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------------------------------------------------------------------------------
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-- --
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-- GNAT RUN-TIME COMPONENTS --
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-- --
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-- S Y S T E M . E X P _ M O D --
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-- --
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-- B o d y --
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-- --
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-- Copyright (C) 1992-2009 Free Software Foundation, Inc. --
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-- --
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-- GNAT is free software; you can redistribute it and/or modify it under --
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-- terms of the GNU General Public License as published by the Free Soft- --
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-- ware Foundation; either version 3, or (at your option) any later ver- --
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-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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-- or FITNESS FOR A PARTICULAR PURPOSE. --
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-- --
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-- As a special exception under Section 7 of GPL version 3, you are granted --
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-- additional permissions described in the GCC Runtime Library Exception, --
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-- version 3.1, as published by the Free Software Foundation. --
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-- --
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-- You should have received a copy of the GNU General Public License and --
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-- a copy of the GCC Runtime Library Exception along with this program; --
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-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
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-- <http://www.gnu.org/licenses/>. --
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-- --
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-- GNAT was originally developed by the GNAT team at New York University. --
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-- Extensive contributions were provided by Ada Core Technologies Inc. --
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-- --
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------------------------------------------------------------------------------
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package body System.Exp_Mod is
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-----------------
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-- Exp_Modular --
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-----------------
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function Exp_Modular
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(Left : Integer;
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Modulus : Integer;
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Right : Natural)
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return Integer
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is
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Result : Integer := 1;
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Factor : Integer := Left;
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Exp : Natural := Right;
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function Mult (X, Y : Integer) return Integer;
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pragma Inline (Mult);
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-- Modular multiplication. Note that we can't take advantage of the
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-- compiler's circuit, because the modulus is not known statically.
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function Mult (X, Y : Integer) return Integer is
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begin
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return Integer
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(Long_Long_Integer (X) * Long_Long_Integer (Y)
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mod Long_Long_Integer (Modulus));
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end Mult;
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-- Start of processing for Exp_Modular
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begin
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-- We use the standard logarithmic approach, Exp gets shifted right
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-- testing successive low order bits and Factor is the value of the
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-- base raised to the next power of 2.
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-- Note: it is not worth special casing the cases of base values -1,0,+1
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-- since the expander does this when the base is a literal, and other
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-- cases will be extremely rare.
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if Exp /= 0 then
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loop
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if Exp rem 2 /= 0 then
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Result := Mult (Result, Factor);
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end if;
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Exp := Exp / 2;
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exit when Exp = 0;
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Factor := Mult (Factor, Factor);
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end loop;
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end if;
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return Result;
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end Exp_Modular;
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end System.Exp_Mod;
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