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 --     Copyright (C) 1992,1993,1994,1995 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 2,  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.  See the GNU General Public License --
  17 -- for  more details.  You should have  received  a copy of the GNU General --
  18 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  19 -- to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston, --
  20 -- MA 02111-1307, USA.                                                      --
  21 --                                                                          --
  22 -- As a special exception,  if other files  instantiate  generics from this --
  23 -- unit, or you link  this unit with other files  to produce an executable, --
  24 -- this  unit  does not  by itself cause  the resulting  executable  to  be --
  25 -- covered  by the  GNU  General  Public  License.  This exception does not --
  26 -- however invalidate  any other reasons why  the executable file  might be --
  27 -- covered by the  GNU Public License.                                      --
  28 --                                                                          --
  29 -- GNAT was originally developed  by the GNAT team at  New York University. --
  30 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  31 --                                                                          --
  32 ------------------------------------------------------------------------------
  33
  34 package body System.Exp_Mod is
  35
  36    -----------------
  37    -- Exp_Modular --
  38    -----------------
  39
  40    function Exp_Modular
  41      (Left    : Integer;
  42       Modulus : Integer;
  43       Right   : Natural)
  44       return    Integer
  45    is
  46       Result : Integer := 1;
  47       Factor : Integer := Left;
  48       Exp    : Natural := Right;
  49
  50       function Mult (X, Y : Integer) return Integer;
  51       pragma Inline (Mult);
  52       --  Modular multiplication. Note that we can't take advantage of the
  53       --  compiler's circuit, because the modulus is not known statically.
  54
  55       function Mult (X, Y : Integer) return Integer is
  56       begin
  57          return Integer
  58            (Long_Long_Integer (X) * Long_Long_Integer (Y)
  59              mod Long_Long_Integer (Modulus));
  60       end Mult;
  61
  62    --  Start of processing for Exp_Modular
  63
  64    begin
  65       --  We use the standard logarithmic approach, Exp gets shifted right
  66       --  testing successive low order bits and Factor is the value of the
  67       --  base raised to the next power of 2.
  68
  69       --  Note: it is not worth special casing the cases of base values -1,0,+1
  70       --  since the expander does this when the base is a literal, and other
  71       --  cases will be extremely rare.
  72
  73       if Exp /= 0 then
  74          loop
  75             if Exp rem 2 /= 0 then
  76                Result := Mult (Result, Factor);
  77             end if;
  78
  79             Exp := Exp / 2;
  80             exit when Exp = 0;
  81             Factor := Mult (Factor, Factor);
  82          end loop;
  83       end if;
  84
  85       return Result;
  86
  87    end Exp_Modular;
  88
  89 end System.Exp_Mod;