1 /*
2 * Copyright 2016 gRPC authors.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #ifndef GRPC_CORE_LIB_SLICE_SLICE_HASH_TABLE_H
18 #define GRPC_CORE_LIB_SLICE_SLICE_HASH_TABLE_H
19
20 #include <grpc/support/port_platform.h>
21
22 #include <string.h>
23
24 #include <grpc/support/alloc.h>
25 #include <grpc/support/log.h>
26
27 #include "src/core/lib/gpr/useful.h"
28 #include "src/core/lib/gprpp/memory.h"
29 #include "src/core/lib/gprpp/ref_counted.h"
30 #include "src/core/lib/gprpp/ref_counted_ptr.h"
31 #include "src/core/lib/slice/slice_internal.h"
32
33 /// Hash table implementation.
34 ///
35 /// This implementation uses open addressing
36 /// (https://en.wikipedia.org/wiki/Open_addressing) with linear
37 /// probing (https://en.wikipedia.org/wiki/Linear_probing).
38 ///
39 /// The keys are \a grpc_slice objects. The values can be any type.
40 ///
41 /// Hash tables are intentionally immutable, to avoid the need for locking.
42
43 namespace grpc_core {
44
45 template <typename T>
46 class SliceHashTable : public RefCounted<SliceHashTable<T>> {
47 public:
48 struct Entry {
49 grpc_slice key;
50 T value;
51 bool is_set;
52 };
53
54 // Function for comparing values.
55 // TODO(roth): Eliminate this and the Cmp() method from this API once
56 // grpc_channel_args is redesigned to require that keys are unique.
57 typedef int (*ValueCmp)(const T&, const T&);
58
59 /// Creates a new hash table containing \a entries, which is an array
60 /// of length \a num_entries. Takes ownership of all keys and values in \a
61 /// entries. If not null, \a value_cmp will be used to compare values in
62 /// the context of \a Cmp(). If null, raw pointer (\a GPR_ICMP) comparison
63 /// will be used.
64 static RefCountedPtr<SliceHashTable> Create(size_t num_entries,
65 Entry* entries,
66 ValueCmp value_cmp);
67
68 // Use Create function instead of using this directly.
69 SliceHashTable(size_t num_entries, Entry* entries, ValueCmp value_cmp);
70 virtual ~SliceHashTable();
71
72 /// Returns the value from the table associated with \a key.
73 /// Returns null if \a key is not found.
74 const T* Get(const grpc_slice& key) const;
75
76 /// Compares \a a vs. \a b.
77 /// A table is considered "smaller" (resp. "greater") if:
78 /// - GPR_ICMP(a->value_cmp, b->value_cmp) < 1 (resp. > 1),
79 /// - else, it contains fewer (resp. more) entries,
80 /// - else, if strcmp(a_key, b_key) < 1 (resp. > 1),
81 /// - else, if value_cmp(a_value, b_value) < 1 (resp. > 1).
82 static int Cmp(const SliceHashTable& a, const SliceHashTable& b);
83
84 private:
85 void Add(const grpc_slice& key, T& value);
86
87 // Default value comparison function, if none specified by caller.
DefaultValueCmp(const T & a,const T & b)88 static int DefaultValueCmp(const T& a, const T& b) { return GPR_ICMP(a, b); }
89
90 const ValueCmp value_cmp_;
91 const size_t size_;
92 size_t max_num_probes_;
93 Entry* entries_;
94 };
95
96 //
97 // implementation -- no user-serviceable parts below
98 //
99
100 template <typename T>
Create(size_t num_entries,Entry * entries,ValueCmp value_cmp)101 RefCountedPtr<SliceHashTable<T>> SliceHashTable<T>::Create(size_t num_entries,
102 Entry* entries,
103 ValueCmp value_cmp) {
104 return MakeRefCounted<SliceHashTable<T>>(num_entries, entries, value_cmp);
105 }
106
107 template <typename T>
SliceHashTable(size_t num_entries,Entry * entries,ValueCmp value_cmp)108 SliceHashTable<T>::SliceHashTable(size_t num_entries, Entry* entries,
109 ValueCmp value_cmp)
110 : value_cmp_(value_cmp),
111 // Keep load factor low to improve performance of lookups.
112 size_(num_entries * 2),
113 max_num_probes_(0) {
114 entries_ = static_cast<Entry*>(gpr_zalloc(sizeof(Entry) * size_));
115 for (size_t i = 0; i < num_entries; ++i) {
116 Entry* entry = &entries[i];
117 Add(entry->key, entry->value);
118 }
119 }
120
121 template <typename T>
~SliceHashTable()122 SliceHashTable<T>::~SliceHashTable() {
123 for (size_t i = 0; i < size_; ++i) {
124 Entry& entry = entries_[i];
125 if (entry.is_set) {
126 grpc_slice_unref_internal(entry.key);
127 entry.value.~T();
128 }
129 }
130 gpr_free(entries_);
131 }
132
133 template <typename T>
Add(const grpc_slice & key,T & value)134 void SliceHashTable<T>::Add(const grpc_slice& key, T& value) {
135 const size_t hash = grpc_slice_hash_internal(key);
136 for (size_t offset = 0; offset < size_; ++offset) {
137 const size_t idx = (hash + offset) % size_;
138 if (!entries_[idx].is_set) {
139 entries_[idx].is_set = true;
140 entries_[idx].key = key;
141 entries_[idx].value = std::move(value);
142 // Keep track of the maximum number of probes needed, since this
143 // provides an upper bound for lookups.
144 if (offset > max_num_probes_) max_num_probes_ = offset;
145 return;
146 }
147 }
148 GPR_ASSERT(false); // Table should never be full.
149 }
150
151 template <typename T>
Get(const grpc_slice & key)152 const T* SliceHashTable<T>::Get(const grpc_slice& key) const {
153 const size_t hash = grpc_slice_hash_internal(key);
154 // We cap the number of probes at the max number recorded when
155 // populating the table.
156 for (size_t offset = 0; offset <= max_num_probes_; ++offset) {
157 const size_t idx = (hash + offset) % size_;
158 if (!entries_[idx].is_set) break;
159 if (grpc_slice_eq(entries_[idx].key, key)) {
160 return &entries_[idx].value;
161 }
162 }
163 return nullptr; // Not found.
164 }
165
166 template <typename T>
Cmp(const SliceHashTable & a,const SliceHashTable & b)167 int SliceHashTable<T>::Cmp(const SliceHashTable& a, const SliceHashTable& b) {
168 ValueCmp value_cmp_a =
169 a.value_cmp_ != nullptr ? a.value_cmp_ : DefaultValueCmp;
170 ValueCmp value_cmp_b =
171 b.value_cmp_ != nullptr ? b.value_cmp_ : DefaultValueCmp;
172 // Compare value_fns
173 const int value_fns_cmp = GPR_ICMP((void*)value_cmp_a, (void*)value_cmp_b);
174 if (value_fns_cmp != 0) return value_fns_cmp;
175 // Compare sizes
176 if (a.size_ < b.size_) return -1;
177 if (a.size_ > b.size_) return 1;
178 // Compare rows.
179 for (size_t i = 0; i < a.size_; ++i) {
180 if (!a.entries_[i].is_set) {
181 if (b.entries_[i].is_set) {
182 return -1; // a empty but b non-empty
183 }
184 continue; // both empty, no need to check key or value
185 } else if (!b.entries_[i].is_set) {
186 return 1; // a non-empty but b empty
187 }
188 // neither entry is empty
189 const int key_cmp = grpc_slice_cmp(a.entries_[i].key, b.entries_[i].key);
190 if (key_cmp != 0) return key_cmp;
191 const int value_cmp = value_cmp_a(a.entries_[i].value, b.entries_[i].value);
192 if (value_cmp != 0) return value_cmp;
193 }
194 return 0;
195 }
196
197 } // namespace grpc_core
198
199 #endif /* GRPC_CORE_LIB_SLICE_SLICE_HASH_TABLE_H */
200