code: /*
   * Copyright 2015 Realm Inc.
   *
   * 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 edu.utexas.cs.utopia.cortado.githubbenchmarks.io.realm.internal.async;
  
  import java.io.File;
  import java.io.FileFilter;
  import java.util.concurrent.ArrayBlockingQueue;
  import java.util.concurrent.Future;
  import java.util.concurrent.ThreadPoolExecutor;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.ReentrantLock;
  import java.util.regex.Pattern;
  
  import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
  
  
  /**
   * Custom thread pool settings, instances of this executor can be paused, and resumed, this will also set
   * appropriate number of Threads & wrap submitted tasks to set the thread priority according to
   * <a href="https://developer.android.com/training/multiple-threads/define-runnable.html"> Androids recommendation</a>.
   */
  public class RealmThreadPoolExecutor extends ThreadPoolExecutor implements RealmThreadPoolExecutorInterface {
      private static final String SYS_CPU_DIR = "/sys/devices/system/cpu/";
  
      // Reduces context switching by using a number of thread proportionate to the number of cores.
      private static final int CORE_POOL_SIZE = calculateCorePoolSize();
      private static final int QUEUE_SIZE = 100;
  
      private boolean isPaused;
      private ReentrantLock pauseLock = new ReentrantLock();
      private Condition unpaused = pauseLock.newCondition();
  
      /**
       * Creates a default RealmThreadPool that is bounded by the number of available cores.
       */
      public static RealmThreadPoolExecutor newDefaultExecutor() {
          return new RealmThreadPoolExecutor(CORE_POOL_SIZE, CORE_POOL_SIZE);
      }
  
      /**
       * Creates a RealmThreadPool with only 1 thread. This is primarily useful for testing.
       */
      public static RealmThreadPoolExecutor newSingleThreadExecutor() {
          return new RealmThreadPoolExecutor(1, 1);
      }
  
      public RealmThreadPoolExecutor() {
          super(1, 1,
                  0L, TimeUnit.MILLISECONDS, //terminated idle thread
                  new ArrayBlockingQueue<Runnable>(QUEUE_SIZE));
      }
  
      /**
       * Tries using the number of files named 'cpuNN' in sysfs to figure out the number of
       * processors on this device. `Runtime.getRuntime().availableProcessors()` may return
       * a smaller number when the device is sleeping.
       *
       * @return the number of threads to be allocated for the executor pool
       */
      @SuppressFBWarnings("DMI_HARDCODED_ABSOLUTE_FILENAME")
      private static int calculateCorePoolSize() {
          int cpus = countFilesInDir(SYS_CPU_DIR, "cpu[0-9]+");
          if (cpus <= 0) {
              cpus = Runtime.getRuntime().availableProcessors();
          }
          return (cpus <= 0) ? 1 : (cpus * 2) + 1;
      }
  
      /**
       * @param dirPath a directory path
       * @param pattern a regex
       * @return the number of files, in the `dirPath` directory, whose names match `pattern`
       */
      private static int countFilesInDir(String dirPath, String pattern) {
          final Pattern filePattern = Pattern.compile(pattern);
          try {
              File[] files = new File(dirPath).listFiles(new FileFilter() {
                  @Override
                  public boolean accept(File file) {
                      return filePattern.matcher(file.getName()).matches();
                  }
              });
              return (files == null) ? 0 : files.length;
          } catch (SecurityException ignore) {
          }
          return 0;
      }
  
      private RealmThreadPoolExecutor(int corePoolSize, int maxPoolSize) {
          super(corePoolSize, maxPoolSize,
                  0L, TimeUnit.MILLISECONDS, //terminated idle thread
                  new ArrayBlockingQueue<Runnable>(QUEUE_SIZE));
      }
  
      /**
       * Method invoked prior to executing the given Runnable to pause execution of the thread.
       *
       * @param t the thread that will run task r
       * @param r the task that will be executed
       */
      @Override
      public void beforeExecute(Thread t, Runnable r) {
          super.beforeExecute(t, r);
          pauseLock.lock();
          try {
              while (isPaused) { unpaused.await(); }
          } catch (InterruptedException ie) {
              t.interrupt();
          } finally {
              pauseLock.unlock();
          }
      }
  
      /**
       * Pauses the executor. Pausing means the executor will stop starting new tasks (but complete current ones).
       */
      public void pause() {
          pauseLock.lock();
          try {
              isPaused = true;
          } finally {
              pauseLock.unlock();
          }
      }
  
      /**
       * Resumes executing any scheduled tasks.
       */
      public void resume() {
          pauseLock.lock();
          try {
              isPaused = false;
              unpaused.signalAll();
          } finally {
              pauseLock.unlock();
          }
      }
  }
  