Reset(F, 1); // Record size = 1
    FSize := FileSize(F);
    if (IOResult <> 0) or (Offset >= FSize) then Exit;
    BufSize := Min(Offset, 1024 * 64); // Max 64k - This value could be optimized
    GetMem(Buf, BufSize);
    try
      OffsetFrom := Offset;
      OffsetTo := 0;
      repeat
        Seek(F, OffsetFrom);
        BlockRead(F, Buf^, BufSize, NumRead);
        if NumRead = 0 then Break;
        Seek(F, OffsetTo);
        BlockWrite(F, Buf^, NumRead, NumWrite);
        Inc(OffsetFrom, NumWrite);
        Inc(OffsetTo, NumWrite);
      until (NumRead = 0) or (NumWrite <> NumRead) or (OffsetFrom >= FSize);
      // Truncate and set to EOF
      Seek(F, FSize - Offset);
      Truncate(F);
      Result := IOResult = 0;
    finally
      FreeMem(Buf);
    end;
  finally
    CloseFile(F);
  end;
end;