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

--                                                                          --

--                         GNAT RUN-TIME COMPONENTS                         --

--                                                                          --

--                       S Y S T E M . E X P _ M O D                        --

--                                                                          --

--                                 B o d y                                  --

--                                                                          --

--          Copyright (C) 1992-2009 Free Software Foundation, Inc.          --

--                                                                          --

-- GNAT is free software;  you can  redistribute it  and/or modify it under --

-- terms of the  GNU General Public License as published  by the Free Soft- --

-- ware  Foundation;  either version 3,  or (at your option) any later ver- --

-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --

-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --

-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --

--                                                                          --

-- As a special exception under Section 7 of GPL version 3, you are granted --

-- additional permissions described in the GCC Runtime Library Exception,   --

-- version 3.1, as published by the Free Software Foundation.               --

--                                                                          --

-- You should have received a copy of the GNU General Public License and    --

-- a copy of the GCC Runtime Library Exception along with this program;     --

-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --

-- <http://www.gnu.org/licenses/>.                                          --

--                                                                          --

-- GNAT was originally developed  by the GNAT team at  New York University. --

-- Extensive contributions were provided by Ada Core Technologies Inc.      --

--                                                                          --

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

package body System.Exp_Mod is

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

   -- Exp_Modular --

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

   function Exp_Modular

     (Left    : Integer;

      Modulus : Integer;

      Right   : Natural)

      return    Integer

   is

      Result : Integer := 1;

      Factor : Integer := Left;

      Exp    : Natural := Right;

      function Mult (X, Y : Integer) return Integer;

      pragma Inline (Mult);

      --  Modular multiplication. Note that we can't take advantage of the

      --  compiler's circuit, because the modulus is not known statically.

      function Mult (X, Y : Integer) return Integer is

      begin

         return Integer

           (Long_Long_Integer (X) * Long_Long_Integer (Y)

             mod Long_Long_Integer (Modulus));

      end Mult;

   --  Start of processing for Exp_Modular

   begin

      --  We use the standard logarithmic approach, Exp gets shifted right

      --  testing successive low order bits and Factor is the value of the

      --  base raised to the next power of 2.

      --  Note: it is not worth special casing the cases of base values -1,0,+1

      --  since the expander does this when the base is a literal, and other

      --  cases will be extremely rare.

      if Exp /= 0 then

         loop

            if Exp rem 2 /= 0 then

               Result := Mult (Result, Factor);

            end if;

            Exp := Exp / 2;

            exit when Exp = 0;

            Factor := Mult (Factor, Factor);

         end loop;

      end if;

      return Result;

   end Exp_Modular;

end System.Exp_Mod;