Move diff logic out of helper

app/logical/diff_builder.rb

@@ -0,0 +1,57 @@
+class DiffBuilder
+  attr_reader :this_text, :that_text, :pattern
+
+  def initialize(this_text, that_text, pattern)
+    @this_text = this_text
+    @that_text = that_text
+    @pattern = pattern
+  end
+
+  def build
+    thisarr = this_text.scan(pattern)
+    otharr = that_text.scan(pattern)
+
+    cbo = Diff::LCS::ContextDiffCallbacks.new
+    diffs = thisarr.diff(otharr, cbo)
+
+    escape_html = ->(str) {str.gsub(/&/, '&amp;').gsub(/</, '&lt;').gsub(/>/, '&gt;')}
+
+    output = thisarr
+    output.each { |q| q.replace(escape_html[q]) }
+
+    diffs.reverse_each do |hunk|
+      newchange = hunk.max {|a, b| a.old_position <=> b.old_position}
+      newstart = newchange.old_position
+      oldstart = hunk.min {|a, b| a.old_position <=> b.old_position}.old_position
+
+      if newchange.action == '+'
+        output.insert(newstart, '</ins>')
+      end
+
+      hunk.reverse_each do |chg|
+        case chg.action
+        when '-'
+          oldstart = chg.old_position
+          output[chg.old_position] = '<br>' if chg.old_element.match(/^\r?\n$/)
+        when '+'
+          if chg.new_element.match(/^\r?\n$/)
+            output.insert(chg.old_position, '<br>')
+          else
+            output.insert(chg.old_position, (escape_html[chg.new_element]).to_s)
+          end
+        end
+      end
+
+      if newchange.action == '+'
+        output.insert(newstart, '<ins>')
+      end
+
+      if hunk[0].action == '-'
+        output.insert((newstart == oldstart || newchange.action != '+') ? newstart + 1 : newstart, '</del>')
+        output.insert(oldstart, '<del>')
+      end
+    end
+
+    output.join.gsub(/\r?\n/, '<br>').html_safe
+  end
+end