/*
 * Copyright (c) 2010 Mysema Ltd.
 * All rights reserved.
 * 
 */
package com.mysema.rdfbean.model;

import java.io.Closeable;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * FileIdSequence provides a java.io.File based cacheing implementation of the
 * IdSequence interface
 * 
 * @author tiwe
 * @version $Id$
 */
public class FileIdSequence implements Closeable, IdSequence {

    private static final Logger logger = LoggerFactory.getLogger(FileIdSequence.class);

    private final ByteBuffer buffer = ByteBuffer.allocate(8);

    private final File file;

    private final FileChannel fileChannel;

    private final int cache;

    private volatile long nextId = 1l;

    private volatile long maxId = 100l;

    /**
     * Use the give File for persistence with a cache of 100 increments
     * 
     * @param file
     */
    public FileIdSequence(File file) {
        this(file, 100);
    }

    /**
     * Use the given File for persistence with a cache of 100 increments
     * 
     * @param file
     * @param cache
     */
    public FileIdSequence(File file, int cache) {
        try {
            if (!file.exists()) {
                if (!file.getParentFile().exists()) {
                    if (!file.getParentFile().mkdirs()) {
                        logger.error("Creation of " + file.getParentFile().getPath() + " failed");
                    }
                }
                if (!file.createNewFile()) {
                    logger.error("Creation of " + file.getPath() + " failed");
                }
            }
            this.file = file;
            this.fileChannel = new RandomAccessFile(file, "rwd").getChannel();
            this.cache = cache;
            synchronize();
        } catch (FileNotFoundException e) {
            throw new RepositoryException(e);
        } catch (IOException e) {
            throw new RepositoryException(e);
        }
    }

    private void synchronize() throws IOException {
        // get the next id from file
        if (file.length() > 0l) {
            fileChannel.read(buffer, 0l);
            buffer.rewind();
            nextId = buffer.getLong();
        } else {
            nextId = 1l;
        }
        maxId = nextId + cache - 1l;

        // set the next id to file
        buffer.rewind();
        buffer.putLong(0, nextId + cache);
        fileChannel.write(buffer, 0l);
        buffer.rewind();

    }

    @Override
    public synchronized long getNextId() {
        try {
            if (nextId > maxId) {
                synchronize();
            }
            return nextId++;
        } catch (IOException e) {
            throw new RepositoryException(e);
        }
    }

    public void close() throws IOException {
        fileChannel.close();
    }

}