From e4464711319bd88ce363159e4a09ab402cda7252 Mon Sep 17 00:00:00 2001
From: barlowa124 <barlowa124@users.noreply.github.com>
Date: Sat, 10 Oct 2026 10:57:01 -0400
Subject: [PATCH] Fix death-birth MoranProcess pairing wrong players

In mode="db", _matchup_indices() used the enumerate position in the
dead node's neighbour list as the player index, so score_all() played
and credited the wrong players. Use self.index[source] so both modes
use global player positions. Fixes #1499.
---
 axelrod/moran.py                 |  3 ++-
 axelrod/tests/unit/test_moran.py | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/axelrod/moran.py b/axelrod/moran.py
index 0b7ea7320..a41e42c6b 100644
--- a/axelrod/moran.py
+++ b/axelrod/moran.py
@@ -354,7 +354,8 @@ def _matchup_indices(self) -> Set[Tuple[int, int]]:
         else:
             # birth-death is global
             sources = sorted(self.locations)
-        for i, source in enumerate(sources):
+        for source in sources:
+            i = self.index[source]
             for target in sorted(self.interaction_graph.out_vertices(source)):
                 j = self.index[target]
                 if (self.players[i] is None) or (self.players[j] is None):
diff --git a/axelrod/tests/unit/test_moran.py b/axelrod/tests/unit/test_moran.py
index 2566118af..489bff8ae 100644
--- a/axelrod/tests/unit/test_moran.py
+++ b/axelrod/tests/unit/test_moran.py
@@ -572,6 +572,31 @@ def test_cycle_death_birth(self):
             winner2 = mp.winning_strategy_name
             self.assertEqual((winner == winner2), outcome)
 
+    def test_death_birth_matchup_indices_use_player_positions(self):
+        """Regression test: in death-birth mode the matchup pairs must be
+        player positions, not positions in the dead node's neighbour list.
+
+        On a 5-cycle, when the player on vertex 2 dies its neighbours
+        (vertices 1 and 3) each play their own out-neighbours, so the
+        matchups are (1, 0) and (3, 4).
+        """
+        N = 5
+        players = [axl.Cooperator(), axl.Defector(), axl.TitForTat(),
+                   axl.Cooperator(), axl.Defector()]
+        mp = MoranProcess(
+            players,
+            interaction_graph=axl.graph.cycle(N),
+            mode="db",
+            seed=1,
+        )
+        # Reproduce the state __next__ creates before score_all()
+        mp.dead = 2
+        mp.players[2] = None
+        self.assertEqual(mp._matchup_indices(), {(1, 0), (3, 4)})
+        scores = mp.score_all()
+        # Only the two neighbours of the dead node and their opponents play
+        self.assertEqual(scores, [0.0, 5.0, 0, 0.0, 5.0])
+
 
 class TestApproximateMoranProcess(unittest.TestCase):
     """A suite of tests for the ApproximateMoranProcess"""