test [utils] add Interval tests

Covers:
 - to_json
 - move_end
 - update_duration
 - unhappy path exceptions (mostly init)

test/test_utils.py

@@ -155,6 +155,11 @@ class TestInterval(unittest.TestCase):
         self.assertTrue(interval3 > interval2) # interval3 start is larger
         self.assertTrue(interval3 < interval4) # same start, interval3 has smaller end
 
+    def test_to_json(self):
+        """Test to_json method"""
+        interval = utils.Interval(start=self.start, end=self.end)
+        self.assertEqual(interval.to_json(), {"start": self.start, "end": self.end, "duration": self.duration})
+
     def test_move_start(self):
         """Test the move_start method - changes start time to time specified, keeps end time constant"""
         interval = utils.Interval(start=self.start, end=self.end)
@@ -171,6 +176,44 @@ class TestInterval(unittest.TestCase):
         self.assertEqual(interval.end, 10)
         self.assertEqual(interval.duration, 8)
 
+    def test_move_end(self):
+        """Test the move_end method - changes end time to time specified, keeps start time constant"""
+        interval = utils.Interval(start=self.start, end=self.end)
+        interval.move_end(15)
+        self.assertEqual(interval.start, 0)
+        self.assertEqual(interval.end, 15)
+        self.assertEqual(interval.duration, 15)
+
+    def test_update_duration(self):
+        """Test the update_duration method - changes duration to time specified, keeps start time constant"""
+        interval = utils.Interval(start=self.start, end=self.end)
+        interval.update_duration(15)
+        self.assertEqual(interval.start, 0)
+        self.assertEqual(interval.end, 15)
+        self.assertEqual(interval.duration, 15)
+        # test with relative=True
+        interval.update_duration(5, relative=True)
+        self.assertEqual(interval.start, 0)
+
+    # Unhappy path tests
+
+    def test_init_no_start_end(self):
+        with self.assertRaises(ValueError):
+            utils.Interval()
+
+    def test_init_start_end_duration(self):
+        with self.assertRaises(ValueError):
+            utils.Interval(start=self.start, end=self.end, duration=self.duration)
+
+    def test_init_start_after_end(self):
+        with self.assertRaises(ValueError):
+            utils.Interval(start=10, end=0)
+
+    def test_init_negative_duration(self):
+        with self.assertRaises(ValueError):
+            utils.Interval(start=0, duration=-10)
+        with self.assertRaises(ValueError):
+            utils.Interval(end=0, duration=-10)
 
 
 if __name__ == "__main__":