summaryrefslogtreecommitdiff
path: root/absl/container/internal/hash_function_defaults.h
blob: cb8f03c87aafbd2134db5dbe35e1f79dd45da2a5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
// 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.
//
// 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<std::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;
    }
  };
};

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_