Delphi - THashedStringList incremental search

Question

I have a THashedStringList with almost 200k items on it(strings and objects). This list will be heavy used on searching items on it. In some cases I need an incremental search on it. I've wrote a rudimentary example for an incremental search using the StartsText routine

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs,
  System.IniFiles, Vcl.StdCtrls,
  System.StrUtils;

type
  TForm1 = class(TForm)
    Memo1: TMemo;
    Edit1: TEdit;
    Memo2: TMemo;
    procedure FormCreate(Sender: TObject);
    procedure Edit1KeyUp(Sender: TObject; var Key: Word; Shift: TShiftState);
  private
    { Private declarations }
  public
  ahsStrLst : THashedStringList;
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function generate(cantidad: integer): string;
const
  letras_mi = 'abcdefghijklmnopqrstuvwxyz';
 var
  i: Integer;
begin
  Result := '';
  for I := 0 to cantidad-1 do
    Result := Result + letras_mi[Random(Length(letras_mi)) + 1];
end;

procedure TForm1.Edit1KeyUp(Sender: TObject; var Key: Word; Shift: TShiftState);
var iPos: Integer;
begin
 Memo2.Clear;
 Memo2.Update;
 for ipos := 0 to ahsStrLst.Count-1 do
  if StartsText(Edit1.Text,ahsStrLst[iPos]) then
   Memo2.Lines.Add(ahsStrLst[iPos])
end;

procedure TForm1.FormCreate(Sender: TObject);
var ipos:Integer;
begin
  ahsStrLst := THashedStringList.Create;
  for iPos := 0 to 50000 do
   ahsStrLst.AddObject(generate(4),TObject(ipos));//here there will be diffent type of objects
  for iPos := 0 to 50000 do
   ahsStrLst.AddObject(generate(6),TObject(ipos));
  for iPos := 0 to 50000 do
   ahsStrLst.AddObject(generate(8),TObject(ipos));
  for iPos := 0 to 50000 do
   ahsStrLst.AddObject(generate(10),TObject(ipos));

   Memo1.Lines.AddStrings(ahsStrLst);
end;

end.

Is there any way to speed up this incremental search?

The whole point of hashing structures is that: if you know the **key** you can use the ***entire*** key to generate a number telling you where to find the item. Basically the only way you can meet your requirement with a hashing structure would be to iterate your entire collection; which would be extremely inefficient. — Disillusioned, Jun 04 '15 at 18:49

David Heffernan · Accepted Answer · 2015-06-04T22:21:31.957

For a start, you should stop using THashedStringList. Once you've got a Delphi with generics you should use TDictionary<K,V> instead. It presents a much cleaner interface, and performs better. However, in this case, a demand for partial matching renders hash based lookup helpless. You need a different data structure.

For efficient partial match lookup you can consider maintaining an ordered list. Then you can perform the lookup using bisection. Because you are performing partial matches you'll have to craft your own bisection because all the facilities provided by the RTL search for single items.

Suppose that you are searching for 'abc'. The first of all find the largest value that is < 'abc'. Your partial matches starts with the item that follows. Then find the largest value that begins with 'abc'. Your partial matches end with that item. If no such item exists, there are no matches.

Delphi - THashedStringList incremental search

1 Answers1