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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2017 OVH
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ *
+ */
+
+#ifndef CEPH_COMMON_PERF_HISTOGRAM_H
+#define CEPH_COMMON_PERF_HISTOGRAM_H
+
+#include <array>
+#include <atomic>
+#include <memory>
+
+#include "common/Formatter.h"
+#include "include/int_types.h"
+#include "include/ceph_assert.h"
+
+class PerfHistogramCommon {
+public:
+  enum scale_type_d : uint8_t {
+    SCALE_LINEAR = 1,
+    SCALE_LOG2 = 2,
+  };
+
+  struct axis_config_d {
+    const char *m_name = nullptr;
+    scale_type_d m_scale_type = SCALE_LINEAR;
+    int64_t m_min = 0;
+    int64_t m_quant_size = 0;
+    int32_t m_buckets = 0;
+    axis_config_d() = default;
+    axis_config_d(const char* name,
+		  scale_type_d scale_type,
+		  int64_t min,
+		  int64_t quant_size,
+		  int32_t buckets)
+      : m_name(name),
+	m_scale_type(scale_type),
+	m_min(min),
+	m_quant_size(quant_size),
+	m_buckets(buckets)
+    {}
+  };
+
+protected:
+  /// Dump configuration of one axis to a formatter
+  static void dump_formatted_axis(ceph::Formatter *f, const axis_config_d &ac);
+
+  /// Quantize given value and convert to bucket number on given axis
+  static int64_t get_bucket_for_axis(int64_t value, const axis_config_d &ac);
+
+  /// Calculate inclusive ranges of axis values for each bucket on that axis
+  static std::vector<std::pair<int64_t, int64_t>> get_axis_bucket_ranges(
+      const axis_config_d &ac);
+};
+
+/// PerfHistogram does trace a histogram of input values. It's an extended
+/// version of a standard histogram which does trace characteristics of a single
+/// one value only. In this implementation, values can be traced in multiple
+/// dimensions - i.e. we can create a histogram of input request size (first
+/// dimension) and processing latency (second dimension). Creating standard
+/// histogram out of such multidimensional one is trivial and requires summing
+/// values across dimensions we're not interested in.
+template <int DIM = 2>
+class PerfHistogram : public PerfHistogramCommon {
+public:
+  /// Initialize new histogram object
+  PerfHistogram(std::initializer_list<axis_config_d> axes_config) {
+    ceph_assert(axes_config.size() == DIM &&
+		"Invalid number of axis configuration objects");
+
+    int i = 0;
+    for (const auto &ac : axes_config) {
+      ceph_assertf(ac.m_buckets > 0, "Must have at least one bucket on axis");
+      ceph_assertf(ac.m_quant_size > 0,
+             "Quantization unit must be non-zero positive integer value");
+
+      m_axes_config[i++] = ac;
+    }
+
+    m_rawData.reset(new std::atomic<uint64_t>[get_raw_size()] {});
+  }
+
+  /// Copy from other histogram object
+  PerfHistogram(const PerfHistogram &other)
+      : m_axes_config(other.m_axes_config) {
+    int64_t size = get_raw_size();
+    m_rawData.reset(new std::atomic<uint64_t>[size] {});
+    for (int64_t i = 0; i < size; i++) {
+      m_rawData[i] = other.m_rawData[i].load();
+    }
+  }
+
+  /// Set all histogram values to 0
+  void reset() {
+    auto size = get_raw_size();
+    for (auto i = size; --i >= 0;) {
+      m_rawData[i] = 0;
+    }
+  }
+
+  /// Increase counter for given axis values by one
+  template <typename... T>
+  void inc(T... axis) {
+    auto index = get_raw_index_for_value(axis...);
+    m_rawData[index]++;
+  }
+
+  /// Increase counter for given axis buckets by one
+  template <typename... T>
+  void inc_bucket(T... bucket) {
+    auto index = get_raw_index_for_bucket(bucket...);
+    m_rawData[index]++;
+  }
+
+  /// Read value from given bucket
+  template <typename... T>
+  uint64_t read_bucket(T... bucket) const {
+    auto index = get_raw_index_for_bucket(bucket...);
+    return m_rawData[index];
+  }
+
+  /// Dump data to a Formatter object
+  void dump_formatted(ceph::Formatter *f) const {
+    // Dump axes configuration
+    f->open_array_section("axes");
+    for (auto &ac : m_axes_config) {
+      dump_formatted_axis(f, ac);
+    }
+    f->close_section();
+
+    // Dump histogram values
+    dump_formatted_values(f);
+  }
+
+protected:
+  /// Raw data stored as linear space, internal indexes are calculated on
+  /// demand.
+  std::unique_ptr<std::atomic<uint64_t>[]> m_rawData;
+
+  /// Configuration of axes
+  std::array<axis_config_d, DIM> m_axes_config;
+
+  /// Dump histogram counters to a formatter
+  void dump_formatted_values(ceph::Formatter *f) const {
+    visit_values([f](int) { f->open_array_section("values"); },
+                 [f](int64_t value) { f->dump_unsigned("value", value); },
+                 [f](int) { f->close_section(); });
+  }
+
+  /// Get number of all histogram counters
+  int64_t get_raw_size() {
+    int64_t ret = 1;
+    for (const auto &ac : m_axes_config) {
+      ret *= ac.m_buckets;
+    }
+    return ret;
+  }
+
+  /// Calculate m_rawData index from axis values
+  template <typename... T>
+  int64_t get_raw_index_for_value(T... axes) const {
+    static_assert(sizeof...(T) == DIM, "Incorrect number of arguments");
+    return get_raw_index_internal<0>(get_bucket_for_axis, 0, axes...);
+  }
+
+  /// Calculate m_rawData index from axis bucket numbers
+  template <typename... T>
+  int64_t get_raw_index_for_bucket(T... buckets) const {
+    static_assert(sizeof...(T) == DIM, "Incorrect number of arguments");
+    return get_raw_index_internal<0>(
+        [](int64_t bucket, const axis_config_d &ac) {
+          ceph_assertf(bucket >= 0, "Bucket index can not be negative");
+          ceph_assertf(bucket < ac.m_buckets, "Bucket index too large");
+          return bucket;
+        },
+        0, buckets...);
+  }
+
+  template <int level = 0, typename F, typename... T>
+  int64_t get_raw_index_internal(F bucket_evaluator, int64_t startIndex,
+                                 int64_t value, T... tail) const {
+    static_assert(level + 1 + sizeof...(T) == DIM,
+                  "Internal consistency check");
+    auto &ac = m_axes_config[level];
+    auto bucket = bucket_evaluator(value, ac);
+    return get_raw_index_internal<level + 1>(
+        bucket_evaluator, ac.m_buckets * startIndex + bucket, tail...);
+  }
+
+  template <int level, typename F>
+  int64_t get_raw_index_internal(F, int64_t startIndex) const {
+    static_assert(level == DIM, "Internal consistency check");
+    return startIndex;
+  }
+
+  /// Visit all histogram counters, call onDimensionEnter / onDimensionLeave
+  /// when starting / finishing traversal
+  /// on given axis, call onValue when dumping raw histogram counter value.
+  template <typename FDE, typename FV, typename FDL>
+  void visit_values(FDE onDimensionEnter, FV onValue, FDL onDimensionLeave,
+                    int level = 0, int startIndex = 0) const {
+    if (level == DIM) {
+      onValue(m_rawData[startIndex]);
+      return;
+    }
+
+    onDimensionEnter(level);
+    auto &ac = m_axes_config[level];
+    startIndex *= ac.m_buckets;
+    for (int32_t i = 0; i < ac.m_buckets; ++i, ++startIndex) {
+      visit_values(onDimensionEnter, onValue, onDimensionLeave, level + 1,
+                   startIndex);
+    }
+    onDimensionLeave(level);
+  }
+};
+
+#endif