hashtable_debug.h
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1 // Copyright 2018 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // This library provides APIs to debug the probing behavior of hash tables.
16 //
17 // In general, the probing behavior is a black box for users and only the
18 // side effects can be measured in the form of performance differences.
19 // These APIs give a glimpse on the actual behavior of the probing algorithms in
20 // these hashtables given a specified hash function and a set of elements.
21 //
22 // The probe count distribution can be used to assess the quality of the hash
23 // function for that particular hash table. Note that a hash function that
24 // performs well in one hash table implementation does not necessarily performs
25 // well in a different one.
26 //
27 // This library supports std::unordered_{set,map}, dense_hash_{set,map} and
28 // absl::{flat,node,string}_hash_{set,map}.
29 
30 #ifndef ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
31 #define ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
32 
33 #include <cstddef>
34 #include <algorithm>
35 #include <type_traits>
36 #include <vector>
37 
39 
40 namespace absl {
41 namespace container_internal {
42 
43 // Returns the number of probes required to lookup `key`. Returns 0 for a
44 // search with no collisions. Higher values mean more hash collisions occurred;
45 // however, the exact meaning of this number varies according to the container
46 // type.
47 template <typename C>
49  const C& c, const typename C::key_type& key) {
52 }
53 
54 // Gets a histogram of the number of probes for each elements in the container.
55 // The sum of all the values in the vector is equal to container.size().
56 template <typename C>
57 std::vector<size_t> GetHashtableDebugNumProbesHistogram(const C& container) {
58  std::vector<size_t> v;
59  for (auto it = container.begin(); it != container.end(); ++it) {
60  size_t num_probes = GetHashtableDebugNumProbes(
61  container,
62  absl::container_internal::hashtable_debug_internal::GetKey<C>(*it, 0));
63  v.resize((std::max)(v.size(), num_probes + 1));
64  v[num_probes]++;
65  }
66  return v;
67 }
68 
72  double mean;
73 };
74 
75 // Gets a summary of the probe count distribution for the elements in the
76 // container.
77 template <typename C>
79  auto probes = GetHashtableDebugNumProbesHistogram(container);
80  HashtableDebugProbeSummary summary = {};
81  for (size_t i = 0; i < probes.size(); ++i) {
82  summary.total_elements += probes[i];
83  summary.total_num_probes += probes[i] * i;
84  }
85  summary.mean = 1.0 * summary.total_num_probes / summary.total_elements;
86  return summary;
87 }
88 
89 // Returns the number of bytes requested from the allocator by the container
90 // and not freed.
91 template <typename C>
92 size_t AllocatedByteSize(const C& c) {
95 }
96 
97 // Returns a tight lower bound for AllocatedByteSize(c) where `c` is of type `C`
98 // and `c.size()` is equal to `num_elements`.
99 template <typename C>
100 size_t LowerBoundAllocatedByteSize(size_t num_elements) {
103 }
104 
105 } // namespace container_internal
106 } // namespace absl
107 
108 #endif // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
int v
Definition: variant_test.cc:81
std::vector< size_t > GetHashtableDebugNumProbesHistogram(const C &container)
Definition: algorithm.h:29
size_t LowerBoundAllocatedByteSize(size_t num_elements)
HashtableDebugProbeSummary GetHashtableDebugProbeSummary(const C &container)
size_t GetHashtableDebugNumProbes(const C &c, const typename C::key_type &key)
static size_t GetNumProbes(const Container &c, const typename Container::key_type &key)
size_t AllocatedByteSize(const C &c)


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autogenerated on Tue Jun 18 2019 19:44:36