gcc/ada/s-expmod.adb

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