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79 lines
2.2 KiB
79 lines
2.2 KiB
/*
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* Copyright 2021 Google LLC
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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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#pragma once
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#include <assert.h>
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#include <optional>
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#include "compactor_stack.h"
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namespace dist_proc {
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namespace aggregation {
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namespace internal {
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class CompactorStack;
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// Class that does reservoir sampling to uniformly-at-random select one out of
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// capacity() items that are added to it. The selected item is added to the
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// compactor stack and sampling continues with the next capacity_ items.
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//
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// Serves as an replacement of num_replaced_levels levels of size 2 of the
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// compactor stack, while only using constant memory.
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class KllSampler {
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public:
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KllSampler(CompactorStack* compactor_stack) : compactor_stack_(compactor_stack) {
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assert(compactor_stack != nullptr);
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Reset();
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}
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void Reset();
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// Adds an item to the sampler with weight one.
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void Add(int64_t item);
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// Adds an item to the sampler with weight >= 1. Does nothing if weight <= 0.
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void AddWithWeight(int64_t item, int weight);
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void DoubleCapacity();
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int64_t capacity() const {
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return capacity_;
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}
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std::optional<std::pair<int64_t, int>> sampled_item_and_weight() const {
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if (item_weight_ == 0) {
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return std::nullopt;
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}
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return std::make_pair(sampled_item_, item_weight_);
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}
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int num_replaced_levels() const {
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return num_replaced_levels_;
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}
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private:
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void AddSampleToCompactorStackAndRestart();
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int64_t sampled_item_;
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int64_t item_weight_;
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int64_t capacity_;
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int num_replaced_levels_;
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CompactorStack* compactor_stack_;
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};
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} // namespace internal
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} // namespace aggregation
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} // namespace dist_proc
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