function OnInit(data) {
   var cmd = data.AddCommand();
   cmd.name = "CompareTabs";
   cmd.method = "OnCompareTabs";
   cmd.label = data.name = "Compare Tabs";
   cmd.desc = data.desc = "Compare tabs in dual display mode and select files differing in name or size.";
   cmd.template = "SAME/S";
   data.default_enable = true;
   return false;
}

function OnCompareTabs(data) {
   var lister = data.func.sourcetab.lister; // get lister object from source tab
   if (!lister.dual) return; // do nothing if not in dual panel mode

   var src = {}, dst = {}, dup = {};
   
   var SelCmd1 = "Select FROMSCRIPT DESELECTNOMATCH";
   var SelCmd2 = "Select INVERT TYPE=Files";
   
   // add all files from source tab to src array
   var srcEnum = new Enumerator(lister.activetab.files);
   while (!srcEnum.atEnd()) {
      var item = srcEnum.item();
      src[item.name] = item;
      srcEnum.moveNext();
   }
   
   // add different files from destination tab to dst array and others to dup array
   var dstEnum = new Enumerator(lister.desttab.files);
   while (!dstEnum.atEnd()) {
      item = dstEnum.item();
      if (!(item.name in src) || (src[item.name].size - item.size)) dst[item.name] = item;
      else dup[item.name] = item;
      dstEnum.moveNext();
   }
   
   // add files from src that are not in dup to select command
   data.func.command.ClearFiles();
   for (var k in src) if (!(k in dup)) data.func.command.AddFile(src[k]);
   // Select DIFFERENT files
   data.func.command.RunCommand(SelCmd1);
   // Invert selection if user requested SAME instead of (default) DIFFERENT
   if (data.func.args.got_arg.same) data.func.command.RunCommand(SelCmd2);
   
   // add all files from dst to select command
   data.func.command.SetSourceTab(lister.desttab);
   data.func.command.ClearFiles();
   for (k in dst) data.func.command.AddFile(dst[k]);
   // Select DIFFERENT files
   data.func.command.RunCommand(SelCmd1);
   // Invert selection if user requested SAME instead of (default) DIFFERENT
   if (data.func.args.got_arg.same) data.func.command.RunCommand(SelCmd2);
}