控制鼠标以屏幕中一点为中心螺旋遍历

[取昵称麻烦]

基础没学好，求算法
控制鼠标以屏幕中一点为中心螺旋遍历，如下图：

[wdonghai]

procedure TForm1.DrawPT(AX, AY, ASWidth,ASHeight: Integer);
const
  cJG=20;
  cFX:array[1..4,0..1] of Integer=((+1,-1),(-1,-1),(-1,+1),(+1,+1));
var
  i,k,iC:integer;
  pt:PPoint;
  b:Boolean;
  sList:TStringList;

  procedure ClearList;
  begin
    while sList.Count>0 do
    begin
      pt:=PPoint(sList.Objects[0]);
      Dispose(pt);
      sList.Delete(0);
    end;
  end;

  function CheckPT(APT:PPoint):Boolean;
  var
    x,y:Integer;
  begin
    x:=AX+(APT.X-AX)*cJG;
    y:=AY+(APT.Y-AY)*cJG;

    result:=(x>0) and (x<=ASWidth-cJG*2) and (y>0) and (y<ASHeight-cJG*2);
  end;

  procedure AddPT(APT:PPoint);
  begin
    b:=CheckPT(APT);
    if b then
    begin
      inc(iC);
      sList.AddObject(IntToStr(iC),TObject(APT));
    end;
  end;
begin
  iC:=0;
  sList:=TStringList.Create;
  ClearPT;
  i:=1;
  b:=true;
  while b do
  begin
    new(pt);
    pt.X:=AX+i;
    pt.Y:=AY+(i-1);
    AddPT(pt);

    k:=1;
    while (k<=4) and b do
    begin
      new(pt);
      pt.X:=AX+cFX[k][0]*i;
      pt.Y:=AY+cFX[k][1]*i;
      AddPT(pt);
      inc(k);
    end;
    inc(i);
  end;

  Canvas.Pen.Color:=clBlue;
  Canvas.MoveTo(AX,AY);
  for i:=0 to iC-1 do
  begin
    pt:=PPoint(sList.Objects[i]);
    pt.X:=AX+(pt.X-AX)*cJG;
    pt.Y:=AY+(pt.Y-AY)*cJG;
    Canvas.LineTo(pt.X,pt.Y);
    sList.Add(IntToStr(pt.X)+','+IntToStr(pt.Y));
  end;
  ClearPT;
  sList.Free;
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Canvas.FillRect(ClientRect);
  DrawPT(X,Y,Width,Height);
end;

[取昵称麻烦]

谢谢，我借鉴一下。