1------------------------------------------------------------------------------
2--                                                                          --
3--                         GNAT RUN-TIME COMPONENTS                         --
4--                                                                          --
5--                       S Y S T E M . E X P _ M O D                        --
6--                                                                          --
7--                                 B o d y                                  --
8--                                                                          --
9--          Copyright (C) 1992-2009 Free Software Foundation, Inc.          --
10--                                                                          --
11-- GNAT is free software;  you can  redistribute it  and/or modify it under --
12-- terms of the  GNU General Public License as published  by the Free Soft- --
13-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
14-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
15-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
16-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
17--                                                                          --
18-- As a special exception under Section 7 of GPL version 3, you are granted --
19-- additional permissions described in the GCC Runtime Library Exception,   --
20-- version 3.1, as published by the Free Software Foundation.               --
21--                                                                          --
22-- You should have received a copy of the GNU General Public License and    --
23-- a copy of the GCC Runtime Library Exception along with this program;     --
24-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
25-- <http://www.gnu.org/licenses/>.                                          --
26--                                                                          --
27-- GNAT was originally developed  by the GNAT team at  New York University. --
28-- Extensive contributions were provided by Ada Core Technologies Inc.      --
29--                                                                          --
30------------------------------------------------------------------------------
31
32package body System.Exp_Mod is
33
34   -----------------
35   -- Exp_Modular --
36   -----------------
37
38   function Exp_Modular
39     (Left    : Integer;
40      Modulus : Integer;
41      Right   : Natural)
42      return    Integer
43   is
44      Result : Integer := 1;
45      Factor : Integer := Left;
46      Exp    : Natural := Right;
47
48      function Mult (X, Y : Integer) return Integer;
49      pragma Inline (Mult);
50      --  Modular multiplication. Note that we can't take advantage of the
51      --  compiler's circuit, because the modulus is not known statically.
52
53      function Mult (X, Y : Integer) return Integer is
54      begin
55         return Integer
56           (Long_Long_Integer (X) * Long_Long_Integer (Y)
57             mod Long_Long_Integer (Modulus));
58      end Mult;
59
60   --  Start of processing for Exp_Modular
61
62   begin
63      --  We use the standard logarithmic approach, Exp gets shifted right
64      --  testing successive low order bits and Factor is the value of the
65      --  base raised to the next power of 2.
66
67      --  Note: it is not worth special casing the cases of base values -1,0,+1
68      --  since the expander does this when the base is a literal, and other
69      --  cases will be extremely rare.
70
71      if Exp /= 0 then
72         loop
73            if Exp rem 2 /= 0 then
74               Result := Mult (Result, Factor);
75            end if;
76
77            Exp := Exp / 2;
78            exit when Exp = 0;
79            Factor := Mult (Factor, Factor);
80         end loop;
81      end if;
82
83      return Result;
84
85   end Exp_Modular;
86
87end System.Exp_Mod;
88