[SDK] Add AdaptingCircularBufferCounter for exponential histograms

sdk/include/opentelemetry/sdk/metrics/data/circular_buffer.h

@@ -0,0 +1,150 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+#pragma once
+
+#include "opentelemetry/nostd/variant.h"
+
+#include <limits>
+#include <vector>
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace sdk
+{
+namespace metrics
+{
+
+/**
+ * An integer array that automatically expands its memory consumption (via copy/allocation) when
+ * reaching limits. This assumes counts remain low, to lower memory overhead.
+ *
+ * This class is NOT thread-safe. It is expected to be behind a synchronized incrementer.
+ *
+ * Instances start by attempting to store one-byte per-cell in the integer array. As values grow,
+ * this will automatically instantiate the next-size integer array (uint8_t -> uint16_t -> uint32_t
+ * -> uint64_t) and copy over values into the larger array. This class expects most usage to remain
+ * within the uint8_t boundary (e.g. cell values < 256).
+ */
+class AdaptingIntegerArray
+{
+public:
+  // Construct an adapting integer array of a given size.
+  explicit AdaptingIntegerArray(size_t size) : backing_(std::vector<uint8_t>(size, 0)) {}
+  AdaptingIntegerArray(const AdaptingIntegerArray &other) = default;
+  AdaptingIntegerArray &operator=(const AdaptingIntegerArray &other) = default;
+
+  /**
+   * Increments the value at the specified index by the given count in the array.
+   *
+   * @param index The index of the value to increment.
+   * @param count The count by which to increment the value.
+   */
+  void Increment(size_t index, uint64_t count);
+
+  /**
+   * Returns the value at the specified index from the array.
+   *
+   * @param index The index of the value to retrieve.
+   * @return The value at the specified index.
+   */
+  uint64_t Get(size_t index) const;
+
+  /**
+   * Returns the size of the array.
+   *
+   * @return The size of the array.
+   */
+  size_t Size() const;
+
+  /**
+   * Clears the array, resetting all values to zero.
+   */
+  void Clear();
+
+private:
+  void EnlargeToFit(uint64_t value);
+
+  nostd::variant<std::vector<uint8_t>,
+                 std::vector<uint16_t>,
+                 std::vector<uint32_t>,
+                 std::vector<uint64_t>>
+      backing_;
+};
+
+/**
+ * A circle-buffer-backed exponential counter.
+ *
+ * The first recorded value becomes the 'base_index'. Going backwards leads to start/stop index.
+ *
+ * This expand start/end index as it sees values.
+ *
+ * This class is NOT thread-safe. It is expected to be behind a synchronized incrementer.
+ */
+class AdaptingCircularBufferCounter
+{
+public:
+  explicit AdaptingCircularBufferCounter(size_t max_size) : backing_(max_size) {}
+  AdaptingCircularBufferCounter(const AdaptingCircularBufferCounter &other) = default;
+  AdaptingCircularBufferCounter &operator=(const AdaptingCircularBufferCounter &other) = default;
+
+  /**
+   * The first index with a recording. May be negative.
+   *
+   * Note: the returned value is not meaningful when Empty returns true.
+   *
+   * @return the first index with a recording.
+   */
+  size_t StartIndex() const { return start_index_; }
+
+  /**
+   * The last index with a recording. May be negative.
+   *
+   * Note: the returned value is not meaningful when Empty returns true.
+   *
+   * @return The last index with a recording.
+   */
+  size_t EndIndex() const { return end_index_; }
+
+  /**
+   * Returns true if no recordings, false if at least one recording.
+   */
+  bool Empty() const { return base_index_ == kNullIndex; }
+
+  /**
+   * Returns the maximum number of buckets allowed in this counter.
+   */
+  size_t MaxSize() const { return backing_.Size(); }
+
+  /** Resets all bucket counts to zero and resets index start/end tracking. **/
+  void Clear();
+
+  /**
+   * Persist new data at index, incrementing by delta amount.
+   *
+   * @param index The index of where to perform the incrementation.
+   * @param delta How much to increment the index by.
+   * @return success status.
+   */
+  bool Increment(size_t index, uint64_t delta);
+
+  /**
+   * Get the number of recordings for the given index.
+   *
+   * @return the number of recordings for the index, or 0 if the index is out of bounds.
+   */
+  uint64_t Get(size_t index);
+
+private:
+  size_t ToBufferIndex(size_t index) const;
+
+  static constexpr size_t kNullIndex = std::numeric_limits<size_t>::max();
+
+  size_t start_index_ = kNullIndex;
+  size_t end_index_   = kNullIndex;
+  size_t base_index_  = kNullIndex;
+  AdaptingIntegerArray backing_;
+};
+
+}  // namespace metrics
+}  // namespace sdk
+OPENTELEMETRY_END_NAMESPACE

sdk/src/metrics/CMakeLists.txt

@@ -17,6 +17,7 @@ add_library(
   aggregation/histogram_aggregation.cc
   aggregation/lastvalue_aggregation.cc
   aggregation/sum_aggregation.cc
+  data/circular_buffer.cc
   exemplar/filter.cc
   exemplar/reservoir.cc
   sync_instruments.cc)

sdk/src/metrics/data/circular_buffer.cc

@@ -0,0 +1,182 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+#include "opentelemetry/sdk/metrics/data/circular_buffer.h"
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace sdk
+{
+namespace metrics
+{
+
+namespace
+{
+
+struct AdaptingIntegerArrayIncrement
+{
+  size_t index;
+  uint64_t count;
+
+  template <typename T>
+  uint64_t operator()(std::vector<T> &backing)
+  {
+    const uint64_t result = backing[index] + count;
+    OPENTELEMETRY_LIKELY_IF(result <= uint64_t(std::numeric_limits<T>::max()))
+    {
+      backing[index] = static_cast<T>(result);
+      return 0;
+    }
+    return result;
+  }
+};
+
+struct AdaptingIntegerArrayGet
+{
+  size_t index;
+
+  template <typename T>
+  uint64_t operator()(const std::vector<T> &backing)
+  {
+    return backing[index];
+  }
+};
+
+struct AdaptingIntegerArraySize
+{
+  template <typename T>
+  uint64_t operator()(const std::vector<T> &backing)
+  {
+    return backing.size();
+  }
+};
+
+struct AdaptingIntegerArrayClear
+{
+  template <typename T>
+  void operator()(std::vector<T> &backing)
+  {
+    std::fill(backing.begin(), backing.end(), 0);
+  }
+};
+
+struct AdaptingIntegerArrayCopy
+{
+  template <class T1, class T2>
+  void operator()(const std::vector<T1> &from, std::vector<T2> &to)
+  {
+    for (size_t i = 0; i < from.size(); i++)
+    {
+      to[i] = static_cast<T2>(from[i]);
+    }
+  }
+};
+
+}  // namespace
+
+void AdaptingIntegerArray::Increment(size_t index, uint64_t count)
+{
+  const uint64_t result = nostd::visit(AdaptingIntegerArrayIncrement{index, count}, backing_);
+  OPENTELEMETRY_LIKELY_IF(result == 0) { return; }
+  EnlargeToFit(result);
+  Increment(index, count);
+}
+
+uint64_t AdaptingIntegerArray::Get(size_t index) const
+{
+  return nostd::visit(AdaptingIntegerArrayGet{index}, backing_);
+}
+
+size_t AdaptingIntegerArray::Size() const
+{
+  return nostd::visit(AdaptingIntegerArraySize{}, backing_);
+}
+
+void AdaptingIntegerArray::Clear()
+{
+  nostd::visit(AdaptingIntegerArrayClear{}, backing_);
+}
+
+void AdaptingIntegerArray::EnlargeToFit(uint64_t value)
+{
+  const size_t backing_size = Size();
+  decltype(backing_) backing;
+  if (value <= std::numeric_limits<uint16_t>::max())
+  {
+    backing = std::vector<uint16_t>(backing_size, 0);
+  }
+  else if (value <= std::numeric_limits<uint32_t>::max())
+  {
+    backing = std::vector<uint32_t>(backing_size, 0);
+  }
+  else
+  {
+    backing = std::vector<uint64_t>(backing_size, 0);
+  }
+  std::swap(backing_, backing);
+  nostd::visit(AdaptingIntegerArrayCopy{}, backing, backing_);
+}
+
+void AdaptingCircularBufferCounter::Clear()
+{
+  start_index_ = kNullIndex;
+  end_index_   = kNullIndex;
+  base_index_  = kNullIndex;
+  backing_.Clear();
+}
+
+bool AdaptingCircularBufferCounter::Increment(size_t index, uint64_t delta)
+{
+  if (Empty())
+  {
+    start_index_ = index;
+    end_index_   = index;
+    base_index_  = index;
+    backing_.Increment(0, delta);
+    return true;
+  }
+
+  if (index > end_index_)
+  {
+    // Move end, check max size.
+    if (index + 1 > backing_.Size() + start_index_)
+    {
+      return false;
+    }
+    end_index_ = index;
+  }
+  else if (index < start_index_)
+  {
+    // Move end, check max size.
+    if (end_index_ + 1 > backing_.Size() + index)
+    {
+      return false;
+    }
+    start_index_ = index;
+  }
+  backing_.Increment(ToBufferIndex(index), delta);
+  return true;
+}
+
+uint64_t AdaptingCircularBufferCounter::Get(size_t index)
+{
+  if (index < start_index_ || index > end_index_)
+  {
+    return 0;
+  }
+  return backing_.Get(ToBufferIndex(index));
+}
+
+size_t AdaptingCircularBufferCounter::ToBufferIndex(size_t index) const
+{
+  // Figure out the index relative to the start of the circular buffer.
+  if (index < base_index_)
+  {
+    // If index is before the base one, wrap around.
+    return index + backing_.Size() - base_index_;
+  }
+  return index - base_index_;
+}
+
+}  // namespace metrics
+}  // namespace sdk
+OPENTELEMETRY_END_NAMESPACE

sdk/test/metrics/BUILD

@@ -270,6 +270,21 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "circular_buffer_counter_test",
+    srcs = [
+        "circular_buffer_counter_test.cc",
+    ],
+    tags = [
+        "metrics",
+        "test",
+    ],
+    deps = [
+        "//sdk/src/metrics",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "histogram_aggregation_test",
     srcs = [

sdk/test/metrics/CMakeLists.txt

@@ -14,6 +14,7 @@ foreach(
   histogram_aggregation_test
   attributes_processor_test
   attributes_hashmap_test
+  circular_buffer_counter_test
   histogram_test
   sync_metric_storage_counter_test
   sync_metric_storage_histogram_test