gcc/ada/libgnat/g-mbdira.adb

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT RUN-TIME COMPONENTS                         --
   4 --                                                                          --
   5 --            G N A T . M B B S _ D I S C R E T E _ R A N D O M             --
   6 --                                                                          --
   7 --                                 B o d y                                  --
   8 --                                                                          --
   9 --          Copyright (C) 1992-2020, Free Software Foundation, Inc.         --
  10 --                                                                          --
  11 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
  12 -- terms of the  GNU General Public License as published  by the Free Soft- --
  13 -- ware  Foundation;  either version 3,  or (at your option) any later ver- --
  14 -- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
  15 -- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
  16 -- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
  17 --                                                                          --
  18 -- As a special exception under Section 7 of GPL version 3, you are granted --
  19 -- additional permissions described in the GCC Runtime Library Exception,   --
  20 -- version 3.1, as published by the Free Software Foundation.               --
  21 --                                                                          --
  22 -- You should have received a copy of the GNU General Public License and    --
  23 -- a copy of the GCC Runtime Library Exception along with this program;     --
  24 -- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
  25 -- <http://www.gnu.org/licenses/>.                                          --
  26 --                                                                          --
  27 -- GNAT was originally developed  by the GNAT team at  New York University. --
  28 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  29 --                                                                          --
  30 ------------------------------------------------------------------------------
  31
  32 with Ada.Calendar;
  33
  34 with Interfaces; use Interfaces;
  35
  36 package body GNAT.MBBS_Discrete_Random is
  37
  38    package Calendar renames Ada.Calendar;
  39
  40    Fits_In_32_Bits : constant Boolean :=
  41                        Rst'Size < 31
  42                          or else (Rst'Size = 31
  43                                   and then Rst'Pos (Rst'First) < 0);
  44    --  This is set True if we do not need more than 32 bits in the result. If
  45    --  we need 64-bits, we will only use the meaningful 48 bits of any 64-bit
  46    --  number generated, since if more than 48 bits are required, we split the
  47    --  computation into two separate parts, since the algorithm does not behave
  48    --  above 48 bits.
  49
  50    --  The way this expression works is that obviously if the size is 31 bits,
  51    --  it fits in 32 bits. In the 32-bit case, it fits in 32-bit signed if the
  52    --  range has negative values. It is too conservative in the case that the
  53    --  programmer has set a size greater than the default, e.g. a size of 33
  54    --  for an integer type with a range of 1..10, but an over-conservative
  55    --  result is OK. The important thing is that the value is only True if
  56    --  we know the result will fit in 32-bits signed. If the value is False
  57    --  when it could be True, the behavior will be correct, just a bit less
  58    --  efficient than it could have been in some unusual cases.
  59    --
  60    --  One might assume that we could get a more accurate result by testing
  61    --  the lower and upper bounds of the type Rst against the bounds of 32-bit
  62    --  Integer. However, there is no easy way to do that. Why? Because in the
  63    --  relatively rare case where this expression has to be evaluated at run
  64    --  time rather than compile time (when the bounds are dynamic), we need a
  65    --  type to use for the computation. But the possible range of upper bound
  66    --  values for Rst (remembering the possibility of 64-bit modular types) is
  67    --  from -2**63 to 2**64-1, and no run-time type has a big enough range.
  68
  69    -----------------------
  70    -- Local Subprograms --
  71    -----------------------
  72
  73    function Square_Mod_N (X, N : Int) return Int;
  74    pragma Inline (Square_Mod_N);
  75    --  Computes X**2 mod N avoiding intermediate overflow
  76
  77    -----------
  78    -- Image --
  79    -----------
  80
  81    function Image (Of_State : State) return String is
  82    begin
  83       return Int'Image (Of_State.X1) &
  84              ','                     &
  85              Int'Image (Of_State.X2) &
  86              ','                     &
  87              Int'Image (Of_State.Q);
  88    end Image;
  89
  90    ------------
  91    -- Random --
  92    ------------
  93
  94    function Random (Gen : Generator) return Rst is
  95       S    : State renames Gen.Writable.Self.Gen_State;
  96       Temp : Int;
  97       TF   : Flt;
  98
  99    begin
 100       --  Check for flat range here, since we are typically run with checks
 101       --  off, note that in practice, this condition will usually be static
 102       --  so we will not actually generate any code for the normal case.
 103
 104       if Rst'Last < Rst'First then
 105          raise Constraint_Error;
 106       end if;
 107
 108       --  Continue with computation if non-flat range
 109
 110       S.X1 := Square_Mod_N (S.X1, S.P);
 111       S.X2 := Square_Mod_N (S.X2, S.Q);
 112       Temp := S.X2 - S.X1;
 113
 114       --  Following duplication is not an error, it is a loop unwinding
 115
 116       if Temp < 0 then
 117          Temp := Temp + S.Q;
 118       end if;
 119
 120       if Temp < 0 then
 121          Temp := Temp + S.Q;
 122       end if;
 123
 124       TF := Offs + (Flt (Temp) * Flt (S.P) + Flt (S.X1)) * S.Scl;
 125
 126       --  Pathological, but there do exist cases where the rounding implicit
 127       --  in calculating the scale factor will cause rounding to 'Last + 1.
 128       --  In those cases, returning 'First results in the least bias.
 129
 130       if TF >= Flt (Rst'Pos (Rst'Last)) + 0.5 then
 131          return Rst'First;
 132
 133       elsif not Fits_In_32_Bits then
 134          return Rst'Val (Interfaces.Integer_64 (TF));
 135
 136       else
 137          return Rst'Val (Int (TF));
 138       end if;
 139    end Random;
 140
 141    -----------
 142    -- Reset --
 143    -----------
 144
 145    procedure Reset (Gen : Generator; Initiator : Integer) is
 146       S      : State renames Gen.Writable.Self.Gen_State;
 147       X1, X2 : Int;
 148
 149    begin
 150       X1 := 2 + Int (Initiator) mod (K1 - 3);
 151       X2 := 2 + Int (Initiator) mod (K2 - 3);
 152
 153       for J in 1 .. 5 loop
 154          X1 := Square_Mod_N (X1, K1);
 155          X2 := Square_Mod_N (X2, K2);
 156       end loop;
 157
 158       --  Eliminate effects of small Initiators
 159
 160       S :=
 161         (X1  => X1,
 162          X2  => X2,
 163          P   => K1,
 164          Q   => K2,
 165          FP  => K1F,
 166          Scl => Scal);
 167    end Reset;
 168
 169    -----------
 170    -- Reset --
 171    -----------
 172
 173    procedure Reset (Gen : Generator) is
 174       S    : State renames Gen.Writable.Self.Gen_State;
 175       Now  : constant Calendar.Time := Calendar.Clock;
 176       X1   : Int;
 177       X2   : Int;
 178
 179    begin
 180       X1 := Int (Calendar.Year    (Now)) * 12 * 31 +
 181             Int (Calendar.Month   (Now) * 31)      +
 182             Int (Calendar.Day     (Now));
 183
 184       X2 := Int (Calendar.Seconds (Now) * Duration (1000.0));
 185
 186       X1 := 2 + X1 mod (K1 - 3);
 187       X2 := 2 + X2 mod (K2 - 3);
 188
 189       --  Eliminate visible effects of same day starts
 190
 191       for J in 1 .. 5 loop
 192          X1 := Square_Mod_N (X1, K1);
 193          X2 := Square_Mod_N (X2, K2);
 194       end loop;
 195
 196       S :=
 197         (X1  => X1,
 198          X2  => X2,
 199          P   => K1,
 200          Q   => K2,
 201          FP  => K1F,
 202          Scl => Scal);
 203
 204    end Reset;
 205
 206    -----------
 207    -- Reset --
 208    -----------
 209
 210    procedure Reset (Gen : Generator; From_State : State) is
 211    begin
 212       Gen.Writable.Self.Gen_State := From_State;
 213    end Reset;
 214
 215    ----------
 216    -- Save --
 217    ----------
 218
 219    procedure Save (Gen : Generator; To_State : out State) is
 220    begin
 221       To_State := Gen.Gen_State;
 222    end Save;
 223
 224    ------------------
 225    -- Square_Mod_N --
 226    ------------------
 227
 228    function Square_Mod_N (X, N : Int) return Int is
 229    begin
 230       return Int ((Integer_64 (X) ** 2) mod (Integer_64 (N)));
 231    end Square_Mod_N;
 232
 233    -----------
 234    -- Value --
 235    -----------
 236
 237    function Value (Coded_State : String) return State is
 238       Last  : constant Natural := Coded_State'Last;
 239       Start : Positive := Coded_State'First;
 240       Stop  : Positive := Coded_State'First;
 241       Outs  : State;
 242
 243    begin
 244       while Stop <= Last and then Coded_State (Stop) /= ',' loop
 245          Stop := Stop + 1;
 246       end loop;
 247
 248       if Stop > Last then
 249          raise Constraint_Error;
 250       end if;
 251
 252       Outs.X1 := Int'Value (Coded_State (Start .. Stop - 1));
 253       Start := Stop + 1;
 254
 255       loop
 256          Stop := Stop + 1;
 257          exit when Stop > Last or else Coded_State (Stop) = ',';
 258       end loop;
 259
 260       if Stop > Last then
 261          raise Constraint_Error;
 262       end if;
 263
 264       Outs.X2  := Int'Value (Coded_State (Start .. Stop - 1));
 265       Outs.Q   := Int'Value (Coded_State (Stop + 1 .. Last));
 266       Outs.P   := Outs.Q * 2 + 1;
 267       Outs.FP  := Flt (Outs.P);
 268       Outs.Scl := (RstL - RstF + 1.0) / (Flt (Outs.P) * Flt (Outs.Q));
 269
 270       --  Now do *some* sanity checks
 271
 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
 276          raise Constraint_Error;
 277       end if;
 278
 279       return Outs;
 280    end Value;
 281
 282 end GNAT.MBBS_Discrete_Random;