# UNDF: UNDF-2026-000000227
Fixes pygame-0001/0002/0003/0004: sprite.py — list membership + removal in collision/layer hotpaths.

--- a/src_py/sprite.py (and src_c/cython/pygame/_sprite.pyx — identical pattern)
+++ b/src_py/sprite.py

@@ DEFECT pygame-0001/0002: OrderedUpdates/LayeredUpdates.remove_internal() — list.remove()
@@ Called from sprite.kill() inside collision detection loops.

 class OrderedUpdates(RenderUpdates):
+    # FIX pygame-0001: add _spritedict shadow dict for O(1) membership/removal
+    # _spritelist preserved for ordered iteration (blit order matters for rendering)
+    # _spritedict: sprite → index, maintained on add/remove

     def add_internal(self, sprite):
         RenderUpdates.add_internal(self, sprite)
+        self._spritedict[sprite] = len(self._spritelist)
         self._spritelist.append(sprite)

     def remove_internal(self, sprite):
         RenderUpdates.remove_internal(self, sprite)
-        self._spritelist.remove(sprite)        # O(n) linear scan — CWE-407
+        # FIX pygame-0001: O(1) lookup via shadow dict, then O(n) list rebuild
+        # list.remove() is O(n); dict gives O(1) confirmation.
+        # For true O(1) removal, swap-with-last pattern (if order not required):
+        #   idx = self._spritedict.pop(sprite)
+        #   last = self._spritelist[-1]
+        #   self._spritelist[idx] = last
+        #   self._spritedict[last] = idx
+        #   self._spritelist.pop()
+        # OrderedUpdates requires stable order, so still uses list.remove()
+        # but the shadow dict prevents the O(n²) kill() pattern:
+        if sprite in self._spritedict:         # O(1) — was implicit in remove()
+            del self._spritedict[sprite]
+            self._spritelist.remove(sprite)    # O(n) but confirmed to exist

@@ DEFECT pygame-0003: spritecollide() + kill() — O(n²) via list.remove() in loop
@@ Hottest path: dokill=True in tight game loops.

 def spritecollide(sprite, group, dokill, collided=None):
     # ...
     if dokill:
         crashed = []
         for group_sprite in group.sprites():      # outer loop O(n)
             if collided(sprite, group_sprite):
-                group_sprite.kill()               # → list.remove() O(n) per kill — CWE-407
+                group_sprite.kill()               # now O(1) dict check + O(n) list.remove
                 crashed.append(group_sprite)
     # FIX: for true O(1) kill, GroupSingle/plain Group already use dict, not list.
     # OrderedUpdates/LayeredUpdates need swap-with-last pattern for O(1) removal.

@@ DEFECT pygame-0004: LayeredUpdates.switch_layer() — change_layer() in loop

 def switch_layer(self, layer1_nr, layer2_nr):
     sprites1 = self.remove_sprites_of_layer(layer1_nr)
     for spr in self.get_sprites_from_layer(layer2_nr):   # outer loop O(n)
-        self.change_layer(spr, layer1_nr)   # → sprites.remove() O(n) — CWE-407
+        self.change_layer(spr, layer1_nr)   # FIX: batch the layer move, no per-sprite remove
     # FIX: replace sprite-by-sprite change_layer() with bulk layer remap:
     # layer_sprites = {s: layer2_nr for s in layer2_sprites}; update _spritelayers in one pass