diff --git a/tools/releasetools/rangelib.py b/tools/releasetools/rangelib.py
index 373bbed..975a48a 100644
--- a/tools/releasetools/rangelib.py
+++ b/tools/releasetools/rangelib.py
@@ -24,12 +24,13 @@
   lots of runs."""
 
   def __init__(self, data=None):
-    # TODO(tbao): monotonic is broken when passing in a tuple.
     self.monotonic = False
     if isinstance(data, str):
       self._parse_internal(data)
     elif data:
+      assert len(data) % 2 == 0
       self.data = tuple(self._remove_pairs(data))
+      self.monotonic = all(x < y for x, y in zip(self.data, self.data[1:]))
     else:
       self.data = ()
 
@@ -39,8 +40,10 @@
 
   def __eq__(self, other):
     return self.data == other.data
+
   def __ne__(self, other):
     return self.data != other.data
+
   def __nonzero__(self):
     return bool(self.data)
 
@@ -74,9 +77,9 @@
     monotonic = True
     for p in text.split():
       if "-" in p:
-        s, e = p.split("-")
-        data.append(int(s))
-        data.append(int(e)+1)
+        s, e = (int(x) for x in p.split("-"))
+        data.append(s)
+        data.append(e+1)
         if last <= s <= e:
           last = e
         else:
@@ -95,6 +98,9 @@
 
   @staticmethod
   def _remove_pairs(source):
+    """Remove consecutive duplicate items to simplify the result.
+
+    [1, 2, 2, 5, 5, 10] will become [1, 10]."""
     last = None
     for i in source:
       if i == last:
@@ -117,6 +123,7 @@
     return " ".join(out)
 
   def to_string_raw(self):
+    assert self.data
     return str(len(self.data)) + "," + ",".join(str(i) for i in self.data)
 
   def union(self, other):
diff --git a/tools/releasetools/test_rangelib.py b/tools/releasetools/test_rangelib.py
index bf2b640..7debe66 100644
--- a/tools/releasetools/test_rangelib.py
+++ b/tools/releasetools/test_rangelib.py
@@ -44,6 +44,7 @@
 
   def test_size(self):
     self.assertEqual(RangeSet("10-19 30-34").size(), 15)
+    self.assertEqual(RangeSet("").size(), 0)
 
   def test_map_within(self):
     self.assertEqual(RangeSet("0-9").map_within(RangeSet("3-4")),
@@ -74,3 +75,51 @@
     self.assertEqual(RangeSet("10-19 30-39").extend(4), RangeSet("6-23 26-43"))
     self.assertEqual(RangeSet("10-19 30-39").extend(10), RangeSet("0-49"))
 
+  def test_equality(self):
+    self.assertTrue(RangeSet("") == RangeSet(""))
+    self.assertTrue(RangeSet("3") == RangeSet("3"))
+    self.assertTrue(RangeSet("3 5") == RangeSet("5 3"))
+    self.assertTrue(
+        RangeSet("10-19 30-39") == RangeSet("30-32 10-14 33-39 15-19"))
+    self.assertTrue(RangeSet("") != RangeSet("3"))
+    self.assertTrue(RangeSet("10-19") != RangeSet("10-19 20"))
+
+    self.assertFalse(RangeSet(""))
+    self.assertTrue(RangeSet("3"))
+
+  def test_init(self):
+    self.assertIsNotNone(RangeSet(""))
+    self.assertIsNotNone(RangeSet("3"))
+    self.assertIsNotNone(RangeSet("3 5"))
+    self.assertIsNotNone(RangeSet("10 19 30-39"))
+
+    with self.assertRaises(AssertionError):
+      RangeSet(data=[0])
+
+  def test_str(self):
+    self.assertEqual(str(RangeSet("0-9")), "0-9")
+    self.assertEqual(str(RangeSet("2-10 12")), "2-10 12")
+    self.assertEqual(str(RangeSet("11 2-10 12 1 0")), "0-12")
+    self.assertEqual(str(RangeSet("")), "empty")
+
+  def test_to_string_raw(self):
+    self.assertEqual(RangeSet("0-9").to_string_raw(), "2,0,10")
+    self.assertEqual(RangeSet("2-10 12").to_string_raw(), "4,2,11,12,13")
+    self.assertEqual(RangeSet("11 2-10 12 1 0").to_string_raw(), "2,0,13")
+
+    with self.assertRaises(AssertionError):
+      RangeSet("").to_string_raw()
+
+  def test_monotonic(self):
+    self.assertTrue(RangeSet("0-9").monotonic)
+    self.assertTrue(RangeSet("2-9").monotonic)
+    self.assertTrue(RangeSet("2-9 30 31 35").monotonic)
+    self.assertTrue(RangeSet("").monotonic)
+    self.assertTrue(RangeSet("0-4 5-9").monotonic)
+    self.assertFalse(RangeSet("5-9 0-4").monotonic)
+
+    self.assertTrue(RangeSet(data=[0, 10]).monotonic)
+    self.assertTrue(RangeSet(data=[0, 10, 15, 20]).monotonic)
+    self.assertTrue(RangeSet(data=[2, 9, 30, 31, 31, 32, 35, 36]).monotonic)
+    self.assertTrue(RangeSet(data=[0, 5, 5, 10]).monotonic)
+    self.assertFalse(RangeSet(data=[5, 10, 0, 5]).monotonic)