File : s-expmod.ads
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) 1992-2015, 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 -- --
19 -- --
20 -- --
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
38 with System.Unsigned_Types;
39
40 package 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
56 end System.Exp_Mod;