/*
 *  RapidMiner
 *
 *  Copyright (C) 2001-2008 by Rapid-I and the contributors
 *
 *  Complete list of developers available at our web site:
 *
 *       http://rapid-i.com
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU Affero General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  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 Affero General Public License for more details.
 *
 *  You should have received a copy of the GNU Affero General Public License
 *  along with this program.  If not, see http://www.gnu.org/licenses/.
 */
package com.rapidminer.example.table;

import java.sql.ResultSet;
import java.sql.SQLException;

import com.rapidminer.example.Attribute;
import com.rapidminer.tools.LogService;


/**
 * Unlike a {@link FileDataRowReader} that reads examples from a file, objects
 * of this class read examples from a {@link ResultSet}, a data structure that
 * is returned from a database query.
 * 
 * @see com.rapidminer.tools.jdbc.DatabaseHandler
 * @see com.rapidminer.operator.io.DatabaseExampleSource
 * @see com.rapidminer.operator.io.KDBExampleSource
 * @author Simon Fischer, Ingo Mierswa
 * @version $Id: DatabaseDataRowReader.java,v 2.15 2006/03/21 15:35:39
 *          ingomierswa Exp $
 */
public class DatabaseDataRowReader implements DataRowReader {

	private ResultSet resultSet;
	
	private static final int DONT_KNOW_YET = 0;

	private static final int YES = 1;

	private static final int NO = 2;

	private int hasNext = DONT_KNOW_YET;

	/**
	 * Creates a datarow reader from a ResultSet. The column meta data must be
	 * tranformed to an {@link Attribute} array.
	 */
	public DatabaseDataRowReader(ResultSet resultSet) throws SQLException {
		this.resultSet = resultSet;
		this.resultSet.beforeFirst();
	}

	public boolean hasNext() {
		switch (hasNext) {
			case YES:
				return true;
			case NO:
				return false;
			case DONT_KNOW_YET:
				try {
					if (resultSet.next()) {
						hasNext = YES;
						return true;
					} else {
						hasNext = NO;
						return false;
					}
				} catch (SQLException e) {
					LogService.getGlobal().logError("While reading examples from result set: " + e.getMessage());
					return false;
				}
			default:
				// impossible
				return false;
		}
	}

	public DataRow next() {
		if (hasNext()) {
			hasNext = DONT_KNOW_YET;
			try {
				return new DatabaseDataRow(resultSet);
			} catch (SQLException sqle) {
				throw new RuntimeException("Error accessing the result of a query:" + sqle.toString(), sqle);
			}
		} else {
			return null;
		}
	}
	
	/** Will throw a new {@link UnsupportedOperationException} since {@link DataRowReader} does not have
	 *  to implement remove. */
	public void remove() {
		throw new UnsupportedOperationException("The method 'remove' is not supported by DataRowReaders on databases!");
	}
}