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+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://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.
+//
+// Define the default Hash and Eq functions for SwissTable containers.
+//
+// std::hash<T> and std::equal_to<T> are not appropriate hash and equal
+// functions for SwissTable containers. There are two reasons for this.
+//
+// SwissTable containers are power of 2 sized containers:
+//
+// This means they use the lower bits of the hash value to find the slot for
+// each entry. The typical hash function for integral types is the identity.
+// This is a very weak hash function for SwissTable and any power of 2 sized
+// hashtable implementation which will lead to excessive collisions. For
+// SwissTable we use murmur3 style mixing to reduce collisions to a minimum.
+//
+// SwissTable containers support heterogeneous lookup:
+//
+// In order to make heterogeneous lookup work, hash and equal functions must be
+// polymorphic. At the same time they have to satisfy the same requirements the
+// C++ standard imposes on hash functions and equality operators. That is:
+//
+//   if hash_default_eq<T>(a, b) returns true for any a and b of type T, then
+//   hash_default_hash<T>(a) must equal hash_default_hash<T>(b)
+//
+// For SwissTable containers this requirement is relaxed to allow a and b of
+// any and possibly different types. Note that like the standard the hash and
+// equal functions are still bound to T. This is important because some type U
+// can be hashed by/tested for equality differently depending on T. A notable
+// example is `const char*`. `const char*` is treated as a c-style string when
+// the hash function is hash<string> but as a pointer when the hash function is
+// hash<void*>.
+//
+#ifndef ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
+#define ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
+
+#include <stdint.h>
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/config.h"
+#include "absl/hash/hash.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+namespace container_internal {
+
+// The hash of an object of type T is computed by using absl::Hash.
+template <class T, class E = void>
+struct HashEq {
+  using Hash = absl::Hash<T>;
+  using Eq = std::equal_to<T>;
+};
+
+struct StringHash {
+  using is_transparent = void;
+
+  size_t operator()(absl::string_view v) const {
+    return absl::Hash<absl::string_view>{}(v);
+  }
+};
+
+// Supports heterogeneous lookup for string-like elements.
+struct StringHashEq {
+  using Hash = StringHash;
+  struct Eq {
+    using is_transparent = void;
+    bool operator()(absl::string_view lhs, absl::string_view rhs) const {
+      return lhs == rhs;
+    }
+  };
+};
+
+#if defined(HAS_GLOBAL_STRING)
+template <>
+struct HashEq<std::string> : StringHashEq {};
+#endif
+template <>
+struct HashEq<std::string> : StringHashEq {};
+template <>
+struct HashEq<absl::string_view> : StringHashEq {};
+
+// Supports heterogeneous lookup for pointers and smart pointers.
+template <class T>
+struct HashEq<T*> {
+  struct Hash {
+    using is_transparent = void;
+    template <class U>
+    size_t operator()(const U& ptr) const {
+      return absl::Hash<const T*>{}(HashEq::ToPtr(ptr));
+    }
+  };
+  struct Eq {
+    using is_transparent = void;
+    template <class A, class B>
+    bool operator()(const A& a, const B& b) const {
+      return HashEq::ToPtr(a) == HashEq::ToPtr(b);
+    }
+  };
+
+ private:
+  static const T* ToPtr(const T* ptr) { return ptr; }
+  template <class U, class D>
+  static const T* ToPtr(const std::unique_ptr<U, D>& ptr) {
+    return ptr.get();
+  }
+  template <class U>
+  static const T* ToPtr(const std::shared_ptr<U>& ptr) {
+    return ptr.get();
+  }
+};
+
+template <class T, class D>
+struct HashEq<std::unique_ptr<T, D>> : HashEq<T*> {};
+template <class T>
+struct HashEq<std::shared_ptr<T>> : HashEq<T*> {};
+
+// This header's visibility is restricted.  If you need to access the default
+// hasher please use the container's ::hasher alias instead.
+//
+// Example: typename Hash = typename absl::flat_hash_map<K, V>::hasher
+template <class T>
+using hash_default_hash = typename container_internal::HashEq<T>::Hash;
+
+// This header's visibility is restricted.  If you need to access the default
+// key equal please use the container's ::key_equal alias instead.
+//
+// Example: typename Eq = typename absl::flat_hash_map<K, V, Hash>::key_equal
+template <class T>
+using hash_default_eq = typename container_internal::HashEq<T>::Eq;
+
+}  // namespace container_internal
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_