64-битное кодирование

{ 64-битное декодирование файлов }
{ Arne de Bruijn }
uses dos;
var

  Base64: array[43..122] of byte;
var

  T: text;
  Chars: set of char;
  S: string;
  K, I, J: word;
  Buf: pointer;
  DShift: integer;
  F: file;
  B, B1: byte;
  Decode: array[0..63] of byte;
  Shift2: byte;
  Size, W: word;
begin
  FillChar(Base64, SizeOf(Base64), 255);
  J := 0;
  for I := 65 to 90 do
  begin
    Base64[I] := J;
    Inc(J);
  end;
  for I := 97 to 122 do
  begin
    Base64[I] := J;
    Inc(J);
  end;
  for I := 48 to 57 do
  begin
    Base64[I] := J;
    Inc(J);
  end;
  Base64[43] := J;
  Inc(J);
  Base64[47] := J;
  Inc(J);
  if ParamCount = 0 then
  begin
    WriteLn('UNBASE64 <mime-файл> [<выходной файл>]');
    Halt(1);
  end;
  S := ParamStr(1);
  assign(T, S);
  GetMem(Buf, 32768);
  SetTextBuf(T, Buf^, 32768);
{$I-}reset(T);
{$I+}
  if IOResult <> 0 then
  begin
    WriteLn('Ошибка считывания ', S);
    Halt(1);
  end;
  if ParamCount >= 2 then
    S := ParamStr(2)
  else
  begin
    write('Расположение:');
    ReadLn(S);
  end;
  assign(F, S);
{$I-}rewrite(F, 1);
{$I+}
  if IOResult <> 0 then
  begin
    WriteLn('Ошибка создания ', S);
    Halt(1);
  end;
  while not eof(T) do
  begin
    ReadLn(T, S);
    if (S <> '') and (pos(' ', S) = 0) and (S[1] >= #43) and (S[1] <= #122) and
      (Base64[byte(S[1])] <> 255) then
    begin
      FillChar(Decode, SizeOf(Decode), 0);
      DShift := 0;
      J := 0;
      Shift2 := 1;
      Size := 255;
      B := 0;
      for I := 1 to Length(S) do
      begin
        case S[I] of
          #43..#122: B1 := Base64[Ord(S[I])];
        else
          B1 := 255;
        end;
        if B1 = 255 then
          if S[I] = '=' then
          begin
            B1 := 0;
            if Size = 255 then
              Size := J;
          end
          else
            WriteLn('Ошибка символа:', S[I], ' (', Ord(S[I]), ')');
        if DShift and 7 = 0 then
        begin
          Decode[J] := byte(B1 shl 2);
          DShift := 2;
        end
        else
        begin
          Decode[J] := Decode[J] or Hi(word(B1) shl (DShift + 2));
          Decode[J + 1] := Lo(word(B1) shl (DShift + 2));
          Inc(J);
          Inc(DShift, 2);
        end;
      end;
      if Size = 255 then
        Size := J;
      BlockWrite(F, Decode, Size);
    end;
  end;
  Close(F);
  close(T);
end.