diff --git a/lib/LIBS b/lib/LIBS
index e226736..c4a2782 100644
--- a/lib/LIBS
+++ b/lib/LIBS
@@ -1,3 +1,4 @@
 samskivert.jar
 ant.jar
 commons-io.jar
+javaws.jar
diff --git a/src/java/com/threerings/getdown/tools/Differ.java b/src/java/com/threerings/getdown/tools/Differ.java
new file mode 100644
index 0000000..5f03dad
--- /dev/null
+++ b/src/java/com/threerings/getdown/tools/Differ.java
@@ -0,0 +1,176 @@
+//
+// $Id: Differ.java,v 1.1 2004/07/13 10:41:52 mdb Exp $
+
+package com.threerings.getdown.tools;
+
+import java.io.BufferedOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+
+import java.util.ArrayList;
+import java.util.jar.JarOutputStream;
+import java.util.zip.ZipEntry;
+
+import com.sun.javaws.jardiff.JarDiff;
+
+import com.samskivert.io.StreamUtil;
+import com.samskivert.util.CollectionUtil;
+
+import com.threerings.getdown.data.Application;
+import com.threerings.getdown.data.Resource;
+
+/**
+ * Generates patch files between two particular revisions of an
+ * application. The differences between all the files in the two
+ * revisions are bundled into a single patch file which is placed into the
+ * target version directory.
+ */
+public class Differ
+{
+    /** A suffix appended to file names to indicate that a file should be
+     * newly created. */
+    public static final String CREATE = ".create";
+
+    /** A suffix appended to file names to indicate that a file should be
+     * patched. */
+    public static final String PATCH = ".patch";
+
+    /** A suffix appended to file names to indicate that a file should be
+     * deleted. */
+    public static final String DELETE = ".delete";
+
+    /**
+     * Creates a single patch file that contains the differences between
+     * the two specified application directories. The patch file will be
+     * created in the <code>nvdir</code> directory with name
+     * <code>patchV.dat</code> where V is the old application version.
+     */
+    public void createDiff (File nvdir, File ovdir, boolean verbose)
+    {
+        // sanity check
+        String nvers = nvdir.getName();
+        String overs = ovdir.getName();
+        try {
+            if (Integer.parseInt(nvers) <= Integer.parseInt(overs)) {
+                System.err.println("New version (" + nvers + ") must be " +
+                                   "greater than old version (" + overs + ").");
+                System.exit(-1);
+            }
+        } catch (Exception e) {
+            System.err.println("Non-numeric versions? [nvers=" + nvers +
+                               ", overs=" + overs + "].");
+            System.exit(-1);
+        }
+
+        Application oapp = createApplication(ovdir);
+        ArrayList orsrcs = new ArrayList();
+        CollectionUtil.addAll(orsrcs, oapp.getCodeResources().iterator());
+        CollectionUtil.addAll(orsrcs, oapp.getResources().iterator());
+        if (verbose) {
+            System.out.println(orsrcs.size() + " old resources.");
+        }
+
+        Application napp = createApplication(nvdir);
+        ArrayList nrsrcs = new ArrayList();
+        CollectionUtil.addAll(nrsrcs, napp.getCodeResources().iterator());
+        CollectionUtil.addAll(nrsrcs, napp.getResources().iterator());
+        if (verbose) {
+            System.out.println(nrsrcs.size() + " new resources.");
+        }
+
+        File patch = new File(nvdir, "patch" + overs + ".dat");
+        JarOutputStream jout = null;
+        try {
+            jout = new JarOutputStream(
+                new BufferedOutputStream(new FileOutputStream(patch)));
+
+            // for each file in the new application, it either already
+            // exists in the old application, or it is new
+            for (int ii = 0; ii < nrsrcs.size(); ii++) {
+                Resource rsrc = (Resource)nrsrcs.get(ii);
+                int oidx = orsrcs.indexOf(rsrc);
+                Resource orsrc = (oidx == -1) ?
+                    null : (Resource)orsrcs.remove(oidx);
+                if (orsrc != null && rsrc.getPath().endsWith(".jar")) {
+                    if (verbose) {
+                        System.out.println(
+                            "Generating jardiff: " + rsrc.getPath());
+                    }
+                    jout.putNextEntry(new ZipEntry(rsrc.getPath() + PATCH));
+                    jarDiff(orsrc.getLocal(), rsrc.getLocal(), jout);
+                } else {
+                    if (verbose) {
+                        System.out.println("New entry: " + rsrc.getPath());
+                    }
+                    jout.putNextEntry(new ZipEntry(rsrc.getPath() + CREATE));
+                    pipe(rsrc.getLocal(), jout);
+                }
+            }
+
+            // now any file remaining in orsrcs needs to be removed
+            for (int ii = 0; ii < orsrcs.size(); ii++) {
+                Resource rsrc = (Resource)orsrcs.get(ii);
+                // simply add an entry with the resource name and the
+                // deletion suffix
+                if (verbose) {
+                    System.out.println("Removing entry: " + rsrc.getPath());
+                }
+                jout.putNextEntry(new ZipEntry(rsrc.getPath() + DELETE));
+            }
+
+            StreamUtil.close(jout);
+
+        } catch (Exception ioe) {
+            System.err.println("Failure creating patch file [patch=" + patch +
+                               ", error=" + ioe + "].");
+            StreamUtil.close(jout);
+            patch.delete();
+        }
+    }
+
+    protected Application createApplication (File dir)
+    {
+        Application app = new Application(dir);
+        try {
+            app.init();
+        } catch (IOException ioe) {
+            System.err.println("Invalid app directory '" + dir + "': " + ioe);
+            System.exit(-1);
+        }
+        return app;
+    }
+
+    protected void pipe (File file, JarOutputStream jout)
+        throws IOException
+    {
+        FileInputStream fin = null;
+        try {
+            fin = new FileInputStream(file);
+            byte[] buffer = new byte[4096];
+            int read;
+            while ((read = fin.read(buffer)) >= 0) {
+                jout.write(buffer, 0, read);
+            }
+        } finally {
+            StreamUtil.close(fin);
+        }
+    }
+
+    protected void jarDiff (File ofile, File nfile, JarOutputStream jout)
+        throws IOException
+    {
+        JarDiff.createPatch(ofile.getPath(), nfile.getPath(), jout, false);
+    }
+
+    public static void main (String[] args)
+    {
+        if (args.length != 2) {
+            System.err.println("Usage: Differ new_vers_dir old_vers_dir");
+            System.exit(-1);
+        }
+        Differ differ = new Differ();
+        differ.createDiff(new File(args[0]), new File(args[1]), false);
+    }
+}