Check if a credit card number is valid using Luhn algorithm

mod10.pas

{This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation, either version 3 of the License, or
 (at your option) any later version.

 This program is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.}
program mod10;
uses crt;
var creditnum: int64;
    num: array [1..100] of longint;
    i, chks, i2: longint;
    stri: string;
    tmp, sum: int64;
begin
 clrscr;
 write('Enter credit card number: ');
 readln(creditnum);
 chks:=creditnum mod 10;
 creditnum:=creditnum div 10;
 str(creditnum, stri);
 for i:=length(stri) downto 1 do begin
  if i2 mod 2=0 then begin
   num[i]:=creditnum mod 10;
   num[i]:=num[i]*2;
  end else
   num[i]:=creditnum mod 10;
  i2:=i2+1;
  creditnum:=creditnum div 10;
 end;
 for i:=1 to length(stri) do begin
  if num[i] div 10<>0 then begin
   tmp:=num[i] mod 10;
   num[i]:=num[i] div 10;
   num[i]:=num[i]+tmp;
   tmp:=0;
  end;
 end;
 for i:=1 to length(stri) do sum:=sum+num[i];
 sum:=sum*9;
 textcolor(yellow);
 if sum mod 10=chks then
  writeln('The credit card number (', stri, ') is a valid credit card number')
 else
  writeln('The credit card number (', stri, ') is NOT a valid credit card number');
 readln;
end.