1// Class template uniform_int_distribution -*- C++ -*-
2
3// Copyright (C) 2009-2021 Free Software Foundation, Inc.
4//
5// This file is part of the GNU ISO C++ Library. This library is free
6// software; you can redistribute it and/or modify it under the
7// terms of the GNU General Public License as published by the
8// Free Software Foundation; either version 3, or (at your option)
9// any later version.
10
11// This library is distributed in the hope that it will be useful,
12// but WITHOUT ANY WARRANTY; without even the implied warranty of
13// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14// GNU General Public License for more details.
15
16// Under Section 7 of GPL version 3, you are granted additional
17// permissions described in the GCC Runtime Library Exception, version
18// 3.1, as published by the Free Software Foundation.
19
20// You should have received a copy of the GNU General Public License and
21// a copy of the GCC Runtime Library Exception along with this program;
22// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23// <http://www.gnu.org/licenses/>.
24
25/**
26 * @file bits/uniform_int_dist.h
27 * This is an internal header file, included by other library headers.
28 * Do not attempt to use it directly. @headername{random}
29 */
30
31#ifndef _GLIBCXX_BITS_UNIFORM_INT_DIST_H
32#define _GLIBCXX_BITS_UNIFORM_INT_DIST_H
33
34#include <type_traits>
35#include <ext/numeric_traits.h>
36#if __cplusplus > 201703L
37# include <concepts>
38#endif
39#include <bits/concept_check.h> // __glibcxx_function_requires
40
41namespace std _GLIBCXX_VISIBILITY(default)
42{
43_GLIBCXX_BEGIN_NAMESPACE_VERSION
44
45#ifdef __cpp_lib_concepts
46 /// Requirements for a uniform random bit generator.
47 template<typename _Gen>
48 concept uniform_random_bit_generator
49 = invocable<_Gen&> && unsigned_integral<invoke_result_t<_Gen&>>
50 && requires
51 {
52 { _Gen::min() } -> same_as<invoke_result_t<_Gen&>>;
53 { _Gen::max() } -> same_as<invoke_result_t<_Gen&>>;
54 requires bool_constant<(_Gen::min() < _Gen::max())>::value;
55 };
56#endif
57
58 namespace __detail
59 {
60 // Determine whether number is a power of two.
61 // This is true for zero, which is OK because we want _Power_of_2(n+1)
62 // to be true if n==numeric_limits<_Tp>::max() and so n+1 wraps around.
63 template<typename _Tp>
64 constexpr bool
65 _Power_of_2(_Tp __x)
66 {
67 return ((__x - 1) & __x) == 0;
68 }
69 }
70
71 /**
72 * @brief Uniform discrete distribution for random numbers.
73 * A discrete random distribution on the range @f$[min, max]@f$ with equal
74 * probability throughout the range.
75 */
76 template<typename _IntType = int>
77 class uniform_int_distribution
78 {
79 static_assert(std::is_integral<_IntType>::value,
80 "template argument must be an integral type");
81
82 public:
83 /** The type of the range of the distribution. */
84 typedef _IntType result_type;
85 /** Parameter type. */
86 struct param_type
87 {
88 typedef uniform_int_distribution<_IntType> distribution_type;
89
90 param_type() : param_type(0) { }
91
92 explicit
93 param_type(_IntType __a,
94 _IntType __b = __gnu_cxx::__int_traits<_IntType>::__max)
95 : _M_a(__a), _M_b(__b)
96 {
97 __glibcxx_assert(_M_a <= _M_b);
98 }
99
100 result_type
101 a() const
102 { return _M_a; }
103
104 result_type
105 b() const
106 { return _M_b; }
107
108 friend bool
109 operator==(const param_type& __p1, const param_type& __p2)
110 { return __p1._M_a == __p2._M_a && __p1._M_b == __p2._M_b; }
111
112 friend bool
113 operator!=(const param_type& __p1, const param_type& __p2)
114 { return !(__p1 == __p2); }
115
116 private:
117 _IntType _M_a;
118 _IntType _M_b;
119 };
120
121 public:
122 /**
123 * @brief Constructs a uniform distribution object.
124 */
125 uniform_int_distribution() : uniform_int_distribution(0) { }
126
127 /**
128 * @brief Constructs a uniform distribution object.
129 */
130 explicit
131 uniform_int_distribution(_IntType __a,
132 _IntType __b
133 = __gnu_cxx::__int_traits<_IntType>::__max)
134 : _M_param(__a, __b)
135 { }
136
137 explicit
138 uniform_int_distribution(const param_type& __p)
139 : _M_param(__p)
140 { }
141
142 /**
143 * @brief Resets the distribution state.
144 *
145 * Does nothing for the uniform integer distribution.
146 */
147 void
148 reset() { }
149
150 result_type
151 a() const
152 { return _M_param.a(); }
153
154 result_type
155 b() const
156 { return _M_param.b(); }
157
158 /**
159 * @brief Returns the parameter set of the distribution.
160 */
161 param_type
162 param() const
163 { return _M_param; }
164
165 /**
166 * @brief Sets the parameter set of the distribution.
167 * @param __param The new parameter set of the distribution.
168 */
169 void
170 param(const param_type& __param)
171 { _M_param = __param; }
172
173 /**
174 * @brief Returns the inclusive lower bound of the distribution range.
175 */
176 result_type
177 min() const
178 { return this->a(); }
179
180 /**
181 * @brief Returns the inclusive upper bound of the distribution range.
182 */
183 result_type
184 max() const
185 { return this->b(); }
186
187 /**
188 * @brief Generating functions.
189 */
190 template<typename _UniformRandomBitGenerator>
191 result_type
192 operator()(_UniformRandomBitGenerator& __urng)
193 { return this->operator()(__urng, _M_param); }
194
195 template<typename _UniformRandomBitGenerator>
196 result_type
197 operator()(_UniformRandomBitGenerator& __urng,
198 const param_type& __p);
199
200 template<typename _ForwardIterator,
201 typename _UniformRandomBitGenerator>
202 void
203 __generate(_ForwardIterator __f, _ForwardIterator __t,
204 _UniformRandomBitGenerator& __urng)
205 { this->__generate(__f, __t, __urng, _M_param); }
206
207 template<typename _ForwardIterator,
208 typename _UniformRandomBitGenerator>
209 void
210 __generate(_ForwardIterator __f, _ForwardIterator __t,
211 _UniformRandomBitGenerator& __urng,
212 const param_type& __p)
213 { this->__generate_impl(__f, __t, __urng, __p); }
214
215 template<typename _UniformRandomBitGenerator>
216 void
217 __generate(result_type* __f, result_type* __t,
218 _UniformRandomBitGenerator& __urng,
219 const param_type& __p)
220 { this->__generate_impl(__f, __t, __urng, __p); }
221
222 /**
223 * @brief Return true if two uniform integer distributions have
224 * the same parameters.
225 */
226 friend bool
227 operator==(const uniform_int_distribution& __d1,
228 const uniform_int_distribution& __d2)
229 { return __d1._M_param == __d2._M_param; }
230
231 private:
232 template<typename _ForwardIterator,
233 typename _UniformRandomBitGenerator>
234 void
235 __generate_impl(_ForwardIterator __f, _ForwardIterator __t,
236 _UniformRandomBitGenerator& __urng,
237 const param_type& __p);
238
239 param_type _M_param;
240
241 // Lemire's nearly divisionless algorithm.
242 // Returns an unbiased random number from __g downscaled to [0,__range)
243 // using an unsigned type _Wp twice as wide as unsigned type _Up.
244 template<typename _Wp, typename _Urbg, typename _Up>
245 static _Up
246 _S_nd(_Urbg& __g, _Up __range)
247 {
248 using _Up_traits = __gnu_cxx::__int_traits<_Up>;
249 using _Wp_traits = __gnu_cxx::__int_traits<_Wp>;
250 static_assert(!_Up_traits::__is_signed, "U must be unsigned");
251 static_assert(!_Wp_traits::__is_signed, "W must be unsigned");
252 static_assert(_Wp_traits::__digits == (2 * _Up_traits::__digits),
253 "W must be twice as wide as U");
254
255 // reference: Fast Random Integer Generation in an Interval
256 // ACM Transactions on Modeling and Computer Simulation 29 (1), 2019
257 // https://arxiv.org/abs/1805.10941
258 _Wp __product = _Wp(__g()) * _Wp(__range);
259 _Up __low = _Up(__product);
260 if (__low < __range)
261 {
262 _Up __threshold = -__range % __range;
263 while (__low < __threshold)
264 {
265 __product = _Wp(__g()) * _Wp(__range);
266 __low = _Up(__product);
267 }
268 }
269 return __product >> _Up_traits::__digits;
270 }
271 };
272
273 template<typename _IntType>
274 template<typename _UniformRandomBitGenerator>
275 typename uniform_int_distribution<_IntType>::result_type
276 uniform_int_distribution<_IntType>::
277 operator()(_UniformRandomBitGenerator& __urng,
278 const param_type& __param)
279 {
280 typedef typename _UniformRandomBitGenerator::result_type _Gresult_type;
281 typedef typename make_unsigned<result_type>::type __utype;
282 typedef typename common_type<_Gresult_type, __utype>::type __uctype;
283
284 constexpr __uctype __urngmin = _UniformRandomBitGenerator::min();
285 constexpr __uctype __urngmax = _UniformRandomBitGenerator::max();
286 static_assert( __urngmin < __urngmax,
287 "Uniform random bit generator must define min() < max()");
288 constexpr __uctype __urngrange = __urngmax - __urngmin;
289
290 const __uctype __urange
291 = __uctype(__param.b()) - __uctype(__param.a());
292
293 __uctype __ret;
294 if (__urngrange > __urange)
295 {
296 // downscaling
297
298 const __uctype __uerange = __urange + 1; // __urange can be zero
299
300#if defined __UINT64_TYPE__ && defined __UINT32_TYPE__
301#if __SIZEOF_INT128__
302 if _GLIBCXX17_CONSTEXPR (__urngrange == __UINT64_MAX__)
303 {
304 // __urng produces values that use exactly 64-bits,
305 // so use 128-bit integers to downscale to desired range.
306 __UINT64_TYPE__ __u64erange = __uerange;
307 __ret = _S_nd<unsigned __int128>(__urng, __u64erange);
308 }
309 else
310#endif
311 if _GLIBCXX17_CONSTEXPR (__urngrange == __UINT32_MAX__)
312 {
313 // __urng produces values that use exactly 32-bits,
314 // so use 64-bit integers to downscale to desired range.
315 __UINT32_TYPE__ __u32erange = __uerange;
316 __ret = _S_nd<__UINT64_TYPE__>(__urng, __u32erange);
317 }
318 else
319#endif
320 {
321 // fallback case (2 divisions)
322 const __uctype __scaling = __urngrange / __uerange;
323 const __uctype __past = __uerange * __scaling;
324 do
325 __ret = __uctype(__urng()) - __urngmin;
326 while (__ret >= __past);
327 __ret /= __scaling;
328 }
329 }
330 else if (__urngrange < __urange)
331 {
332 // upscaling
333 /*
334 Note that every value in [0, urange]
335 can be written uniquely as
336
337 (urngrange + 1) * high + low
338
339 where
340
341 high in [0, urange / (urngrange + 1)]
342
343 and
344
345 low in [0, urngrange].
346 */
347 __uctype __tmp; // wraparound control
348 do
349 {
350 const __uctype __uerngrange = __urngrange + 1;
351 __tmp = (__uerngrange * operator()
352 (__urng, param_type(0, __urange / __uerngrange)));
353 __ret = __tmp + (__uctype(__urng()) - __urngmin);
354 }
355 while (__ret > __urange || __ret < __tmp);
356 }
357 else
358 __ret = __uctype(__urng()) - __urngmin;
359
360 return __ret + __param.a();
361 }
362
363
364 template<typename _IntType>
365 template<typename _ForwardIterator,
366 typename _UniformRandomBitGenerator>
367 void
368 uniform_int_distribution<_IntType>::
369 __generate_impl(_ForwardIterator __f, _ForwardIterator __t,
370 _UniformRandomBitGenerator& __urng,
371 const param_type& __param)
372 {
373 __glibcxx_function_requires(_ForwardIteratorConcept<_ForwardIterator>)
374 typedef typename _UniformRandomBitGenerator::result_type _Gresult_type;
375 typedef typename make_unsigned<result_type>::type __utype;
376 typedef typename common_type<_Gresult_type, __utype>::type __uctype;
377
378 static_assert( __urng.min() < __urng.max(),
379 "Uniform random bit generator must define min() < max()");
380
381 constexpr __uctype __urngmin = __urng.min();
382 constexpr __uctype __urngmax = __urng.max();
383 constexpr __uctype __urngrange = __urngmax - __urngmin;
384 const __uctype __urange
385 = __uctype(__param.b()) - __uctype(__param.a());
386
387 __uctype __ret;
388
389 if (__urngrange > __urange)
390 {
391 if (__detail::_Power_of_2(__urngrange + 1)
392 && __detail::_Power_of_2(__urange + 1))
393 {
394 while (__f != __t)
395 {
396 __ret = __uctype(__urng()) - __urngmin;
397 *__f++ = (__ret & __urange) + __param.a();
398 }
399 }
400 else
401 {
402 // downscaling
403 const __uctype __uerange = __urange + 1; // __urange can be zero
404 const __uctype __scaling = __urngrange / __uerange;
405 const __uctype __past = __uerange * __scaling;
406 while (__f != __t)
407 {
408 do
409 __ret = __uctype(__urng()) - __urngmin;
410 while (__ret >= __past);
411 *__f++ = __ret / __scaling + __param.a();
412 }
413 }
414 }
415 else if (__urngrange < __urange)
416 {
417 // upscaling
418 /*
419 Note that every value in [0, urange]
420 can be written uniquely as
421
422 (urngrange + 1) * high + low
423
424 where
425
426 high in [0, urange / (urngrange + 1)]
427
428 and
429
430 low in [0, urngrange].
431 */
432 __uctype __tmp; // wraparound control
433 while (__f != __t)
434 {
435 do
436 {
437 constexpr __uctype __uerngrange = __urngrange + 1;
438 __tmp = (__uerngrange * operator()
439 (__urng, param_type(0, __urange / __uerngrange)));
440 __ret = __tmp + (__uctype(__urng()) - __urngmin);
441 }
442 while (__ret > __urange || __ret < __tmp);
443 *__f++ = __ret;
444 }
445 }
446 else
447 while (__f != __t)
448 *__f++ = __uctype(__urng()) - __urngmin + __param.a();
449 }
450
451 // operator!= and operator<< and operator>> are defined in <bits/random.h>
452
453_GLIBCXX_END_NAMESPACE_VERSION
454} // namespace std
455
456#endif
457