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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 {
inline namespace lts_2018_12_18 {
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;
}
};
};
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
} // inline namespace lts_2018_12_18
} // namespace absl
#endif // ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
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