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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// -----------------------------------------------------------------------------
// mocking_bit_gen.h
// -----------------------------------------------------------------------------
// This file includes an `absl::MockingBitGen` class to use as a mock within the
// Googletest testing framework. Such a mock is useful to provide deterministic
// values as return values within (otherwise random) Abseil distribution
// functions. Such determinism within a mock is useful within testing frameworks
// to test otherwise indeterminate APIs.
// More information about the Googletest testing framework is available at
#include <iterator>
#include <limits>
#include <memory>
#include <tuple>
#include <type_traits>
#include <utility>
#include "gmock/gmock.h"
#include "gtest/gtest.h"
#include "absl/base/internal/fast_type_id.h"
#include "absl/container/flat_hash_map.h"
#include "absl/meta/type_traits.h"
#include "absl/random/distributions.h"
#include "absl/random/internal/distribution_caller.h"
#include "absl/random/random.h"
#include "absl/strings/str_cat.h"
#include "absl/strings/str_join.h"
#include "absl/types/span.h"
#include "absl/types/variant.h"
#include "absl/utility/utility.h"
namespace absl {
namespace random_internal {
template <typename>
struct DistributionCaller;
class MockHelpers;
} // namespace random_internal
class BitGenRef;
// MockingBitGen
// `absl::MockingBitGen` is a mock Uniform Random Bit Generator (URBG) class
// which can act in place of an `absl::BitGen` URBG within tests using the
// Googletest testing framework.
// Usage:
// Use an `absl::MockingBitGen` along with a mock distribution object (within
// mock_distributions.h) inside Googletest constructs such as ON_CALL(),
// EXPECT_TRUE(), etc. to produce deterministic results conforming to the
// distribution's API contract.
// Example:
// // Mock a call to an `absl::Bernoulli` distribution using Googletest
// absl::MockingBitGen bitgen;
// ON_CALL(absl::MockBernoulli(), Call(bitgen, 0.5))
// .WillByDefault(testing::Return(true));
// EXPECT_TRUE(absl::Bernoulli(bitgen, 0.5));
// // Mock a call to an `absl::Uniform` distribution within Googletest
// absl::MockingBitGen bitgen;
// ON_CALL(absl::MockUniform<int>(), Call(bitgen, testing::_, testing::_))
// .WillByDefault([] (int low, int high) {
// return low + (high - low) / 2;
// });
// EXPECT_EQ(absl::Uniform<int>(gen, 0, 10), 5);
// EXPECT_EQ(absl::Uniform<int>(gen, 30, 40), 35);
// At this time, only mock distributions supplied within the Abseil random
// library are officially supported.
// EXPECT_CALL and ON_CALL need to be made within the same DLL component as
// the call to absl::Uniform and related methods, otherwise mocking will fail
// since the underlying implementation creates a type-specific pointer which
// will be distinct across different DLL boundaries.
class MockingBitGen {
MockingBitGen() = default;
~MockingBitGen() = default;
// URBG interface
using result_type = absl::BitGen::result_type;
static constexpr result_type(min)() { return (absl::BitGen::min)(); }
static constexpr result_type(max)() { return (absl::BitGen::max)(); }
result_type operator()() { return gen_(); }
// GetMockFnType returns the testing::MockFunction for a result and tuple.
// This method only exists for type deduction and is otherwise unimplemented.
template <typename ResultT, typename... Args>
static auto GetMockFnType(ResultT, std::tuple<Args...>)
-> ::testing::MockFunction<ResultT(Args...)>;
// MockFnCaller is a helper method for use with absl::apply to
// apply an ArgTupleT to a compatible MockFunction.
// NOTE: MockFnCaller is essentially equivalent to the lambda:
// [fn](auto... args) { return fn->Call(std::move(args)...)}
// however that fails to build on some supported platforms.
template <typename MockFnType, typename ResultT, typename Tuple>
struct MockFnCaller;
// specialization for std::tuple.
template <typename MockFnType, typename ResultT, typename... Args>
struct MockFnCaller<MockFnType, ResultT, std::tuple<Args...>> {
MockFnType* fn;
inline ResultT operator()(Args... args) {
return fn->Call(std::move(args)...);
// FunctionHolder owns a particular ::testing::MockFunction associated with
// a mocked type signature, and implement the type-erased Apply call, which
// applies type-erased arguments to the mock.
class FunctionHolder {
virtual ~FunctionHolder() = default;
// Call is a dispatch function which converts the
// generic type-erased parameters into a specific mock invocation call.
virtual void Apply(/*ArgTupleT*/ void* args_tuple,
/*ResultT*/ void* result) = 0;
template <typename MockFnType, typename ResultT, typename ArgTupleT>
class FunctionHolderImpl final : public FunctionHolder {
void Apply(void* args_tuple, void* result) override {
// Requires tuple_args to point to a ArgTupleT, which is a
// std::tuple<Args...> used to invoke the mock function. Requires result
// to point to a ResultT, which is the result of the call.
*static_cast<ResultT*>(result) =
absl::apply(MockFnCaller<MockFnType, ResultT, ArgTupleT>{&mock_fn_},
MockFnType mock_fn_;
// MockingBitGen::RegisterMock
// RegisterMock<ResultT, ArgTupleT>(FastTypeIdType) is the main extension
// point for extending the MockingBitGen framework. It provides a mechanism to
// install a mock expectation for a function like ResultT(Args...) keyed by
// type_idex onto the MockingBitGen context. The key is that the type_index
// used to register must match the type index used to call the mock.
// The returned MockFunction<...> type can be used to setup additional
// distribution parameters of the expectation.
template <typename ResultT, typename ArgTupleT, typename SelfT>
auto RegisterMock(SelfT&, base_internal::FastTypeIdType type)
-> decltype(GetMockFnType(std::declval<ResultT>(),
std::declval<ArgTupleT>()))& {
using MockFnType = decltype(GetMockFnType(std::declval<ResultT>(),
using WrappedFnType = absl::conditional_t<
std::is_same<SelfT, ::testing::NiceMock<absl::MockingBitGen>>::value,
::testing::StrictMock<MockFnType>, MockFnType>>>;
using ImplT = FunctionHolderImpl<WrappedFnType, ResultT, ArgTupleT>;
auto& mock = mocks_[type];
if (!mock) {
mock = absl::make_unique<ImplT>();
return static_cast<ImplT*>(mock.get())->mock_fn_;
// MockingBitGen::InvokeMock
// InvokeMock(FastTypeIdType, args, result) is the entrypoint for invoking
// mocks registered on MockingBitGen.
// When no mocks are registered on the provided FastTypeIdType, returns false.
// Otherwise attempts to invoke the mock function ResultT(Args...) that
// was previously registered via the type_index.
// Requires tuple_args to point to a ArgTupleT, which is a std::tuple<Args...>
// used to invoke the mock function.
// Requires result to point to a ResultT, which is the result of the call.
inline bool InvokeMock(base_internal::FastTypeIdType type, void* args_tuple,
void* result) {
// Trigger a mock, if there exists one that matches `param`.
auto it = mocks_.find(type);
if (it == mocks_.end()) return false;
it->second->Apply(args_tuple, result);
return true;
absl::BitGen gen_;
template <typename>
friend struct ::absl::random_internal::DistributionCaller; // for InvokeMock
friend class ::absl::BitGenRef; // for InvokeMock
friend class ::absl::random_internal::MockHelpers; // for RegisterMock,
// InvokeMock
} // namespace absl