Display basically diff

app/assets/stylesheets/common.css.scss

@@ -169,3 +169,11 @@ label {
 .datetime {
   white-space: nowrap;
 }
+.diff {
+  .deleted {
+    background-color: #fdd;
+  }
+  .added {
+    background-color: #dfd;
+  }
+}

app/controllers/concerns/setting_history_concern.rb

@@ -8,6 +8,9 @@ module SettingHistoryConcern
   end
 
   def show
+    @current = @fluentd.agent.config
+    target = @backup_file.content
+    @sdiff = Diff::LCS.sdiff(@current.split("\n").map(&:rstrip), target.split("\n").map(&:rstrip))
   end
 
   def reuse

app/views/fluentd/settings/histories/_diff.html.erb

@@ -0,0 +1,15 @@
+<pre>
+<% @sdiff.each do |diff| %>
+<% case diff.action %>
+<% when "=" %>
+<span class="stay"><%= diff.new_element %></span>
+<% when "!" %>
+<span class="deleted">- <%= diff.old_element %></span>
+<span class="added">+ <%= diff.new_element %></span>
+<% when "-" %>
+<span class="deleted">- <%= diff.old_element %></span>
+<% when "+" %>
+<span class="added">+ <%= diff.new_element %></span>
+<% end %>
+<% end %>
+</pre>

app/views/fluentd/settings/histories/show.html.haml

@@ -14,8 +14,21 @@
       %p= @backup_file.note.content
 
 .row
-  .col-xs-12
+  .col-xs-6
+    %h2 Current
+    %pre
+      = preserve do
+        = @current
+  .col-xs-6
+    %h2 Target
     %pre
       = preserve do
         = @backup_file.content
 
+  .col-xs-12.diff
+    %h2
+      Diff: 
+      current
+      = ".."
+      = params[:id]
+    = preserve render "diff"