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------------------------------------------------------------------------------
-- --
-- GNAT RUN-TIME COMPONENTS --
-- --
-- S Y S T E M . E X P _ M O D --
-- --
-- S p e c --
-- --
-- Copyright (C) 1992-2025, 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 --
-- <http://www.gnu.org/licenses/>. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
-- This function performs exponentiation of a modular type with nonbinary
-- modulus values. Arithmetic is done in Long_Long_Unsigned, with explicit
-- accounting for the modulus value which is passed as the second argument.
--
-- Note that 1 is a binary modulus (2**0), so the compiler should not (and
-- will not) call this function with Modulus equal to 1.
with System.Unsigned_Types;
package System.Exp_Mod
with Pure, SPARK_Mode
is
use type System.Unsigned_Types.Unsigned;
subtype Unsigned is System.Unsigned_Types.Unsigned;
function Exp_Modular
(Left : Unsigned;
Modulus : Unsigned;
Right : Natural) return Unsigned;
-- Return the power of ``Left`` by ``Right` modulo ``Modulus``.
--
-- This function is implemented using the standard logarithmic approach:
-- ``Right`` gets shifted right testing successive low order bits, and
-- ``Left`` is raised to the next power of 2. The multiplications are
-- performed using modular multiplications.
end System.Exp_Mod;
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