| ------------------------------------------------------------------------------ |
| -- -- |
| -- 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; |
