repositionableinputstream.java

爬虫

/* RepositionableInputStream.java
 *
 * $Id: RepositionableInputStream.java,v 1.3 2006/01/19 02:05:33 stack-sf Exp $
 *
 * Created Dec 20, 2005
 *
 * Copyright (C) 2005 Internet Archive.
 *
 * This file is part of the Heritrix web crawler (crawler.archive.org).
 *
 * Heritrix is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or
 * any later version.
 *
 * Heritrix 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 Lesser Public License for more details.
 *
 * You should have received a copy of the GNU Lesser Public License
 * along with Heritrix; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
package org.archive.io;

import it.unimi.dsi.fastutil.io.RepositionableStream;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * Wrapper around an {@link InputStream} to make a primitive Repositionable
 * stream. Uses a {@link BufferedInputStream}. Has limitations as we come
 * across buffer boundaries.  At least if the access is over the top end of
 * the buffer, could be fixed by calling a buffer fill.  Going back is
 * probably not possible.  Mitigate against buffer boundary problems by
 * having the inflater buffer -- if stream is gzipped -- be at least half
 * size of the backing buffer used here.
 * <p>TODO: More robust implementation.  Tried to use the it.unimi.dsi.io
 * FastBufferdInputStream but relies on FileChannel ByteBuffers and if not
 * present -- as would be the case reading from a network stream, the main
 * application for this instance -- then it expects the underlying stream 
 * implements RepositionableStream interface.
 * @author stack
 */
public class RepositionableInputStream extends BufferedInputStream implements
        RepositionableStream {
    private long position = 0;
    private long markPosition = 0;
    
    public RepositionableInputStream(InputStream in) {
        super(in);
    }
    
    public RepositionableInputStream(InputStream in, int size) {
        super(in, size);
    }

    public int read(byte[] b) throws IOException {
        int read = super.read(b);
        if (read != -1) {
            position += read;
        }
        return read;
    }
    
    public synchronized int read(byte[] b, int offset, int ct)
    throws IOException {
        int read = super.read(b, offset, ct);
        if (read != -1) {
            position += read;
        }
        return read;
    }
    
    public int read() throws IOException {
        int c = super.read();
        if (c != -1) {
            position++;
        }
        return c;
    }

    /**
     * @param offset Offset is ignored.  We call reset on underlying stream. Use
     * sparingly and only after a call to position.
     */
    public void position(final long offset) {
        if (this.position == offset) {
            return;
        }
        int diff =  (int)(offset - this.position);
        long lowerBound = this.position - this.pos;
        long upperBound = lowerBound + this.count;
        if (offset < lowerBound || offset >= upperBound) {
            throw new IllegalAccessError("Offset outside goes outside " +
                "current this.buf (TODO: At least do fills if positive)");
        }
        this.position = offset;
        this.pos += diff;
    }

    public void mark(int readlimit) {
        this.markPosition = this.position;
        super.mark(readlimit);
    }

    public void reset() throws IOException {
        super.reset();
        this.position = this.markPosition;
    }

    public long position() {
        return this.position;
    }
}