base/metrics/histogram_samples.h

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_METRICS_HISTOGRAM_SAMPLES_H_
#define BASE_METRICS_HISTOGRAM_SAMPLES_H_

#include <stddef.h>
#include <stdint.h>

#include <memory>

#include "base/atomicops.h"
#include "base/macros.h"
#include "base/metrics/histogram_base.h"

namespace base {

class Pickle;
class PickleIterator;
class SampleCountIterator;

// HistogramSamples is a container storing all samples of a histogram. All
// elements must be of a fixed width to ensure 32/64-bit interoperability.
// If this structure changes, bump the version number for kTypeIdHistogram
// in persistent_histogram_allocator.cc.
class BASE_EXPORT HistogramSamples {
 public:
  struct Metadata {
    // Expected size for 32/64-bit check.
    static constexpr size_t kExpectedInstanceSize = 24;

    // Initialized when the sample-set is first created with a value provided
    // by the caller. It is generally used to identify the sample-set across
    // threads and processes, though not necessarily uniquely as it is possible
    // to have multiple sample-sets representing subsets of the data.
    uint64_t id;

    // The sum of all the entries, effectivly the sum(sample * count) for
    // all samples. Despite being atomic, no guarantees are made on the
    // accuracy of this value; there may be races during histogram
    // accumulation and snapshotting that we choose to accept. It should
    // be treated as approximate.
#ifdef ARCH_CPU_64_BITS
    subtle::Atomic64 sum;
#else
    // 32-bit systems don't have atomic 64-bit operations. Use a basic type
    // and don't worry about "shearing".
    int64_t sum;
#endif

    // A "redundant" count helps identify memory corruption. It redundantly
    // stores the total number of samples accumulated in the histogram. We
    // can compare this count to the sum of the counts (TotalCount() function),
    // and detect problems. Note, depending on the implementation of different
    // histogram types, there might be races during histogram accumulation
    // and snapshotting that we choose to accept. In this case, the tallies
    // might mismatch even when no memory corruption has happened.
    HistogramBase::AtomicCount redundant_count;

    // 4 bytes of padding to explicitly extend this structure to a multiple of
    // 64-bits. This is required to ensure the structure is the same size on
    // both 32-bit and 64-bit builds.
    char padding[4];
  };

  // Because sturctures held in persistent memory must be POD, there can be no
  // default constructor to clear the fields. This derived class exists just
  // to clear them when being allocated on the heap.
  struct LocalMetadata : Metadata {
    LocalMetadata() {
      id = 0;
      sum = 0;
      redundant_count = 0;
    }
  };

  explicit HistogramSamples(uint64_t id);
  HistogramSamples(uint64_t id, Metadata* meta);
  virtual ~HistogramSamples();

  virtual void Accumulate(HistogramBase::Sample value,
                          HistogramBase::Count count) = 0;
  virtual HistogramBase::Count GetCount(HistogramBase::Sample value) const = 0;
  virtual HistogramBase::Count TotalCount() const = 0;

  virtual void Add(const HistogramSamples& other);

  // Add from serialized samples.
  virtual bool AddFromPickle(PickleIterator* iter);

  virtual void Subtract(const HistogramSamples& other);

  virtual std::unique_ptr<SampleCountIterator> Iterator() const = 0;
  virtual bool Serialize(Pickle* pickle) const;

  // Accessor fuctions.
  uint64_t id() const { return meta_->id; }
  int64_t sum() const {
#ifdef ARCH_CPU_64_BITS
    return subtle::NoBarrier_Load(&meta_->sum);
#else
    return meta_->sum;
#endif
  }
  HistogramBase::Count redundant_count() const {
    return subtle::NoBarrier_Load(&meta_->redundant_count);
  }

 protected:
  // Based on |op| type, add or subtract sample counts data from the iterator.
  enum Operator { ADD, SUBTRACT };
  virtual bool AddSubtractImpl(SampleCountIterator* iter, Operator op) = 0;

  void IncreaseSum(int64_t diff);
  void IncreaseRedundantCount(HistogramBase::Count diff);

 private:
  // In order to support histograms shared through an external memory segment,
  // meta values may be the local storage or external storage depending on the
  // wishes of the derived class.
  LocalMetadata local_meta_;
  Metadata* meta_;

  DISALLOW_COPY_AND_ASSIGN(HistogramSamples);
};

class BASE_EXPORT SampleCountIterator {
 public:
  virtual ~SampleCountIterator();

  virtual bool Done() const = 0;
  virtual void Next() = 0;

  // Get the sample and count at current position.
  // |min| |max| and |count| can be NULL if the value is not of interest.
  // Requires: !Done();
  virtual void Get(HistogramBase::Sample* min,
                   HistogramBase::Sample* max,
                   HistogramBase::Count* count) const = 0;

  // Get the index of current histogram bucket.
  // For histograms that don't use predefined buckets, it returns false.
  // Requires: !Done();
  virtual bool GetBucketIndex(size_t* index) const;
};

}  // namespace base

#endif  // BASE_METRICS_HISTOGRAM_SAMPLES_H_