// Copyright 2023 The Bazel Authors. All rights reserved.
//
// Licensed 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.
package com.google.devtools.build.lib.remote.circuitbreaker;

import com.google.devtools.build.lib.remote.Retrier;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * The {@link FailureCircuitBreaker} implementation of the {@link Retrier.CircuitBreaker} prevents further calls to
 * a remote cache once the failures rate within a given window exceeds a specified threshold for a build.
 * In the context of Bazel, a new instance of {@link Retrier.CircuitBreaker} is created for each build. Therefore, if
 * the circuit breaker trips during a build, the remote cache will be disabled for that build. However, it will be
 * enabled again for the next build as a new instance of {@link Retrier.CircuitBreaker} will be created.
 */
public class FailureCircuitBreaker implements Retrier.CircuitBreaker {

  private State state;
  private final AtomicInteger successes;
  private final AtomicInteger failures;
  private final int failureRateThreshold;
  private final int slidingWindowSize;
  private final int minCallCountToComputeFailureRate;
  private final ScheduledExecutorService scheduledExecutor;

  /**
   * Creates a {@link FailureCircuitBreaker}.
   *
   * @param failureRateThreshold  is used to set the min percentage of failure required to trip the circuit breaker in
   *                             given time window.
   * @param slidingWindowSize the size of the sliding window in milliseconds to calculate the number of failures.
   */
  public FailureCircuitBreaker(int failureRateThreshold, int slidingWindowSize) {
    this.failures = new AtomicInteger(0);
    this.successes = new AtomicInteger(0);
    this.failureRateThreshold = failureRateThreshold;
    this.slidingWindowSize = slidingWindowSize;
    this.minCallCountToComputeFailureRate = CircuitBreakerFactory.DEFAULT_MIN_CALL_COUNT_TO_COMPUTE_FAILURE_RATE;
    this.state = State.ACCEPT_CALLS;
    this.scheduledExecutor =
        slidingWindowSize > 0 ? Executors.newSingleThreadScheduledExecutor() : null;
  }

  @Override
  public State state() {
    return this.state;
  }

  @Override
  public void recordFailure() {
    int failureCount = failures.incrementAndGet();
    int totalCallCount = successes.get() + failureCount;
    if (slidingWindowSize > 0) {
      var unused =
          scheduledExecutor.schedule(
              failures::decrementAndGet, slidingWindowSize, TimeUnit.MILLISECONDS);
    }

    if (totalCallCount < minCallCountToComputeFailureRate) {
      // The remote call count is below the threshold required to calculate the failure rate.
      return;
    }
    double failureRate = (failureCount * 100.0) / totalCallCount;

    // Since the state can only be changed to the open state, synchronization is not required.
    if (failureRate > this.failureRateThreshold) {
      this.state = State.REJECT_CALLS;
    }
  }

  @Override
  public void recordSuccess() {
    successes.incrementAndGet();
    if (slidingWindowSize > 0) {
      scheduledExecutor.schedule(successes::decrementAndGet, slidingWindowSize, TimeUnit.MILLISECONDS);
    }
  }
}