------------------------------------------------------------------------------ -- -- -- GNAT RUN-TIME COMPONENTS -- -- -- -- S Y S T E M . E X P _ M O D -- -- -- -- B o d y -- -- -- -- Copyright (C) 1992-2019, Free Software Foundation, Inc. -- -- -- -- GNAT is free software; you can redistribute it and/or modify it under -- -- terms of the GNU General Public License as published by the Free Soft- -- -- ware Foundation; either version 3, or (at your option) any later ver- -- -- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- -- or FITNESS FOR A PARTICULAR PURPOSE. -- -- -- -- As a special exception under Section 7 of GPL version 3, you are granted -- -- additional permissions described in the GCC Runtime Library Exception, -- -- version 3.1, as published by the Free Software Foundation. -- -- -- -- You should have received a copy of the GNU General Public License and -- -- a copy of the GCC Runtime Library Exception along with this program; -- -- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see -- -- . -- -- -- -- GNAT was originally developed by the GNAT team at New York University. -- -- Extensive contributions were provided by Ada Core Technologies Inc. -- -- -- ------------------------------------------------------------------------------ package body System.Exp_Mod is use System.Unsigned_Types; ----------------- -- Exp_Modular -- ----------------- function Exp_Modular (Left : Unsigned; Modulus : Unsigned; Right : Natural) return Unsigned is Result : Unsigned := 1; Factor : Unsigned := Left; Exp : Natural := Right; function Mult (X, Y : Unsigned) return Unsigned is (Unsigned (Long_Long_Unsigned (X) * Long_Long_Unsigned (Y) mod Long_Long_Unsigned (Modulus))); -- Modular multiplication. Note that we can't take advantage of the -- compiler's circuit, because the modulus is not known statically. begin -- We use the standard logarithmic approach, Exp gets shifted right -- testing successive low order bits and Factor is the value of the -- base raised to the next power of 2. -- Note: it is not worth special casing the cases of base values -1,0,+1 -- since the expander does this when the base is a literal, and other -- cases will be extremely rare. if Exp /= 0 then loop if Exp rem 2 /= 0 then Result := Mult (Result, Factor); end if; Exp := Exp / 2; exit when Exp = 0; Factor := Mult (Factor, Factor); end loop; end if; return Result; end Exp_Modular; end System.Exp_Mod;