1------------------------------------------------------------------------------
2--                                                                          --
3--                         GNAT RUN-TIME COMPONENTS                         --
4--                                                                          --
5--                       S Y S T E M . E X P _ M O D                        --
6--                                                                          --
7--                                 S p e c                                  --
8--                                                                          --
9--           Copyright (C) 1992-2019, 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
32--  This function performs exponentiation of a modular type with nonbinary
33--  modulus values. Arithmetic is done in Long_Long_Unsigned, with explicit
34--  accounting for the modulus value which is passed as the second argument.
35--  Note that 1 is a binary modulus (2**0), so the compiler should not (and
36--  will not) call this function with Modulus equal to 1.
37
38with System.Unsigned_Types;
39
40package System.Exp_Mod is
41   pragma Pure;
42   use type System.Unsigned_Types.Unsigned;
43
44   subtype Power_Of_2 is System.Unsigned_Types.Unsigned with
45     Dynamic_Predicate =>
46        Power_Of_2 /= 0 and then (Power_Of_2 and (Power_Of_2 - 1)) = 0;
47
48   function Exp_Modular
49     (Left    : System.Unsigned_Types.Unsigned;
50      Modulus : System.Unsigned_Types.Unsigned;
51      Right   : Natural) return System.Unsigned_Types.Unsigned
52   with
53       Pre  => Modulus /= 0 and then Modulus not in Power_Of_2,
54       Post => Exp_Modular'Result = Left ** Right mod Modulus;
55
56end System.Exp_Mod;
57