package org.bukkit.metadata;

import org.apache.commons.lang.Validate;
import org.bukkit.plugin.Plugin;

import java.lang.ref.SoftReference;
import java.util.concurrent.Callable;

/**
 * The LazyMetadataValue class implements a type of metadata that is not
 * computed until another plugin asks for it.
 * <p>
 * By making metadata values lazy, no computation is done by the providing
 * plugin until absolutely necessary (if ever). Additionally,
 * LazyMetadataValue objects cache their values internally unless overridden
 * by a {@link CacheStrategy} or invalidated at the individual or plugin
 * level. Once invalidated, the LazyMetadataValue will recompute its value
 * when asked.
 */
public class LazyMetadataValue extends MetadataValueAdapter implements MetadataValue
{
	private Callable<Object> lazyValue;
	private CacheStrategy cacheStrategy;
	private SoftReference<Object> internalValue;
	private static final Object ACTUALLY_NULL = new Object();

	/**
	 * Initialized a LazyMetadataValue object with the default
	 * CACHE_AFTER_FIRST_EVAL cache strategy.
	 *
	 * @param owningPlugin the {@link Plugin} that created this metadata
	 *                     value.
	 * @param lazyValue    the lazy value assigned to this metadata value.
	 */
	public LazyMetadataValue(Plugin owningPlugin, Callable<Object> lazyValue)
	{
		this(owningPlugin, CacheStrategy.CACHE_AFTER_FIRST_EVAL, lazyValue);
	}

	/**
	 * Initializes a LazyMetadataValue object with a specific cache strategy.
	 *
	 * @param owningPlugin  the {@link Plugin} that created this metadata
	 *                      value.
	 * @param cacheStrategy determines the rules for caching this metadata
	 *                      value.
	 * @param lazyValue     the lazy value assigned to this metadata value.
	 */
	public LazyMetadataValue(Plugin owningPlugin, CacheStrategy cacheStrategy, Callable<Object> lazyValue)
	{
		super(owningPlugin);
		Validate.notNull(cacheStrategy, "cacheStrategy cannot be null");
		Validate.notNull(lazyValue, "lazyValue cannot be null");
		this.internalValue = new SoftReference<Object>(null);
		this.lazyValue = lazyValue;
		this.cacheStrategy = cacheStrategy;
	}

	/**
	 * Protected special constructor used by FixedMetadataValue to bypass
	 * standard setup.
	 */
	protected LazyMetadataValue(Plugin owningPlugin)
	{
		super(owningPlugin);
	}

	public Object value()
	{
		eval();
		Object value = internalValue.get();
		if(value == ACTUALLY_NULL)
		{
			return null;
		}
		return value;
	}

	/**
	 * Lazily evaluates the value of this metadata item.
	 *
	 * @throws MetadataEvaluationException if computing the metadata value
	 *                                     fails.
	 */
	private synchronized void eval() throws MetadataEvaluationException
	{
		if(cacheStrategy == CacheStrategy.NEVER_CACHE || internalValue.get() == null)
		{
			try
			{
				Object value = lazyValue.call();
				if(value == null)
				{
					value = ACTUALLY_NULL;
				}
				internalValue = new SoftReference<Object>(value);
			} catch(Exception e)
			{
				throw new MetadataEvaluationException(e);
			}
		}
	}

	public synchronized void invalidate()
	{
		if(cacheStrategy != CacheStrategy.CACHE_ETERNALLY)
		{
			internalValue.clear();
		}
	}

	/**
	 * Describes possible caching strategies for metadata.
	 */
	public enum CacheStrategy
	{
		/**
		 * Once the metadata value has been evaluated, do not re-evaluate the
		 * value until it is manually invalidated.
		 */
		CACHE_AFTER_FIRST_EVAL,

		/**
		 * Re-evaluate the metadata item every time it is requested
		 */
		NEVER_CACHE,

		/**
		 * Once the metadata value has been evaluated, do not re-evaluate the
		 * value in spite of manual invalidation.
		 */
		CACHE_ETERNALLY
	}
}