Ada.Containers.Prime_Numbers

package Ada.Containers.Prime_Numbers is
   pragma Pure;

   type Primes_Type is array (Positive range <>) of Hash_Type;

   Primes : constant Primes_Type :=
     (53,         97,         193,       389,       769,
      1543,       3079,       6151,      12289,     24593,
      49157,      98317,      196613,    393241,    786433,
      1572869,    3145739,    6291469,   12582917,  25165843,
      50331653,   100663319,  201326611, 402653189, 805306457,
      1610612741, 3221225473, 4294967291);

   function To_Prime (Length : Count_Type) return Hash_Type;

end Ada.Containers.Prime_Numbers;

package body Ada.Containers.Prime_Numbers is

   --------------
   -- To_Prime --
   --------------

   function To_Prime (Length : Count_Type) return Hash_Type is
      I, J, K : Integer'Base;
      Index   : Integer'Base;

   begin
      I := Primes'Last - Primes'First;
      Index := Primes'First;
      while I > 0 loop
         J := I / 2;
         K := Index + J;

         if Primes (K) < Hash_Type (Length) then
            Index := K + 1;
            I := I - J - 1;
         else
            I := J;
         end if;
      end loop;

      return Primes (Index);
   end To_Prime;

end Ada.Containers.Prime_Numbers;