Webster fix (#9)

* deallocate seats properly

* simplify

* webster test

* missed conflict

* black

tests/test_apportionment.py

@@ -11,6 +11,10 @@ def test_apportionment_values(apportionment_method, votes, seats_val):
     assert sum(apportionment_method(votes, seats_val)) == seats_val
 
 
+def test_webster():
+    assert all([a >= 0 for a in webster([2057, 2496, 2059, 181], 15)])
+
+
 def test_adams():
     assert adams([10, 1], 2) == [1, 1]
 

voting/apportionment.py

@@ -183,13 +183,19 @@ def webster(votes, seats):
         for k in range(unallocated):
             assigned[divs[k]] += 1
     elif unallocated < 0:
-        unallocated = abs(unallocated)
+        overallocated = abs(unallocated)
         # divisors that would subtract a seat from each group
         diffs = [
             1.0 * vote / (i + 0.5) if dec >= 0.5 else 1.0 * vote / (i - 0.5) for i, dec, vote in zip(lower, decs, votes)
         ]
         # argsort
         divs = [i[0] for i in sorted(enumerate(diffs), key=itemgetter(1))]
-        for k in range(unallocated):
-            assigned[divs[k]] -= 1
+        # deallocate seats without bringing seat allocation below zero
+        div_idx = 0
+        while overallocated > 0:
+            if assigned[divs[div_idx]] > 0:
+                assigned[divs[div_idx]] -= 1
+                overallocated -= 1
+            else:
+                div_idx += 1
     return assigned