gcc/ada/s-expmod.adb

   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
  32 package 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
  87 end System.Exp_Mod;