------------------------------------------------------------------------------ -- -- -- GNAT RUN-TIME COMPONENTS -- -- -- -- S Y S T E M . E X P _ M O D -- -- -- -- B o d y -- -- -- -- Copyright (C) 1992-2005 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 2, 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. See the GNU General Public License -- -- for more details. You should have received a copy of the GNU General -- -- Public License distributed with GNAT; see file COPYING. If not, write -- -- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, -- -- Boston, MA 02110-1301, USA. -- -- -- -- As a special exception, if other files instantiate generics from this -- -- unit, or you link this unit with other files to produce an executable, -- -- this unit does not by itself cause the resulting executable to be -- -- covered by the GNU General Public License. This exception does not -- -- however invalidate any other reasons why the executable file might be -- -- covered by the GNU Public License. -- -- -- -- 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;