// 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.

#include <cstdint>
#include <memory>
#include <thread>
#include <vector>

#include <gflags/gflags.h>
#include <gtest/gtest.h>

#include "kudu/util/hdr_histogram.h"
#include "kudu/util/monotime.h"
#include "kudu/util/test_util.h"

DEFINE_int32(histogram_test_num_threads, 16,
    "Number of threads to spawn for mt-hdr_histogram test");
DEFINE_uint64(histogram_test_num_increments_per_thread, 100000LU,
    "Number of times to call Increment() per thread in mt-hdr_histogram test");

using std::thread;
using std::unique_ptr;
using std::vector;

namespace kudu {

class MtHdrHistogramTest : public KuduTest {
 public:
  MtHdrHistogramTest() {
    num_threads_ = FLAGS_histogram_test_num_threads;
    num_times_ = FLAGS_histogram_test_num_increments_per_thread;
  }

 protected:
  int num_threads_;
  uint64_t num_times_;
};

// Increment a counter a bunch of times in the same bucket
static void IncrementSameHistValue(HdrHistogram* hist, uint64_t value, uint64_t times) {
  for (uint64_t i = 0; i < times; i++) {
    hist->Increment(value);
  }
}

TEST_F(MtHdrHistogramTest, ConcurrentWriteTest) {
  const uint64_t kValue = 1LU;

  HdrHistogram hist(100000LU, 3);

  vector<thread> threads;
  threads.reserve(num_threads_);
  for (int i = 0; i < num_threads_; i++) {
    threads.emplace_back([this, &hist]() {
      IncrementSameHistValue(&hist, kValue, this->num_times_);
    });
  }
  for (auto& t : threads) {
    t.join();
  }

  HdrHistogram snapshot(hist);
  ASSERT_EQ(num_threads_ * num_times_, snapshot.CountInBucketForValue(kValue));
}

// Copy while writing, then iterate to ensure copies are consistent.
TEST_F(MtHdrHistogramTest, ConcurrentCopyWhileWritingTest) {
  const int kNumCopies = 10;
  const uint64_t kValue = 1;

  HdrHistogram hist(100000LU, 3);

  vector<thread> threads;
  threads.reserve(num_threads_);
  for (int i = 0; i < num_threads_; i++) {
    threads.emplace_back([this, &hist]() {
      IncrementSameHistValue(&hist, kValue, this->num_times_);
    });
  }

  // This is somewhat racy but the goal is to catch this issue at least
  // most of the time. At the time of this writing, before fixing a bug where
  // the total count stored in a copied histogram may not match its internal
  // counts (under concurrent writes), this test fails for me on 100/100 runs.
  vector<unique_ptr<HdrHistogram>> snapshots;
  for (int i = 0; i < kNumCopies; i++) {
    snapshots.emplace_back(new HdrHistogram(hist));
    SleepFor(MonoDelta::FromMicroseconds(100));
  }
  for (int i = 0; i < kNumCopies; i++) {
    snapshots[i]->MeanValue(); // Will crash if underlying iterator is inconsistent.
  }

  for (auto& t : threads) {
    t.join();
  }
}

} // namespace kudu