// This software is released into the Public Domain.  See copying.txt for details.
package org.openstreetmap.osmosis.core.store;

import java.io.IOException;
import java.util.Iterator;
import java.util.logging.Level;
import java.util.logging.Logger;

import org.openstreetmap.osmosis.core.OsmosisRuntimeException;
import org.openstreetmap.osmosis.core.lifecycle.Closeable;


/**
 * Provides read-only access to a random access object store. Each thread
 * accessing the object store must create its own reader. The reader maintains
 * all references to heavyweight resources such as file handles used to access
 * the store eliminating the need for objects such as object iterators to be
 * cleaned up explicitly.
 * 
 * @param <T>
 *            The object type being stored.
 * @author Brett Henderson
 */
public class RandomAccessObjectStoreReader<T> implements Closeable {
	private static final Logger LOG = Logger.getLogger(RandomAccessObjectStoreReader.class.getName());
	
	private BufferedRandomAccessFileInputStream randomFile;
	private ObjectReader objectReader;
	
	
	/**
	 * Creates a new instance.
	 * 
	 * @param randomFile
	 *            A read-only random access file opened on the store file.
	 * @param objectReader
	 *            The reader containing the objects to be deserialized.
	 */
	public RandomAccessObjectStoreReader(BufferedRandomAccessFileInputStream randomFile, ObjectReader objectReader) {
		this.randomFile = randomFile;
		this.objectReader = objectReader;
	}
	
	
	/**
	 * Seeks to the specified location in the storage file.
	 * 
	 * @param offset
	 *            The file offset to seek to.
	 */
	private void seek(long offset) {
		try {
			randomFile.seek(offset);
		} catch (IOException e) {
			throw new OsmosisRuntimeException("Unable to seek to position " + offset + " in the storage file.", e);
		}
	}
	
	
	/**
	 * Reads the object at the specified file offset.
	 * 
	 * @param offset
	 *            The file offset to read an object from.
	 * @return The requested object.
	 */
	@SuppressWarnings("unchecked")
	public T get(long offset) {
		seek(offset);
		
		return (T) objectReader.readObject();
	}
	
	
	/**
	 * Returns the length of data.
	 * 
	 * @return The data length in bytes.
	 */
	public long length() {
		try {
			return randomFile.length();
		} catch (IOException e) {
			throw new OsmosisRuntimeException("Unable to obtain the length of the storage file.", e);
		}
	}
	
	
	/**
	 * Returns the current read position.
	 * 
	 * @return The current offset offset in bytes.
	 */
	public long position() {
		try {
			return randomFile.position();
		} catch (IOException e) {
			throw new OsmosisRuntimeException("Unable to obtain the current position in the storage file.", e);
		}
	}
	
	
	/**
	 * Iterates over the entire stream of data.
	 * 
	 * @param offset
	 *            The location in the storage file to begin reading.
	 * @return An iterator for reading objects from the data store. This
	 *         iterator must be released after use.
	 */
	public Iterator<T> iterate(long offset) {
		seek(offset);
		
		return new ObjectDataInputIterator<T>(objectReader);
	}
	
	
	/**
	 * Iterates over the entire stream of data.
	 * 
	 * @return An iterator for reading objects from the data store. This
	 *         iterator must be released after use.
	 */
	public Iterator<T> iterate() {
		return iterate(0);
	}
	
	
	/**
	 * {@inheritDoc}
	 */
	@Override
	public void close() {
		if (randomFile != null) {
			try {
				randomFile.close();
			} catch (IOException e) {
				// We cannot throw an exception within a release statement.
				LOG.log(Level.WARNING, "Unable to close random access file.", e);
			}
			randomFile = null;
		}
	}
}