Đảo bit

program GiaiMa;
uses crt;
type mang=array[1..20] of string;
var
  F,F2: Text;
  path,path2,temp: string;
  s: mang;
  ch:char;
  i,j,lech:byte;
{*****DAO NGUOC CHUOI*****}
function nguoc(str: string):string;
var
  i:byte;
begin
  nguoc:='';
  for i:=length(str) downto 1  do nguoc:=nguoc+str[i];
end;
{*****QUAY VONG *****}
function vong(str: string; v:byte):string;
var i,so:byte;
begin
  vong:='';
  for i:=1 to length(str) do
    begin
      so:=ord(str[i]);
      if (so>=65) and (so<=90) then
        begin
          so:=(so-65-v+26) mod 26 + 65;
          vong:=vong+chr(so+32);
        end
      else
        begin
          vong:=vong+chr(so);
        end;
    end;
end;
{***** DOI BIT *****}
function doibit(str: string):string;
var i:byte;
begin
  doibit:='';
  for i:=1 to length(str) do
    begin
      if (str[i]='0') then
        doibit:=doibit+'1'
      else
        begin
          if (str[i]='1')
            then doibit:=doibit+'0'
          else
            doibit:=doibit+str[i];
        end;
    end;
end;
{***** CHUONG TRINH CHINH *****}
begin
  clrscr;
  {write('Nhap path file: ');='mao.txt';
  readln(path);}
  path:='/home/thieumao/Desktop/input.txt';
  assign(F, path);
  reset(F);
  i:=0;
  writeln('***FILE INPUT***');
  while (not eof(f)) do
    begin
      while (not eoln(F)) do
        begin
          i:=i+1;
          readln(F,s[i]); writeln('Dong ',i:2,': ',s[i]);
        end;
      {readln(F); writeln;}
    end;
  close(F);
  writeln;

  writeln('***FILE OUTPUT***');
  path2:='/home/thieumao/Desktop/output.txt';
  assign(F2, path2);
  rewrite(F2);

  for j:=1 to i do
    begin
      writeln(F2, doibit(s[j]));
      writeln('Dong',j:2,': ', doibit(s[j]) );
    end;
  close(F2);
end.