From 2c4327c5009c9bde6a9143989a62baa8ece8d4b5 Mon Sep 17 00:00:00 2001
From: Igor Stadnyk <33496176+fallenmi@users.noreply.github.com>
Date: Mon, 12 Oct 2026 09:21:48 +0200
Subject: [PATCH] Fix IndexedSet indices after trimming a fragmented dead tail

---
 boltons/setutils.py    |  5 +++--
 tests/test_setutils.py | 24 ++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/boltons/setutils.py b/boltons/setutils.py
index c9a02815..8a884a62 100644
--- a/boltons/setutils.py
+++ b/boltons/setutils.py
@@ -152,9 +152,10 @@ def _cull(self):
             num_dead = 1
             while items[-(num_dead + 1)] is _MISSING:
                 num_dead += 1
-            if ded and ded[-1][1] == len(items):
-                del ded[-1]
             del items[-num_dead:]
+            # The dead tail may span several adjacent, unmerged intervals.
+            while ded and ded[-1][0] >= len(items):
+                del ded[-1]
 
     def _get_real_index(self, index):
         if index < 0:
diff --git a/tests/test_setutils.py b/tests/test_setutils.py
index cfed65ff..ce5a868a 100644
--- a/tests/test_setutils.py
+++ b/tests/test_setutils.py
@@ -331,3 +331,27 @@ def test_small_difference_update_unchanged():
     thing = IndexedSet(range(100))
     thing.difference_update({1, 3, 5})
     assert list(thing)[:5] == [0, 2, 4, 6, 7]
+
+
+@mark.parametrize('method', ['remove', 'discard'])
+@mark.parametrize('size, removed', [
+    (39, (0, 37, 36, 38)),
+    (80, (0, 78, 77, 76, 79)),
+    (39, (0, 36, 37, 38)),
+])
+def test_iset_add_after_trimming_dead_tail(method, size, removed):
+    iset = IndexedSet(range(size))
+    expected = list(range(size))
+    for value in removed:
+        getattr(iset, method)(value)
+        expected.remove(value)
+    assert list(iset) == expected
+
+    for value in (size, size + 1):
+        iset.add(value)
+        expected.append(value)
+        assert iset.index(value) == len(expected) - 1
+        assert list(iset) == expected
+        assert [iset[i] for i in range(len(iset))] == expected
+        assert [iset[i] for i in range(-len(iset), 0)] == expected
+        assert [iset.index(item) for item in expected] == list(range(len(expected)))