IBAN szám ellenőrzése Delphi -vel

Az eredeti ellenőrző algoritmust nem nagyon írtam át, csak beletettem az országkódokat.

function ChangeAlpha(input: string): string;
var a: Char;
begin
  Result := input;
  for a := 'A' to 'Z' do
  begin
    Result := StringReplace(Result, a, IntToStr(Ord(a) - 55), [rfReplaceAll]);
  end;
end;

function CalculateDigits(iban: string): Integer;
var v, l: Integer;
    alpha: string;
    number: Longint;
    rest: Integer;
begin
  iban := UpperCase(iban);
  if Pos('IBAN', iban) > 0 then
  Delete(iban, Pos('IBAN', iban), 4);
  iban := iban + Copy(iban, 1, 4);
  Delete(iban, 1, 4);
  iban := ChangeAlpha(iban);
  v := 1;
  l := 9;
  rest := 0;
  alpha := '';
  try
  while v <= Length(iban) do
  begin
    if l > Length(iban) then
    l := Length(iban);
    alpha := alpha + Copy(iban, v, l);
    number := StrToInt(alpha);
    rest := number mod 97;
    v := v + l;
    alpha := IntToStr(rest);
    l := 9 - Length(alpha);
  end;
  except
  rest := 0;
  end;
  Result := rest;
end;

function IndexOf(stringArray: array of string; partString: String): integer;
var i: Integer;
begin
  Result := -1;
  for i:=0 to Length(stringArray) do
  begin
    if stringArray[i] = partString then
    begin
      Result := i;
      break;
    end;
  end;
end;

function CheckIBAN(iban: string; justcountrycode: boolean): Boolean;
const countrycodes: array[1..54] of string = ('AD','AE','AL','AT','BA','BE','BG','CH','CY','CZ','DE','DK',
       'EE','ES','FI','FO','FR','GB','GE','GI','GL','GR','HR','HU','IE','IL','IS',
       'IT','KW','KZ','LB','LI','LT','LU','LV','MC','ME','MK','MR','MT','MU','NL',
       'NO','PL','PT','RO','RS','SA','SE','SI','SK','SM','TN','TR');

begin
  Result := true;
  iban := StringReplace(iban, ' ', '', [rfReplaceAll]);
  iban := StringReplace(iban, '-', '', [rfReplaceAll]);
  if IndexOf(countrycodes, Copy(iban,1,2)) < 0 then
  begin
    Result := false;
    Exit;
  end;

  if justcountrycode then Exit;
  if CalculateDigits(iban) = 1 then
  Result := True
  else
  Result := False;
end;