Move time to complete stats into separate class

This separates out the calculations from the queries so we can get
decent tests around them.

app/models/stats/time_to_complete.rb

@@ -0,0 +1,59 @@
+module Stats
+  class TimeToComplete
+
+    SECONDS_PER_DAY = 86400;
+
+    attr_reader :actions
+    def initialize(actions)
+      @actions = actions
+    end
+
+    def avg
+      @avg ||= (sum / count) / SECONDS_PER_DAY
+    end
+
+    def max
+      @max ||= max_in_seconds / SECONDS_PER_DAY
+    end
+
+    def min
+      @min ||= min_in_seconds / SECONDS_PER_DAY
+    end
+
+    def min_sec
+      min_sec = arbitrary_day + min_in_seconds # convert to a datetime
+      @min_sec = min_sec.strftime("%H:%M:%S")
+      @min_sec = min.round.to_s + " days " + @min_sec if min >= 1
+      @min_sec
+    end
+
+    private
+
+    def min_in_seconds
+      @min_in_seconds ||= durations.min || 0
+    end
+
+    def max_in_seconds
+      @max_in_seconds ||= durations.max || 0
+    end
+
+    def count
+      actions.empty? ? 1 : actions.size
+    end
+
+    def durations
+      @durations ||= actions.map do |r|
+        (r.completed_at - r.created_at)
+      end
+    end
+
+    def sum
+      @sum ||= durations.inject(0) {|sum, d| sum + d}
+    end
+
+    def arbitrary_day
+      @arbitrary_day ||= Time.utc(2000, 1, 1, 0, 0)
+    end
+
+  end
+end