source: trunk/src/gcc/gcc/ada/s-expmod.adb@ 2

Last change on this file since 2 was 2, checked in by bird, 22 years ago

Initial revision
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1------------------------------------------------------------------------------
2-- --
3-- GNAT RUNTIME COMPONENTS --
4-- --
5-- S Y S T E M . E X P _ M O D --
6-- --
7-- B o d y --
8-- --
9-- $Revision: 1.1.16.2 $
10-- --
11-- Copyright (C) 1992,1993,1994,1995 Free Software Foundation, Inc. --
12-- --
13-- GNAT is free software; you can redistribute it and/or modify it under --
14-- terms of the GNU General Public License as published by the Free Soft- --
15-- ware Foundation; either version 2, or (at your option) any later ver- --
16-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
17-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
18-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
19-- for more details. You should have received a copy of the GNU General --
20-- Public License distributed with GNAT; see file COPYING. If not, write --
21-- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, --
22-- MA 02111-1307, USA. --
23-- --
24-- As a special exception, if other files instantiate generics from this --
25-- unit, or you link this unit with other files to produce an executable, --
26-- this unit does not by itself cause the resulting executable to be --
27-- covered by the GNU General Public License. This exception does not --
28-- however invalidate any other reasons why the executable file might be --
29-- covered by the GNU Public License. --
30-- --
31-- GNAT was originally developed by the GNAT team at New York University. --
32-- Extensive contributions were provided by Ada Core Technologies Inc. --
33-- --
34------------------------------------------------------------------------------
35
36package body System.Exp_Mod is
37
38 -----------------
39 -- Exp_Modular --
40 -----------------
41
42 function Exp_Modular
43 (Left : Integer;
44 Modulus : Integer;
45 Right : Natural)
46 return Integer
47 is
48 Result : Integer := 1;
49 Factor : Integer := Left;
50 Exp : Natural := Right;
51
52 function Mult (X, Y : Integer) return Integer;
53 pragma Inline (Mult);
54 -- Modular multiplication. Note that we can't take advantage of the
55 -- compiler's circuit, because the modulus is not known statically.
56
57 function Mult (X, Y : Integer) return Integer is
58 begin
59 return Integer
60 (Long_Long_Integer (X) * Long_Long_Integer (Y)
61 mod Long_Long_Integer (Modulus));
62 end Mult;
63
64 -- Start of processing for Exp_Modular
65
66 begin
67 -- We use the standard logarithmic approach, Exp gets shifted right
68 -- testing successive low order bits and Factor is the value of the
69 -- base raised to the next power of 2.
70
71 -- Note: it is not worth special casing the cases of base values -1,0,+1
72 -- since the expander does this when the base is a literal, and other
73 -- cases will be extremely rare.
74
75 if Exp /= 0 then
76 loop
77 if Exp rem 2 /= 0 then
78 Result := Mult (Result, Factor);
79 end if;
80
81 Exp := Exp / 2;
82 exit when Exp = 0;
83 Factor := Mult (Factor, Factor);
84 end loop;
85 end if;
86
87 return Result;
88
89 end Exp_Modular;
90
91end System.Exp_Mod;
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