Fixed f-resilient uniform strategy bug (#8).

When constructing an f-resilient uniform strategy, the read quorums were
being used for the write quorums. I added a unit test that fails because
of this bug and then fixed the bug to make the test pass.

Thanks so much @samueleresca for finding this bug!

quoracle/quorum_system.py

@@ -176,7 +176,7 @@ class QuorumSystem(Generic[T]):
         else:
             xs = list(self.elements())
             read_quorums = list(self._f_resilient_quorums(f, xs, self.reads))
-            write_quorums = list(self._f_resilient_quorums(f, xs, self.reads))
+            write_quorums = list(self._f_resilient_quorums(f, xs, self.writes))
             if len(read_quorums) == 0:
                 raise NoStrategyFoundError(
                     f'There are no {f}-resilient read quorums')

tests/test_quorum_system.py

@@ -44,6 +44,8 @@ class TestQuorumSystem(unittest.TestCase):
         b = Node('b')
         c = Node('c')
         d = Node('d')
+        e = Node('e')
+        f = Node('f')
 
         sigma = QuorumSystem(reads=a).uniform_strategy()
         self.assertEqual(sigma.sigma_r, {
@@ -128,6 +130,16 @@ class TestQuorumSystem(unittest.TestCase):
             frozenset({'b', 'd'}): 1 / 4,
         })
 
+        sigma = QuorumSystem(reads=a*b+c*d+e*f).uniform_strategy(f=1)
+        self.assertEqual(sigma.sigma_r, {
+            frozenset({'a', 'b', 'c', 'd'}): 1 / 3,
+            frozenset({'a', 'b', 'e', 'f'}): 1 / 3,
+            frozenset({'c', 'd', 'e', 'f'}): 1 / 3,
+        })
+        self.assertEqual(sigma.sigma_w, {
+            frozenset({'a', 'b', 'c', 'd', 'e', 'f'}): 1,
+        })
+
     def test_make_strategy(self):
         a = Node('a')
         b = Node('b')