{ Although a very trivial program : still copyrighted ;-) }
{ Copyright 1999, Henk Postma				  } 
PROGRAM Prime;
   TYPE
      Primenumber = ^PrimeRec;
      PrimeRec    = RECORD
                       Number : Longint;
                       Next   : Primenumber;
                    END;

   VAR
      Einde,
      Flag   : Boolean;
      Last,
      First,
      Index  : Primenumber;
      Noofprimes,
      Cutoff,
      Count  : Longint;
      F2,
      F      : Text;

   BEGIN
      Write ('Do you have any idea how far you would like to go ? [Y/N] ');
      IF upcase (readkey) = 'Y' THEN
      BEGIN
         Writeln;
         Write (' Until which number ? ');
         Readln (cutoff);
         IF CutOff < 0 THEN
            cutoff := 2147483640;
      END
      ELSE
         Cutoff := 0;
      Writeln;

      Assign (F, 'prime.dat');
      Assign (F2, 'prime2.dat');
      Rewrite (F);
      Rewrite (F2);
      ClrScr;
      New (First);
      First^.Number := 2;
      Noofprimes    := 1;
      First^.Next   := NIL;
      Last          := First;
      Count         := 3;
      IF Cutoff <> 0 THEN
         CutOff        := Round (1+sqrt(Cutoff));
      TextColor (15);
      Write (2 : 10);
      Writeln (F, First^.Number);
      Writeln (F2, First^.number, #9, noofprimes);
      Einde := FALSE;

      REPEAT
         Index := First;
         Flag  := FALSE;
         WHILE (Index <> NIL) AND (Index^.Number*Index^.Number <= Count) AND NOT Flag DO
         BEGIN
            Flag  := (count MOD Index^.number = 0);
            Index := Index^.Next;
         END;
         IF NOT Flag THEN
         BEGIN
            TextColor (7);
            IF (Cutoff = 0) OR (Count <= Cutoff) THEN
            BEGIN
               New (Last^.Next);
               Last := Last^.Next;
               Last^.number := Count;
               Last^.Next := NIL;
               TextColor (15);
            END;
            Write (Count : 10);
            Inc (Noofprimes);
            Writeln (F, Count);
            Writeln (F2, Count, #9, Noofprimes);
         END;
         Inc (Count);
         IF Keypressed THEN
         BEGIN
            Writeln;
            WHILE Keypressed DO
               Readkey;
            Write (' Are you sure you want to terminate this calculation ? [Y/N] ');
            Einde := (UpCase (Readkey) = 'Y');
            Writeln;
         END;
      UNTIL Einde OR ((Count >= Cutoff*Cutoff) AND (Cutoff <> 0));

      WHILE First <> NIL DO
      BEGIN
         Index := First;
         First := First^.Next;
         Dispose (Index);
      END;
      Close (F);
      Close (F2);
   END.