/**
 * Copyright (C) 2001-2017 by RapidMiner and the contributors
 * 
 * Complete list of developers available at our web site:
 * 
 * http://rapidminer.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 com.rapidminer.example.Attribute;
import com.rapidminer.tools.LogService;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.logging.Level;


/**
 * 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.DatabaseDataReader
 * @see com.rapidminer.operator.io.KDBExampleSource
 * @author Simon Fischer, Ingo Mierswa 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();
	}

	@Override
	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());
					LogService
							.getRoot()
							.log(Level.SEVERE,
									"com.rapidminer.example.table.DatabaseDataRowReader.error_while_reading_examples_from_result_set",
									e.getMessage());
					return false;
				}
			default:
				// impossible
				return false;
		}
	}

	@Override
	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.
	 */
	@Override
	public void remove() {
		throw new UnsupportedOperationException("The method 'remove' is not supported by DataRowReaders on databases!");
	}
}