Fix for bug #3, intersection of multiple range sets not working corre…

…ctly

doc/changelog.rst

@@ -4,6 +4,13 @@ Version are structured like the following: ``<major>.<minor>.<bugfix>``. The
 first `0.1` release does not properly adhere to this. Unless explicitly stated,
 changes are made by `Andreas Runfalk <https://github.com/runfalk>`_.
 
+Version 0.2.2
+-------------
+Unreleased
+
+- Fixed `bug #3 <https://github.com/runfalk/spans/issues/3>`_, intersection of
+  multiple range sets not working correctly.
+
 Version 0.2.1
 -------------
 Released on 27th June, 2016

spans/settypes.py

@@ -440,14 +440,34 @@ def intersection(self, *others):
                  `other`.
         """
 
-        # TODO: Optimize this
+        # Initialize output with a reference to this rangeset. When
+        # intersecting against multiple rangesets at once this will be replaced
+        # after each iteration.
+        output = self
 
-        intersection = self.__class__([])
         for other in others:
-            for a in self:
+            # Intermediate rangeset containing intersection for this current
+            # iteration.
+            intersection = self.__class__([])
+
+            # Intersect every range within the current output with every range
+            # within the currently processed other rangeset. All intersecting
+            # parts are added to the intermediate intersection set.
+            for a in output:
                 for b in other:
                     intersection.add(a.intersection(b))
-        return intersection
+
+            # If the intermediate intersection rangeset is still empty, there
+            # where no intersections with at least one of the arguments and
+            # we can quit early, since any intersection with the empty set will
+            # always be empty.
+            if not intersection:
+                return intersection
+
+            # Update output with intersection for the current iteration.
+            output = intersection
+
+        return output
 
     __contains__ = contains
 

spans/tests/test_settypes.py

@@ -151,3 +151,20 @@ def test_greater_than(self):
             intrangeset([range_b]) > intrangeset([range_a, range_b]))
         self.assertTrue(
             intrangeset([range_a, range_b]) >= intrangeset([range_a]))
+
+    def test_bug3_intersection(self):
+        """
+        `Bug #3 <https://github.com/runfalk/spans/issues/3>`_
+        """
+
+        range_a = intrange(1, 5)
+        range_b = intrange(5, 10)
+        range_c = intrange(10, 15)
+
+        rangeset_a = intrangeset([range_a, range_c])
+        rangeset_b = intrangeset([range_b])
+        rangeset_c = intrangeset([range_c])
+        rangeset_empty = intrangeset([])
+
+        self.assertEqual(
+            rangeset_a.intersection(rangeset_b, rangeset_c), rangeset_empty)