Diff from RB for issue 717

bug717-1.diff

diff --git a/public/javascripts/Grader/file_selector.js b/public/javascripts/Grader/file_selector.js
--- a/public/javascripts/Grader/file_selector.js
+++ b/public/javascripts/Grader/file_selector.js
@@ -8,3 +8,9 @@ function bump_select(select_node, bump_amount) {
   $(select_node).selectedIndex = result;
   load_submitted_file($F(select_node));
 }
+
+function validate_bump_buttons(bump_prev, bump_next, select) {
+  var fileSelect = $(select);
+  $(bump_prev).disabled = (fileSelect.selectedIndex == 0);
+  $(bump_next).disabled = (fileSelect.selectedIndex == fileSelect.length - 1);
+}
diff --git a/app/views/results/common/_file_selector.html.erb b/app/views/results/common/_file_selector.html.erb
--- a/app/views/results/common/_file_selector.html.erb
+++ b/app/views/results/common/_file_selector.html.erb
@@ -1,9 +1,24 @@
 <% # Make sure to include the public/javascripts/Grader/file_selector.js
    # file when using this partial
 %>
+
+<script type="text/javascript">
+function validate()
+{
+  validate_bump_buttons('bump_prev', 'bump_next', 'select_file_id');
+}
+
+Event.observe(window, 'load', function() {
+  validate();
+  Event.observe('select_file_id', 'change', validate);
+  Event.observe('bump_prev', 'click', validate);
+  Event.observe('bump_next', 'click', validate);
+});
+</script>
+
 <%= form_tag({:action => "download"}, {:id => "grader_view_file_selector_form"}) do %>
   <%= label_tag "select_file_id", I18n.t("common.submission_file"), :class => "inline_label" %>
-  <input type="button" onclick="bump_select($('select_file_id'), -1); return false;" value="<-" />
+  <input type="button" id="bump_prev" onclick="bump_select($('select_file_id'), -1); return false;" value="<-" />
   <select id="select_file_id" name="select_file_id" onchange="if ($(this).getValue() != '') { load_submitted_file($(this).getValue()); }">
     <% if files.empty? %>
       <option value=""><%= I18n.t("common.no_file_in_repository") %></option>
@@ -14,7 +29,7 @@
         </option>
     <% end %>
   </select>
-  <input type="button" onclick="bump_select($('select_file_id'), 1); return false;" value="->" />
+  <input type="button" id="bump_next" onclick="bump_select($('select_file_id'), 1); return false;" value="->" />
   <span id="loading_code" style="display:none;">
     <%=image_tag('spinner.gif')%> <%= I18n.t("common.loading_code") %>
   </span>