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/*
 * Copyright (C) the original author or authors.
 *
 * 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 net.derquinse.common.orm.hib;

import java.lang.reflect.ParameterizedType;

import net.derquinse.common.orm.Sequence;
import net.derquinse.common.orm.SequenceDAO;
import net.derquinse.common.orm.SequenceNotFoundException;

import org.hibernate.LockOptions;
import org.hibernate.Session;
import org.hibernate.SessionFactory;

import com.google.common.base.Preconditions;

/**
 * Abstract DAO a sequence entity. WARNING: Transactional sequences are a source of locks and may
 * hurt scalability.
 * @author Andres Rodriguez
 * @param  Sequence type.
 */
public abstract class AbstractHibernateSequenceDAO implements SequenceDAO {
	/** Persistent class. */
	private Class persistentClass;
	/** Hibernate Session factory. */
	private final SessionFactory sessionFactory;

	/**
	 * Initialized the DAO
	 * @param sessionFactory Hibernate Session factory.
	 */
	public AbstractHibernateSequenceDAO(final SessionFactory sessionFactory) {
		this.sessionFactory = Preconditions.checkNotNull(sessionFactory, "The session factory is mandatory.");
		@SuppressWarnings("unchecked")
		final Class type = (Class) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];
		this.persistentClass = type;
	}

	/**
	 * Creates a new sequence.
	 * @param id Sequence name.
	 * @param initial Initial value.
	 * @param increment Increment between values.
	 * @return The newly created sequence.
	 */
	protected abstract T create(String id, long initial, long increment);

	private static String nonNull(String id) {
		return Preconditions.checkNotNull(id, "The sequence name (id) is mandatory.");
	}

	/**
	 * Gets the current value of a sequence.
	 * @param id Sequence name.
	 * @return The current value of the specified sequence.
	 * @throws SequenceNotFoundException if the sequence is not found.
	 */
	public long getCurrentValue(final String id) throws SequenceNotFoundException {
		nonNull(id);
		final Session session = sessionFactory.getCurrentSession();
		@SuppressWarnings("unchecked")
		final T sequence = (T) session.get(persistentClass, id, LockOptions.READ);
		if (sequence == null) {
			throw new SequenceNotFoundException(id);
		}
		return sequence.getCurrent();
	}

	/**
	 * Gets the next value of a sequence. The row is locked in the current transaction.
	 * @param id Sequence name.
	 * @return The next value of the specified sequence.
	 * @throws SequenceNotFoundException if the sequence is not found.
	 */
	public long getNextValue(final String id) throws SequenceNotFoundException {
		nonNull(id);
		final Session session = sessionFactory.getCurrentSession();
		@SuppressWarnings("unchecked")
		final T sequence = (T) session.get(persistentClass, id, LockOptions.UPGRADE);
		if (sequence == null) {
			throw new SequenceNotFoundException(id);
		}
		final long next = sequence.getNext();
		session.update(sequence);
		return next;
	}

	/**
	 * Gets the next value of a sequence, creating it if it does not exist.
	 * @param id Sequence name.
	 * @param initial Initial value.
	 * @param increment Increment between values.
	 * @return The next value of the specified sequence.
	 */
	public long getNextValue(final String id, final long initial, final long increment) {
		nonNull(id);
		final Session session = sessionFactory.getCurrentSession();
		@SuppressWarnings("unchecked")
		T sequence = (T) session.get(persistentClass, id, LockOptions.UPGRADE);
		if (sequence != null) {
			return sequence.getNext();
		}
		// Create a new sequence and save it.
		sequence = create(id, initial, increment);
		session.save(sequence);
		// TODO: handle duplicates.
		return initial;
	}
}