if vBeginning >= fBufferSize then // deletion range outside the buffer.
    Exit;

  if (vBeginning >= fGapStart) and (vBeginning < fGapEnd) then begin { deletions inside the gap are weird, so skip the gap. }
    assert(vBeginning = fGapStart); // postcondition of #PositionFromTextMark
    {Inc(vBeginning, fGapEnd - fGapStart);}
    vBeginning := fGapEnd;
  end;
  
  if (vEnd >= fGapStart) and (vEnd < fGapEnd) then begin { don't shrink the gap. }
    vEnd := fGapStart;
  end;

  assert((vBeginning < fGapStart) or (vBeginning >= fGapEnd));

  if vBeginning >= vEnd then // deletion range empty.
    Exit;
  
  vMovingBlockSize := fBufferSize - vEnd;
  vGapAffectedP := (vBeginning < fGapStart){ must be and (vEnd >= fGapStart)}; // and (vBeginning < fGapEnd and vEnd >= fGapStart), that being the more obvious condition.

  // the gap can only be used when vEnd [>]= fGapStart or vBeginning [<]= fGapEnd, otherwise we need to move it which is associated with a cost proportional to the relative movement of the gap.
  // abcGGGGdef
  //  ^ ^
  //  B E
  
  // abcGGGGdef
  //        ^ ^
  //        B E

  vIntersectionStart := Max(vBeginning, fGapStart);
  vIntersectionEnd1 := Min(vEnd, fGapEnd);

  vOffset := vBeginning - vEnd;

  if vIntersectionStart < vIntersectionEnd1 then begin // there is an intersection
    assert(vGapAffectedP);
  end else begin // there is no intersection with the gap
    vIntersectionStart := vEnd;
    vIntersectionEnd1 := vEnd;
    assert(not vGapAffectedP);
  end;

  { optimized path: when the deletion range is adjacent to the gap. }

  if vIntersectionStart = fGapStart { TODO and some gap size limit is not exceeded } then begin
    fGapStart := vBeginning;
    FillChar(fBuffer[fGapStart], vIntersectionStart - fGapStart, 0); { for better debugging. }
    vIntersectionStart := fGapStart;
  end;
  
  if vIntersectionEnd1 = fGapEnd { TODO and some gap size limit is not exceeded } then begin
    fGapEnd := vEnd;
    FillChar(fBuffer[vIntersectionEnd1], fGapEnd - vIntersectionEnd1, 0); { for better debugging. }
    vIntersectionEnd1 := fGapEnd;
  end;
  
  { normal path. }
  
  { destination:
      [ vBeginning : vIntersectionStart [ is the first range to be deleted, if any.
      [ vIntersectionEnd1 : vEnd [ is the second range to be deleted, if any.
      both do not intersect the gap.
  }
  
  { vOffset is how much data needs to be moved. In case the fast paths did half of the work, update it. }
  
  vOffset := (vIntersectionStart - vBeginning) + (vEnd - vIntersectionEnd1);
  
  // source of the new data is the first chunk that is NOT in the deletion range, i.e. [ fEnd : fBufferSize [ X not [ fGapStart, fGapEnd [