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--  Extended pseudo-random number generation

--  This package provides a type representing pseudo-random number generators,
--  and subprograms to extract various uniform distributions of numbers
--  from them. It also provides types for representing initialization values
--  and snapshots of internal generator state, which permit reproducible
--  pseudo-random streams.

--  The generator currently provided by this package has an extremely long
--  period (at least 2**19937-1), and passes the Big Crush test suite, with the
--  exception of the two linear complexity tests. Therefore, it is suitable
--  for simulations, but should not be used as a cryptographic pseudo-random
--  source without additional processing.

--  Note: this package is in the System hierarchy so that it can be directly
--  used by other predefined packages. User access to this package is via
--  the package GNAT.Random_Numbers (file g-rannum.ads), which also extends
--  its capabilities. The interfaces are different so as to include in
--  System.Random_Numbers only the definitions necessary to implement the
--  standard random-number packages Ada.Numerics.Float_Random and
--  Ada.Numerics.Discrete_Random.

--  Note: this package is marked SPARK_Mode Off, because functions Random work
--  by side-effect to change the value of the generator, hence they should not
--  be called from SPARK code.

with Interfaces;

private with Ada.Strings.Text_Buffers;

package System.Random_Numbers with
  SPARK_Mode => Off
is
   type Generator is limited private;
   --  Generator encodes the current state of a random number stream, it is
   --  provided as input to produce the next random number, and updated so
   --  that it is ready to produce the next one.

   type State is private;
   --  A non-limited version of a Generator's internal state

   function Random (Gen : Generator) return Float;
   function Random (Gen : Generator) return Long_Float;
   --  Return pseudo-random numbers uniformly distributed on [0.0 .. 1.0)

   function Random (Gen : Generator) return Interfaces.Unsigned_32;
   function Random (Gen : Generator) return Interfaces.Unsigned_64;
   function Random (Gen : Generator) return Interfaces.Unsigned_128;
   --  Return pseudo-random numbers uniformly distributed on T'First .. T'Last
   --  for builtin integer types.

   generic
      type Result_Subtype is (<>);
      Default_Min : Result_Subtype := Result_Subtype'Val (0);
   function Random_Discrete
     (Gen : Generator;
      Min : Result_Subtype := Default_Min;
      Max : Result_Subtype := Result_Subtype'Last) return Result_Subtype;
   --  Returns pseudo-random numbers uniformly distributed on Min .. Max

   generic
      type Result_Subtype is digits <>;
   function Random_Float (Gen : Generator) return Result_Subtype;
   --  Returns pseudo-random numbers uniformly distributed on [0 .. 1)

   type Initialization_Vector is
     array (Integer range <>) of Interfaces.Unsigned_32;
   --  Provides the most general initialization values for a generator (used
   --  in Reset).  In general, there is little point in providing more than
   --  a certain number of values (currently 624).

   procedure Reset (Gen : Generator);
   --  Re-initialize the state of Gen from the time of day

   procedure Reset (Gen : Generator; Initiator : Initialization_Vector);
   procedure Reset (Gen : Generator; Initiator : Interfaces.Integer_32);
   procedure Reset (Gen : Generator; Initiator : Interfaces.Unsigned_32);
   procedure Reset (Gen : Generator; Initiator : Integer);
   --  Re-initialize Gen based on the Initiator in various ways. Identical
   --  values of Initiator cause identical sequences of values.

   procedure Reset (Gen : Generator; From_State : Generator);
   --  Causes the state of Gen to be identical to that of From_State; Gen
   --  and From_State will produce identical sequences of values subsequently.

   procedure Reset (Gen : Generator; From_State : State);
   procedure Save  (Gen : Generator; To_State : out State);
   --  The sequence
   --     Save (Gen2, S); Reset (Gen1, S)
   --  has the same effect as Reset (Gen2, Gen1).

   procedure Reset (Gen : Generator; From_Image : String);
   function Image (Gen : Generator) return String;
   --  The call
   --     Reset (Gen2, Image (Gen1))
   --  has the same effect as Reset (Gen2, Gen1);

   Max_Image_Width : constant := 11 * 624;
   --  Maximum possible length of result of Image (...)

   function Image (Of_State : State) return String;
   --  A String representation of Of_State. Identical to the result of
   --  Image (Gen), if Of_State has been set with Save (Gen, Of_State).

   function Value (Coded_State : String) return State;
   --  Inverse of Image on States

private

   N : constant := 624;
   --  The number of 32-bit integers in the shift register

   M : constant := 397;
   --  Feedback distance from the current position

   subtype State_Val is Interfaces.Unsigned_32;
   type State is array (0 .. N - 1) of State_Val with Put_Image => Put_Image;

   procedure Put_Image
     (S : in out Ada.Strings.Text_Buffers.Root_Buffer_Type'Class; V : State);

   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

      S : State := [others => 0];
      --  The shift register, a circular buffer

      I : Integer := N;
      --  Current starting position in shift register S (N means uninitialized)
      --  Naming exception: I is fine to use here as it is the name used in the
      --  original paper describing the Mersenne Twister and in common
      --  descriptions of the algorithm.
   end record;

end System.Random_Numbers;