Make the post versions more like the old format

- "Current" is now most like the old format
-- It is therefore now the default for post versions
- Only show the actual edits in their own column
- Show the current state at that version in another column
- On the "previous" view, don't double-show full list of tags for
  the first post versions, so leave edits blank

app/helpers/post_versions_helper.rb

@@ -1,46 +1,51 @@
 module PostVersionsHelper
   def post_version_diff(post_version, type)
+    return "" if type == "previous" && post_version.version == 1
+
     other = post_version.send(type)
 
-    this_tags = post_version.tag_array
-    this_tags << "rating:#{post_version.rating}" if post_version.rating.present?
-    this_tags << "parent:#{post_version.parent_id}" if post_version.parent_id.present?
-    this_tags << "source:#{post_version.source}" if post_version.source.present?
+    added_tags = post_version.added_tags
+    added_tags << "rating:#{post_version_value(post_version.rating)}" if post_version.rating_changed
+    added_tags << "parent:#{post_version_value(post_version.parent_id)}" if post_version.parent_changed
+    added_tags << "source:#{post_version_value(post_version.source)}" if post_version.source_changed
 
-    other_tags = other.present? ? other.tag_array : []
-    if other.present?
-      other_tags << "rating:#{other.rating}" if other.rating.present?
-      other_tags << "parent:#{other.parent_id}" if other.parent_id.present?
-      other_tags << "source:#{other.source}" if other.source.present?
-    elsif type == "subsequent"
-      other_tags = this_tags
-    end
+    removed_tags = post_version.removed_tags
 
-    if type == "previous"
-      added_tags = this_tags - other_tags
-      removed_tags = other_tags - this_tags
+    if type == "previous" || other.nil?
+      obsolete_added_tags = []
+      obsolete_removed_tags = []
     else
-      added_tags = other_tags - this_tags
-      removed_tags = this_tags - other_tags
+      other_tags = other.tags.split
+      other_tags << "rating:#{post_version_value(other.rating)}"
+      other_tags << "parent:#{post_version_value(other.parent_id)}"
+      other_tags << "source:#{post_version_value(other.source)}"
+      obsolete_added_tags = added_tags - other_tags
+      obsolete_removed_tags = removed_tags & other_tags
     end
-    unchanged_tags = this_tags & other_tags
-
     html = '<span class="diff-list">'
 
     added_tags.each do |tag|
-      html << '<ins>+' + link_to(wordbreakify(tag), posts_path(:tags => tag)) + '</ins>'
-      html << " "
+      obsolete_class = (obsolete_added_tags.include?(tag) ? "diff-obsolete" : "");
+      html << %(<ins class="#{obsolete_class}">#{link_to(wordbreakify(tag), posts_path(:tags => tag))}</ins> )
     end
     removed_tags.each do |tag|
-      html << '<del>-' + link_to(wordbreakify(tag), posts_path(:tags => tag)) + '</del>'
-      html << " "
-    end
-    unchanged_tags.each do |tag|
-      html << '<span>' + link_to(wordbreakify(tag), posts_path(:tags => tag)) + '</span>'
-      html << " "
+      obsolete_class = (obsolete_removed_tags.include?(tag) ? "diff-obsolete" : "");
+      html << %(<del class="#{obsolete_class}">#{link_to(wordbreakify(tag), posts_path(:tags => tag))}</del> )
     end
 
     html << "</span>"
     html.html_safe
   end
+
+  def post_version_field(post_version, field)
+    value = post_version_value(post_version.send(field))
+    prefix = (field == :parent_id ? "parent" : field.to_s)
+    search = prefix + ":" + value.to_s
+    display = (field == :rating ? post_version.pretty_rating : value)
+    %(<b>#{field.to_s.titleize}:</b> #{link_to(display, posts_path(:tags => search))}).html_safe
+  end
+
+  def post_version_value(value)
+    return (value.present? ? value : "none")
+  end
 end