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/*
 * Copyright (c) 2018 - Manifold Systems LLC
 *
 * 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 manifold.util.concurrent;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public abstract class LockingLazyVar
{
  private final static Object NULL = new Object();
  private volatile T _val = null;
  private final ReadWriteLock _rwLock;

  protected LockingLazyVar()
  {
    this( new ReentrantReadWriteLock() );
  }

  protected LockingLazyVar( ReadWriteLock lock )
  {
    _rwLock = lock;
  }

  /**
   * @return the value of this lazy var, created if necessary
   */
  public final T get()
  {
    T result;
    Lock rLock = _rwLock.readLock();
    rLock.lock();
    try
    {
      result = _val;
    }
    finally
    {
      rLock.unlock();
    }

    if( result == NULL )
    {
      return null;
    }

    if( result != null )
    {
      return result;
    }

    Lock wLock = _rwLock.writeLock();
    wLock.lock();
    try
    {
      result = _val;
      if( result == NULL )
      {
        return null;
      }
      if( result == null )
      {
        result = init();
        if( result == null )
        {
          _val = (T)NULL;
        }
        else
        {
          _val = result;
        }
      }
    }
    finally
    {
      wLock.unlock();
    }
    return result;
  }

  public T set( T value )
  {
    if( value == null )
    {
      value = (T)NULL;
    }
    T prev = get();
    Lock wLock = _rwLock.writeLock();
    wLock.lock();
    try
    {
      _val = value;
    }
    finally
    {
      wLock.unlock();
    }
    return prev;
  }

  protected abstract T init();

  /**
   * Clears the variable, forcing the next call to {@link #get()} to re-calculate
   * the value.
   */
  public final T clear()
  {
    T prev;
    Lock wLock = _rwLock.writeLock();
    wLock.lock();
    try
    {
      prev = _val;
      _val = null;
    }
    finally
    {
      wLock.unlock();
    }
    return prev;
  }

  public final void clearNoLock()
  {
    _val = null;
  }

  public boolean isLoaded()
  {
    Lock rLock = _rwLock.readLock();
    rLock.lock();
    try
    {
      return _val != null;
    }
    finally
    {
      rLock.unlock();
    }
  }

  /**
   * A simple init interface to make LockingLazyVar's easier to construct.
   */
  public interface LazyVarInit
  {
    Q init();
  }

  /**
   * Creates a new LockingLazyVar based on the type of the LazyVarInit passed in.
   */
  public static  LockingLazyVar make( final LazyVarInit init )
  {
    return new LockingLazyVar()
    {
      protected Q init()
      {
        return init.init();
      }
    };
  }

  public static  LockingLazyVar make( ReadWriteLock lock, final LazyVarInit init )
  {
    return new LockingLazyVar( lock )
    {
      protected Q init()
      {
        return init.init();
      }
    };
  }
}