summaryrefslogtreecommitdiff
path: root/absl/random/bit_gen_ref.h
blob: e8771162e5fb5771308119f66a43e4c19723d49c (plain)
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
//
// 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.
//
// -----------------------------------------------------------------------------
// File: bit_gen_ref.h
// -----------------------------------------------------------------------------
//
// This header defines a bit generator "reference" class, for use in interfaces
// that take both Abseil (e.g. `absl::BitGen`) and standard library (e.g.
// `std::mt19937`) bit generators.

#ifndef ABSL_RANDOM_BIT_GEN_REF_H_
#define ABSL_RANDOM_BIT_GEN_REF_H_

#include "absl/base/macros.h"
#include "absl/meta/type_traits.h"
#include "absl/random/internal/distribution_caller.h"
#include "absl/random/internal/fast_uniform_bits.h"
#include "absl/random/internal/mocking_bit_gen_base.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal {

template <typename URBG, typename = void, typename = void, typename = void>
struct is_urbg : std::false_type {};

template <typename URBG>
struct is_urbg<
    URBG,
    absl::enable_if_t<std::is_same<
        typename URBG::result_type,
        typename std::decay<decltype((URBG::min)())>::type>::value>,
    absl::enable_if_t<std::is_same<
        typename URBG::result_type,
        typename std::decay<decltype((URBG::max)())>::type>::value>,
    absl::enable_if_t<std::is_same<
        typename URBG::result_type,
        typename std::decay<decltype(std::declval<URBG>()())>::type>::value>>
    : std::true_type {};

}  // namespace random_internal

// -----------------------------------------------------------------------------
// absl::BitGenRef
// -----------------------------------------------------------------------------
//
// `absl::BitGenRef` is a type-erasing class that provides a generator-agnostic
// non-owning "reference" interface for use in place of any specific uniform
// random bit generator (URBG). This class may be used for both Abseil
// (e.g. `absl::BitGen`, `absl::InsecureBitGen`) and Standard library (e.g
// `std::mt19937`, `std::minstd_rand`) bit generators.
//
// Like other reference classes, `absl::BitGenRef` does not own the
// underlying bit generator, and the underlying instance must outlive the
// `absl::BitGenRef`.
//
// `absl::BitGenRef` is particularly useful when used with an
// `absl::MockingBitGen` to test specific paths in functions which use random
// values.
//
// Example:
//    void TakesBitGenRef(absl::BitGenRef gen) {
//      int x = absl::Uniform<int>(gen, 0, 1000);
//    }
//
class BitGenRef {
 public:
  using result_type = uint64_t;

  BitGenRef(const absl::BitGenRef&) = default;
  BitGenRef(absl::BitGenRef&&) = default;
  BitGenRef& operator=(const absl::BitGenRef&) = default;
  BitGenRef& operator=(absl::BitGenRef&&) = default;

  template <typename URBG,
            typename absl::enable_if_t<
                (!std::is_same<URBG, BitGenRef>::value &&
                 random_internal::is_urbg<URBG>::value)>* = nullptr>
  BitGenRef(URBG& gen)  // NOLINT
      : mocked_gen_ptr_(MakeMockPointer(&gen)),
        t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
        generate_impl_fn_(ImplFn<URBG>) {
  }

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

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

  result_type operator()() { return generate_impl_fn_(t_erased_gen_ptr_); }

 private:
  friend struct absl::random_internal::DistributionCaller<absl::BitGenRef>;
  using impl_fn = result_type (*)(uintptr_t);
  using mocker_base_t = absl::random_internal::MockingBitGenBase;

  // Convert an arbitrary URBG pointer into either a valid mocker_base_t
  // pointer or a nullptr.
  static inline mocker_base_t* MakeMockPointer(mocker_base_t* t) { return t; }
  static inline mocker_base_t* MakeMockPointer(void*) { return nullptr; }

  template <typename URBG>
  static result_type ImplFn(uintptr_t ptr) {
    // Ensure that the return values from operator() fill the entire
    // range promised by result_type, min() and max().
    absl::random_internal::FastUniformBits<result_type> fast_uniform_bits;
    return fast_uniform_bits(*reinterpret_cast<URBG*>(ptr));
  }

  mocker_base_t* mocked_gen_ptr_;
  uintptr_t t_erased_gen_ptr_;
  impl_fn generate_impl_fn_;
};

namespace random_internal {

template <>
struct DistributionCaller<absl::BitGenRef> {
  template <typename DistrT, typename FormatT, typename... Args>
  static typename DistrT::result_type Call(absl::BitGenRef* gen_ref,
                                           Args&&... args) {
    auto* mock_ptr = gen_ref->mocked_gen_ptr_;
    if (mock_ptr == nullptr) {
      DistrT dist(std::forward<Args>(args)...);
      return dist(*gen_ref);
    } else {
      return mock_ptr->template Call<DistrT, FormatT>(
          std::forward<Args>(args)...);
    }
  }
};

}  // namespace random_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_BIT_GEN_REF_H_