------------------------------------------------------------------------------
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------------------------------------------------------------------------------

--  This function performs exponentiation of a modular type with nonbinary
--  modulus values. Arithmetic is done in Long_Long_Unsigned, with explicit
--  accounting for the modulus value which is passed as the second argument.
--  Note that 1 is a binary modulus (2**0), so the compiler should not (and
--  will not) call this function with Modulus equal to 1.

with System.Unsigned_Types;

package System.Exp_Mod is
   pragma Pure;
   use type System.Unsigned_Types.Unsigned;

   subtype Power_Of_2 is System.Unsigned_Types.Unsigned with
     Dynamic_Predicate =>
        Power_Of_2 /= 0 and then (Power_Of_2 and (Power_Of_2 - 1)) = 0;

   function Exp_Modular
     (Left    : System.Unsigned_Types.Unsigned;
      Modulus : System.Unsigned_Types.Unsigned;
      Right   : Natural) return System.Unsigned_Types.Unsigned
   with
       Pre  => Modulus /= 0 and then Modulus not in Power_Of_2,
       Post => Exp_Modular'Result = Left ** Right mod Modulus;

end System.Exp_Mod;