1 ------------------------------------------------------------------------------
3 -- GNAT RUN-TIME COMPONENTS --
5 -- S Y S T E M . R A N D O M _ N U M B E R S --
9 -- Copyright (C) 2007-2010, 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 ------------------------------------------------------------------------------
32 ------------------------------------------------------------------------------
34 -- The implementation here is derived from a C-program for MT19937, with --
35 -- initialization improved 2002/1/26. As required, the following notice is --
36 -- copied from the original program. --
38 -- Copyright (C) 1997 - 2002, Makoto Matsumoto and Takuji Nishimura, --
39 -- All rights reserved. --
41 -- Redistribution and use in source and binary forms, with or without --
42 -- modification, are permitted provided that the following conditions --
45 -- 1. Redistributions of source code must retain the above copyright --
46 -- notice, this list of conditions and the following disclaimer. --
48 -- 2. Redistributions in binary form must reproduce the above copyright --
49 -- notice, this list of conditions and the following disclaimer in the --
50 -- documentation and/or other materials provided with the distribution.--
52 -- 3. The names of its contributors may not be used to endorse or promote --
53 -- products derived from this software without specific prior written --
56 -- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS --
57 -- "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT --
58 -- LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR --
59 -- A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT --
60 -- OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, --
61 -- SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED --
62 -- TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR --
63 -- PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF --
64 -- LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING --
65 -- NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS --
66 -- SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. --
68 ------------------------------------------------------------------------------
70 ------------------------------------------------------------------------------
72 -- This is an implementation of the Mersenne Twister, twisted generalized --
73 -- feedback shift register of rational normal form, with state-bit --
74 -- reflection and tempering. This version generates 32-bit integers with a --
75 -- period of 2**19937 - 1 (a Mersenne prime, hence the name). For --
76 -- applications requiring more than 32 bits (up to 64), we concatenate two --
79 -- See http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html for --
82 -- In contrast to the original code, we do not generate random numbers in --
83 -- batches of N. Measurement seems to show this has very little if any --
84 -- effect on performance, and it may be marginally better for real-time --
85 -- applications with hard deadlines. --
87 ------------------------------------------------------------------------------
89 with Ada.Calendar; use Ada.Calendar;
90 with Ada.Unchecked_Conversion;
91 with Interfaces; use Interfaces;
95 package body System.Random_Numbers is
97 -------------------------
98 -- Implementation Note --
99 -------------------------
101 -- The design of this spec is very awkward, as a result of Ada 95 not
102 -- permitting in-out parameters for function formals (most naturally,
103 -- Generator values would be passed this way). In pure Ada 95, the only
104 -- solution is to use the heap and pointers, and, to avoid memory leaks,
107 -- This is awfully heavy, so what we do is to use Unrestricted_Access to
108 -- get a pointer to the state in the passed Generator. This works because
109 -- Generator is a limited type and will thus always be passed by reference.
111 Low31_Mask : constant := 2**31-1;
112 Bit31_Mask : constant := 2**31;
114 Matrix_A_X : constant array (State_Val range 0 .. 1) of State_Val :=
117 Y2K : constant Calendar.Time :=
119 (Year => 2000, Month => 1, Day => 1, Seconds => 0.0);
120 -- First Year 2000 day
122 Image_Numeral_Length : constant := Max_Image_Width / N;
123 subtype Image_String is String (1 .. Max_Image_Width);
127 procedure Init (Gen : out Generator; Initiator : Unsigned_32);
128 -- Perform a default initialization of the state of Gen. The resulting
129 -- state is identical for identical values of Initiator.
131 procedure Insert_Image
132 (S : in out Image_String;
135 -- Insert image of V into S, in the Index'th 11-character substring
137 function Extract_Value (S : String; Index : Integer) return State_Val;
138 -- Treat S as a sequence of 11-character decimal numerals and return
139 -- the result of converting numeral #Index (numbering from 0)
141 function To_Unsigned is
142 new Unchecked_Conversion (Integer_32, Unsigned_32);
143 function To_Unsigned is
144 new Unchecked_Conversion (Integer_64, Unsigned_64);
150 function Random (Gen : Generator) return Unsigned_32 is
151 G : Generator renames Gen'Unrestricted_Access.all;
159 Y := (G.S (I) and Bit31_Mask) or (G.S (I + 1) and Low31_Mask);
160 Y := G.S (I + M) xor Shift_Right (Y, 1) xor Matrix_A_X (Y and 1);
164 Y := (G.S (I) and Bit31_Mask) or (G.S (I + 1) and Low31_Mask);
165 Y := G.S (I + (M - N))
166 xor Shift_Right (Y, 1)
167 xor Matrix_A_X (Y and 1);
171 Y := (G.S (I) and Bit31_Mask) or (G.S (0) and Low31_Mask);
172 Y := G.S (M - 1) xor Shift_Right (Y, 1) xor Matrix_A_X (Y and 1);
183 Y := Y xor Shift_Right (Y, 11);
184 Y := Y xor (Shift_Left (Y, 7) and 16#9d2c5680#);
185 Y := Y xor (Shift_Left (Y, 15) and 16#efc60000#);
186 Y := Y xor Shift_Right (Y, 18);
192 type Unsigned is mod <>;
193 type Real is digits <>;
194 with function Random (G : Generator) return Unsigned is <>;
195 function Random_Float_Template (Gen : Generator) return Real;
196 pragma Inline (Random_Float_Template);
197 -- Template for a random-number generator implementation that delivers
198 -- values of type Real in the range [0 .. 1], using values from Gen,
199 -- assuming that Unsigned is large enough to hold the bits of a mantissa
202 function Random_Float_Template (Gen : Generator) return Real is
204 pragma Compile_Time_Error
205 (Unsigned'Last <= 2**(Real'Machine_Mantissa - 1),
206 "insufficiently large modular type used to hold mantissa");
209 -- This code generates random floating-point numbers from unsigned
210 -- integers. Assuming that Real'Machine_Radix = 2, it can deliver all
211 -- machine values of type Real (as implied by Real'Machine_Mantissa and
212 -- Real'Machine_Emin), which is not true of the standard method (to
213 -- which we fall back for non-binary radix): computing Real(<random
214 -- integer>) / (<max random integer>+1). To do so, we first extract an
215 -- (M-1)-bit significand (where M is Real'Machine_Mantissa), and then
216 -- decide on a normalized exponent by repeated coin flips, decrementing
217 -- from 0 as long as we flip heads (1 bits). This process yields the
218 -- proper geometric distribution for the exponent: in a uniformly
219 -- distributed set of floating-point numbers, 1/2 of them will be in
220 -- (0.5, 1], 1/4 will be in (0.25, 0.5], and so forth. It makes a
221 -- further adjustment at binade boundaries (see comments below) to give
222 -- the effect of selecting a uniformly distributed real deviate in
223 -- [0..1] and then rounding to the nearest representable floating-point
224 -- number. The algorithm attempts to be stingy with random integers. In
225 -- the worst case, it can consume roughly -Real'Machine_Emin/32 32-bit
226 -- integers, but this case occurs with probability around
227 -- 2**Machine_Emin, and the expected number of calls to integer-valued
228 -- Random is 1. For another discussion of the issues addressed by this
229 -- process, see Allen Downey's unpublished paper at
230 -- http://allendowney.com/research/rand/downey07randfloat.pdf.
232 if Real'Machine_Radix /= 2 then
234 (Real (Unsigned'(Random (Gen))) * 2.0**(-Unsigned'Size));
237 type Bit_Count is range 0 .. 4;
239 subtype T is Real'Base;
241 Trailing_Ones : constant array (Unsigned_32 range 0 .. 15)
243 := (2#00000# => 0, 2#00001# => 1, 2#00010# => 0, 2#00011# => 2,
244 2#00100# => 0, 2#00101# => 1, 2#00110# => 0, 2#00111# => 3,
245 2#01000# => 0, 2#01001# => 1, 2#01010# => 0, 2#01011# => 2,
246 2#01100# => 0, 2#01101# => 1, 2#01110# => 0, 2#01111# => 4);
248 Pow_Tab : constant array (Bit_Count range 0 .. 3) of Real
249 := (0 => 2.0**(0 - T'Machine_Mantissa),
250 1 => 2.0**(-1 - T'Machine_Mantissa),
251 2 => 2.0**(-2 - T'Machine_Mantissa),
252 3 => 2.0**(-3 - T'Machine_Mantissa));
254 Extra_Bits : constant Natural :=
255 (Unsigned'Size - T'Machine_Mantissa + 1);
256 -- Random bits left over after selecting mantissa
259 X : Real; -- Scaled mantissa
260 R : Unsigned_32; -- Supply of random bits
261 R_Bits : Natural; -- Number of bits left in R
263 K : Bit_Count; -- Next decrement to exponent
266 Mantissa := Random (Gen) / 2**Extra_Bits;
267 R := Unsigned_32 (Mantissa mod 2**Extra_Bits);
268 R_Bits := Extra_Bits;
269 X := Real (2**(T'Machine_Mantissa - 1) + Mantissa); -- Exact
271 if Extra_Bits < 4 and then R < 2**Extra_Bits - 1 then
272 -- We got lucky and got a zero in our few extra bits
273 K := Trailing_Ones (R);
278 -- R has R_Bits unprocessed random bits, a multiple of 4.
279 -- X needs to be halved for each trailing one bit. The
280 -- process stops as soon as a 0 bit is found. If R_Bits
281 -- becomes zero, reload R.
283 -- Process 4 bits at a time for speed: the two iterations
284 -- on average with three tests each was still too slow,
285 -- probably because the branches are not predictable.
286 -- This loop now will only execute once 94% of the cases,
287 -- doing more bits at a time will not help.
289 while R_Bits >= 4 loop
290 K := Trailing_Ones (R mod 16);
292 exit Find_Zero when K < 4; -- Exits 94% of the time
294 R_Bits := R_Bits - 4;
299 -- Do not allow us to loop endlessly even in the (very
300 -- unlikely) case that Random (Gen) keeps yielding all ones.
302 exit Find_Zero when X = 0.0;
308 -- K has the count of trailing ones not reflected yet in X.
309 -- The following multiplication takes care of that, as well
310 -- as the correction to move the radix point to the left of
311 -- the mantissa. Doing it at the end avoids repeated rounding
312 -- errors in the exceedingly unlikely case of ever having
313 -- a subnormal result.
315 X := X * Pow_Tab (K);
317 -- The smallest value in each binade is rounded to by 0.75 of
318 -- the span of real numbers as its next larger neighbor, and
319 -- 1.0 is rounded to by half of the span of real numbers as its
320 -- next smaller neighbor. To account for this, when we encounter
321 -- the smallest number in a binade, we substitute the smallest
322 -- value in the next larger binade with probability 1/2.
324 if Mantissa = 0 and then Unsigned_32'(Random (Gen)) mod 2 = 0 then
331 end Random_Float_Template;
333 function Random (Gen : Generator) return Float is
334 function F is new Random_Float_Template (Unsigned_32, Float);
339 function Random (Gen : Generator) return Long_Float is
340 function F is new Random_Float_Template (Unsigned_64, Long_Float);
345 function Random (Gen : Generator) return Unsigned_64 is
347 return Shift_Left (Unsigned_64 (Unsigned_32'(Random (Gen))), 32)
348 or Unsigned_64 (Unsigned_32'(Random (Gen)));
351 ---------------------
352 -- Random_Discrete --
353 ---------------------
355 function Random_Discrete
357 Min : Result_Subtype := Default_Min;
358 Max : Result_Subtype := Result_Subtype'Last) return Result_Subtype
365 raise Constraint_Error;
367 elsif Result_Subtype'Base'Size > 32 then
369 -- In the 64-bit case, we have to be careful, since not all 64-bit
370 -- unsigned values are representable in GNAT's root_integer type.
371 -- Ignore different-size warnings here; since GNAT's handling
374 pragma Warnings ("Z");
375 function Conv_To_Unsigned is
376 new Unchecked_Conversion (Result_Subtype'Base, Unsigned_64);
377 function Conv_To_Result is
378 new Unchecked_Conversion (Unsigned_64, Result_Subtype'Base);
379 pragma Warnings ("z");
381 N : constant Unsigned_64 :=
382 Conv_To_Unsigned (Max) - Conv_To_Unsigned (Min) + 1;
384 X, Slop : Unsigned_64;
388 return Conv_To_Result (Conv_To_Unsigned (Min) + Random (Gen));
391 Slop := Unsigned_64'Last rem N + 1;
395 exit when Slop = N or else X <= Unsigned_64'Last - Slop;
398 return Conv_To_Result (Conv_To_Unsigned (Min) + X rem N);
402 elsif Result_Subtype'Pos (Max) - Result_Subtype'Pos (Min) =
405 return Result_Subtype'Val
406 (Result_Subtype'Pos (Min) + Unsigned_32'Pos (Random (Gen)));
409 N : constant Unsigned_32 :=
410 Unsigned_32 (Result_Subtype'Pos (Max) -
411 Result_Subtype'Pos (Min) + 1);
412 Slop : constant Unsigned_32 := Unsigned_32'Last rem N + 1;
418 exit when Slop = N or else X <= Unsigned_32'Last - Slop;
423 (Result_Subtype'Pos (Min) + Unsigned_32'Pos (X rem N));
432 function Random_Float (Gen : Generator) return Result_Subtype is
434 if Result_Subtype'Base'Digits > Float'Digits then
435 return Result_Subtype'Machine (Result_Subtype
436 (Long_Float'(Random (Gen))));
438 return Result_Subtype'Machine (Result_Subtype
439 (Float'(Random (Gen))));
447 procedure Reset (Gen : out Generator) is
448 X : constant Unsigned_32 := Unsigned_32 ((Calendar.Clock - Y2K) * 64.0);
453 procedure Reset (Gen : out Generator; Initiator : Integer_32) is
455 Init (Gen, To_Unsigned (Initiator));
458 procedure Reset (Gen : out Generator; Initiator : Unsigned_32) is
460 Init (Gen, Initiator);
463 procedure Reset (Gen : out Generator; Initiator : Integer) is
465 pragma Warnings ("C");
466 -- This is probably an unnecessary precaution against future change, but
467 -- since the test is a static expression, no extra code is involved.
469 if Integer'Size <= 32 then
470 Init (Gen, To_Unsigned (Integer_32 (Initiator)));
474 Initiator1 : constant Unsigned_64 :=
475 To_Unsigned (Integer_64 (Initiator));
476 Init0 : constant Unsigned_32 :=
477 Unsigned_32 (Initiator1 mod 2 ** 32);
478 Init1 : constant Unsigned_32 :=
479 Unsigned_32 (Shift_Right (Initiator1, 32));
481 Reset (Gen, Initialization_Vector'(Init0, Init1));
485 pragma Warnings ("c");
488 procedure Reset (Gen : out Generator; Initiator : Initialization_Vector) is
492 Init (Gen, 19650218);
496 if Initiator'Length > 0 then
497 for K in reverse 1 .. Integer'Max (N, Initiator'Length) loop
500 xor ((Gen.S (I - 1) xor Shift_Right (Gen.S (I - 1), 30))
502 + Initiator (J + Initiator'First) + Unsigned_32 (J);
508 Gen.S (0) := Gen.S (N - 1);
512 if J >= Initiator'Length then
518 for K in reverse 1 .. N - 1 loop
520 (Gen.S (I) xor ((Gen.S (I - 1)
521 xor Shift_Right (Gen.S (I - 1), 30)) * 1566083941))
526 Gen.S (0) := Gen.S (N - 1);
531 Gen.S (0) := Bit31_Mask;
534 procedure Reset (Gen : out Generator; From_State : Generator) is
536 Gen.S := From_State.S;
537 Gen.I := From_State.I;
540 procedure Reset (Gen : out Generator; From_State : State) is
546 procedure Reset (Gen : out Generator; From_Image : String) is
550 for J in 0 .. N - 1 loop
551 Gen.S (J) := Extract_Value (From_Image, J);
559 procedure Save (Gen : Generator; To_State : out State) is
568 To_State (0 .. N - 1 - Gen.I) := Gen.S (Gen.I .. N - 1);
569 To_State (N - Gen.I .. N - 1) := Gen.S (0 .. Gen.I - 1);
577 function Image (Of_State : State) return String is
578 Result : Image_String;
581 Result := (others => ' ');
583 for J in Of_State'Range loop
584 Insert_Image (Result, J, Of_State (J));
590 function Image (Gen : Generator) return String is
591 Result : Image_String;
594 Result := (others => ' ');
596 for J in 0 .. N - 1 loop
597 Insert_Image (Result, J, Gen.S ((J + Gen.I) mod N));
607 function Value (Coded_State : String) return State is
611 Reset (Gen, Coded_State);
620 procedure Init (Gen : out Generator; Initiator : Unsigned_32) is
622 Gen.S (0) := Initiator;
624 for I in 1 .. N - 1 loop
627 * (Gen.S (I - 1) xor Shift_Right (Gen.S (I - 1), 30))
638 procedure Insert_Image
639 (S : in out Image_String;
643 Value : constant String := State_Val'Image (V);
645 S (Index * 11 + 1 .. Index * 11 + Value'Length) := Value;
652 function Extract_Value (S : String; Index : Integer) return State_Val is
653 Start : constant Integer := S'First + Index * Image_Numeral_Length;
655 return State_Val'Value (S (Start .. Start + Image_Numeral_Length - 1));
658 end System.Random_Numbers;