1 ------------------------------------------------------------------------------
3 -- GNAT RUN-TIME COMPONENTS --
5 -- G N A T . M B B S _ D I S C R E T E _ R A N D O M --
9 -- Copyright (C) 1992-2024, Free Software Foundation, Inc. --
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. --
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. --
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/>. --
27 -- GNAT was originally developed by the GNAT team at New York University. --
28 -- Extensive contributions were provided by Ada Core Technologies Inc. --
30 ------------------------------------------------------------------------------
34 with Interfaces
; use Interfaces
;
36 package body GNAT
.MBBS_Discrete_Random
is
38 package Calendar
renames Ada
.Calendar
;
40 Fits_In_32_Bits
: constant Boolean :=
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
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.
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.
69 -----------------------
70 -- Local Subprograms --
71 -----------------------
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
81 function Image
(Of_State
: State
) return String is
83 return Int
'Image (Of_State
.X1
) &
85 Int
'Image (Of_State
.X2
) &
87 Int
'Image (Of_State
.Q
);
94 function Random
(Gen
: Generator
) return Rst
is
95 S
: State
renames Gen
.Writable
.Self
.Gen_State
;
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.
104 if Rst
'Last < Rst
'First then
105 raise Constraint_Error
;
108 -- Continue with computation if non-flat range
110 S
.X1
:= Square_Mod_N
(S
.X1
, S
.P
);
111 S
.X2
:= Square_Mod_N
(S
.X2
, S
.Q
);
114 -- Following duplication is not an error, it is a loop unwinding
124 TF
:= Offs
+ (Flt
(Temp
) * Flt
(S
.P
) + Flt
(S
.X1
)) * S
.Scl
;
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.
130 if TF
>= Flt
(Rst
'Pos (Rst
'Last)) + 0.5 then
133 elsif not Fits_In_32_Bits
then
134 return Rst
'Val (Interfaces
.Integer_64
(TF
));
137 return Rst
'Val (Int
(TF
));
145 procedure Reset
(Gen
: Generator
; Initiator
: Integer) is
146 S
: State
renames Gen
.Writable
.Self
.Gen_State
;
150 X1
:= 2 + Int
(Initiator
) mod (K1
- 3);
151 X2
:= 2 + Int
(Initiator
) mod (K2
- 3);
154 X1
:= Square_Mod_N
(X1
, K1
);
155 X2
:= Square_Mod_N
(X2
, K2
);
158 -- Eliminate effects of small Initiators
173 procedure Reset
(Gen
: Generator
) is
174 S
: State
renames Gen
.Writable
.Self
.Gen_State
;
175 Now
: constant Calendar
.Time
:= Calendar
.Clock
;
180 X1
:= Int
(Calendar
.Year
(Now
)) * 12 * 31 +
181 Int
(Calendar
.Month
(Now
) * 31) +
182 Int
(Calendar
.Day
(Now
));
184 X2
:= Int
(Calendar
.Seconds
(Now
) * Duration (1000.0));
186 X1
:= 2 + X1
mod (K1
- 3);
187 X2
:= 2 + X2
mod (K2
- 3);
189 -- Eliminate visible effects of same day starts
192 X1
:= Square_Mod_N
(X1
, K1
);
193 X2
:= Square_Mod_N
(X2
, K2
);
210 procedure Reset
(Gen
: Generator
; From_State
: State
) is
212 Gen
.Writable
.Self
.Gen_State
:= From_State
;
219 procedure Save
(Gen
: Generator
; To_State
: out State
) is
221 To_State
:= Gen
.Gen_State
;
228 function Square_Mod_N
(X
, N
: Int
) return Int
is
230 return Int
((Integer_64
(X
) ** 2) mod (Integer_64
(N
)));
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;
244 while Stop
<= Last
and then Coded_State
(Stop
) /= ',' loop
249 raise Constraint_Error
;
252 Outs
.X1
:= Int
'Value (Coded_State
(Start
.. Stop
- 1));
257 exit when Stop
> Last
or else Coded_State
(Stop
) = ',';
261 raise Constraint_Error
;
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
));
270 -- Now do *some* sanity checks
273 or else Outs
.X1
not in 2 .. Outs
.P
- 1
274 or else Outs
.X2
not in 2 .. Outs
.Q
- 1
276 raise Constraint_Error
;
282 end GNAT
.MBBS_Discrete_Random
;