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) 1992-2010, 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 32with Ada.Calendar; 33 34with Interfaces; use Interfaces; 35 36package 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 282end GNAT.MBBS_Discrete_Random; 283