How to synchronize equal lines in two stringlists

Question

I have two stringlists that I wish to synchronize, so that equal lines get the same index, while different lines will be kept in the list where they originally were, and the other stringlist should get a "filler" for that index. Consider this example:

SL1:  1,1,2,3,5,8 
SL2:  1,3,5,7,9

procedure SyncStringlists(aSL1,aSL2 : TStringList; aFill : string = '-');

The procedure should change the lists to this

SL1:  1,1,2,3,5,8,-,-
SL2:  1,-,-,3,5,-,7,9

or, if the lists are sorted, to this

SL1:  1,1,2,3,5,-,8,-
SL2:  1,-,-,3,5,7,',9

How should I go about doing this?

Answer 1

Try this for the case where your lists are monotone increasing.

procedure SyncStringlists(SL1, SL2: TStringList; const Fill: string='-');
var
  i1, i2: Integer;
begin
  i1 := 0;
  i2 := 0;
  while (i1<SL1.Count) and (i2<SL2.Count) do begin
    if SL1[i1]<SL2[i2] then begin
      SL2.Insert(i2, Fill);
    end else if SL1[i1]>SL2[i2] then begin
      SL1.Insert(i1, Fill);
    end;
    inc(i1);
    inc(i2);
  end;
  while SL1.Count<SL2.Count do begin
    SL1.Add(Fill);
  end;
  while SL2.Count<SL1.Count do begin
    SL2.Add(Fill);
  end;
end;