/*
 * SSSync, a Simple and Stupid Synchronizer for data with multi-valued attributes
 * Copyright (C) 2014  Ludovic Pouzenc <ludovic@pouzenc.fr>
 *  
 * This file is part of SSSync.
 *
 *  SSSync is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  SSSync 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 General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with SSSync.  If not, see <http://www.gnu.org/licenses/>
 */

package data.io.sql;

import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Iterator;

import data.MVDataEntry;
import data.io.AbstractMVDataReader;

/**
 * Stream-oriented reader from a particular RDBMS source.
 * 
 * @author lpouzenc
 */
public class SQLRelDataReader extends AbstractMVDataReader {

	private final Connection conn;
	private final String request;
	
	private transient String columnNames[];
	private transient ResultSet rs;
	private transient boolean didNext;
	private transient boolean hasNext;

	/**
	 * Build a new reader from an existing connection and a File containing a SELECT statement.
	 * @param dataSourceName A short string representing this reader (for logging) 
	 * @param conn A pre-established SQL data connection
	 * @param queryFile An SQL file containing an SQL SELECT statement
	 * @throws IOException 
	 */
	public SQLRelDataReader(String dataSourceName, Connection conn, File queryFile) throws IOException  {
		this.dataSourceName = dataSourceName;
		this.conn = conn;
		this.request = readEntireFile(queryFile);
	}
	
	/**
	 * Build a new reader from an existing connection and a String containing a SELECT statement.
	 * @param dataSourceName A short string representing this reader (for logging)
	 * @param conn A pre-established SQL data connection
	 * @param query A String containing an SQL SELECT statement
	 * @throws IOException 
	 */
	public SQLRelDataReader(String dataSourceName, Connection conn, String query) {
		this.dataSourceName = dataSourceName;
		this.conn = conn;
		this.request = query;
	}

	/**
	 * {@inheritDoc}
	 * Note : multiple iterators on the same instance are not supported
	 */
	@Override
	public Iterator<MVDataEntry> iterator() {
		try {
			// Reset iterator-related attributes
			hasNext = false;
			didNext = false;
			
			// Close and free any previous request result
			if ( rs != null ) {
				rs.close();
			}
			// (Re-)Execute the SQL request
			Statement stmt = conn.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
			rs = stmt.executeQuery(request);
			
			// Get the column names
			ResultSetMetaData rsmd = rs.getMetaData();
			columnNames = new String[rsmd.getColumnCount()];
			for (int i = 0; i < columnNames.length ; i++) {
				// Java SQL : all indices starts at 1 (it sucks !)
				columnNames[i] = rsmd.getColumnName(i+1); 
			}
		} catch (SQLException e) {
			throw new RuntimeException("Could not execute query : " + e.getMessage() + "\n" + request );
		}
		
		return this;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public boolean hasNext() {
		// java.sql.ResultSet don't implement Iterable interface at all
		// It's next() don't return anything except hasNext() result but it moves the cursor !  
        if (!didNext) {
            try {
				hasNext = rs.next();
			} catch (SQLException e) {
				throw new RuntimeException(e);
			}
            didNext = true;
        }
        return hasNext;
	}

	/**
	 * {@inheritDoc}
	 */
	@Override
	public MVDataEntry next() {
		MVDataEntry result = null;
		try {
	        if (!didNext) {
	            rs.next();
	        }
	        didNext = false;
	        //TODO Instead of always use the first col, user could choose a specific columnName like in LDAP
			String key = rs.getString(1); 
			result = new MVDataEntry(key);
			for (int i = 0; i < columnNames.length ; i++) {
				// Java SQL : all indices starts at 1 (it sucks !)
				result.splitAndPut(columnNames[i], rs.getString(i+1), ";"); // TODO regex should be an option
			}
			
		} catch (SQLException e) {
			throw new RuntimeException("Exception while reading next line in SQL resultset", e);
		}		
		
		return result;
	}
	
	/**
	 * Helper function to load and entire file as a String.
	 * @param file
	 * @return
	 * @throws IOException
	 */
    private static String readEntireFile(File file) throws IOException {
        FileReader input = new FileReader(file);
        StringBuilder contents = new StringBuilder();
        char[] buffer = new char[4096];
        int read = 0;
        do {
            contents.append(buffer, 0, read);
            read = input.read(buffer);
        } while (read >= 0);
        input.close();
        
        return contents.toString();
    }
}