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+// 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_DISTRIBUTIONS_H_
+#define ABSL_RANDOM_INTERNAL_DISTRIBUTIONS_H_
+
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/distribution_caller.h"
+#include "absl/random/internal/traits.h"
+#include "absl/random/internal/uniform_helper.h"
+
+namespace absl {
+namespace random_internal {
+template <typename D>
+struct DistributionFormatTraits;
+
+// UniformImpl implements the core logic of the Uniform<T> call, which is to
+// select the correct distribution type, compute the bounds based on the
+// interval tag, and then generate a value.
+template <typename NumType, typename TagType, typename URBG>
+NumType UniformImpl(TagType tag,
+                    URBG& urbg,  // NOLINT(runtime/references)
+                    NumType lo, NumType hi) {
+  static_assert(
+      std::is_arithmetic<NumType>::value,
+      "absl::Uniform<T>() must use an integer or real parameter type.");
+
+  using distribution_t =
+      typename std::conditional<std::is_integral<NumType>::value,
+                                absl::uniform_int_distribution<NumType>,
+                                absl::uniform_real_distribution<NumType>>::type;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
+
+  auto a = random_internal::uniform_lower_bound<NumType>(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound<NumType>(tag, lo, hi);
+  // TODO(lar): it doesn't make a lot of sense to ask for a random number in an
+  // empty range.  Right now we just return a boundary--even though that
+  // boundary is not an acceptable value!  Is there something better we can do
+  // here?
+
+  using gen_t = absl::decay_t<URBG>;
+  if (a > b) return a;
+  return DistributionCaller<gen_t>::template Call<distribution_t, format_t>(
+      &urbg, a, b);
+}
+
+// In the absence of an explicitly provided return-type, the template
+// "uniform_inferred_return_t<A, B>" is used to derive a suitable type, based on
+// the data-types of the endpoint-arguments {A lo, B hi}.
+//
+// Given endpoints {A lo, B hi}, one of {A, B} will be chosen as the
+// return-type, if one type can be implicitly converted into the other, in a
+// lossless way. The template "is_widening_convertible" implements the
+// compile-time logic for deciding if such a conversion is possible.
+//
+// If no such conversion between {A, B} exists, then the overload for
+// absl::Uniform() will be discarded, and the call will be ill-formed.
+// Return-type for absl::Uniform() when the return-type is inferred.
+template <typename A, typename B>
+using uniform_inferred_return_t =
+    absl::enable_if_t<absl::disjunction<is_widening_convertible<A, B>,
+                                        is_widening_convertible<B, A>>::value,
+                      typename std::conditional<
+                          is_widening_convertible<A, B>::value, B, A>::type>;
+
+}  // namespace random_internal
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTIONS_H_