1 files changed, 40 insertions, 2 deletions
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