------------------------------------------------------------------------------
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--                         GNAT RUN-TIME COMPONENTS                         --
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------------------------------------------------------------------------------

--  Signed integer exponentiation (checks off)

with Ada.Numerics.Big_Numbers.Big_Integers_Ghost;

generic

   type Int is range <>;

package System.Exponn
  with Pure, SPARK_Mode
is
   --  Preconditions in this unit are meant for analysis only, not for run-time
   --  checking, so that the expected exceptions are raised. This is enforced
   --  by setting the corresponding assertion policy to Ignore. Postconditions
   --  and contract cases should not be executed at runtime as well, in order
   --  not to slow down the execution of these functions.

   pragma Assertion_Policy (Pre            => Ignore,
                            Post           => Ignore,
                            Contract_Cases => Ignore,
                            Ghost          => Ignore);

   package BI_Ghost renames Ada.Numerics.Big_Numbers.Big_Integers_Ghost;
   subtype Big_Integer is BI_Ghost.Big_Integer with Ghost;
   use type BI_Ghost.Big_Integer;

   package Signed_Conversion is new BI_Ghost.Signed_Conversions (Int => Int);

   function Big (Arg : Int) return Big_Integer is
     (Signed_Conversion.To_Big_Integer (Arg))
   with Ghost;

   function In_Int_Range (Arg : Big_Integer) return Boolean is
     (BI_Ghost.In_Range (Arg, Big (Int'First), Big (Int'Last)))
   with Ghost;

   function Expon (Left : Int; Right : Natural) return Int
   with
     Pre  => In_Int_Range (Big (Left) ** Right),
     Post => Expon'Result = Left ** Right;

end System.Exponn;
