function TEditForm.IsCorrectTextData(CurrentEdit: TEdit): boolean;
var
    IsCorrectInput: boolean;
    TextLength, i: integer;
begin
    isCorrectInput := false;
    TextLength := length(CurrentEdit.text);
    i := 2;
    if TextLength <= CurrentEdit.MaxLength then
    begin
        if TextLength = 0 then
            IsCorrectInput := true
        else
        begin
            if (TextLength = 1) and (CurrentEdit.text[1] in BIG_LETTERS) then
                IsCorrectInput := True
            else
            begin
                IsCorrectInput := CurrentEdit.text[1] in BIG_LETTERS;
                while IsCorrectInput and (i <= TextLength) do
                begin
                    if not(CurrentEdit.text[i] in SMALL_LETTERS) then
                        IsCorrectInput := false;
                    inc(i);
                end;
            end;
        end;
    end;
    Result := IsCorrectInput;
end;

procedure TEditForm.fillStringGrid(SubscribersStringGrid: TStringGrid;
                            TelephoneEdit, SurnameEdit, NameEdit,
                                PatronymicEdit, CityEdit: TEdit);
var
    i, j, ListLength: Integer;
    PCurrentSubscriber: TSubscriberPointer;
    RequiredSubscriber: TRequiredSubscriber;
begin
    i := 1;
    j := 0;
    SubscriberList.deleteList();
    getRequiredSubscriber(RequiredSubscriber, TelephoneEdit.text, SurnameEdit.text, NameEdit.text,
                                PatronymicEdit.text, CityEdit.text);
    readSubscribers(RequiredSubscriber);
    sortList();
    ListLength := SubscriberList.length();
    setlength(NumberArray, ListLength);
    PCurrentSubscriber := SubscriberList.getHeader()^.next;
    while (PCurrentSubscriber <> nil) do
    begin
        SubscribersStringGrid.RowCount := SubscribersStringGrid.RowCount + 1;
        SubscribersStringGrid.Cells[0, i] := intToStr(i);
        SubscribersStringGrid.Cells[1, i] := PCurrentSubscriber^.telephoneNumber;
        SubscribersStringGrid.Cells[2, i] := PCurrentSubscriber^.surname;
        SubscribersStringGrid.Cells[3, i] := PCurrentSubscriber^.name;
        SubscribersStringGrid.Cells[4, i] := PCurrentSubscriber^.patronymic;
        SubscribersStringGrid.Cells[5, i] := PCurrentSubscriber^.city;
        SubscribersStringGrid.Cells[6, i] := intToStr(PCurrentSubscriber^.numberOfDays);
        NumberArray[j] := PCurrentSubscriber^.number;
        Inc(i);
        inc(j);
        PCurrentSubscriber := PCurrentSubscriber^.next;
    end;
end;

procedure TEditForm.ClearButtonClick(Sender: TObject);
begin
    TelephoneEdit.text := '';
    NameEdit.text := '';
    SurnameEdit.text := '';
    PatronymicEdit.text := '';
    CityEdit.text := '';
    TelephoneRButton.Checked := false;
    SurnameRButton.Checked := false;
    NameRButton.Checked := false;
    PatronymicRButton.Checked := false;
    CityRButton.Checked := false;
    DateRButton.Checked := false;
    CurrentRadioButton := 0;
    SubscribersStringGrid.RowCount := 1;
    EditButton.Enabled := false;
    DeleteButton.Enabled := false;
end;

procedure TEditForm.DataEditKeyPress(Sender: TObject; var Key: Char);
begin
    if (Key <> #08) then
    begin
        if length((Sender as TEdit).text) = 0 then
        begin
            if not (Key in BIG_LETTERS) then
                Key := #0
        end
        else
            if not(Key in SMALL_LETTERS) then
                Key := #0;
    end;
end;

procedure TEditForm.ShowButtonClick(Sender: TObject);
begin
    if areEditsCorrect() then
    begin
        clearStringGrid();
        if not isFileEmpty() then
        begin
            fillStringGrid(SubscribersStringGrid, TelephoneEdit, SurnameEdit, NameEdit,
                            PatronymicEdit, CityEdit);
            setEditButtonsCondition();
        end
        else
            MessageDlg('Subscriber file is empty.', mtError, [mbOk], 0, mbOk);
    end
    else
    begin
        MessageDlg('Wrong data.', mtError, [mbCancel], 0);
        clearButtonClick(Sender);
    end;
end;