Don't rely on .index to locate pairs in main chain

Just in case an object might override __eq__, use object identity.

Author: haukex

merge_insertion/__init__.py

@@ -122,6 +122,12 @@ async def _bin_insert_index(array :Sequence[T], item :T, comp :Comparator) -> in
             left = mid + 1
     return left
 
+def _ident_find(array :Sequence[T], item :T) -> int:
+    for i,e in enumerate(array):
+        if e is item:
+            return i
+    raise IndexError(f"failed to find item {item!r} in array")
+
 async def merge_insertion_sort(array :Sequence[T], comparator :Comparator) -> Sequence[T]:
     """Merge-Insertion Sort (Ford-Johnson algorithm) with async comparison.
 
@@ -156,7 +162,7 @@ async def merge_insertion_sort(array :Sequence[T], comparator :Comparator) -> Se
             idx = await _bin_insert_index([ i[0] for i in main_chain ], item, comparator)
         else:
             assert len(pair)==2
-            pair_idx = main_chain.index(pair)
+            pair_idx = _ident_find(main_chain, pair)
             item = pair.pop()
             idx = await _bin_insert_index([ i[0] for i in main_chain[:pair_idx] ], item, comparator)
         main_chain.insert(idx, [item])

pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "merge-insertion"
 description = "The merge-insertion sort (aka the Ford-Johnson algorithm) is optimized for using few comparisons."
-version = "1.0.0"
+version = "1.1.0"
 authors = [ { name="Hauke Dämpfling", email="haukex@zero-g.net" } ]
 readme = "README.md"
 requires-python = ">=3.10"

tests/test_merge_insertion.py

@@ -30,6 +30,10 @@
 async def _comp(ab :tuple[str,str]) -> Literal[0,1]:
     return 0 if ab[0] > ab[1] else 1
 
+class AlwaysEqual:
+    def __eq__(self, other):
+        return isinstance(other, AlwaysEqual)
+
 class TestMergeInsertionSort(unittest.IsolatedAsyncioTestCase):
 
     def _test_comp(self, comp :uut.Comparator, max_calls :int, log :Optional[list[tuple[uut.T,uut.T]]] = None) -> uut.Comparator:
@@ -115,6 +119,16 @@ async def test_bin_insert_index(self):
         self.assertEqual( await bin_insert_index(a[:7], 'G', self._test_comp(_comp,3)), 3 )
         self.assertEqual( await bin_insert_index(a[:7], 'O', self._test_comp(_comp,3)), 7 )
 
+    def test_ident_find(self):
+        ident_find = uut._ident_find  # pyright: ignore[reportPrivateUsage]  # pylint: disable=protected-access
+        o = AlwaysEqual()
+        a = (1, 'foo', AlwaysEqual(), o)
+        self.assertEqual( a.index(AlwaysEqual()), 2 )
+        self.assertEqual( a.index(o), 2 )
+        self.assertEqual( ident_find(a, o), 3 )
+        with self.assertRaises(IndexError):
+            ident_find(a, AlwaysEqual())
+
     async def test_merge_insertion_sort_detail(self):
         log :list[tuple[str,str]] = []
         self.assertEqual( await uut.merge_insertion_sort('ABCDE', self._test_comp(_comp, 7, log)), ['A','B','C','D','E'] )