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--                                                                          --

--                         GNAT RUN-TIME COMPONENTS                         --

--                                                                          --

--            G N A T . M B B S _ D I S C R E T E _ R A N D O M             --

--                                                                          --

--                                 S p e c                                  --

--                                                                          --

--          Copyright (C) 1992-2010, Free Software Foundation, Inc.         --

--                                                                          --

-- This specification is derived from the Ada Reference Manual for use with --

-- GNAT. The copyright notice above, and the license provisions that follow --

-- apply solely to the  contents of the part following the private keyword. --

--                                                                          --

-- 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.      --

--                                                                          --

------------------------------------------------------------------------------

--  The implementation used in this package was contributed by Robert

--  Eachus. It is based on the work of L. Blum, M. Blum, and M. Shub, SIAM

--  Journal of Computing, Vol 15. No 2, May 1986. The particular choices for P

--  and Q chosen here guarantee a period of 562,085,314,430,582 (about 2**49),

--  and the generated sequence has excellent randomness properties. For further

--  details, see the paper "Fast Generation of Trustworthy Random Numbers", by

--  Robert Eachus, which describes both the algorithm and the efficient

--  implementation approach used here.

--  Formerly, this package was Ada.Numerics.Discrete_Random. It is retained

--  here in part to allow users to reconstruct number sequences generated

--  by previous versions.

with Interfaces;

generic

   type Result_Subtype is (<>);

package GNAT.MBBS_Discrete_Random is

   --  The algorithm used here is reliable from a required statistical point of

   --  view only up to 48 bits. We try to behave reasonably in the case of

   --  larger types, but we can't guarantee the required properties. So

   --  generate a warning for these (slightly) dubious cases.

   pragma Compile_Time_Warning

     (Result_Subtype'Size > 48,

      "statistical properties not guaranteed for size > 48");

   --  Basic facilities

   type Generator is limited private;

   function Random (Gen : Generator) return Result_Subtype;

   procedure Reset (Gen : Generator);

   procedure Reset (Gen : Generator; Initiator : Integer);

   --  Advanced facilities

   type State is private;

   procedure Save  (Gen : Generator; To_State   : out State);

   procedure Reset (Gen : Generator; From_State : State);

   Max_Image_Width : constant := 80;

   function Image (Of_State    : State)  return String;

   function Value (Coded_State : String) return State;

private

   subtype Int is Interfaces.Integer_32;

   subtype Rst is Result_Subtype;

   --  We prefer to use 14 digits for Flt, but some targets are more limited

   type Flt is digits Positive'Min (14, Long_Long_Float'Digits);

   RstF : constant Flt := Flt (Rst'Pos (Rst'First));

   RstL : constant Flt := Flt (Rst'Pos (Rst'Last));

   Offs : constant Flt := RstF - 0.5;

   K1   : constant := 94_833_359;

   K1F  : constant := 94_833_359.0;

   K2   : constant := 47_416_679;

   K2F  : constant := 47_416_679.0;

   Scal : constant Flt := (RstL - RstF + 1.0) / (K1F * K2F);

   type State is record

      X1  : Int := Int (2999 ** 2);

      X2  : Int := Int (1439 ** 2);

      P   : Int := K1;

      Q   : Int := K2;

      FP  : Flt := K1F;

      Scl : Flt := Scal;

   end record;

   type Writable_Access (Self : access Generator) is limited null record;

   --  Auxiliary type to make Generator a self-referential type

   type Generator is limited record

      Writable  : Writable_Access (Generator'Access);

      --  This self reference allows functions to modify Generator arguments

      Gen_State : State;

   end record;

end GNAT.MBBS_Discrete_Random;