type

  TDBMathLib = class(TMathLib)
  private
    { Private declarations }
    FDataSource: TDataSource;
  protected
    { Protected declarations }
    procedure Notification(AComponent: TComponent; AOperation: TOperation); override;
  public
    { Public declarations }
    function HandlesStrFunction(FuncName:string):Boolean; override;
    function CalcStrFunction(FuncName:string;Params:TStringList;var ErrType,ErrParam: Integer):string; override;
  published
    { Published declarations }
    property DataSource: TDataSource read FDataSource write FDataSource;
  end;


implementation

{ TDBMathLib }

function TDBMathLib.CalcStrFunction(FuncName: string; Params: TStringList;
  var ErrType, ErrParam: Integer): string;
var
  s: string;
  n,e: integer;
  fld: TField;
begin
  if (FuncName = ''DBV'') then
  begin
    if Params.Count <> 2 then
    begin
      ErrType := Error_InvalidNrOfParams;
      Exit;
    end;

    if not Assigned(DataSource) then
    begin
      ErrType := Error_NoDataSource;
      Exit;
    end;

    if not Assigned(DataSource.DataSet) then
    begin
      ErrType := Error_NoDataSet;
      Exit;
    end;

    if not DataSource.DataSet.Active then
    begin
      ErrType := Error_NoDataSetActive;
      Exit;
    end;

    s := Params.Strings[0]; // DB FIELD value

    fld := DataSource.DataSet.FieldByName(s);

    if not Assigned(fld) then
    begin
      ErrType := Error_InvalidValue;
      ErrParam := 1;
    end
    else
    begin
      val(Params.Strings[1],n, e);

      DataSource.DataSet.First;
      DataSource.DataSet.MoveBy(n);

      Result := fld.AsString;
    end;
  end;
end;

function TDBMathLib.HandlesStrFunction(FuncName: string): Boolean;
begin
  Result := FuncName = ''DBV'';
end;

procedure TDBMathLib.Notification(AComponent: TComponent;
  AOperation: TOperation);
begin
  inherited;
  if (AOperation = opRemove) and (AComponent = FDataSource) then
    FDataSource := nil;
end;