// concurrency/ReaderWriterList.java
// ©2015 MindView LLC: see Copyright.txt
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import java.util.*;
import static com.mindviewinc.util.Print.*;

public class ReaderWriterList<T> {
  private ArrayList<T> lockedList;
  // Make the ordering fair:
  private ReentrantReadWriteLock lock =
    new ReentrantReadWriteLock(true);
  public ReaderWriterList(int size, T initialValue) {
    lockedList = new ArrayList<>(
      Collections.nCopies(size, initialValue));
  }
  public T set(int index, T element) {
    Lock wlock = lock.writeLock();
    wlock.lock();
    try {
      return lockedList.set(index, element);
    } finally {
      wlock.unlock();
    }
  }
  public T get(int index) {
    Lock rlock = lock.readLock();
    rlock.lock();
    try {
      // Show that multiple readers
      // can acquire the read lock:
      if(lock.getReadLockCount() > 1)
        print(lock.getReadLockCount());
      return lockedList.get(index);
    } finally {
      rlock.unlock();
    }
  }
  public static void main(String[] args) throws Exception {
    new ReaderWriterListTest(30, 1);
  }
}

class ReaderWriterListTest {
  ExecutorService exec = Executors.newCachedThreadPool();
  private final static int SIZE = 100;
  private static Random rand = new Random(47);
  private ReaderWriterList<Integer> list =
    new ReaderWriterList<>(SIZE, 0);
  private class Writer implements Runnable {
    @Override
    public void run() {
      try {
        for(int i = 0; i < 20; i++) { // 2 second test
          list.set(i, rand.nextInt());
          TimeUnit.MILLISECONDS.sleep(100);
        }
      } catch(InterruptedException e) {
        // Acceptable way to exit
      }
      print("Writer finished, shutting down");
      exec.shutdownNow();
    }
  }
  private class Reader implements Runnable {
    @Override
    public void run() {
      try {
        while(!Thread.interrupted()) {
          for(int i = 0; i < SIZE; i++) {
            list.get(i);
            TimeUnit.MILLISECONDS.sleep(1);
          }
        }
      } catch(InterruptedException e) {
        // Acceptable way to exit
      }
    }
  }
  public ReaderWriterListTest(int readers, int writers) {
    for(int i = 0; i < readers; i++)
      exec.execute(new Reader());
    for(int i = 0; i < writers; i++)
      exec.execute(new Writer());
  }
}
/* Output: (First and last 10 Lines)
6
15
14
13
12
11
10
9
8
5
________...________...________...________...________
21
30
29
28
28
30
29
28
27
26
*/