Change "percentage change" function and add comments/test for it.

tools/bisect-perf-regression_test.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import math
 import unittest
 
 # Special import necessary because filename contains dash characters.
 bisect_perf_module = __import__('bisect-perf-regression')
 
 
 class BisectPerfRegressionTest(unittest.TestCase):
   """Test case for top-level functions in the bisect-perf-regrssion module."""
 
   def setUp(self):
@@ skipping 53 matching lines @@
         0, bisect_perf_module.CalculateConfidence(bad_values, good_values))
 
   def testCalculateConfidence100(self):
     """Tests the confidence calculation when it's expected to be 100."""
     bad_values = [[1, 1], [1, 1]]
     good_values = [[1.2, 1.2], [1.2, 1.2]]
     # Standard deviation in both groups is zero, so confidence is 100.
     self.assertEqual(
         100, bisect_perf_module.CalculateConfidence(bad_values, good_values))
 
+  def testCalculateRelativeChange(self):
+    """Tests the common cases for calculating relative change."""
+    # The change is relative to the first value, regardless of which is bigger.
+    self.assertEqual(0.5, bisect_perf_module.CalculateRelativeChange(1.0, 1.5))
+    self.assertEqual(0.5, bisect_perf_module.CalculateRelativeChange(2.0, 1.0))
+
+  def testCalculateRelativeChangeFromZero(self):
+    """Tests what happens when relative change from zero is calculated."""
+    # If the first number is zero, then the result is not a number.
+    self.assertEqual(0, bisect_perf_module.CalculateRelativeChange(0, 0))
+    self.assertTrue(
+        math.isnan(bisect_perf_module.CalculateRelativeChange(0, 1)))
+    self.assertTrue(
+        math.isnan(bisect_perf_module.CalculateRelativeChange(0, -1)))
+
+  def testCalculateRelativeChangeWithNegatives(self):
+    """Tests that relative change given is always positive."""
+    self.assertEqual(3.0, bisect_perf_module.CalculateRelativeChange(-1, 2))
+    self.assertEqual(3.0, bisect_perf_module.CalculateRelativeChange(1, -2))
+    self.assertEqual(1.0, bisect_perf_module.CalculateRelativeChange(-1, -2))
+
+
 if __name__ == '__main__':
   unittest.main()