From ac1a532a9ed1c05c23143c47e85b4b06aabca2d5 Mon Sep 17 00:00:00 2001
From: Felix Sargent <felix.sargent@gmail.com>
Date: Fri, 17 Jul 2026 23:04:18 +0100
Subject: [PATCH] Fix IRV results contract for strategy polling

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 src/vse_sim/methods.py    | 42 ++++++++++++++++-----------------------
 tests/test_regressions.py | 18 ++++++++++++++++-
 2 files changed, 34 insertions(+), 26 deletions(-)

diff --git a/src/vse_sim/methods.py b/src/vse_sim/methods.py
index 33a3cae..8270a2f 100644
--- a/src/vse_sim/methods.py
+++ b/src/vse_sim/methods.py
@@ -525,37 +525,27 @@ def runIrv(self, remaining, ncand):
             remaining = self.eliminateCandidate(remaining, toEliminate)
         return results
 
-    def resultsFor(self, voters, chooser, tally=None, **kwargs):
-        """Tabulate the rank-vector ballots produced by this simulator.
-
-        IRV's public ``results`` API accepts candidate IDs in preference
-        order, including partial rankings.  The simulator's honest and
-        strategic ballot producers instead return rank vectors, so translate
-        those ballots before invoking the tabulator.
-        """
-        def orderingChooser(cls, voter, chooserTally):
-            ballot = chooser(cls, voter, chooserTally)
-            return sorted(range(len(ballot)), key=lambda candidate: ballot[candidate],
-                          reverse=True)
-
-        orderingChooser.__name__ = chooser.__name__
-        return super().resultsFor(voters, orderingChooser, tally, **kwargs)
-
     @staticmethod
-    def winner(results):
-        """Return the winner from IRV's winner-first finish ordering.
+    def rankVectorToPreference(ballot):
+        """Return candidate IDs in descending preference order from a rank vector."""
+        return sorted(range(len(ballot)), key=lambda candidate: ballot[candidate],
+                      reverse=True)
 
-        Unlike score methods, :meth:`runIrv` returns candidate IDs ordered by
-        finish, with the winner at index zero.
-        """
-        return results[0]
+    @staticmethod
+    def finishOrderToResults(finishOrder):
+        """Convert winner-first finish order to high-is-better candidate scores."""
+        ncand = len(finishOrder)
+        results = [-1] * ncand
+        for score, candidate in enumerate(reversed(finishOrder)):
+            results[candidate] = score
+        return results
 
     def results(self, ballots, **kwargs):
         """IRV results.
 
         >>> Irv().resultsFor(DeterministicModel(3)(5,3),Irv().honBallot)["results"]
-        [2, 1, 0]
-        >>> Irv().winner([2,0,1])
+        [0, 1, 2]
+        >>> Irv().results([[0,1,2]])[2]
         2
         >>> Irv().results([[0,1,2],[2,1,0]])[1]
         0
@@ -564,7 +554,9 @@ def results(self, ballots, **kwargs):
         """
         if type(ballots) is not list:
             ballots = list(ballots)
-        return self.runIrv(self.buildPreferenceSchedule(ballots), len(ballots[0]))
+        rankings = [self.rankVectorToPreference(ballot) for ballot in ballots]
+        finishOrder = self.runIrv(self.buildPreferenceSchedule(rankings), len(ballots[0]))
+        return self.finishOrderToResults(finishOrder)
 
     @staticmethod #cls is provided explicitly, not through binding
     @rememberBallot
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
index 7b0b890..21030a6 100644
--- a/tests/test_regressions.py
+++ b/tests/test_regressions.py
@@ -8,7 +8,7 @@
 from scripts.recalculate_irv_pages import recalculate
 from vse_sim.core import SideTally
 from vse_sim.diagnostics import TRACE, setDebug, trace
-from vse_sim.methods import Mav, Schulze, Score
+from vse_sim.methods import Irv, Mav, Schulze, Score
 from vse_sim.simulation import CsvBatch, seedRandomGenerators
 from vse_sim.strategies import ProbChooser, beHon, beStrat
 from vse_sim.voter_models import Electorate, Voter
@@ -75,6 +75,22 @@ def test_mav_cutoffs_are_scoped_to_generated_ballot_function():
     assert low_ballot(Mav, Voter([-2, -1]), SideTally()) == expected
 
 
+def test_irv_results_keep_simulator_score_contract_for_strategy():
+    method = Irv()
+    voters = Electorate(
+        [Voter([0, 1, 2])] * 4
+        + [Voter([2, 1, 0])] * 3
+        + [Voter([1, 2, 0])] * 2
+    )
+
+    results = method.resultsFor(voters, method.honBallot)["results"]
+    polls = sorted(enumerate(results), key=lambda candidate_result: -candidate_result[1])
+
+    assert results == [2, 0, 1]
+    assert method.winner(results) == 0
+    assert polls[0][0] == 0
+
+
 @pytest.mark.parametrize(
     "probabilities",
     [