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The core O/RM functionality as provided by Hibernate

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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * Copyright (c) 2010, 2013, Red Hat Inc. or third-party contributors as
 * indicated by the @author tags or express copyright attribution
 * statements applied by the authors.  All third-party contributions are
 * distributed under license by Red Hat Inc.
 *
 * This copyrighted material is made available to anyone wishing to use, modify,
 * copy, or redistribute it subject to the terms and conditions of the GNU
 * Lesser General Public License, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 * for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this distribution; if not, write to:
 * Free Software Foundation, Inc.
 * 51 Franklin Street, Fifth Floor
 * Boston, MA  02110-1301  USA
 */
package org.hibernate.criterion;

import java.util.List;

import org.hibernate.Criteria;
import org.hibernate.HibernateException;
import org.hibernate.dialect.function.SQLFunction;
import org.hibernate.dialect.function.SQLFunctionRegistry;
import org.hibernate.type.Type;

/**
 * A row count
 *
 * @author Gavin King
 */
public class RowCountProjection extends SimpleProjection {
	private static final List ARGS = java.util.Collections.singletonList( "*" );

	@Override
	public Type[] getTypes(Criteria criteria, CriteriaQuery criteriaQuery) throws HibernateException {
		final Type countFunctionReturnType = getFunction( criteriaQuery ).getReturnType( null, criteriaQuery.getFactory() );
		return new Type[] { countFunctionReturnType };
	}

	@Override
	public String toSqlString(Criteria criteria, int position, CriteriaQuery criteriaQuery) throws HibernateException {
		return getFunction( criteriaQuery ).render( null, ARGS, criteriaQuery.getFactory() ) + " as y" + position + '_';
	}

	protected SQLFunction getFunction(CriteriaQuery criteriaQuery) {
		final SQLFunctionRegistry sqlFunctionRegistry = criteriaQuery.getFactory().getSqlFunctionRegistry();
		final SQLFunction function = sqlFunctionRegistry.findSQLFunction( "count" );
		if ( function == null ) {
			throw new HibernateException( "Unable to locate count function mapping" );
		}
		return function;
	}

	@Override
	public String toString() {
		return "count(*)";
	}
}