------------------------------------------------------------------------------
--                                                                          --
--                         GNAT RUN-TIME COMPONENTS                         --
--                                                                          --
--         A D A . N U M E R I C S . D I S C R E T E _ R A N D O M          --
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------------------------------------------------------------------------------

--  Note: 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.

with Interfaces;

generic
   type Result_Subtype is (<>);

package Ada.Numerics.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 Generator is limited record
      Gen_State : State;
   end record;

end Ada.Numerics.Discrete_Random;