//
// 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 <utility>
#include <type_traits>

#include "absl/base/config.h"
#include "absl/base/internal/fast_type_id.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.");
            // SFINAE to detect whether the URBG type includes a member matching
            // bool InvokeMock(base_internal::FastTypeIdType, void*, void*).
            //
            // These live inside BitGenRef so that they have friend access
            // to MockingBitGen. (see similar methods in DistributionCaller).
            template<template<class...> class Trait, class AlwaysVoid, class... Args>
            struct detector : std::false_type
            {
            };
            template<template<class...> class Trait, class... Args>
            struct detector<Trait, absl::void_t<Trait<Args...>>, Args...> : std::true_type
            {
            };

            template<class T>
            using invoke_mock_t = decltype(std::declval<T*>()->InvokeMock(
                std::declval<::absl::base_internal::FastTypeIdType>(),
                std::declval<void*>(),
                std::declval<void*>()
            ));

            using HasInvokeMock = typename detector<invoke_mock_t, void, URBG>::type;

            // 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<absl::decay_t<Args>...>;
                using KeyT = ResultT(DistrT, ArgTupleT);

                ArgTupleT arg_tuple(std::forward<Args>(args)...);
                ResultT result;
                if (!urbg->InvokeMock(::absl::base_internal::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...>(HasInvokeMock{}, urbg, std::forward<Args>(args)...);
            }
        };

    }  // namespace random_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_