Move time to complete stats into separate class

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

test/unit/time_to_complete_test.rb

@@ -0,0 +1,75 @@
+require 'date'
+require 'time'
+require './test/minimal_test_helper'
+require 'app/models/stats/time_to_complete'
+
+FakeTask = Struct.new(:created_at, :completed_at)
+
+class TimeToCompleteTest < Test::Unit::TestCase
+
+  def now
+    @now ||= Time.utc(2013, 1, 2, 3, 4, 5)
+  end
+
+  def day0
+    @day0 ||= Time.utc(2013, 1, 2, 0, 0, 0)
+  end
+
+  def day2
+    @day2 ||= Time.utc(2012, 12, 31, 0, 0, 0)
+  end
+
+  def day4
+    @day4 ||= Time.utc(2012, 12, 29, 0, 0, 0)
+  end
+
+  def day6
+    @day6 ||= Time.utc(2012, 12, 27, 0, 0, 0)
+  end
+
+  def fake_tasks
+    @fake_tasks ||= [
+      FakeTask.new(day2, now),
+      FakeTask.new(day4, now),
+      FakeTask.new(day6, now)
+    ]
+  end
+
+  def test_empty_minimum
+    assert_equal 0, Stats::TimeToComplete.new([]).min
+  end
+
+  def test_empty_maximum
+    assert_equal 0, Stats::TimeToComplete.new([]).max
+  end
+
+  def test_empty_avg
+    assert_equal 0, Stats::TimeToComplete.new([]).avg
+  end
+
+  def test_empty_min_sec
+    assert_equal '00:00:00', Stats::TimeToComplete.new([]).min_sec
+  end
+
+  def test_minimum
+    assert_equal 2.127835648148148, Stats::TimeToComplete.new(fake_tasks).min
+  end
+
+  def test_maximum
+    assert_equal 6.127835648148148, Stats::TimeToComplete.new(fake_tasks).max
+  end
+
+  def test_avg
+    assert_equal 4.127835648148148, Stats::TimeToComplete.new(fake_tasks).avg
+  end
+
+  def test_min_sec
+    assert_equal '2 days 03:04:05', Stats::TimeToComplete.new(fake_tasks).min_sec
+  end
+
+  def test_min_sec_with_less_than_a_day
+    task = FakeTask.new(day0, now)
+    assert_equal '03:04:05', Stats::TimeToComplete.new([task]).min_sec
+  end
+end
+