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1 ------------------------------------------------------------------------------
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2 -- --
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3 -- GNAT RUN-TIME COMPONENTS --
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4 -- --
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5 -- S Y S T E M . E X P _ M O D --
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6 -- --
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7 -- S p e c --
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8 -- --
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131
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9 -- Copyright (C) 1992-2018, Free Software Foundation, Inc. --
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111
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10 -- --
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11 -- GNAT is free software; you can redistribute it and/or modify it under --
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12 -- terms of the GNU General Public License as published by the Free Soft- --
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13 -- ware Foundation; either version 3, or (at your option) any later ver- --
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14 -- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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15 -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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16 -- or FITNESS FOR A PARTICULAR PURPOSE. --
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17 -- --
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18 -- As a special exception under Section 7 of GPL version 3, you are granted --
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19 -- additional permissions described in the GCC Runtime Library Exception, --
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20 -- version 3.1, as published by the Free Software Foundation. --
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21 -- --
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22 -- You should have received a copy of the GNU General Public License and --
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23 -- a copy of the GCC Runtime Library Exception along with this program; --
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24 -- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
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25 -- <http://www.gnu.org/licenses/>. --
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26 -- --
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27 -- GNAT was originally developed by the GNAT team at New York University. --
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28 -- Extensive contributions were provided by Ada Core Technologies Inc. --
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29 -- --
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30 ------------------------------------------------------------------------------
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31
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32 -- This function performs exponentiation of a modular type with nonbinary
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33 -- modulus values. Arithmetic is done in Long_Long_Unsigned, with explicit
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34 -- accounting for the modulus value which is passed as the second argument.
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35 -- Note that 1 is a binary modulus (2**0), so the compiler should not (and
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36 -- will not) call this function with Modulus equal to 1.
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37
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38 with System.Unsigned_Types;
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39
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40 package System.Exp_Mod is
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41 pragma Pure;
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42 use type System.Unsigned_Types.Unsigned;
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43
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44 subtype Power_Of_2 is System.Unsigned_Types.Unsigned with
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45 Dynamic_Predicate =>
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46 Power_Of_2 /= 0 and then (Power_Of_2 and (Power_Of_2 - 1)) = 0;
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47
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48 function Exp_Modular
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49 (Left : System.Unsigned_Types.Unsigned;
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50 Modulus : System.Unsigned_Types.Unsigned;
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51 Right : Natural) return System.Unsigned_Types.Unsigned
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52 with
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53 Pre => Modulus /= 0 and then Modulus not in Power_Of_2,
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54 Post => Exp_Modular'Result = Left ** Right mod Modulus;
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55
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56 end System.Exp_Mod;
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