Buffer.java

import java.io.IOException;
import java.io.RandomAccessFile;

// -------------------------------------------------------------------------
/**
 * Write a one-sentence summary of your class here. Follow it with additional
 * details about its purpose, what abstraction it represents, and how to use it.
 *
 * @author wenfeng ren
 * @version Nov 3, 2014
 */
public class Buffer
{
    private int              block;
    private byte[]           data;
    private RandomAccessFile disk;
    private int              position;
    private boolean          dirty;
    private final static int BLOCK_SIZE = 4096;


    // ----------------------------------------------------------
    /**
     * Create a new Buffer object.
     *
     * @param disk the disk file
     * @param block the index of the block
     */
    public Buffer(RandomAccessFile disk, int block)
    {
        data = new byte[BLOCK_SIZE];
        this.disk = disk;
        this.block = block;
        position = block * BLOCK_SIZE;
        dirty = false;
    }


    // ----------------------------------------------------------
    /**
     * get data from buffer.
     *
     * @return data
     */
    public byte[] read()
    {
        Stat.cacheHits++;
        return data;
    }


    // ----------------------------------------------------------
    /**
     * set data.
     *
     * @param newData to be set
     */
    public void write(byte[] newData)
    {
        data = newData;
        dirty = true;
    }


    // ----------------------------------------------------------
    /**
     * write data back to disk.
     *
     * @throws IOException
     */
    public void writeBack()
        throws IOException
    {
        if (dirty)
        {
            disk.seek(position);
            disk.write(data);
            dirty = false;
            // update stat info.
            Stat.diskWrites++;
        }
    }


    // ----------------------------------------------------------
    /**
     * read from disk.
     *
     * @throws IOException
     */
    public void diskRead()
        throws IOException
    {

        data = new byte[BLOCK_SIZE];
        disk.seek(position);
        disk.read(data);
        // update stat info.
        Stat.diskReads++;
    }


    // ----------------------------------------------------------
    /**
     * block index.
     *
     * @return the number of block
     */
    public int block()
    {
        return block;
    }
}