1 files changed, 17 insertions, 0 deletions

diff --git a/app/helpers/revisions_helper.rb b/app/helpers/revisions_helper.rb
index a6290133..25a2aa38 100644
--- a/app/helpers/revisions_helper.rb
+++ b/app/helpers/revisions_helper.rb
@@ -4,4 +4,21 @@ module RevisionsHelper
   def describe_page_reference(ref)
     %w[head draft autosave].include?(ref.to_s) ? ref.to_s.capitalize : "revision #{ref}"
   end
+
+  def neighbour_revision_pairs node, start_ref, end_ref
+    return [nil, nil] unless start_ref.to_s.match?(/\A\d+\z/) &&
+                             end_ref.to_s.match?(/\A\d+\z/)
+
+    a, b = start_ref.to_i, end_ref.to_i
+    return [nil, nil] unless (a - b).abs == 1
+
+    lo, hi  = [a, b].minmax
+    forward = a < b
+    earlier = forward ? [lo - 1, lo] : [lo, lo - 1]
+    later   = forward ? [hi, hi + 1] : [hi + 1, hi]
+
+    [earlier, later].map do |pair|
+      pair if pair.all? { |rev| node.pages.exists?(:revision => rev) }
+    end
+  end
 end