File : 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) 1992-2015, 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 -- --
19 -- --
20 -- --
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;