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-rw-r--r--absl/random/BUILD.bazel1
-rw-r--r--absl/random/CMakeLists.txt2
-rw-r--r--absl/random/bit_gen_ref.h5
-rw-r--r--absl/random/distribution_format_traits.h278
-rw-r--r--absl/random/distributions.h41
-rw-r--r--absl/random/internal/BUILD.bazel1
-rw-r--r--absl/random/internal/distribution_caller.h18
-rw-r--r--absl/random/internal/mocking_bit_gen_base.h42
-rw-r--r--absl/random/mocking_bit_gen.cc1
-rw-r--r--absl/random/mocking_bit_gen.h6
-rw-r--r--absl/random/mocking_bit_gen_test.cc6
11 files changed, 375 insertions, 26 deletions
diff --git a/absl/random/BUILD.bazel b/absl/random/BUILD.bazel
index 43ed9840a035..2585b39742e2 100644
--- a/absl/random/BUILD.bazel
+++ b/absl/random/BUILD.bazel
@@ -53,6 +53,7 @@ cc_library(
         "bernoulli_distribution.h",
         "beta_distribution.h",
         "discrete_distribution.h",
+        "distribution_format_traits.h",
         "distributions.h",
         "exponential_distribution.h",
         "gaussian_distribution.h",
diff --git a/absl/random/CMakeLists.txt b/absl/random/CMakeLists.txt
index 53f1aa5c775c..46dbc3efbc83 100644
--- a/absl/random/CMakeLists.txt
+++ b/absl/random/CMakeLists.txt
@@ -78,6 +78,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_random
+    absl::strings
 )
 
 # Internal-only target, do not depend on directly.
@@ -167,6 +168,7 @@ absl_cc_library(
     "bernoulli_distribution.h"
     "beta_distribution.h"
     "discrete_distribution.h"
+    "distribution_format_traits.h"
     "distributions.h"
     "exponential_distribution.h"
     "gaussian_distribution.h"
diff --git a/absl/random/bit_gen_ref.h b/absl/random/bit_gen_ref.h
index 59591a479d8c..e8771162e5fb 100644
--- a/absl/random/bit_gen_ref.h
+++ b/absl/random/bit_gen_ref.h
@@ -132,7 +132,7 @@ namespace random_internal {
 
 template <>
 struct DistributionCaller<absl::BitGenRef> {
-  template <typename DistrT, typename... Args>
+  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_;
@@ -140,7 +140,8 @@ struct DistributionCaller<absl::BitGenRef> {
       DistrT dist(std::forward<Args>(args)...);
       return dist(*gen_ref);
     } else {
-      return mock_ptr->template Call<DistrT>(std::forward<Args>(args)...);
+      return mock_ptr->template Call<DistrT, FormatT>(
+          std::forward<Args>(args)...);
     }
   }
 };
diff --git a/absl/random/distribution_format_traits.h b/absl/random/distribution_format_traits.h
new file mode 100644
index 000000000000..22b358cc8c37
--- /dev/null
+++ b/absl/random/distribution_format_traits.h
@@ -0,0 +1,278 @@
+//
+// 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_DISTRIBUTION_FORMAT_TRAITS_H_
+#define ABSL_RANDOM_DISTRIBUTION_FORMAT_TRAITS_H_
+
+#include <string>
+#include <tuple>
+#include <typeinfo>
+
+#include "absl/meta/type_traits.h"
+#include "absl/random/bernoulli_distribution.h"
+#include "absl/random/beta_distribution.h"
+#include "absl/random/exponential_distribution.h"
+#include "absl/random/gaussian_distribution.h"
+#include "absl/random/log_uniform_int_distribution.h"
+#include "absl/random/poisson_distribution.h"
+#include "absl/random/uniform_int_distribution.h"
+#include "absl/random/uniform_real_distribution.h"
+#include "absl/random/zipf_distribution.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_join.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+struct IntervalClosedClosedTag;
+struct IntervalClosedOpenTag;
+struct IntervalOpenClosedTag;
+struct IntervalOpenOpenTag;
+
+namespace random_internal {
+
+// ScalarTypeName defines a preferred hierarchy of preferred type names for
+// scalars, and is evaluated at compile time for the specific type
+// specialization.
+template <typename T>
+constexpr const char* ScalarTypeName() {
+  static_assert(std::is_integral<T>() || std::is_floating_point<T>(), "");
+  // clang-format off
+    return
+        std::is_same<T, float>::value ? "float" :
+        std::is_same<T, double>::value ? "double" :
+        std::is_same<T, long double>::value ? "long double" :
+        std::is_same<T, bool>::value ? "bool" :
+        std::is_signed<T>::value && sizeof(T) == 1 ? "int8_t" :
+        std::is_signed<T>::value && sizeof(T) == 2 ? "int16_t" :
+        std::is_signed<T>::value && sizeof(T) == 4 ? "int32_t" :
+        std::is_signed<T>::value && sizeof(T) == 8 ? "int64_t" :
+        std::is_unsigned<T>::value && sizeof(T) == 1 ? "uint8_t" :
+        std::is_unsigned<T>::value && sizeof(T) == 2 ? "uint16_t" :
+        std::is_unsigned<T>::value && sizeof(T) == 4 ? "uint32_t" :
+        std::is_unsigned<T>::value && sizeof(T) == 8 ? "uint64_t" :
+            "undefined";
+  // clang-format on
+
+  // NOTE: It would be nice to use typeid(T).name(), but that's an
+  // implementation-defined attribute which does not necessarily
+  // correspond to a name. We could potentially demangle it
+  // using, e.g. abi::__cxa_demangle.
+}
+
+// Distribution traits used by DistributionCaller and internal implementation
+// details of the mocking framework.
+/*
+struct DistributionFormatTraits {
+   // Returns the parameterized name of the distribution function.
+   static constexpr const char* FunctionName()
+   // Format DistrT parameters.
+   static std::string FormatArgs(DistrT& dist);
+   // Format DistrT::result_type results.
+   static std::string FormatResults(DistrT& dist);
+};
+*/
+template <typename DistrT>
+struct DistributionFormatTraits;
+
+template <typename R>
+struct DistributionFormatTraits<absl::uniform_int_distribution<R>> {
+  using distribution_t = absl::uniform_int_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Uniform"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat("absl::IntervalClosedClosed, ", (d.min)(), ", ",
+                        (d.max)());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::uniform_real_distribution<R>> {
+  using distribution_t = absl::uniform_real_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Uniform"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat((d.min)(), ", ", (d.max)());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::exponential_distribution<R>> {
+  using distribution_t = absl::exponential_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Exponential"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat(d.lambda());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::poisson_distribution<R>> {
+  using distribution_t = absl::poisson_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Poisson"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat(d.mean());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <>
+struct DistributionFormatTraits<absl::bernoulli_distribution> {
+  using distribution_t = absl::bernoulli_distribution;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Bernoulli"; }
+
+  static constexpr const char* FunctionName() { return Name(); }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat(d.p());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::beta_distribution<R>> {
+  using distribution_t = absl::beta_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Beta"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat(d.alpha(), ", ", d.beta());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::zipf_distribution<R>> {
+  using distribution_t = absl::zipf_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Zipf"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat(d.k(), ", ", d.v(), ", ", d.q());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::gaussian_distribution<R>> {
+  using distribution_t = absl::gaussian_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "Gaussian"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrJoin(std::make_tuple(d.mean(), d.stddev()), ", ");
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename R>
+struct DistributionFormatTraits<absl::log_uniform_int_distribution<R>> {
+  using distribution_t = absl::log_uniform_int_distribution<R>;
+  using result_t = typename distribution_t::result_type;
+
+  static constexpr const char* Name() { return "LogUniform"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<R>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrJoin(std::make_tuple((d.min)(), (d.max)(), d.base()), ", ");
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+template <typename NumType>
+struct UniformDistributionWrapper;
+
+template <typename NumType>
+struct DistributionFormatTraits<UniformDistributionWrapper<NumType>> {
+  using distribution_t = UniformDistributionWrapper<NumType>;
+  using result_t = NumType;
+
+  static constexpr const char* Name() { return "Uniform"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<NumType>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat((d.min)(), ", ", (d.max)());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_DISTRIBUTION_FORMAT_TRAITS_H_
diff --git a/absl/random/distributions.h b/absl/random/distributions.h
index 7abdfa8f202a..c1fb66501593 100644
--- a/absl/random/distributions.h
+++ b/absl/random/distributions.h
@@ -55,6 +55,7 @@
 #include "absl/base/internal/inline_variable.h"
 #include "absl/random/bernoulli_distribution.h"
 #include "absl/random/beta_distribution.h"
+#include "absl/random/distribution_format_traits.h"
 #include "absl/random/exponential_distribution.h"
 #include "absl/random/gaussian_distribution.h"
 #include "absl/random/internal/distributions.h"  // IWYU pragma: export
@@ -125,13 +126,14 @@ Uniform(TagType tag,
         R lo, R hi) {
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = random_internal::UniformDistributionWrapper<R>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   auto a = random_internal::uniform_lower_bound(tag, lo, hi);
   auto b = random_internal::uniform_upper_bound(tag, lo, hi);
   if (a > b) return a;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, tag, lo, hi);
+      distribution_t, format_t>(&urbg, tag, lo, hi);
 }
 
 // absl::Uniform<T>(bitgen, lo, hi)
@@ -144,6 +146,7 @@ Uniform(URBG&& urbg,  // NOLINT(runtime/references)
         R lo, R hi) {
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = random_internal::UniformDistributionWrapper<R>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   constexpr auto tag = absl::IntervalClosedOpen;
   auto a = random_internal::uniform_lower_bound(tag, lo, hi);
@@ -151,7 +154,7 @@ Uniform(URBG&& urbg,  // NOLINT(runtime/references)
   if (a > b) return a;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, lo, hi);
+      distribution_t, format_t>(&urbg, lo, hi);
 }
 
 // absl::Uniform(tag, bitgen, lo, hi)
@@ -169,14 +172,15 @@ Uniform(TagType tag,
   using gen_t = absl::decay_t<URBG>;
   using return_t = typename random_internal::uniform_inferred_return_t<A, B>;
   using distribution_t = random_internal::UniformDistributionWrapper<return_t>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   auto a = random_internal::uniform_lower_bound<return_t>(tag, lo, hi);
   auto b = random_internal::uniform_upper_bound<return_t>(tag, lo, hi);
   if (a > b) return a;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, tag, static_cast<return_t>(lo),
-                      static_cast<return_t>(hi));
+      distribution_t, format_t>(&urbg, tag, static_cast<return_t>(lo),
+                                static_cast<return_t>(hi));
 }
 
 // absl::Uniform(bitgen, lo, hi)
@@ -192,6 +196,7 @@ Uniform(URBG&& urbg,  // NOLINT(runtime/references)
   using gen_t = absl::decay_t<URBG>;
   using return_t = typename random_internal::uniform_inferred_return_t<A, B>;
   using distribution_t = random_internal::UniformDistributionWrapper<return_t>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   constexpr auto tag = absl::IntervalClosedOpen;
   auto a = random_internal::uniform_lower_bound<return_t>(tag, lo, hi);
@@ -199,8 +204,8 @@ Uniform(URBG&& urbg,  // NOLINT(runtime/references)
   if (a > b) return a;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, static_cast<return_t>(lo),
-                      static_cast<return_t>(hi));
+      distribution_t, format_t>(&urbg, static_cast<return_t>(lo),
+                                static_cast<return_t>(hi));
 }
 
 // absl::Uniform<unsigned T>(bitgen)
@@ -212,9 +217,10 @@ typename absl::enable_if_t<!std::is_signed<R>::value, R>  //
 Uniform(URBG&& urbg) {  // NOLINT(runtime/references)
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = random_internal::UniformDistributionWrapper<R>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg);
+      distribution_t, format_t>(&urbg);
 }
 
 // -----------------------------------------------------------------------------
@@ -242,9 +248,10 @@ bool Bernoulli(URBG&& urbg,  // NOLINT(runtime/references)
                double p) {
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = absl::bernoulli_distribution;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, p);
+      distribution_t, format_t>(&urbg, p);
 }
 
 // -----------------------------------------------------------------------------
@@ -274,9 +281,10 @@ RealType Beta(URBG&& urbg,  // NOLINT(runtime/references)
 
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = typename absl::beta_distribution<RealType>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, alpha, beta);
+      distribution_t, format_t>(&urbg, alpha, beta);
 }
 
 // -----------------------------------------------------------------------------
@@ -306,9 +314,10 @@ RealType Exponential(URBG&& urbg,  // NOLINT(runtime/references)
 
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = typename absl::exponential_distribution<RealType>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, lambda);
+      distribution_t, format_t>(&urbg, lambda);
 }
 
 // -----------------------------------------------------------------------------
@@ -337,9 +346,10 @@ RealType Gaussian(URBG&& urbg,  // NOLINT(runtime/references)
 
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = typename absl::gaussian_distribution<RealType>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, mean, stddev);
+      distribution_t, format_t>(&urbg, mean, stddev);
 }
 
 // -----------------------------------------------------------------------------
@@ -379,9 +389,10 @@ IntType LogUniform(URBG&& urbg,  // NOLINT(runtime/references)
 
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = typename absl::log_uniform_int_distribution<IntType>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, lo, hi, base);
+      distribution_t, format_t>(&urbg, lo, hi, base);
 }
 
 // -----------------------------------------------------------------------------
@@ -409,9 +420,10 @@ IntType Poisson(URBG&& urbg,  // NOLINT(runtime/references)
 
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = typename absl::poisson_distribution<IntType>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, mean);
+      distribution_t, format_t>(&urbg, mean);
 }
 
 // -----------------------------------------------------------------------------
@@ -441,9 +453,10 @@ IntType Zipf(URBG&& urbg,  // NOLINT(runtime/references)
 
   using gen_t = absl::decay_t<URBG>;
   using distribution_t = typename absl::zipf_distribution<IntType>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
   return random_internal::DistributionCaller<gen_t>::template Call<
-      distribution_t>(&urbg, hi, q, v);
+      distribution_t, format_t>(&urbg, hi, q, v);
 }
 
 ABSL_NAMESPACE_END
diff --git a/absl/random/internal/BUILD.bazel b/absl/random/internal/BUILD.bazel
index 8839078f5f9e..d7ad4efec989 100644
--- a/absl/random/internal/BUILD.bazel
+++ b/absl/random/internal/BUILD.bazel
@@ -509,6 +509,7 @@ cc_library(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         "//absl/random",
+        "//absl/strings",
     ],
 )
 
diff --git a/absl/random/internal/distribution_caller.h b/absl/random/internal/distribution_caller.h
index ae2680ddd40a..02603cf84355 100644
--- a/absl/random/internal/distribution_caller.h
+++ b/absl/random/internal/distribution_caller.h
@@ -31,8 +31,22 @@ namespace random_internal {
 template <typename URBG>
 struct DistributionCaller {
   // Call the provided distribution type. The parameters are expected
-  // to be explicitly specified.  DistrT is the distribution type.
-  template <typename DistrT, typename... Args>
+  // to be explicitly specified.
+  // DistrT is the distribution type.
+  // FormatT is the formatter type:
+  //
+  // struct FormatT {
+  //   using result_type = distribution_t::result_type;
+  //   static std::string FormatCall(
+  //       const distribution_t& distr,
+  //       absl::Span<const result_type>);
+  //
+  //   static std::string FormatExpectation(
+  //       absl::string_view match_args,
+  //       absl::Span<const result_t> results);
+  // }
+  //
+  template <typename DistrT, typename FormatT, typename... Args>
   static typename DistrT::result_type Call(URBG* urbg, Args&&... args) {
     DistrT dist(std::forward<Args>(args)...);
     return dist(*urbg);
diff --git a/absl/random/internal/mocking_bit_gen_base.h b/absl/random/internal/mocking_bit_gen_base.h
index acd6387204aa..eeeae9d295b2 100644
--- a/absl/random/internal/mocking_bit_gen_base.h
+++ b/absl/random/internal/mocking_bit_gen_base.h
@@ -16,14 +16,39 @@
 #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 {
 ABSL_NAMESPACE_BEGIN
 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;
@@ -36,9 +61,14 @@ class MockingBitGenBase {
   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.
@@ -51,9 +81,10 @@ class MockingBitGenBase {
   }
 
   // Call the generating distribution function.
-  // Invoked by DistributionCaller<>::Call<DistT>.
+  // Invoked by DistributionCaller<>::Call<DistT, FormatT>.
   // DistT is the distribution type.
-  template <typename DistrT, typename... Args>
+  // 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>...>;
@@ -68,11 +99,18 @@ class MockingBitGenBase {
     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
diff --git a/absl/random/mocking_bit_gen.cc b/absl/random/mocking_bit_gen.cc
index 022091154541..6bb1e414aeab 100644
--- a/absl/random/mocking_bit_gen.cc
+++ b/absl/random/mocking_bit_gen.cc
@@ -20,6 +20,7 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 MockingBitGen::~MockingBitGen() {
+
   for (const auto& del : deleters_) {
     del();
   }
diff --git a/absl/random/mocking_bit_gen.h b/absl/random/mocking_bit_gen.h
index 246c5b1e035a..36cef91113e3 100644
--- a/absl/random/mocking_bit_gen.h
+++ b/absl/random/mocking_bit_gen.h
@@ -109,7 +109,7 @@ class MockingBitGen : public absl::random_internal::MockingBitGenBase {
 
   // MockingBitGen::Register
   //
-  // Register<DistrT, ArgTupleT> is the main extension point for
+  // 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.
@@ -182,10 +182,10 @@ namespace random_internal {
 
 template <>
 struct DistributionCaller<absl::MockingBitGen> {
-  template <typename DistrT, typename... Args>
+  template <typename DistrT, typename FormatT, typename... Args>
   static typename DistrT::result_type Call(absl::MockingBitGen* gen,
                                            Args&&... args) {
-    return gen->template Call<DistrT>(std::forward<Args>(args)...);
+    return gen->template Call<DistrT, FormatT>(std::forward<Args>(args)...);
   }
 };
 
diff --git a/absl/random/mocking_bit_gen_test.cc b/absl/random/mocking_bit_gen_test.cc
index dcf74fd6db85..f0ffc9ac9283 100644
--- a/absl/random/mocking_bit_gen_test.cc
+++ b/absl/random/mocking_bit_gen_test.cc
@@ -66,10 +66,10 @@ TEST(BasicMocking, AllDistributionsAreOverridable) {
       .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_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 500000);
   EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
-      .WillOnce(Return(2040));
-  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+      .WillOnce(Return(500000));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 500000);
 }
 
 TEST(BasicMocking, OnDistribution) {
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