[GitHub] [nifi-minifi-cpp] adamdebreceni opened a new pull request #1038: MINIFICPP-1525 - Support flow file swapping in Connection

[GitBox <gi...@apache.org>]

adamdebreceni opened a new pull request #1038:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1038


   Thank you for submitting a contribution to Apache NiFi - MiNiFi C++.
   
   In order to streamline the review of the contribution we ask you
   to ensure the following steps have been taken:
   
   ### For all changes:
   - [ ] Is there a JIRA ticket associated with this PR? Is it referenced
        in the commit message?
   
   - [ ] Does your PR title start with MINIFICPP-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.
   
   - [ ] Has your PR been rebased against the latest commit within the target branch (typically main)?
   
   - [ ] Is your initial contribution a single, squashed commit?
   
   ### For code changes:
   - [ ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under [ASF 2.0](http://www.apache.org/legal/resolved.html#category-a)?
   - [ ] If applicable, have you updated the LICENSE file?
   - [ ] If applicable, have you updated the NOTICE file?
   
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[GitHub] [nifi-minifi-cpp] martinzink commented on a change in pull request #1038: MINIFICPP-1525 - Support flow file swapping in Connection

[GitBox <gi...@apache.org>]

martinzink commented on a change in pull request #1038:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1038#discussion_r767141667



##########
File path: libminifi/include/utils/MinMaxHeap.h
##########
@@ -0,0 +1,320 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <functional>
+#include <vector>
+#include <cmath>
+#include <utility>
+
+struct MinMaxHeapTestAccessor;
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+namespace utils {
+
+template<typename T, typename Comparator = std::less<T>>
+class MinMaxHeap {
+ public:
+  void clear() {
+    data_.clear();
+  }
+
+  const T& min() const {
+    return data_[0];
+  }
+
+  const T& max() const {
+    // the element at index 0, 1 or 2
+    if (data_.size() == 1) {
+      return data_[0];
+    }
+    if (data_.size() == 2) {
+      return data_[1];
+    }
+    if (less_(data_[2], data_[1])) {
+      return data_[1];
+    }
+    return data_[2];
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+  bool empty() const {
+    return data_.empty();
+  }
+
+  void push(T item) {
+    data_.push_back(std::move(item));
+    pushUp(data_.size() - 1);
+  }
+
+  T popMin() {
+    std::swap(data_[0], data_[data_.size() - 1]);
+    T item = std::move(data_.back());
+    data_.pop_back();
+    pushDown(0);
+    return item;
+  }
+
+  T popMax() {
+    if (data_.size() <= 2) {
+      T item = std::move(data_.back());
+      data_.pop_back();
+      return item;
+    }
+    if (less_(data_[2], data_[1])) {
+      std::swap(data_[1], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(1);
+      return item;
+    } else {
+      std::swap(data_[2], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(2);
+      return item;
+    }
+  }
+
+ private:
+  friend struct ::MinMaxHeapTestAccessor;
+
+  static size_t getLevel(size_t index) {
+    // more performant solutions are possible
+    // investigate if this turns out to be a bottleneck
+    size_t level = 0;
+    ++index;
+    while (index >>= 1) {
+      ++level;
+    }
+    return level;
+  }
+
+  static bool isOnMinLevel(size_t index) {
+    return getLevel(index) % 2 == 0;
+  }
+
+  static size_t getParent(size_t index) {
+    return (index - 1) / 2;
+  }
+
+  /**
+   * WARNING! must only be called when index is on a min-level.

Review comment:
       I think we should throw if this is violated.

##########
File path: libminifi/include/utils/MinMaxHeap.h
##########
@@ -0,0 +1,320 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <functional>
+#include <vector>
+#include <cmath>
+#include <utility>
+
+struct MinMaxHeapTestAccessor;
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+namespace utils {
+
+template<typename T, typename Comparator = std::less<T>>
+class MinMaxHeap {
+ public:
+  void clear() {
+    data_.clear();
+  }
+
+  const T& min() const {
+    return data_[0];
+  }
+
+  const T& max() const {
+    // the element at index 0, 1 or 2
+    if (data_.size() == 1) {
+      return data_[0];
+    }
+    if (data_.size() == 2) {
+      return data_[1];
+    }
+    if (less_(data_[2], data_[1])) {
+      return data_[1];
+    }
+    return data_[2];
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+  bool empty() const {
+    return data_.empty();
+  }
+
+  void push(T item) {
+    data_.push_back(std::move(item));
+    pushUp(data_.size() - 1);
+  }
+
+  T popMin() {
+    std::swap(data_[0], data_[data_.size() - 1]);
+    T item = std::move(data_.back());
+    data_.pop_back();
+    pushDown(0);
+    return item;
+  }
+
+  T popMax() {
+    if (data_.size() <= 2) {
+      T item = std::move(data_.back());
+      data_.pop_back();
+      return item;
+    }
+    if (less_(data_[2], data_[1])) {
+      std::swap(data_[1], data_[data_.size() - 1]);

Review comment:
       I think if we were to extract the max() logic into a getMaxIndex() then this and max() could be simplified.




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[GitHub] [nifi-minifi-cpp] martinzink commented on a change in pull request #1038: MINIFICPP-1525 - Support flow file swapping in Connection

[GitBox <gi...@apache.org>]

martinzink commented on a change in pull request #1038:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1038#discussion_r767141667



##########
File path: libminifi/include/utils/MinMaxHeap.h
##########
@@ -0,0 +1,320 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <functional>
+#include <vector>
+#include <cmath>
+#include <utility>
+
+struct MinMaxHeapTestAccessor;
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+namespace utils {
+
+template<typename T, typename Comparator = std::less<T>>
+class MinMaxHeap {
+ public:
+  void clear() {
+    data_.clear();
+  }
+
+  const T& min() const {
+    return data_[0];
+  }
+
+  const T& max() const {
+    // the element at index 0, 1 or 2
+    if (data_.size() == 1) {
+      return data_[0];
+    }
+    if (data_.size() == 2) {
+      return data_[1];
+    }
+    if (less_(data_[2], data_[1])) {
+      return data_[1];
+    }
+    return data_[2];
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+  bool empty() const {
+    return data_.empty();
+  }
+
+  void push(T item) {
+    data_.push_back(std::move(item));
+    pushUp(data_.size() - 1);
+  }
+
+  T popMin() {
+    std::swap(data_[0], data_[data_.size() - 1]);
+    T item = std::move(data_.back());
+    data_.pop_back();
+    pushDown(0);
+    return item;
+  }
+
+  T popMax() {
+    if (data_.size() <= 2) {
+      T item = std::move(data_.back());
+      data_.pop_back();
+      return item;
+    }
+    if (less_(data_[2], data_[1])) {
+      std::swap(data_[1], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(1);
+      return item;
+    } else {
+      std::swap(data_[2], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(2);
+      return item;
+    }
+  }
+
+ private:
+  friend struct ::MinMaxHeapTestAccessor;
+
+  static size_t getLevel(size_t index) {
+    // more performant solutions are possible
+    // investigate if this turns out to be a bottleneck
+    size_t level = 0;
+    ++index;
+    while (index >>= 1) {
+      ++level;
+    }
+    return level;
+  }
+
+  static bool isOnMinLevel(size_t index) {
+    return getLevel(index) % 2 == 0;
+  }
+
+  static size_t getParent(size_t index) {
+    return (index - 1) / 2;
+  }
+
+  /**
+   * WARNING! must only be called when index is on a min-level.

Review comment:
       I think we should throw if this is violated.

##########
File path: libminifi/include/utils/MinMaxHeap.h
##########
@@ -0,0 +1,320 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <functional>
+#include <vector>
+#include <cmath>
+#include <utility>
+
+struct MinMaxHeapTestAccessor;
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+namespace utils {
+
+template<typename T, typename Comparator = std::less<T>>
+class MinMaxHeap {
+ public:
+  void clear() {
+    data_.clear();
+  }
+
+  const T& min() const {
+    return data_[0];
+  }
+
+  const T& max() const {
+    // the element at index 0, 1 or 2
+    if (data_.size() == 1) {
+      return data_[0];
+    }
+    if (data_.size() == 2) {
+      return data_[1];
+    }
+    if (less_(data_[2], data_[1])) {
+      return data_[1];
+    }
+    return data_[2];
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+  bool empty() const {
+    return data_.empty();
+  }
+
+  void push(T item) {
+    data_.push_back(std::move(item));
+    pushUp(data_.size() - 1);
+  }
+
+  T popMin() {
+    std::swap(data_[0], data_[data_.size() - 1]);
+    T item = std::move(data_.back());
+    data_.pop_back();
+    pushDown(0);
+    return item;
+  }
+
+  T popMax() {
+    if (data_.size() <= 2) {
+      T item = std::move(data_.back());
+      data_.pop_back();
+      return item;
+    }
+    if (less_(data_[2], data_[1])) {
+      std::swap(data_[1], data_[data_.size() - 1]);

Review comment:
       I think if we were to extract the max() logic into a getMaxIndex() then this and max() could be simplified.




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[GitHub] [nifi-minifi-cpp] szaszm commented on a change in pull request #1038: MINIFICPP-1525 - Support flow file swapping in Connection

[GitBox <gi...@apache.org>]

szaszm commented on a change in pull request #1038:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1038#discussion_r704567374



##########
File path: extensions/rocksdb-repos/FlowFileLoader.h
##########
@@ -0,0 +1,68 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <future>
+#include <list>
+
+#include "RocksDatabase.h"
+#include "FlowFile.h"
+#include "gsl.h"
+#include "core/ContentRepository.h"
+#include "SwapManager.h"
+#include "utils/ThreadPool.h"
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+
+class FlowFileLoader {
+  using FlowFilePtr = std::shared_ptr<core::FlowFile>;
+  using FlowFilePtrVec = std::vector<FlowFilePtr>;
+
+  static constexpr size_t thread_count_ = 2;
+
+ public:
+  FlowFileLoader();
+
+  ~FlowFileLoader();
+
+  void initialize(gsl::not_null<minifi::internal::RocksDatabase*> db, std::shared_ptr<core::ContentRepository> content_repo);
+
+  void start();
+
+  void stop();
+
+  std::future<FlowFilePtrVec> load(std::vector<SwappedFlowFile> flow_files);
+
+ private:
+  utils::TaskRescheduleInfo loadImpl(const std::vector<SwappedFlowFile>& flow_files, std::shared_ptr<std::promise<FlowFilePtrVec>>& output);
+
+  utils::ThreadPool<utils::TaskRescheduleInfo> thread_pool_{thread_count_, false, nullptr, "FlowFileLoaderThreadPool"};
+
+  minifi::internal::RocksDatabase* db_{nullptr};
+
+  std::shared_ptr<core::ContentRepository> content_repo_;
+  std::shared_ptr<logging::Logger> logger_;

Review comment:
       Missing `#include "core/logging/Logger.h"`

##########
File path: libminifi/include/utils/MinMaxHeap.h
##########
@@ -0,0 +1,320 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <functional>
+#include <vector>
+#include <cmath>
+#include <utility>
+
+struct MinMaxHeapTestAccessor;
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+namespace utils {
+
+template<typename T, typename Comparator = std::less<T>>
+class MinMaxHeap {
+ public:
+  void clear() {
+    data_.clear();
+  }
+
+  const T& min() const {
+    return data_[0];
+  }
+
+  const T& max() const {
+    // the element at index 0, 1 or 2
+    if (data_.size() == 1) {
+      return data_[0];
+    }
+    if (data_.size() == 2) {
+      return data_[1];
+    }
+    if (less_(data_[2], data_[1])) {
+      return data_[1];
+    }
+    return data_[2];
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+  bool empty() const {
+    return data_.empty();
+  }
+
+  void push(T item) {
+    data_.push_back(std::move(item));
+    pushUp(data_.size() - 1);
+  }
+
+  T popMin() {
+    std::swap(data_[0], data_[data_.size() - 1]);
+    T item = std::move(data_.back());
+    data_.pop_back();
+    pushDown(0);
+    return item;
+  }
+
+  T popMax() {
+    if (data_.size() <= 2) {
+      T item = std::move(data_.back());
+      data_.pop_back();
+      return item;
+    }
+    if (less_(data_[2], data_[1])) {
+      std::swap(data_[1], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(1);
+      return item;
+    } else {
+      std::swap(data_[2], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(2);
+      return item;
+    }
+  }
+
+ private:
+  friend struct ::MinMaxHeapTestAccessor;
+
+  static size_t getLevel(size_t index) {
+    // more performant solutions are possible
+    // investigate if this turns out to be a bottleneck
+    size_t level = 0;
+    ++index;
+    while (index >>= 1) {
+      ++level;
+    }
+    return level;
+  }
+
+  static bool isOnMinLevel(size_t index) {
+    return getLevel(index) % 2 == 0;
+  }
+
+  static size_t getParent(size_t index) {
+    return (index - 1) / 2;
+  }
+
+  /**
+   * WARNING! must only be called when index is on a min-level.

Review comment:
       This sounds like a precondition, so I think `gsl_Expects` or `gsl_ExpectsAudit` (if checking is expensive) fits the problem better.




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[GitHub] [nifi-minifi-cpp] szaszm commented on a change in pull request #1038: MINIFICPP-1525 - Support flow file swapping in Connection

[GitBox <gi...@apache.org>]

szaszm commented on a change in pull request #1038:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1038#discussion_r704567374



##########
File path: extensions/rocksdb-repos/FlowFileLoader.h
##########
@@ -0,0 +1,68 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <future>
+#include <list>
+
+#include "RocksDatabase.h"
+#include "FlowFile.h"
+#include "gsl.h"
+#include "core/ContentRepository.h"
+#include "SwapManager.h"
+#include "utils/ThreadPool.h"
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+
+class FlowFileLoader {
+  using FlowFilePtr = std::shared_ptr<core::FlowFile>;
+  using FlowFilePtrVec = std::vector<FlowFilePtr>;
+
+  static constexpr size_t thread_count_ = 2;
+
+ public:
+  FlowFileLoader();
+
+  ~FlowFileLoader();
+
+  void initialize(gsl::not_null<minifi::internal::RocksDatabase*> db, std::shared_ptr<core::ContentRepository> content_repo);
+
+  void start();
+
+  void stop();
+
+  std::future<FlowFilePtrVec> load(std::vector<SwappedFlowFile> flow_files);
+
+ private:
+  utils::TaskRescheduleInfo loadImpl(const std::vector<SwappedFlowFile>& flow_files, std::shared_ptr<std::promise<FlowFilePtrVec>>& output);
+
+  utils::ThreadPool<utils::TaskRescheduleInfo> thread_pool_{thread_count_, false, nullptr, "FlowFileLoaderThreadPool"};
+
+  minifi::internal::RocksDatabase* db_{nullptr};
+
+  std::shared_ptr<core::ContentRepository> content_repo_;
+  std::shared_ptr<logging::Logger> logger_;

Review comment:
       Missing `#include "core/logging/Logger.h"`




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[GitHub] [nifi-minifi-cpp] szaszm commented on a change in pull request #1038: MINIFICPP-1525 - Support flow file swapping in Connection

[GitBox <gi...@apache.org>]

szaszm commented on a change in pull request #1038:
URL: https://github.com/apache/nifi-minifi-cpp/pull/1038#discussion_r767163102



##########
File path: libminifi/include/utils/MinMaxHeap.h
##########
@@ -0,0 +1,320 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+
+#pragma once
+
+#include <functional>
+#include <vector>
+#include <cmath>
+#include <utility>
+
+struct MinMaxHeapTestAccessor;
+
+namespace org {
+namespace apache {
+namespace nifi {
+namespace minifi {
+namespace utils {
+
+template<typename T, typename Comparator = std::less<T>>
+class MinMaxHeap {
+ public:
+  void clear() {
+    data_.clear();
+  }
+
+  const T& min() const {
+    return data_[0];
+  }
+
+  const T& max() const {
+    // the element at index 0, 1 or 2
+    if (data_.size() == 1) {
+      return data_[0];
+    }
+    if (data_.size() == 2) {
+      return data_[1];
+    }
+    if (less_(data_[2], data_[1])) {
+      return data_[1];
+    }
+    return data_[2];
+  }
+
+  size_t size() const {
+    return data_.size();
+  }
+
+  bool empty() const {
+    return data_.empty();
+  }
+
+  void push(T item) {
+    data_.push_back(std::move(item));
+    pushUp(data_.size() - 1);
+  }
+
+  T popMin() {
+    std::swap(data_[0], data_[data_.size() - 1]);
+    T item = std::move(data_.back());
+    data_.pop_back();
+    pushDown(0);
+    return item;
+  }
+
+  T popMax() {
+    if (data_.size() <= 2) {
+      T item = std::move(data_.back());
+      data_.pop_back();
+      return item;
+    }
+    if (less_(data_[2], data_[1])) {
+      std::swap(data_[1], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(1);
+      return item;
+    } else {
+      std::swap(data_[2], data_[data_.size() - 1]);
+      T item = std::move(data_.back());
+      data_.pop_back();
+      pushDown(2);
+      return item;
+    }
+  }
+
+ private:
+  friend struct ::MinMaxHeapTestAccessor;
+
+  static size_t getLevel(size_t index) {
+    // more performant solutions are possible
+    // investigate if this turns out to be a bottleneck
+    size_t level = 0;
+    ++index;
+    while (index >>= 1) {
+      ++level;
+    }
+    return level;
+  }
+
+  static bool isOnMinLevel(size_t index) {
+    return getLevel(index) % 2 == 0;
+  }
+
+  static size_t getParent(size_t index) {
+    return (index - 1) / 2;
+  }
+
+  /**
+   * WARNING! must only be called when index is on a min-level.

Review comment:
       This sounds like a precondition, so I think `gsl_Expects` or `gsl_ExpectsAudit` (if checking is expensive) fits the problem better.




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