about summary refs log tree commit diff
path: root/absl/random/internal/pcg_engine.h
blob: 53c23fe1b492a6c3d30829a500d9706f0e4c027d (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_
#define ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_

#include <type_traits>

#include "absl/base/config.h"
#include "absl/meta/type_traits.h"
#include "absl/numeric/int128.h"
#include "absl/random/internal/fastmath.h"
#include "absl/random/internal/iostream_state_saver.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal {

// pcg_engine is a simplified implementation of Melissa O'Neil's PCG engine in
// C++.  PCG combines a linear congruential generator (LCG) with output state
// mixing functions to generate each random variate.  pcg_engine supports only a
// single sequence (oneseq), and does not support streams.
//
// pcg_engine is parameterized by two types:
//   Params, which provides the multiplier and increment values;
//   Mix, which mixes the state into the result.
//
template <typename Params, typename Mix>
class pcg_engine {
  static_assert(std::is_same<typename Params::state_type,
                             typename Mix::state_type>::value,
                "Class-template absl::pcg_engine must be parameterized by "
                "Params and Mix with identical state_type");

  static_assert(std::is_unsigned<typename Mix::result_type>::value,
                "Class-template absl::pcg_engine must be parameterized by "
                "an unsigned Mix::result_type");

  using params_type = Params;
  using mix_type = Mix;
  using state_type = typename Mix::state_type;

 public:
  // C++11 URBG interface:
  using result_type = typename Mix::result_type;

  static constexpr result_type(min)() {
    return (std::numeric_limits<result_type>::min)();
  }

  static constexpr result_type(max)() {
    return (std::numeric_limits<result_type>::max)();
  }

  explicit pcg_engine(uint64_t seed_value = 0) { seed(seed_value); }

  template <class SeedSequence,
            typename = typename absl::enable_if_t<
                !std::is_same<SeedSequence, pcg_engine>::value>>
  explicit pcg_engine(SeedSequence&& seq) {
    seed(seq);
  }

  pcg_engine(const pcg_engine&) = default;
  pcg_engine& operator=(const pcg_engine&) = default;
  pcg_engine(pcg_engine&&) = default;
  pcg_engine& operator=(pcg_engine&&) = default;

  result_type operator()() {
    // Advance the LCG state, always using the new value to generate the output.
    state_ = lcg(state_);
    return Mix{}(state_);
  }

  void seed(uint64_t seed_value = 0) {
    state_type tmp = seed_value;
    state_ = lcg(tmp + Params::increment());
  }

  template <class SeedSequence>
  typename absl::enable_if_t<
      !std::is_convertible<SeedSequence, uint64_t>::value, void>
  seed(SeedSequence&& seq) {
    reseed(seq);
  }

  void discard(uint64_t count) { state_ = advance(state_, count); }

  bool operator==(const pcg_engine& other) const {
    return state_ == other.state_;
  }

  bool operator!=(const pcg_engine& other) const { return !(*this == other); }

  template <class CharT, class Traits>
  friend typename absl::enable_if_t<(sizeof(state_type) == 16),
                                    std::basic_ostream<CharT, Traits>&>
  operator<<(
      std::basic_ostream<CharT, Traits>& os,  // NOLINT(runtime/references)
      const pcg_engine& engine) {
    auto saver = random_internal::make_ostream_state_saver(os);
    random_internal::stream_u128_helper<state_type> helper;
    helper.write(pcg_engine::params_type::multiplier(), os);
    os << os.fill();
    helper.write(pcg_engine::params_type::increment(), os);
    os << os.fill();
    helper.write(engine.state_, os);
    return os;
  }

  template <class CharT, class Traits>
  friend typename absl::enable_if_t<(sizeof(state_type) <= 8),
                                    std::basic_ostream<CharT, Traits>&>
  operator<<(
      std::basic_ostream<CharT, Traits>& os,  // NOLINT(runtime/references)
      const pcg_engine& engine) {
    auto saver = random_internal::make_ostream_state_saver(os);
    os << pcg_engine::params_type::multiplier() << os.fill();
    os << pcg_engine::params_type::increment() << os.fill();
    os << engine.state_;
    return os;
  }

  template <class CharT, class Traits>
  friend typename absl::enable_if_t<(sizeof(state_type) == 16),
                                    std::basic_istream<CharT, Traits>&>
  operator>>(
      std::basic_istream<CharT, Traits>& is,  // NOLINT(runtime/references)
      pcg_engine& engine) {                   // NOLINT(runtime/references)
    random_internal::stream_u128_helper<state_type> helper;
    auto mult = helper.read(is);
    auto inc = helper.read(is);
    auto tmp = helper.read(is);
    if (mult != pcg_engine::params_type::multiplier() ||
        inc != pcg_engine::params_type::increment()) {
      // signal failure by setting the failbit.
      is.setstate(is.rdstate() | std::ios_base::failbit);
    }
    if (!is.fail()) {
      engine.state_ = tmp;
    }
    return is;
  }

  template <class CharT, class Traits>
  friend typename absl::enable_if_t<(sizeof(state_type) <= 8),
                                    std::basic_istream<CharT, Traits>&>
  operator>>(
      std::basic_istream<CharT, Traits>& is,  // NOLINT(runtime/references)
      pcg_engine& engine) {                   // NOLINT(runtime/references)
    state_type mult{}, inc{}, tmp{};
    is >> mult >> inc >> tmp;
    if (mult != pcg_engine::params_type::multiplier() ||
        inc != pcg_engine::params_type::increment()) {
      // signal failure by setting the failbit.
      is.setstate(is.rdstate() | std::ios_base::failbit);
    }
    if (!is.fail()) {
      engine.state_ = tmp;
    }
    return is;
  }

 private:
  state_type state_;

  // Returns the linear-congruential generator next state.
  static inline constexpr state_type lcg(state_type s) {
    return s * Params::multiplier() + Params::increment();
  }

  // Returns the linear-congruential arbitrary seek state.
  inline state_type advance(state_type s, uint64_t n) const {
    state_type mult = Params::multiplier();
    state_type inc = Params::increment();
    state_type m = 1;
    state_type i = 0;
    while (n > 0) {
      if (n & 1) {
        m *= mult;
        i = i * mult + inc;
      }
      inc = (mult + 1) * inc;
      mult *= mult;
      n >>= 1;
    }
    return m * s + i;
  }

  template <class SeedSequence>
  void reseed(SeedSequence& seq) {
    using sequence_result_type = typename SeedSequence::result_type;
    constexpr size_t kBufferSize =
        sizeof(state_type) / sizeof(sequence_result_type);
    sequence_result_type buffer[kBufferSize];
    seq.generate(std::begin(buffer), std::end(buffer));
    // Convert the seed output to a single state value.
    state_type tmp = buffer[0];
    for (size_t i = 1; i < kBufferSize; i++) {
      tmp <<= (sizeof(sequence_result_type) * 8);
      tmp |= buffer[i];
    }
    state_ = lcg(tmp + params_type::increment());
  }
};

// Parameterized implementation of the PCG 128-bit oneseq state.
// This provides state_type, multiplier, and increment for pcg_engine.
template <uint64_t kMultA, uint64_t kMultB, uint64_t kIncA, uint64_t kIncB>
class pcg128_params {
 public:
#if ABSL_HAVE_INTRINSIC_INT128
  using state_type = __uint128_t;
  static inline constexpr state_type make_u128(uint64_t a, uint64_t b) {
    return (static_cast<__uint128_t>(a) << 64) | b;
  }
#else
  using state_type = absl::uint128;
  static inline constexpr state_type make_u128(uint64_t a, uint64_t b) {
    return absl::MakeUint128(a, b);
  }
#endif

  static inline constexpr state_type multiplier() {
    return make_u128(kMultA, kMultB);
  }
  static inline constexpr state_type increment() {
    return make_u128(kIncA, kIncB);
  }
};

// Implementation of the PCG xsl_rr_128_64 128-bit mixing function, which
// accepts an input of state_type and mixes it into an output of result_type.
struct pcg_xsl_rr_128_64 {
#if ABSL_HAVE_INTRINSIC_INT128
  using state_type = __uint128_t;
#else
  using state_type = absl::uint128;
#endif
  using result_type = uint64_t;

  inline uint64_t operator()(state_type state) {
    // This is equivalent to the xsl_rr_128_64 mixing function.
#if ABSL_HAVE_INTRINSIC_INT128
    uint64_t rotate = static_cast<uint64_t>(state >> 122u);
    state ^= state >> 64;
    uint64_t s = static_cast<uint64_t>(state);
#else
    uint64_t h = Uint128High64(state);
    uint64_t rotate = h >> 58u;
    uint64_t s = Uint128Low64(state) ^ h;
#endif
    return random_internal::rotr(s, rotate);
  }
};

// Parameterized implementation of the PCG 64-bit oneseq state.
// This provides state_type, multiplier, and increment for pcg_engine.
template <uint64_t kMult, uint64_t kInc>
class pcg64_params {
 public:
  using state_type = uint64_t;
  static inline constexpr state_type multiplier() { return kMult; }
  static inline constexpr state_type increment() { return kInc; }
};

// Implementation of the PCG xsh_rr_64_32 64-bit mixing function, which accepts
// an input of state_type and mixes it into an output of result_type.
struct pcg_xsh_rr_64_32 {
  using state_type = uint64_t;
  using result_type = uint32_t;
  inline uint32_t operator()(uint64_t state) {
    return random_internal::rotr(
        static_cast<uint32_t>(((state >> 18) ^ state) >> 27), state >> 59);
  }
};

// Stable pcg_engine implementations:
// This is a 64-bit generator using 128-bits of state.
// The output sequence is equivalent to Melissa O'Neil's pcg64_oneseq.
using pcg64_2018_engine = pcg_engine<
    random_internal::pcg128_params<0x2360ed051fc65da4ull, 0x4385df649fccf645ull,
                                   0x5851f42d4c957f2d, 0x14057b7ef767814f>,
    random_internal::pcg_xsl_rr_128_64>;

// This is a 32-bit generator using 64-bits of state.
// This is equivalent to Melissa O'Neil's pcg32_oneseq.
using pcg32_2018_engine = pcg_engine<
    random_internal::pcg64_params<0x5851f42d4c957f2dull, 0x14057b7ef767814full>,
    random_internal::pcg_xsh_rr_64_32>;

}  // namespace random_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_