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1 706 jeremybenn
------------------------------------------------------------------------------
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--                                                                          --
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--                         GNAT RUN-TIME COMPONENTS                         --
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--                                                                          --
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--            G N A T . M B B S _ D I S C R E T E _ R A N D O M             --
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--                                                                          --
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--                                 B o d y                                  --
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--                                                                          --
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--          Copyright (C) 1992-2010, Free Software Foundation, Inc.         --
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--                                                                          --
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-- GNAT is free software;  you can  redistribute it  and/or modify it under --
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-- terms of the  GNU General Public License as published  by the Free Soft- --
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-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
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-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
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-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
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-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
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--                                                                          --
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-- As a special exception under Section 7 of GPL version 3, you are granted --
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-- additional permissions described in the GCC Runtime Library Exception,   --
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-- version 3.1, as published by the Free Software Foundation.               --
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--                                                                          --
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-- You should have received a copy of the GNU General Public License and    --
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-- a copy of the GCC Runtime Library Exception along with this program;     --
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-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
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-- <http://www.gnu.org/licenses/>.                                          --
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--                                                                          --
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-- GNAT was originally developed  by the GNAT team at  New York University. --
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-- Extensive contributions were provided by Ada Core Technologies Inc.      --
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--                                                                          --
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------------------------------------------------------------------------------
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with Ada.Calendar;
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with Interfaces; use Interfaces;
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package body GNAT.MBBS_Discrete_Random is
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   package Calendar renames Ada.Calendar;
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   Fits_In_32_Bits : constant Boolean :=
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                       Rst'Size < 31
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                         or else (Rst'Size = 31
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                                  and then Rst'Pos (Rst'First) < 0);
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   --  This is set True if we do not need more than 32 bits in the result. If
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   --  we need 64-bits, we will only use the meaningful 48 bits of any 64-bit
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   --  number generated, since if more than 48 bits are required, we split the
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   --  computation into two separate parts, since the algorithm does not behave
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   --  above 48 bits.
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   --  The way this expression works is that obviously if the size is 31 bits,
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   --  it fits in 32 bits. In the 32-bit case, it fits in 32-bit signed if the
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   --  range has negative values. It is too conservative in the case that the
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   --  programmer has set a size greater than the default, e.g. a size of 33
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   --  for an integer type with a range of 1..10, but an over-conservative
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   --  result is OK. The important thing is that the value is only True if
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   --  we know the result will fit in 32-bits signed. If the value is False
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   --  when it could be True, the behavior will be correct, just a bit less
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   --  efficient than it could have been in some unusual cases.
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   --
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   --  One might assume that we could get a more accurate result by testing
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   --  the lower and upper bounds of the type Rst against the bounds of 32-bit
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   --  Integer. However, there is no easy way to do that. Why? Because in the
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   --  relatively rare case where this expression has to be evaluated at run
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   --  time rather than compile time (when the bounds are dynamic), we need a
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   --  type to use for the computation. But the possible range of upper bound
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   --  values for Rst (remembering the possibility of 64-bit modular types) is
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   --  from -2**63 to 2**64-1, and no run-time type has a big enough range.
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   -----------------------
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   -- Local Subprograms --
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   -----------------------
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   function Square_Mod_N (X, N : Int) return Int;
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   pragma Inline (Square_Mod_N);
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   --  Computes X**2 mod N avoiding intermediate overflow
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   -----------
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   -- Image --
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   -----------
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   function Image (Of_State : State) return String is
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   begin
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      return Int'Image (Of_State.X1) &
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             ','                     &
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             Int'Image (Of_State.X2) &
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             ','                     &
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             Int'Image (Of_State.Q);
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   end Image;
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   ------------
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   -- Random --
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   ------------
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   function Random (Gen : Generator) return Rst is
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      S    : State renames Gen.Writable.Self.Gen_State;
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      Temp : Int;
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      TF   : Flt;
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   begin
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      --  Check for flat range here, since we are typically run with checks
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      --  off, note that in practice, this condition will usually be static
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      --  so we will not actually generate any code for the normal case.
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      if Rst'Last < Rst'First then
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         raise Constraint_Error;
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      end if;
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      --  Continue with computation if non-flat range
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      S.X1 := Square_Mod_N (S.X1, S.P);
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      S.X2 := Square_Mod_N (S.X2, S.Q);
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      Temp := S.X2 - S.X1;
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      --  Following duplication is not an error, it is a loop unwinding!
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      if Temp < 0 then
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         Temp := Temp + S.Q;
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      end if;
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      if Temp < 0 then
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         Temp := Temp + S.Q;
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      end if;
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      TF := Offs + (Flt (Temp) * Flt (S.P) + Flt (S.X1)) * S.Scl;
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      --  Pathological, but there do exist cases where the rounding implicit
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      --  in calculating the scale factor will cause rounding to 'Last + 1.
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      --  In those cases, returning 'First results in the least bias.
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      if TF >= Flt (Rst'Pos (Rst'Last)) + 0.5 then
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         return Rst'First;
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      elsif not Fits_In_32_Bits then
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         return Rst'Val (Interfaces.Integer_64 (TF));
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      else
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         return Rst'Val (Int (TF));
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      end if;
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   end Random;
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   -----------
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   -- Reset --
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   -----------
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   procedure Reset (Gen : Generator; Initiator : Integer) is
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      S      : State renames Gen.Writable.Self.Gen_State;
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      X1, X2 : Int;
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   begin
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      X1 := 2 + Int (Initiator) mod (K1 - 3);
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      X2 := 2 + Int (Initiator) mod (K2 - 3);
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      for J in 1 .. 5 loop
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         X1 := Square_Mod_N (X1, K1);
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         X2 := Square_Mod_N (X2, K2);
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      end loop;
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      --  Eliminate effects of small Initiators
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      S :=
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        (X1  => X1,
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         X2  => X2,
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         P   => K1,
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         Q   => K2,
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         FP  => K1F,
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         Scl => Scal);
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   end Reset;
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   -----------
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   -- Reset --
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   -----------
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   procedure Reset (Gen : Generator) is
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      S    : State renames Gen.Writable.Self.Gen_State;
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      Now  : constant Calendar.Time := Calendar.Clock;
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      X1   : Int;
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      X2   : Int;
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   begin
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      X1 := Int (Calendar.Year    (Now)) * 12 * 31 +
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            Int (Calendar.Month   (Now) * 31)     +
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            Int (Calendar.Day     (Now));
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      X2 := Int (Calendar.Seconds (Now) * Duration (1000.0));
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      X1 := 2 + X1 mod (K1 - 3);
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      X2 := 2 + X2 mod (K2 - 3);
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      --  Eliminate visible effects of same day starts
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      for J in 1 .. 5 loop
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         X1 := Square_Mod_N (X1, K1);
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         X2 := Square_Mod_N (X2, K2);
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      end loop;
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      S :=
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        (X1  => X1,
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         X2  => X2,
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         P   => K1,
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         Q   => K2,
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         FP  => K1F,
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         Scl => Scal);
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204
   end Reset;
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   -----------
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   -- Reset --
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   -----------
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   procedure Reset (Gen : Generator; From_State : State) is
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   begin
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      Gen.Writable.Self.Gen_State := From_State;
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   end Reset;
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   ----------
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   -- Save --
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   ----------
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   procedure Save (Gen : Generator; To_State : out State) is
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   begin
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      To_State := Gen.Gen_State;
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   end Save;
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   ------------------
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   -- Square_Mod_N --
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   ------------------
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   function Square_Mod_N (X, N : Int) return Int is
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   begin
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      return Int ((Integer_64 (X) ** 2) mod (Integer_64 (N)));
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   end Square_Mod_N;
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   -----------
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   -- Value --
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   -----------
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   function Value (Coded_State : String) return State is
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      Last  : constant Natural := Coded_State'Last;
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      Start : Positive := Coded_State'First;
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      Stop  : Positive := Coded_State'First;
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      Outs  : State;
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243
   begin
244
      while Stop <= Last and then Coded_State (Stop) /= ',' loop
245
         Stop := Stop + 1;
246
      end loop;
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248
      if Stop > Last then
249
         raise Constraint_Error;
250
      end if;
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      Outs.X1 := Int'Value (Coded_State (Start .. Stop - 1));
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      Start := Stop + 1;
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255
      loop
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         Stop := Stop + 1;
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         exit when Stop > Last or else Coded_State (Stop) = ',';
258
      end loop;
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260
      if Stop > Last then
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         raise Constraint_Error;
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      end if;
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      Outs.X2  := Int'Value (Coded_State (Start .. Stop - 1));
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      Outs.Q   := Int'Value (Coded_State (Stop + 1 .. Last));
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      Outs.P   := Outs.Q * 2 + 1;
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      Outs.FP  := Flt (Outs.P);
268
      Outs.Scl := (RstL - RstF + 1.0) / (Flt (Outs.P) * Flt (Outs.Q));
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      --  Now do *some* sanity checks
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272
      if Outs.Q < 31
273
        or else Outs.X1 not in 2 .. Outs.P - 1
274
        or else Outs.X2 not in 2 .. Outs.Q - 1
275
      then
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         raise Constraint_Error;
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      end if;
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      return Outs;
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   end Value;
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end GNAT.MBBS_Discrete_Random;

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