// Copyright 2016 Twitter. 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.twitter.heron.common.basics;

import java.io.IOException;
import java.nio.channels.ClosedChannelException;
import java.nio.channels.SelectableChannel;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.time.Duration;
import java.util.Iterator;
import java.util.Set;

/**
 * A NIOLooper, implementing WakeableLooper, is a class wrapping a Java NIO selector to dispatch events.
 * It extends WakeableLooper, so it will execute in a while loop unless the exitLoop() is called.
 * And in every execution, in tasksOnWakeup(), it will handle the selected keys.
 * The NIOLooper should start by calling {@code loop()}
 */

public class NIOLooper extends WakeableLooper {
  private final Selector selector;

  public NIOLooper() throws IOException {
    selector = Selector.open();

    addNIOLooperTasks();
  }

  private void addNIOLooperTasks() {
    Runnable task = new Runnable() {
      @Override
      public void run() {
        handleSelectedKeys();
      }
    };
    addTasksOnWakeup(task);
  }

  @Override
  public void doWait() {
    // If timer task exists, the doWait() should wait not later than the time timer to execute
    // It no timer exists, we consider it will wait forever until other threads call wakeUp()
    Duration nextTimeoutInterval = getNextTimeoutInterval();

    // doWait(timeout), which in fact is implemented by selector.select(timeout), and it will
    // wake up, if other threads wake it up, it meets the timeout, one channel is selected, or
    // the current thread is interrupted.
    try {
      if (nextTimeoutInterval.toMillis() > 0) {
        // The select will take the timeout in unit of milli-seconds
        selector.select(nextTimeoutInterval.toMillis());
      } else {
        selector.selectNow();
      }
    } catch (IOException e) {
      e.printStackTrace();
      throw new RuntimeException(e);
    }
  }

  @Override
  public void wakeUp() {
    selector.wakeup();
  }

  public void removeAllInterest(SelectableChannel channel) {
    SelectionKey key = channel.keyFor(selector);
    if (key != null) {
      key.cancel();
    }
  }

  // Handle the selected keys
  private void handleSelectedKeys() {
    Set<SelectionKey> selectedKeys = selector.selectedKeys();
    Iterator<SelectionKey> keyIterator = selectedKeys.iterator();
    while (keyIterator.hasNext()) {
      SelectionKey key = keyIterator.next();
      keyIterator.remove();

      ISelectHandler callback = (ISelectHandler) key.attachment();

      if (!key.isValid()) {
        // This method key.channel() will continue to return the channel even after the
        // key is cancelled.
        callback.handleError(key.channel());
        continue;
      }

      // We need to check whether the key is still valid since:
      // 1. The key could be cancelled by last operation
      // 2. The process might not fail-fast or throw exceptions after the key is cancelled
      if (key.isValid() && key.isWritable()) {
        callback.handleWrite(key.channel());
      }

      if (key.isValid() && key.isReadable()) {
        callback.handleRead(key.channel());
      }

      if (key.isValid() && key.isConnectable()) {
        callback.handleConnect(key.channel());
      }

      if (key.isValid() && key.isAcceptable()) {
        callback.handleAccept(key.channel());
      }

    }
  }

  public boolean isChannelValid(SelectableChannel channel) {
    SelectionKey key = channel.keyFor(selector);
    return key != null && key.isValid();
  }

  /**
   * Followings are the register, unregister, isRegister for different operations for the selector and channel
   */
  public void registerRead(SelectableChannel channel, ISelectHandler callback)
      throws ClosedChannelException {
    assert channel.keyFor(selector) == null
        || (channel.keyFor(selector).interestOps() & SelectionKey.OP_CONNECT) == 0;
    addInterest(channel, SelectionKey.OP_READ, callback);
  }

  public void unregisterRead(SelectableChannel channel) {
    removeInterest(channel, SelectionKey.OP_READ);
  }

  public boolean isReadRegistered(SelectableChannel channel) {
    return isInterestRegistered(channel, SelectionKey.OP_READ);
  }

  public void registerConnect(SelectableChannel channel, ISelectHandler callback)
      throws ClosedChannelException {
    // This channel should be first use
    assert channel.keyFor(selector) == null;
    addInterest(channel, SelectionKey.OP_CONNECT, callback);
  }

  public void unregisterConnect(SelectableChannel channel) {
    removeInterest(channel, SelectionKey.OP_CONNECT);
  }

  public boolean isConnectRegistered(SelectableChannel channel) {
    return isInterestRegistered(channel, SelectionKey.OP_CONNECT);
  }

  public void registerAccept(SelectableChannel channel, ISelectHandler callback)
      throws ClosedChannelException {
    addInterest(channel, SelectionKey.OP_ACCEPT, callback);
  }

  public void unregisterAccept(SelectableChannel channel) {
    removeInterest(channel, SelectionKey.OP_ACCEPT);
  }

  public boolean isAcceptRegistered(SelectableChannel channel) {
    return isInterestRegistered(channel, SelectionKey.OP_ACCEPT);
  }

  public void registerWrite(SelectableChannel channel, ISelectHandler callback)
      throws ClosedChannelException {
    addInterest(channel, SelectionKey.OP_WRITE, callback);
  }

  public void unregisterWrite(SelectableChannel channel) {
    removeInterest(channel, SelectionKey.OP_WRITE);
  }

  public boolean isWriteRegistered(SelectableChannel channel) {
    return isInterestRegistered(channel, SelectionKey.OP_WRITE);
  }

  /**
   * Register an operation interest on a SelectableChannel, with ISelectHandler as callback attachment
   * There are two cases when trying to register an interest
   * 1. The whole key does not exist; no interests ever registered for this channel
   * 2. The key exists due to other interests registered but not the one we are adding
   * <p>
   * In 1st case, we just register this channel with operation on the given Selector
   * In 2nd case, we have to make sure the state of NIOLooper is clean:
   * 1. Key has to be valid
   * 2. The interest has not yet been registered
   * 3. If old attached ISelectHandler exists, it has to be the same as new one
   * If any one of above 3 conditions are not met, RuntimeException would be thrown.
   *
   * @param channel The Selectable to register operation interest
   * @param operation The operation interest to register
   * @param callback The Callback to handle
   * @throws ClosedChannelException if Channel is closed when trying to register an interest
   */
  private void addInterest(SelectableChannel channel,
                           int operation,
                           ISelectHandler callback)
      throws ClosedChannelException {

    SelectionKey key = channel.keyFor(selector);

    if (key == null) {
      channel.register(selector, operation, callback);
    } else if (!key.isValid()) {
      throw new RuntimeException(
          String.format("Unable to add %d in %s due to key is invalid", operation, channel));
    } else {
      // Key is not null and key is valid
      if ((key.interestOps() & operation) != 0) {
        throw new RuntimeException(
            String.format("%d has been registered in %s", operation, channel));
      }
      if (key.attachment() == null) {
        key.attach(callback);
      } else {
        if (callback != key.attachment()) {
          throw new RuntimeException("Unmatched SelectHandler has already been attached"
              + " for other operation");
        }
        // If call == key.attachment
        // Just skip
      }
      key.interestOps(key.interestOps() | operation);
    }
  }

  /**
   * Remove one operation interest on a SelectableChannel.
   * The SelectableChannel has to be registered with Selector ahead.
   * Otherwise, NullPointerExceptions would throw
   * The key for SelectableChannel has to be valid.
   * Otherwise, InvalidValid Exception would throw.
   *
   * @param channel the SelectableChannel to remove operation interest
   * @param operation the interest to remove
   */
  private void removeInterest(SelectableChannel channel, int operation) {
    SelectionKey key = channel.keyFor(selector);

    // Exception would be thrown if key is null or key is inValid
    // We do not need double check it ahead
    key.interestOps(key.interestOps() & (~operation));
  }

  /**
   * Check whether an operation interest was registered on a SelectableChannel
   * There are two cases that interest is not registered
   * 1. The whole key does not exist; no interests ever registered for this channel
   * 2. The key exists due to other interests registered but not the one we are adding
   * If the key exists, the key for SelectableChannel has to be valid.
   * Otherwise, InvalidValid Exception would throw.
   *
   * @param channel The Selectable to check
   * @param operation The operation interest to check
   */
  private boolean isInterestRegistered(SelectableChannel channel, int operation) {
    SelectionKey key = channel.keyFor(selector);

    return key != null && (key.interestOps() & operation) != 0;
  }
}