absl/random/internal/distribution_caller.h

//
// 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_DISTRIBUTION_CALLER_H_
#define ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_

#include <tuple>
#include <type_traits>
#include <utility>

#include "absl/base/config.h"
#include "absl/base/fast_type_id.h"
#include "absl/meta/type_traits.h"
#include "absl/random/mocking_access.h"
#include "absl/utility/utility.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal {

// DistributionCaller provides an opportunity to overload the general
// mechanism for calling a distribution, allowing for mock-RNG classes
// to intercept such calls.
template <typename URBG>
struct DistributionCaller {
  static_assert(!std::is_pointer<URBG>::value,
                "You must pass a reference, not a pointer.");

  using RandomMockingAccess = ::absl::RandomMockingAccess;

  // Default implementation of distribution caller.
  template <typename DistrT, typename... Args>
  static typename DistrT::result_type Impl(std::false_type, URBG* urbg,
                                           Args&&... args) {
    DistrT dist(std::forward<Args>(args)...);
    return dist(*urbg);
  }

  // Mock implementation of distribution caller.
  // The underlying KeyT must match the KeyT constructed by MockOverloadSet.
  template <typename DistrT, typename... Args>
  static typename DistrT::result_type Impl(std::true_type, URBG* urbg,
                                           Args&&... args) {
    using ResultT = typename DistrT::result_type;
    using ArgTupleT = std::tuple<std::decay_t<Args>...>;
    using KeyT = ResultT(DistrT, ArgTupleT);

    ArgTupleT arg_tuple(std::forward<Args>(args)...);
    ResultT result;
    if (!RandomMockingAccess::InvokeMock<URBG>(urbg, FastTypeId<KeyT>(),
                                               &arg_tuple, &result)) {
      auto dist = absl::make_from_tuple<DistrT>(arg_tuple);
      result = dist(*urbg);
    }
    return result;
  }

  // Default implementation of distribution caller.
  template <typename DistrT, typename... Args>
  static typename DistrT::result_type Call(URBG* urbg, Args&&... args) {
    return Impl<DistrT, Args...>(RandomMockingAccess::HasInvokeMock<URBG>{},
                                 urbg, std::forward<Args>(args)...);
  }
};

}  // namespace random_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_