Export of internal Abseil changes

--
4503a1945d61540d3f7dd9ee2399297fbd423b82 by Andy Soffer <asoffer@google.com>:

Releasing absl::MockingBitGen and absl::BitGenRef

PiperOrigin-RevId: 281594380

--
2cb926fd9e39b5fa24c9541bfafcf18916cf7df2 by Abseil Team <absl-team@google.com>:

Explicitly export files needed by other packages

PiperOrigin-RevId: 281482304
GitOrigin-RevId: 4503a1945d61540d3f7dd9ee2399297fbd423b82
Change-Id: Id9f694eaaa23f42de817c8e8a28e6f86444f5637

12 files changed, 1598 insertions, 0 deletions

diff --git a/absl/random/BUILD.bazel b/absl/random/BUILD.bazel
index c3520df8dccb..2585b39742e2 100644
--- a/absl/random/BUILD.bazel
+++ b/absl/random/BUILD.bazel
@@ -110,6 +110,58 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "bit_gen_ref",
+    hdrs = ["bit_gen_ref.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:distribution_caller",
+        "//absl/random/internal:fast_uniform_bits",
+        "//absl/random/internal:mocking_bit_gen_base",
+    ],
+)
+
+cc_library(
+    name = "mock_distributions",
+    testonly = 1,
+    hdrs = ["mock_distributions.h"],
+    deps = [
+        ":distributions",
+        ":mocking_bit_gen",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:mock_overload_set",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "mocking_bit_gen",
+    testonly = 1,
+    srcs = [
+        "mocking_bit_gen.cc",
+    ],
+    hdrs = [
+        "mocking_bit_gen.h",
+    ],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":distributions",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:flat_hash_map",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:distribution_caller",
+        "//absl/random/internal:mocking_bit_gen_base",
+        "//absl/strings",
+        "//absl/types:span",
+        "//absl/types:variant",
+        "//absl/utility",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
 cc_test(
     name = "bernoulli_distribution_test",
     size = "small",
@@ -346,6 +398,46 @@ cc_test(
 )
 
 cc_test(
+    name = "bit_gen_ref_test",
+    size = "small",
+    srcs = ["bit_gen_ref_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bit_gen_ref",
+        ":random",
+        "//absl/random/internal:sequence_urbg",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mocking_bit_gen_test",
+    size = "small",
+    srcs = ["mocking_bit_gen_test.cc"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bit_gen_ref",
+        ":mock_distributions",
+        ":mocking_bit_gen",
+        ":random",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mock_distributions_test",
+    size = "small",
+    srcs = ["mock_distributions_test.cc"],
+    deps = [
+        ":mock_distributions",
+        ":mocking_bit_gen",
+        ":random",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
     name = "examples_test",
     size = "small",
     srcs = ["examples_test.cc"],
diff --git a/absl/random/CMakeLists.txt b/absl/random/CMakeLists.txt
index 264a6f3c6abc..13e96357ffdd 100644
--- a/absl/random/CMakeLists.txt
+++ b/absl/random/CMakeLists.txt
@@ -34,6 +34,132 @@ absl_cc_library(
 
 absl_cc_library(
   NAME
+    random_bit_gen_ref
+  HDRS
+    "bit_gen_ref.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::random_internal_distribution_caller
+    absl::random_internal_fast_uniform_bits
+    absl::random_internal_mocking_bit_gen_base
+    absl::type_traits
+)
+
+absl_cc_test(
+  NAME
+    random_bit_gen_ref_test
+  SRCS
+    "bit_gen_ref_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_bit_gen_ref
+    absl::random_random
+    absl::random_internal_sequence_urbg
+    gmock
+    gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mocking_bit_gen_base
+  HDRS
+    "internal/mocking_bit_gen_base.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_random
+    absl::strings
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mock_overload_set
+  HDRS
+    "internal/mock_overload_set.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_mocking_bit_gen
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    random_mocking_bit_gen
+  HDRS
+    "mock_distributions.h"
+    "mocking_bit_gen.h"
+  SRCS
+    "mocking_bit_gen.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::flat_hash_map
+    absl::raw_logging_internal
+    absl::random_distributions
+    absl::random_internal_distribution_caller
+    absl::random_internal_mocking_bit_gen_base
+    absl::random_internal_mock_overload_set
+    absl::strings
+    absl::span
+    absl::type_traits
+    absl::utility
+    absl::variant
+    gmock
+    gtest
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    random_mock_distributions_test
+  SRCS
+    "mock_distributions_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_mocking_bit_gen
+    absl::random_random
+    gmock
+    gtest_main
+)
+
+absl_cc_test(
+  NAME
+    random_mocking_bit_gen_test
+  SRCS
+    "mocking_bit_gen_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_bit_gen_ref
+    absl::random_mocking_bit_gen
+    absl::random_random
+    gmock
+    gtest_main
+)
+
+absl_cc_library(
+  NAME
     random_distributions
   SRCS
     "discrete_distribution.cc"
diff --git a/absl/random/bit_gen_ref.h b/absl/random/bit_gen_ref.h
new file mode 100644
index 000000000000..00e904f85ec3
--- /dev/null
+++ b/absl/random/bit_gen_ref.h
@@ -0,0 +1,151 @@
+//
+// 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 {
+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
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_BIT_GEN_REF_H_
diff --git a/absl/random/bit_gen_ref_test.cc b/absl/random/bit_gen_ref_test.cc
new file mode 100644
index 000000000000..bc02ca5c1621
--- /dev/null
+++ b/absl/random/bit_gen_ref_test.cc
@@ -0,0 +1,99 @@
+//
+// 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.
+//
+#include "absl/random/bit_gen_ref.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/random/internal/sequence_urbg.h"
+#include "absl/random/random.h"
+
+namespace absl {
+
+class ConstBitGen : public absl::random_internal::MockingBitGenBase {
+  bool CallImpl(const std::type_info&, void*, void* result) override {
+    *static_cast<int*>(result) = 42;
+    return true;
+  }
+};
+
+namespace random_internal {
+template <>
+struct DistributionCaller<ConstBitGen> {
+  template <typename DistrT, typename FormatT, typename... Args>
+  static typename DistrT::result_type Call(ConstBitGen* gen, Args&&... args) {
+    return gen->template Call<DistrT, FormatT>(std::forward<Args>(args)...);
+  }
+};
+}  // namespace random_internal
+
+namespace {
+int FnTest(absl::BitGenRef gen_ref) { return absl::Uniform(gen_ref, 1, 7); }
+
+template <typename T>
+class BitGenRefTest : public testing::Test {};
+
+using BitGenTypes =
+    ::testing::Types<absl::BitGen, absl::InsecureBitGen, std::mt19937,
+                     std::mt19937_64, std::minstd_rand>;
+TYPED_TEST_SUITE(BitGenRefTest, BitGenTypes);
+
+TYPED_TEST(BitGenRefTest, BasicTest) {
+  TypeParam gen;
+  auto x = FnTest(gen);
+  EXPECT_NEAR(x, 4, 3);
+}
+
+TYPED_TEST(BitGenRefTest, Copyable) {
+  TypeParam gen;
+  absl::BitGenRef gen_ref(gen);
+  FnTest(gen_ref);  // Copy
+}
+
+TEST(BitGenRefTest, PassThroughEquivalence) {
+  // sequence_urbg returns 64-bit results.
+  absl::random_internal::sequence_urbg urbg(
+      {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
+       0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
+       0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
+       0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
+
+  std::vector<uint64_t> output(12);
+
+  {
+    absl::BitGenRef view(urbg);
+    for (auto& v : output) {
+      v = view();
+    }
+  }
+
+  std::vector<uint64_t> expected(
+      {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
+       0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
+       0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
+       0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
+
+  EXPECT_THAT(output, testing::Eq(expected));
+}
+
+TEST(BitGenRefTest, MockingBitGenBaseOverrides) {
+  ConstBitGen const_gen;
+  EXPECT_EQ(FnTest(const_gen), 42);
+
+  absl::BitGenRef gen_ref(const_gen);
+  EXPECT_EQ(FnTest(gen_ref), 42);  // Copy
+}
+}  // namespace
+}  // namespace absl
diff --git a/absl/random/internal/BUILD.bazel b/absl/random/internal/BUILD.bazel
index cc9bc013bc90..5026e2b27220 100644
--- a/absl/random/internal/BUILD.bazel
+++ b/absl/random/internal/BUILD.bazel
@@ -493,6 +493,29 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "mocking_bit_gen_base",
+    hdrs = ["mocking_bit_gen_base.h"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/random",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "mock_overload_set",
+    testonly = 1,
+    hdrs = ["mock_overload_set.h"],
+    visibility = [
+        "//absl/random:__pkg__",
+    ],
+    deps = [
+        "//absl/random:mocking_bit_gen",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
 cc_test(
     name = "nonsecure_base_test",
     size = "small",
diff --git a/absl/random/internal/mock_overload_set.h b/absl/random/internal/mock_overload_set.h
new file mode 100644
index 000000000000..539313d73c95
--- /dev/null
+++ b/absl/random/internal/mock_overload_set.h
@@ -0,0 +1,89 @@
+//
+// Copyright 2019 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_MOCK_OVERLOAD_SET_H_
+#define ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
+
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/random/mocking_bit_gen.h"
+
+namespace absl {
+namespace random_internal {
+
+template <typename DistrT, typename Fn>
+struct MockSingleOverload;
+
+// MockSingleOverload
+//
+// MockSingleOverload hooks in to gMock's `ON_CALL` and `EXPECT_CALL` macros.
+// EXPECT_CALL(mock_single_overload, Call(...))` will expand to a call to
+// `mock_single_overload.gmock_Call(...)`. Because expectations are stored on
+// the MockingBitGen (an argument passed inside `Call(...)`), this forwards to
+// arguments to Mocking::Register.
+template <typename DistrT, typename Ret, typename... Args>
+struct MockSingleOverload<DistrT, Ret(MockingBitGen&, Args...)> {
+  static_assert(std::is_same<typename DistrT::result_type, Ret>::value,
+                "Overload signature must have return type matching the "
+                "distributions result type.");
+  auto gmock_Call(
+      absl::MockingBitGen& gen,  // NOLINT(google-runtime-references)
+      const ::testing::Matcher<Args>&... args)
+      -> decltype(gen.Register<DistrT, Args...>(args...)) {
+    return gen.Register<DistrT, Args...>(args...);
+  }
+};
+
+template <typename DistrT, typename Ret, typename Arg, typename... Args>
+struct MockSingleOverload<DistrT, Ret(Arg, MockingBitGen&, Args...)> {
+  static_assert(std::is_same<typename DistrT::result_type, Ret>::value,
+                "Overload signature must have return type matching the "
+                "distributions result type.");
+  auto gmock_Call(
+      const ::testing::Matcher<Arg>& arg,
+      absl::MockingBitGen& gen,  // NOLINT(google-runtime-references)
+      const ::testing::Matcher<Args>&... args)
+      -> decltype(gen.Register<DistrT, Arg, Args...>(arg, args...)) {
+    return gen.Register<DistrT, Arg, Args...>(arg, args...);
+  }
+};
+
+// MockOverloadSet
+//
+// MockOverloadSet takes a distribution and a collection of signatures and
+// performs overload resolution amongst all the overloads. This makes
+// `EXPECT_CALL(mock_overload_set, Call(...))` expand and do overload resolution
+// correctly.
+template <typename DistrT, typename... Signatures>
+struct MockOverloadSet;
+
+template <typename DistrT, typename Sig>
+struct MockOverloadSet<DistrT, Sig> : public MockSingleOverload<DistrT, Sig> {
+  using MockSingleOverload<DistrT, Sig>::gmock_Call;
+};
+
+template <typename DistrT, typename FirstSig, typename... Rest>
+struct MockOverloadSet<DistrT, FirstSig, Rest...>
+    : public MockSingleOverload<DistrT, FirstSig>,
+      public MockOverloadSet<DistrT, Rest...> {
+  using MockSingleOverload<DistrT, FirstSig>::gmock_Call;
+  using MockOverloadSet<DistrT, Rest...>::gmock_Call;
+};
+
+}  // namespace random_internal
+}  // namespace absl
+#endif  // ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
diff --git a/absl/random/internal/mocking_bit_gen_base.h b/absl/random/internal/mocking_bit_gen_base.h
new file mode 100644
index 000000000000..aff2ba6df3e7
--- /dev/null
+++ b/absl/random/internal/mocking_bit_gen_base.h
@@ -0,0 +1,118 @@
+//
+// 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_MOCKING_BIT_GEN_BASE_H_
+#define ABSL_RANDOM_INTERNAL_MOCKING_BIT_GEN_BASE_H_
+
+#include <atomic>
+#include <deque>
+#include <string>
+#include <typeinfo>
+
+#include "absl/random/random.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+namespace random_internal {
+
+// MockingBitGenExpectationFormatter is invoked to format unsatisfied mocks
+// and remaining results into a description string.
+template <typename DistrT, typename FormatT>
+struct MockingBitGenExpectationFormatter {
+  std::string operator()(absl::string_view args) {
+    return absl::StrCat(FormatT::FunctionName(), "(", args, ")");
+  }
+};
+
+// MockingBitGenCallFormatter is invoked to format each distribution call
+// into a description string for the mock log.
+template <typename DistrT, typename FormatT>
+struct MockingBitGenCallFormatter {
+  std::string operator()(const DistrT& dist,
+                         const typename DistrT::result_type& result) {
+    return absl::StrCat(
+        FormatT::FunctionName(), "(", FormatT::FormatArgs(dist), ") => {",
+        FormatT::FormatResults(absl::MakeSpan(&result, 1)), "}");
+  }
+};
+
+class MockingBitGenBase {
+  template <typename>
+  friend struct DistributionCaller;
+  using generator_type = absl::BitGen;
+
+ public:
+  // URBG interface
+  using result_type = generator_type::result_type;
+  static constexpr result_type(min)() { return (generator_type::min)(); }
+  static constexpr result_type(max)() { return (generator_type::max)(); }
+  result_type operator()() { return gen_(); }
+
+  MockingBitGenBase() : gen_(), observed_call_log_() {}
+  virtual ~MockingBitGenBase() = default;
+
+ protected:
+  const std::deque<std::string>& observed_call_log() {
+    return observed_call_log_;
+  }
+
+  // CallImpl is the type-erased virtual dispatch.
+  // The type of dist is always distribution<T>,
+  // The type of result is always distribution<T>::result_type.
+  virtual bool CallImpl(const std::type_info& distr_type, void* dist_args,
+                        void* result) = 0;
+
+  template <typename DistrT, typename ArgTupleT>
+  static const std::type_info& GetTypeId() {
+    return typeid(std::pair<absl::decay_t<DistrT>, absl::decay_t<ArgTupleT>>);
+  }
+
+  // Call the generating distribution function.
+  // Invoked by DistributionCaller<>::Call<DistT, FormatT>.
+  // DistT is the distribution type.
+  // FormatT is the distribution formatter traits type.
+  template <typename DistrT, typename FormatT, typename... Args>
+  typename DistrT::result_type Call(Args&&... args) {
+    using distr_result_type = typename DistrT::result_type;
+    using ArgTupleT = std::tuple<absl::decay_t<Args>...>;
+
+    ArgTupleT arg_tuple(std::forward<Args>(args)...);
+    auto dist = absl::make_from_tuple<DistrT>(arg_tuple);
+
+    distr_result_type result{};
+    bool found_match =
+        CallImpl(GetTypeId<DistrT, ArgTupleT>(), &arg_tuple, &result);
+
+    if (!found_match) {
+      result = dist(gen_);
+    }
+
+    // TODO(asoffer): Forwarding the args through means we no longer need to
+    // extract them from the from the distribution in formatter traits. We can
+    // just StrJoin them.
+    observed_call_log_.push_back(
+        MockingBitGenCallFormatter<DistrT, FormatT>{}(dist, result));
+    return result;
+  }
+
+ private:
+  generator_type gen_;
+  std::deque<std::string> observed_call_log_;
+};  // namespace random_internal
+
+}  // namespace random_internal
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_MOCKING_BIT_GEN_BASE_H_
diff --git a/absl/random/mock_distributions.h b/absl/random/mock_distributions.h
new file mode 100644
index 000000000000..1af98a24a7f0
--- /dev/null
+++ b/absl/random/mock_distributions.h
@@ -0,0 +1,259 @@
+// 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: mock_distributions.h
+// -----------------------------------------------------------------------------
+//
+// This file contains mock distribution functions for use alongside an
+// `absl::MockingBitGen` object within the Googletest testing framework. Such
+// mocks are useful to provide deterministic values as return values within
+// (otherwise random) Abseil distribution functions.
+//
+// The return type of each function is a mock expectation object which
+// is used to set the match result.
+//
+// More information about the Googletest testing framework is available at
+// https://github.com/google/googletest
+//
+// Example:
+//
+//   absl::MockingBitGen mock;
+//   EXPECT_CALL(absl::MockUniform<int>(), Call(mock, 1, 1000))
+//     .WillRepeatedly(testing::ReturnRoundRobin({20, 40}));
+//
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 20);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 40);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 20);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 40);
+
+#ifndef ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
+#define ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
+
+#include <limits>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/distributions.h"
+#include "absl/random/internal/mock_overload_set.h"
+#include "absl/random/mocking_bit_gen.h"
+
+namespace absl {
+
+// -----------------------------------------------------------------------------
+// absl::MockUniform
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Uniform.
+//
+// `absl::MockUniform` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(mock))
+//     .WillOnce(Return(123456));
+//  auto x = absl::Uniform<uint32_t>(mock);
+//  assert(x == 123456)
+//
+template <typename R>
+using MockUniform = random_internal::MockOverloadSet<
+    random_internal::UniformDistributionWrapper<R>,
+    R(IntervalClosedOpenTag, MockingBitGen&, R, R),
+    R(IntervalClosedClosedTag, MockingBitGen&, R, R),
+    R(IntervalOpenOpenTag, MockingBitGen&, R, R),
+    R(IntervalOpenClosedTag, MockingBitGen&, R, R), R(MockingBitGen&, R, R),
+    R(MockingBitGen&)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockBernoulli
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Bernoulli.
+//
+// `absl::MockBernoulli` is a class used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockBernoulli(), Call(mock, testing::_))
+//     .WillOnce(Return(false));
+//  assert(absl::Bernoulli(mock, 0.5) == false);
+//
+using MockBernoulli =
+    random_internal::MockOverloadSet<absl::bernoulli_distribution,
+                                     bool(MockingBitGen&, double)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockBeta
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Beta.
+//
+// `absl::MockBeta` is a class used in conjunction with Googletest's `ON_CALL()`
+// and `EXPECT_CALL()` macros. To use it, default-construct an instance of it
+// inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the same way one
+// would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockBeta(), Call(mock, 3.0, 2.0))
+//     .WillOnce(Return(0.567));
+//  auto x = absl::Beta<double>(mock, 3.0, 2.0);
+//  assert(x == 0.567);
+//
+template <typename RealType>
+using MockBeta =
+    random_internal::MockOverloadSet<absl::beta_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType,
+                                              RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockExponential
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Exponential.
+//
+// `absl::MockExponential` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockExponential<double>(), Call(mock, 0.5))
+//     .WillOnce(Return(12.3456789));
+//  auto x = absl::Exponential<double>(mock, 0.5);
+//  assert(x == 12.3456789)
+//
+template <typename RealType>
+using MockExponential =
+    random_internal::MockOverloadSet<absl::exponential_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockGaussian
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Gaussian.
+//
+// `absl::MockGaussian` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockGaussian<double>(), Call(mock, 16.3, 3.3))
+//     .WillOnce(Return(12.3456789));
+//  auto x = absl::Gaussian<double>(mock, 16.3, 3.3);
+//  assert(x == 12.3456789)
+//
+template <typename RealType>
+using MockGaussian =
+    random_internal::MockOverloadSet<absl::gaussian_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType,
+                                              RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockLogUniform
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::LogUniform.
+//
+// `absl::MockLogUniform` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockLogUniform<int>(), Call(mock, 10, 10000, 10))
+//     .WillOnce(Return(1221));
+//  auto x = absl::LogUniform<int>(mock, 10, 10000, 10);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockLogUniform = random_internal::MockOverloadSet<
+    absl::log_uniform_int_distribution<IntType>,
+    IntType(MockingBitGen&, IntType, IntType, IntType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockPoisson
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Poisson.
+//
+// `absl::MockPoisson` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockPoisson<int>(), Call(mock, 2.0))
+//     .WillOnce(Return(1221));
+//  auto x = absl::Poisson<int>(mock, 2.0);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockPoisson =
+    random_internal::MockOverloadSet<absl::poisson_distribution<IntType>,
+                                     IntType(MockingBitGen&, double)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockZipf
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Zipf.
+//
+// `absl::MockZipf` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockZipf<int>(), Call(mock, 1000000, 2.0, 1.0))
+//     .WillOnce(Return(1221));
+//  auto x = absl::Zipf<int>(mock, 1000000, 2.0, 1.0);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockZipf =
+    random_internal::MockOverloadSet<absl::zipf_distribution<IntType>,
+                                     IntType(MockingBitGen&, IntType, double,
+                                             double)>;
+
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
diff --git a/absl/random/mock_distributions_test.cc b/absl/random/mock_distributions_test.cc
new file mode 100644
index 000000000000..de23bafe1e55
--- /dev/null
+++ b/absl/random/mock_distributions_test.cc
@@ -0,0 +1,72 @@
+// 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.
+
+#include "absl/random/mock_distributions.h"
+
+#include "gtest/gtest.h"
+#include "absl/random/mocking_bit_gen.h"
+#include "absl/random/random.h"
+
+namespace {
+using ::testing::Return;
+
+TEST(MockDistributions, Examples) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillOnce(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillOnce(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillOnce(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillOnce(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Beta<double>(gen, 3.0, 2.0), 0.567);
+  EXPECT_CALL(absl::MockBeta<double>(), Call(gen, 3.0, 2.0))
+      .WillOnce(Return(0.567));
+  EXPECT_EQ(absl::Beta<double>(gen, 3.0, 2.0), 0.567);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  EXPECT_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillOnce(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  EXPECT_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillOnce(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+  EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillOnce(Return(2040));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+}
+
+}  // namespace
diff --git a/absl/random/mocking_bit_gen.cc b/absl/random/mocking_bit_gen.cc
new file mode 100644
index 000000000000..73144528a78e
--- /dev/null
+++ b/absl/random/mocking_bit_gen.cc
@@ -0,0 +1,28 @@
+//
+// 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.
+//
+#include "absl/random/mocking_bit_gen.h"
+
+#include <string>
+
+namespace absl {
+MockingBitGen::~MockingBitGen() {
+
+  for (const auto& del : deleters_) {
+    del();
+  }
+}
+
+}  // namespace absl
diff --git a/absl/random/mocking_bit_gen.h b/absl/random/mocking_bit_gen.h
new file mode 100644
index 000000000000..d1b524a993dc
--- /dev/null
+++ b/absl/random/mocking_bit_gen.h
@@ -0,0 +1,194 @@
+// 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.
+//
+// -----------------------------------------------------------------------------
+// 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
+// https://github.com/google/googletest
+
+#ifndef ABSL_RANDOM_MOCKING_BIT_GEN_H_
+#define ABSL_RANDOM_MOCKING_BIT_GEN_H_
+
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <typeindex>
+#include <typeinfo>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.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/internal/mocking_bit_gen_base.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, typename>
+struct MockSingleOverload;
+
+}  // namespace random_internal
+
+// 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) / 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.
+//
+class MockingBitGen : public absl::random_internal::MockingBitGenBase {
+ public:
+  MockingBitGen() {}
+
+  ~MockingBitGen() override;
+
+ private:
+  template <typename DistrT, typename... Args>
+  using MockFnType =
+      ::testing::MockFunction<typename DistrT::result_type(Args...)>;
+
+  // MockingBitGen::Register
+  //
+  // Register<DistrT, FormatT, ArgTupleT> is the main extension point for
+  // extending the MockingBitGen framework. It provides a mechanism to install a
+  // mock expectation for the distribution `distr_t` onto the MockingBitGen
+  // context.
+  //
+  // The returned MockFunction<...> type can be used to setup additional
+  // distribution parameters of the expectation.
+  template <typename DistrT, typename... Args, typename... Ms>
+  decltype(std::declval<MockFnType<DistrT, Args...>>().gmock_Call(
+      std::declval<Ms>()...))
+  Register(Ms&&... matchers) {
+    auto& mock =
+        mocks_[std::type_index(GetTypeId<DistrT, std::tuple<Args...>>())];
+
+    if (!mock.mock_fn) {
+      auto* mock_fn = new MockFnType<DistrT, Args...>;
+      mock.mock_fn = mock_fn;
+      mock.match_impl = &MatchImpl<DistrT, Args...>;
+      deleters_.emplace_back([mock_fn] { delete mock_fn; });
+    }
+
+    return static_cast<MockFnType<DistrT, Args...>*>(mock.mock_fn)
+        ->gmock_Call(std::forward<Ms>(matchers)...);
+  }
+
+  mutable std::vector<std::function<void()>> deleters_;
+
+  using match_impl_fn = void (*)(void* mock_fn, void* t_erased_dist_args,
+                                 void* t_erased_result);
+  struct MockData {
+    void* mock_fn = nullptr;
+    match_impl_fn match_impl = nullptr;
+  };
+
+  mutable absl::flat_hash_map<std::type_index, MockData> mocks_;
+
+  template <typename DistrT, typename... Args>
+  static void MatchImpl(void* mock_fn, void* dist_args, void* result) {
+    using result_type = typename DistrT::result_type;
+    *static_cast<result_type*>(result) = absl::apply(
+        [mock_fn](Args... args) -> result_type {
+          return (*static_cast<MockFnType<DistrT, Args...>*>(mock_fn))
+              .Call(std::move(args)...);
+        },
+        *static_cast<std::tuple<Args...>*>(dist_args));
+  }
+
+  // Looks for an appropriate mock - Returns the mocked result if one is found.
+  // Otherwise, returns a random value generated by the underlying URBG.
+  bool CallImpl(const std::type_info& key_type, void* dist_args,
+                void* result) override {
+    // Trigger a mock, if there exists one that matches `param`.
+    auto it = mocks_.find(std::type_index(key_type));
+    if (it == mocks_.end()) return false;
+    auto* mock_data = static_cast<MockData*>(&it->second);
+    mock_data->match_impl(mock_data->mock_fn, dist_args, result);
+    return true;
+  }
+
+  template <typename, typename>
+  friend struct ::absl::random_internal::MockSingleOverload;
+  friend struct ::absl::random_internal::DistributionCaller<
+      absl::MockingBitGen>;
+};
+
+// -----------------------------------------------------------------------------
+// Implementation Details Only Below
+// -----------------------------------------------------------------------------
+
+namespace random_internal {
+
+template <>
+struct DistributionCaller<absl::MockingBitGen> {
+  template <typename DistrT, typename FormatT, typename... Args>
+  static typename DistrT::result_type Call(absl::MockingBitGen* gen,
+                                           Args&&... args) {
+    return gen->template Call<DistrT, FormatT>(std::forward<Args>(args)...);
+  }
+};
+
+}  // namespace random_internal
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCKING_BIT_GEN_H_
diff --git a/absl/random/mocking_bit_gen_test.cc b/absl/random/mocking_bit_gen_test.cc
new file mode 100644
index 000000000000..dcf74fd6db85
--- /dev/null
+++ b/absl/random/mocking_bit_gen_test.cc
@@ -0,0 +1,347 @@
+//
+// 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.
+//
+#include "absl/random/mocking_bit_gen.h"
+
+#include <numeric>
+#include <random>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest-spi.h"
+#include "gtest/gtest.h"
+#include "absl/random/bit_gen_ref.h"
+#include "absl/random/mock_distributions.h"
+#include "absl/random/random.h"
+
+namespace {
+using ::testing::Ne;
+using ::testing::Return;
+
+TEST(BasicMocking, AllDistributionsAreOverridable) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillOnce(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillOnce(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillOnce(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillOnce(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  EXPECT_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillOnce(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  EXPECT_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillOnce(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+  EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillOnce(Return(2040));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+}
+
+TEST(BasicMocking, OnDistribution) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  ON_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillByDefault(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  ON_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillByDefault(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  ON_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillByDefault(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillByDefault(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  ON_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillByDefault(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  ON_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillByDefault(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  ON_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillByDefault(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+  ON_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillByDefault(Return(2040));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+}
+
+TEST(BasicMocking, GMockMatchers) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  ON_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillByDefault(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+}
+
+TEST(BasicMocking, OverridesWithMultipleGMockExpectations) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 10000))
+      .WillOnce(Return(20))
+      .WillOnce(Return(40))
+      .WillOnce(Return(60));
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 20);
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 40);
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 60);
+}
+
+TEST(BasicMocking, DefaultArgument) {
+  absl::MockingBitGen gen;
+
+  ON_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillByDefault(Return(200));
+
+  EXPECT_EQ(absl::Exponential<double>(gen), 200);
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 200);
+}
+
+TEST(BasicMocking, MultipleGenerators) {
+  auto get_value = [](absl::BitGenRef gen_ref) {
+    return absl::Uniform(gen_ref, 1, 1000000);
+  };
+  absl::MockingBitGen unmocked_generator;
+  absl::MockingBitGen mocked_with_3;
+  absl::MockingBitGen mocked_with_11;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(mocked_with_3, 1, 1000000))
+      .WillOnce(Return(3))
+      .WillRepeatedly(Return(17));
+  EXPECT_CALL(absl::MockUniform<int>(), Call(mocked_with_11, 1, 1000000))
+      .WillOnce(Return(11))
+      .WillRepeatedly(Return(17));
+
+  // Ensure that unmocked generator generates neither value.
+  int unmocked_value = get_value(unmocked_generator);
+  EXPECT_NE(unmocked_value, 3);
+  EXPECT_NE(unmocked_value, 11);
+  // Mocked generators should generate their mocked values.
+  EXPECT_EQ(get_value(mocked_with_3), 3);
+  EXPECT_EQ(get_value(mocked_with_11), 11);
+  // Ensure that the mocks have expired.
+  EXPECT_NE(get_value(mocked_with_3), 3);
+  EXPECT_NE(get_value(mocked_with_11), 11);
+}
+
+TEST(BasicMocking, MocksNotTrigeredForIncorrectTypes) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(gen)).WillOnce(Return(42));
+
+  EXPECT_NE(absl::Uniform<uint16_t>(gen), 42);  // Not mocked
+  EXPECT_EQ(absl::Uniform<uint32_t>(gen), 42);  // Mock triggered
+}
+
+TEST(BasicMocking, FailsOnUnsatisfiedMocks) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        absl::MockingBitGen gen;
+        EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+            .WillOnce(Return(3.0));
+        // Does not call absl::Exponential().
+      }(),
+      "unsatisfied and active");
+}
+
+TEST(OnUniform, RespectsUniformIntervalSemantics) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(),
+              Call(absl::IntervalClosed, gen, 1, 1000000))
+      .WillOnce(Return(301));
+  EXPECT_NE(absl::Uniform(gen, 1, 1000000), 301);  // Not mocked
+  EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 1, 1000000), 301);
+}
+
+TEST(OnUniform, RespectsNoArgUnsignedShorthand) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(gen)).WillOnce(Return(42));
+  EXPECT_EQ(absl::Uniform<uint32_t>(gen), 42);
+}
+
+TEST(RepeatedlyModifier, ForceSnakeEyesForManyDice) {
+  auto roll_some_dice = [](absl::BitGenRef gen_ref) {
+    std::vector<int> results(16);
+    for (auto& r : results) {
+      r = absl::Uniform(absl::IntervalClosed, gen_ref, 1, 6);
+    }
+    return results;
+  };
+  std::vector<int> results;
+  absl::MockingBitGen gen;
+
+  // Without any mocked calls, not all dice roll a "6".
+  results = roll_some_dice(gen);
+  EXPECT_LT(std::accumulate(std::begin(results), std::end(results), 0),
+            results.size() * 6);
+
+  // Verify that we can force all "6"-rolls, with mocking.
+  ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosed, gen, 1, 6))
+      .WillByDefault(Return(6));
+  results = roll_some_dice(gen);
+  EXPECT_EQ(std::accumulate(std::begin(results), std::end(results), 0),
+            results.size() * 6);
+}
+
+TEST(WillOnce, DistinctCounters) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .Times(3)
+      .WillRepeatedly(Return(0));
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1000001, 2000000))
+      .Times(3)
+      .WillRepeatedly(Return(1));
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 0);
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 0);
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 0);
+}
+
+TEST(TimesModifier, ModifierSaturatesAndExpires) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        absl::MockingBitGen gen;
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+            .Times(3)
+            .WillRepeatedly(Return(15))
+            .RetiresOnSaturation();
+
+        EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 15);
+        EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 15);
+        EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 15);
+        // Times(3) has expired - Should get a different value now.
+
+        EXPECT_NE(absl::Uniform(gen, 1, 1000000), 15);
+      }(),
+      "");
+}
+
+TEST(TimesModifier, Times0) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.0)).Times(0);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).Times(0);
+}
+
+TEST(AnythingMatcher, MatchesAnyArgument) {
+  using testing::_;
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosed, gen, _, 1000))
+        .WillByDefault(Return(11));
+    ON_CALL(absl::MockUniform<int>(),
+            Call(absl::IntervalClosed, gen, _, Ne(1000)))
+        .WillByDefault(Return(99));
+
+    EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 10, 1000000), 99);
+    EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 10, 1000), 11);
+  }
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(gen, 1, _))
+        .WillByDefault(Return(25));
+    ON_CALL(absl::MockUniform<int>(), Call(gen, Ne(1), _))
+        .WillByDefault(Return(99));
+    EXPECT_EQ(absl::Uniform(gen, 3, 1000000), 99);
+    EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 25);
+  }
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(gen, _, _))
+        .WillByDefault(Return(145));
+    EXPECT_EQ(absl::Uniform(gen, 1, 1000), 145);
+    EXPECT_EQ(absl::Uniform(gen, 10, 1000), 145);
+    EXPECT_EQ(absl::Uniform(gen, 100, 1000), 145);
+  }
+}
+
+TEST(AnythingMatcher, WithWillByDefault) {
+  using testing::_;
+  absl::MockingBitGen gen;
+  std::vector<int> values = {11, 22, 33, 44, 55, 66, 77, 88, 99, 1010};
+
+  ON_CALL(absl::MockUniform<size_t>(), Call(gen, 0, _))
+      .WillByDefault(Return(0));
+  for (int i = 0; i < 100; i++) {
+    auto& elem = values[absl::Uniform(gen, 0u, values.size())];
+    EXPECT_EQ(elem, 11);
+  }
+}
+
+TEST(BasicMocking, WillByDefaultWithArgs) {
+  using testing::_;
+
+  absl::MockingBitGen gen;
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, _))
+      .WillByDefault(
+          [](double lambda) { return static_cast<int>(lambda * 10); });
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.7), 17);
+  EXPECT_EQ(absl::Poisson<int>(gen, 0.03), 0);
+}
+
+TEST(MockingBitGen, InSequenceSucceedsInOrder) {
+  absl::MockingBitGen gen;
+
+  testing::InSequence seq;
+
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(3));
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 2.0)).WillOnce(Return(4));
+
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 3);
+  EXPECT_EQ(absl::Poisson<int>(gen, 2.0), 4);
+}
+
+}  // namespace