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.{:c;'09AZazüÜßß'2/{{}/c<\c>|!}%{|}*}:p,.2%\(;2%+:t;{.p{t(\:t;}@if}%

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const
  LETTER_CHARS = ['a' .. 'z', 'A' .. 'Z', 'ä', 'ö', 'ü', 'Ä', 'Ö', 'Ü',
    '0'..'9', 'ß'];

type
  TCryptAction = (caEncrypt, caDecrypt);

procedure IncPos(var APosition: PChar; AEndPos: PChar; ACount: Integer;
  AIgnoreBrackets: Boolean);
var
  InsideBrackets: Boolean;
begin
  InsideBrackets := False;
  while (ACount >= 0) and (APosition < AEndPos) do
  begin
    while (not CharInSet(APosition^, LETTER_CHARS)
      or (InsideBrackets and not AIgnoreBrackets))
      and (APosition < AEndPos) do
    begin
      if CharInSet(APosition^, ['[', ']']) then
        InsideBrackets := not InsideBrackets;
      Inc(APosition);
    end;
    if (ACount > 0) and (APosition < AEndPos) then
      Inc(APosition);
    Dec(ACount);
  end;
end;

function Crypt(const AValue: string; AAction: TCryptAction;
  AIgnoreBrackets: Boolean): string;
var
  SecondRun: Boolean;
  CurrentEncPos, CurrentDecPos, EncEndPos, DecEndPos, EncStartPos: PChar;
begin
  SetString(Result, PChar(AValue), Length(AValue));
  if AAction = caEncrypt then
  begin
    CurrentEncPos := PChar(Result);
    CurrentDecPos := PChar(AValue);
  end
  else
  begin
    CurrentEncPos := PChar(AValue);
    CurrentDecPos := PChar(Result);
  end;
  EncEndPos := CurrentEncPos + Length(AValue);
  DecEndPos := CurrentDecPos + Length(AValue);
  while not CharInSet(CurrentEncPos^, LETTER_CHARS) do
    Inc(CurrentEncPos);
  while not CharInSet(CurrentDecPos^, LETTER_CHARS) do
    Inc(CurrentDecPos);
  EncStartPos := CurrentEncPos;
  SecondRun := False;
  while CurrentEncPos < EncEndPos do
  begin
    if AAction = caEncrypt then
      CurrentEncPos^ := CurrentDecPos^
    else
      CurrentDecPos^ := CurrentEncPos^;
    IncPos(CurrentEncPos, EncEndPos, 2, AIgnoreBrackets);
    IncPos(CurrentDecPos, DecEndPos, 1, AIgnoreBrackets);
    if not SecondRun and (CurrentEncPos = EncEndPos) then
    begin
      SecondRun := True;
      CurrentEncPos := EncStartPos + 1;
      while not CharInSet(CurrentEncPos^, LETTER_CHARS) do
        Inc(CurrentEncPos);
    end;
  end;
end;

procedure TfrmAprilDF01Main.btnDecryptClick(Sender: TObject);
begin
  memDecrypted.Text := Crypt(memEncrypted.Text, caDecrypt,
    chbIgnoreBrackets.Checked);
end;

procedure TfrmAprilDF01Main.btnEncryptClick(Sender: TObject);
begin
  memEncrypted.Text := Crypt(memDecrypted.Text, caEncrypt,
    chbIgnoreBrackets.Checked);
end;