about summary refs log tree commit diff
path: root/third_party/abseil_cpp/absl/random/internal/pool_urbg.h
blob: 05721929f54e6a5fb1adfa467de3c37d0bf49f23 (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
// Copyright 2017 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_POOL_URBG_H_
#define ABSL_RANDOM_INTERNAL_POOL_URBG_H_

#include <cinttypes>
#include <limits>

#include "absl/random/internal/traits.h"
#include "absl/types/span.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal {

// RandenPool is a thread-safe random number generator [random.req.urbg] that
// uses an underlying pool of Randen generators to generate values.  Each thread
// has affinity to one instance of the underlying pool generators.  Concurrent
// access is guarded by a spin-lock.
template <typename T>
class RandenPool {
 public:
  using result_type = T;
  static_assert(std::is_unsigned<result_type>::value,
                "RandenPool template argument must be a built-in unsigned "
                "integer 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)();
  }

  RandenPool() {}

  // Returns a single value.
  inline result_type operator()() { return Generate(); }

  // Fill data with random values.
  static void Fill(absl::Span<result_type> data);

 protected:
  // Generate returns a single value.
  static result_type Generate();
};

extern template class RandenPool<uint8_t>;
extern template class RandenPool<uint16_t>;
extern template class RandenPool<uint32_t>;
extern template class RandenPool<uint64_t>;

// PoolURBG uses an underlying pool of random generators to implement a
// thread-compatible [random.req.urbg] interface with an internal cache of
// values.
template <typename T, size_t kBufferSize>
class PoolURBG {
  // Inheritance to access the protected static members of RandenPool.
  using unsigned_type = typename make_unsigned_bits<T>::type;
  using PoolType = RandenPool<unsigned_type>;
  using SpanType = absl::Span<unsigned_type>;

  static constexpr size_t kInitialBuffer = kBufferSize + 1;
  static constexpr size_t kHalfBuffer = kBufferSize / 2;

 public:
  using result_type = T;

  static_assert(std::is_unsigned<result_type>::value,
                "PoolURBG must be parameterized by an unsigned integer type");

  static_assert(kBufferSize > 1,
                "PoolURBG must be parameterized by a buffer-size > 1");

  static_assert(kBufferSize <= 256,
                "PoolURBG must be parameterized by a buffer-size <= 256");

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

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

  PoolURBG() : next_(kInitialBuffer) {}

  // copy-constructor does not copy cache.
  PoolURBG(const PoolURBG&) : next_(kInitialBuffer) {}
  const PoolURBG& operator=(const PoolURBG&) {
    next_ = kInitialBuffer;
    return *this;
  }

  // move-constructor does move cache.
  PoolURBG(PoolURBG&&) = default;
  PoolURBG& operator=(PoolURBG&&) = default;

  inline result_type operator()() {
    if (next_ >= kBufferSize) {
      next_ = (kBufferSize > 2 && next_ > kBufferSize) ? kHalfBuffer : 0;
      PoolType::Fill(SpanType(reinterpret_cast<unsigned_type*>(state_ + next_),
                              kBufferSize - next_));
    }
    return state_[next_++];
  }

 private:
  // Buffer size.
  size_t next_;  // index within state_
  result_type state_[kBufferSize];
};

}  // namespace random_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_POOL_URBG_H_