[pascal] multiplication:(Nhân số cực lớn trong pascal)

program this_is_so_hard;
uses crt;
type
    int         =       integer;
var
    l           :       string;
    a,b,c       :       string;

function add(a,b:string):string;
var d,i,j,k:int;
    code:int;
    i1,i2,kt:int;
    s,o     :string;
    c,tg    :string;
begin
     if length(b)>length(a) then
     begin
          c:=a;
          a:=b;
          b:=c;
     end;

     for i:=1 to length(a)-length(b) do
     b:='0'+b;
     c:='';
     d:=length(a);
     j:=length(a);
     k:=length(a);  kt:=0;
     repeat
           val(a[j],i1,code);
           val(b[k],i2,code);
           {write('i1: ',i1,' i2: ',i2,' ');}
              i:=i1+i2+kt; {witeln(i);}
              if i>=10 then begin
                                 i:=i mod 10;
                                 kt:=1;
                                 str(i,s);
                                 insert(s,c,length(c)+1);
                            end
                            else
                            begin
                                 kt:=0;
                                 str(i,s);
                                 insert(s,c,length(c)+1);
                            end;

           d:=d-1;
           j:=j-1;
           k:=k-1;
     until d=0;
     if kt=1 then insert('1',c,length(c)+1);
     o:='';
     for i:=length(c) downto 1 do o:=o+c[i];
     add:=o;
end;

procedure multification;
var  i,j,t1,t2,s,nho:int;
     m,n,is:string;
     code,d:int;

begin
     m:='';
          nho:=0;
          val(b[length(b)],t1,code);
          for j:=length(a) downto 1 do
          begin
               val(a[j],t2,code);
               s:=t1*t2+nho;
               if s>=10 then begin nho:= s div 10; s:=s mod 10; end
               else nho:=0;
               str(s,is);
               insert(is,m,0);
          end;
          if nho >0 then
          begin
               str(nho,is);
               insert(is,m,0);
          end;
     d:=0;
     nho:=0;
     for i:=length(b)-1 downto 1 do
     begin
          n:='';
          val(b[i],t1,code);
          for j:=length(a) downto 1 do
          begin
               val(a[j],t2,code);
               s:=t1*t2+nho;
               if s>=10 then begin
                                  nho:= s div 10;
                                  s:=s mod 10;
                             end
                             else nho:=0;
               str(s,is);
               insert(is,n,0);
          end;
          if nho >0 then
          begin
               str(nho,is);
               insert(is,n,0);
          end;
          d:=d+1;
          for j:=1 to d do n:=n+'0';
          for j:=1 to length(n)-length(m) do m:='0'+m;
          write(m); break;
          m:=add(m,n);
     end;
     {write(m); }
end;

begin
     clrscr;
     write('nhap so a: '); readln(a);
     write('nhap so b: '); readln(b);
     if length(b)>length(a) then
     begin
          c:=a;
          a:=b;
          b:=c;
     end;
     multification;
     readln;
end.