public Object nextElement() {
            if (nextItem == null) {
                throw new NoSuchElementException("PageDB Enumeration");
            }

            Page toReturn = nextItem;
            this.nextItem = new Page();
            try {
                if (! reader.next(nextItem)) {
                    this.nextItem = null;
                }
            } catch (IOException ie) {
                this.nextItem = null;
            }
            return toReturn;
        }
    }

    /**
     * Get all the hyperlinks that link TO the indicated URL.
     */     
    public Link[] getLinks(UTF8 url) throws IOException {
        Vector records = new Vector(3);
        Link l = new Link();
        l.getURL().set(url);

        linksByURL.seek(l);
        while (linksByURL.next(l, NullWritable.get())) {
            if (url.equals(l.getURL())) {
                records.add(l);
                l = new Link();
            } else {
                break;
            }
        }
        
        // Xfer from the vector into an array
        return (Link[]) records.toArray(LINK_RECORDS);
    }

    /**
     * Grab all the links from the given MD5 hash.
     */
    public Link[] getLinks(MD5Hash md5) throws IOException {
        Vector records = new Vector(3);
        Link l = new Link();
        l.getFromID().set(md5);

        linksByMD5.seek(l);
        while (linksByMD5.next(l, NullWritable.get())) {
            if (md5.equals(l.getFromID())) {
                records.add(l);
                l = new Link();
            } else {
                break;
            }
        }
        
        // Xfer from the vector into an array
        return (Link[]) records.toArray(LINK_RECORDS);
    }

    /**
     * Return all the links, by target URL
     */
    public Enumeration links() {
        return new MapEnumerator(linksByURL);
    }

    /**
     * Return the number of links in our db.
     */
    public long numLinks() {
        return totalLinks;
    }

    //
    // Here's the class for the above function
    //
    class MapEnumerator implements Enumeration {
        MapFile.Reader reader;
        Link nextItem;

        /**
         * Start the cursor and find the first item.
         * Store it for later return.
         */
        public MapEnumerator(MapFile.Reader reader) {
            this.reader = reader;
            this.nextItem = new Link();
            try {
                if (! reader.next(this.nextItem, NullWritable.get())) {
                    this.nextItem = null;
                }
            } catch (IOException ie) {
                this.nextItem = null;
            }
        }

        /**
         * If there's no item left in store, we've hit the end.
         */
        public boolean hasMoreElements() {
            return (nextItem != null);
        }

        /**
         * Set aside the item we have in store.  Then retrieve
         * another for the next time we're called.  Finally, return
         * the set-aside item.
         */
        public Object nextElement() {
            if (nextItem == null) {
                throw new NoSuchElementException("PageDB Enumeration");
            }

            Link toReturn = nextItem;
            this.nextItem = new Link();
            try {
                if (! reader.next(nextItem, NullWritable.get())) {
                    this.nextItem = null;
                }
            } catch (IOException ie) {
                this.nextItem = null;
            }
            return toReturn;
        }
    }

    /**
     * The WebDBReader.main() provides some handy utility methods
     * for looking through the contents of the webdb.  Hoo-boy!
     */
    public static void main(String argv[]) throws FileNotFoundException, IOException {
        if (argv.length < 2) {
            System.out.println("Usage: java net.nutch.db.WebDBReader (-local | -ndfs <namenode:port>) <db> [-pageurl url] | [-pagemd5 md5] | [-dumppageurl] | [-dumppagemd5] | [-toppages <k>] | [-linkurl url] | [-linkmd5 md5] | [-dumplinks] | [-stats]");
            return;

        }

        int i = 0;
        NutchFileSystem nfs = NutchFileSystem.parseArgs(argv, i);
        File dbDir = new File(argv[i++]);
        WebDBReader reader = new WebDBReader(nfs, dbDir);
        try {
            String cmd = argv[i++];

            if ("-pageurl".equals(cmd)) {
                String url = argv[i++];
                System.out.println(reader.getPage(url.trim()));
            } else if ("-pagemd5".equals(cmd)) {
                MD5Hash md5 = new MD5Hash(argv[i++]);
                Page pages[] = reader.getPages(md5);
                System.out.println("Found " + pages.length + " pages.");
                for (int j = 0; j < pages.length; j++) {
                    System.out.println("Page " + j + ": " + pages[j]);
                }
            } else if ("-dumppageurl".equals(cmd)) {
                System.out.println(reader);
                System.out.println();
                int j = 1;
                for (Enumeration e = reader.pages(); e.hasMoreElements(); j++) {
                    Page page = (Page) e.nextElement();
                    System.out.println("Page " + j + ": " + page);
                    System.out.println();
                }
            } else if ("-dumppagemd5".equals(cmd)) {
                System.out.println(reader);
                System.out.println();
                int j = 1;
                for (Enumeration e = reader.pagesByMD5(); e.hasMoreElements(); j++) {
                    Page page = (Page) e.nextElement();
                    System.out.println("Page " + j + ": " + page);
                    System.out.println();
                }
            } else if ("-toppages".equals(cmd)) {
                int topSize = Integer.parseInt(argv[i++]);

                // Create a sorted list
                SortedSet topSet = new TreeSet(new Comparator() {
                    public int compare(Object o1, Object o2) {
                        Page p1 = (Page) o1;
                        Page p2 = (Page) o2;
                        if (p1.getScore() < p2.getScore()) {
                            return -1;
                        } else if (p1.getScore() == p2.getScore()) {
                            // If two scores are equal, we will
                            // use regular Page comparison (which
                            // uses URL as the primary key).  We
                            // don't want to uniquify by score!
                            return p1.compareTo(p2);
                        } else {
                            return 1;
                        }
                    }
                }
                    );

                // Find the top "topSize" elts
                Page lowestPage = null;
                for (Enumeration e = reader.pages(); e.hasMoreElements(); ) {
                    Page curPage = (Page) e.nextElement();
                    if (topSet.size() < topSize) {
                        topSet.add(curPage);
                        lowestPage = (Page) topSet.first();
                    } else if (lowestPage.getScore() < curPage.getScore()) {
                        topSet.remove(lowestPage);
                        topSet.add(curPage);
                        lowestPage = (Page) topSet.first();
                    }
                }
            
                // Print them out
                int j = 0;
                for (Iterator it = topSet.iterator(); it.hasNext(); j++) {
                    System.out.println("Page " + j + ": " + (Page) it.next());
                    System.out.println();
                }
            } else if ("-linkurl".equals(cmd)) {
                String url = argv[i++];
                Link links[] = reader.getLinks(new UTF8(url.trim()));
                System.out.println("Found " + links.length + " links.");
                for (int j = 0; j < links.length; j++) {
                    System.out.println("Link " + j + ": " + links[j]);
                }
            } else if ("-linkmd5".equals(cmd)) {
                MD5Hash fromID = new MD5Hash(argv[i++]);
                Link links[] = reader.getLinks(fromID);
                System.out.println("Found " + links.length + " links.");
                for (int j = 0; j < links.length; j++) {
                    System.out.println("Link " + j + ": " + links[j]);
                }
            } else if ("-dumplinks".equals(cmd)) {
                System.out.println(reader);
                System.out.println();
                Enumeration e = reader.pagesByMD5();
                while (e.hasMoreElements()) {
                  Page page = (Page) e.nextElement();
                  Link[] links = reader.getLinks(page.getMD5());
                  if (links.length > 0) {
                    System.out.println("from " + page.getURL());
                    for (int j = 0; j < links.length; j++) {
                      System.out.println(" to " + links[j].getURL());
                    }
                    System.out.println();
                  }
                }
            } else if ("-stats".equals(cmd)) {
                System.out.println("Stats for " + reader);
                System.out.println("-------------------------------");
                System.out.println("Number of pages: " + reader.numPages());
                System.out.println("Number of links: " + reader.numLinks());
            } else {
                System.out.println("Sorry, no command with name " + cmd);
            }
        } finally {
            reader.close();
            nfs.close();
        }
    }
}