108 lines
4 KiB
C++
108 lines
4 KiB
C++
// Copyright 2018 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// This library provides APIs to debug the probing behavior of hash tables.
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//
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// In general, the probing behavior is a black box for users and only the
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// side effects can be measured in the form of performance differences.
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// These APIs give a glimpse on the actual behavior of the probing algorithms in
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// these hashtables given a specified hash function and a set of elements.
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//
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// The probe count distribution can be used to assess the quality of the hash
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// function for that particular hash table. Note that a hash function that
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// performs well in one hash table implementation does not necessarily performs
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// well in a different one.
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//
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// This library supports std::unordered_{set,map}, dense_hash_{set,map} and
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// absl::{flat,node,string}_hash_{set,map}.
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#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
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#define ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
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#include <cstddef>
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#include <algorithm>
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#include <type_traits>
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#include <vector>
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#include "absl/container/internal/hashtable_debug_hooks.h"
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namespace absl {
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namespace container_internal {
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// Returns the number of probes required to lookup `key`. Returns 0 for a
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// search with no collisions. Higher values mean more hash collisions occurred;
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// however, the exact meaning of this number varies according to the container
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// type.
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template <typename C>
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size_t GetHashtableDebugNumProbes(
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const C& c, const typename C::key_type& key) {
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return absl::container_internal::hashtable_debug_internal::
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HashtableDebugAccess<C>::GetNumProbes(c, key);
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}
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// Gets a histogram of the number of probes for each elements in the container.
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// The sum of all the values in the vector is equal to container.size().
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template <typename C>
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std::vector<size_t> GetHashtableDebugNumProbesHistogram(const C& container) {
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std::vector<size_t> v;
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for (auto it = container.begin(); it != container.end(); ++it) {
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size_t num_probes = GetHashtableDebugNumProbes(
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container,
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absl::container_internal::hashtable_debug_internal::GetKey<C>(*it, 0));
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v.resize((std::max)(v.size(), num_probes + 1));
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v[num_probes]++;
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}
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return v;
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}
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struct HashtableDebugProbeSummary {
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size_t total_elements;
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size_t total_num_probes;
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double mean;
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};
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// Gets a summary of the probe count distribution for the elements in the
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// container.
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template <typename C>
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HashtableDebugProbeSummary GetHashtableDebugProbeSummary(const C& container) {
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auto probes = GetHashtableDebugNumProbesHistogram(container);
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HashtableDebugProbeSummary summary = {};
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for (size_t i = 0; i < probes.size(); ++i) {
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summary.total_elements += probes[i];
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summary.total_num_probes += probes[i] * i;
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}
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summary.mean = 1.0 * summary.total_num_probes / summary.total_elements;
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return summary;
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}
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// Returns the number of bytes requested from the allocator by the container
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// and not freed.
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template <typename C>
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size_t AllocatedByteSize(const C& c) {
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return absl::container_internal::hashtable_debug_internal::
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HashtableDebugAccess<C>::AllocatedByteSize(c);
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}
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// Returns a tight lower bound for AllocatedByteSize(c) where `c` is of type `C`
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// and `c.size()` is equal to `num_elements`.
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template <typename C>
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size_t LowerBoundAllocatedByteSize(size_t num_elements) {
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return absl::container_internal::hashtable_debug_internal::
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HashtableDebugAccess<C>::LowerBoundAllocatedByteSize(num_elements);
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}
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} // namespace container_internal
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} // namespace absl
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#endif // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
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