arangodb/arangod/Cache/FrequencyBuffer.h

////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2017 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// 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
///
///     http://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.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Daniel H. Larkin
////////////////////////////////////////////////////////////////////////////////

#ifndef ARANGODB_CACHE_FREQUENCY_BUFFER_H
#define ARANGODB_CACHE_FREQUENCY_BUFFER_H

#include "Basics/Common.h"

#include <stdint.h>
#include <algorithm>
#include <atomic>
#include <memory>
#include <unordered_map>
#include <utility>
#include <vector>

namespace arangodb {
namespace cache {

////////////////////////////////////////////////////////////////////////////////
/// @brief Lockless structure to calculate approximate relative event
/// frequencies.
///
/// Used to record events and then compute the number of occurrences of each
/// within a certain time-frame. The underlying structure is a circular buffer
/// which over-writes itself after it fills up (thus only maintaining a recent
/// window on the records).
////////////////////////////////////////////////////////////////////////////////
template <class T, class Comparator = std::equal_to<T>,
          class Hasher = std::hash<T>>
class FrequencyBuffer {
 public:
  typedef std::vector<std::pair<T, uint64_t>> stats_t;

 private:
  std::atomic<uint64_t> _current;
  uint64_t _capacity;
  uint64_t _mask;
  std::unique_ptr<std::vector<T>> _buffer;
  Comparator _cmp;
  T _empty;

 public:
  //////////////////////////////////////////////////////////////////////////////
  /// @brief Initialize with the given capacity.
  //////////////////////////////////////////////////////////////////////////////
  FrequencyBuffer(uint64_t capacity) : _current(0), _cmp(), _empty() {
    uint64_t i = 0;
    for (; (static_cast<uint64_t>(1) << i) < capacity; i++) {
    }
    _capacity = (static_cast<uint64_t>(1) << i);
    _mask = _capacity - 1;
    _buffer.reset(new std::vector<T>(_capacity));
    TRI_ASSERT(_buffer->capacity() == _capacity);
    TRI_ASSERT(_buffer->size() == _capacity);
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Reports the hidden allocation size (not captured by sizeof).
  //////////////////////////////////////////////////////////////////////////////
  static uint64_t allocationSize(uint64_t capacity) {
    return sizeof(std::vector<T>) + (capacity * sizeof(T));
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Reports the memory usage in bytes.
  //////////////////////////////////////////////////////////////////////////////
  uint64_t memoryUsage() {
    return ((_capacity * sizeof(T)) + sizeof(FrequencyBuffer<T>) +
            sizeof(std::vector<T>));
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Insert an individual event record.
  //////////////////////////////////////////////////////////////////////////////
  void insertRecord(T record) {
    (*_buffer)[_current++ & _mask] = record;
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Remove all occurrences of the specified event record.
  //////////////////////////////////////////////////////////////////////////////
  void purgeRecord(T record) {
    for (size_t i = 0; i < _capacity; i++) {
      if (_cmp((*_buffer)[i], record)) {
        (*_buffer)[i] = _empty;
      }
    }
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Return a list of (event, count) pairs for each recorded event in
  /// ascending order.
  //////////////////////////////////////////////////////////////////////////////
  std::shared_ptr<typename FrequencyBuffer::stats_t> getFrequencies() const {
    // calculate frequencies
    std::unordered_map<T, uint64_t, Hasher, Comparator> frequencies;
    for (size_t i = 0; i < _capacity; i++) {
      T const entry = (*_buffer)[i];
      if (!_cmp(entry, _empty)) {
        frequencies[entry]++;
      }
    }

    // gather and sort frequencies
    std::shared_ptr<stats_t> data(new stats_t());
    data->reserve(frequencies.size());
    for (auto f : frequencies) {
      data->emplace_back(std::pair<T, uint64_t>(f.first, f.second));
    }
    std::sort(data->begin(), data->end(),
              [](std::pair<T, uint64_t>& left, std::pair<T, uint64_t>& right) {
                return left.second < right.second;
              });

    return data;
  }

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Clear the buffer, removing all event records.
  //////////////////////////////////////////////////////////////////////////////
  void clear() {
    for (size_t i = 0; i < _capacity; i++) {
      (*_buffer)[i] = T();
    }
  }
};

};  // end namespace cache
};  // end namespace arangodb

#endif