Fix introsort heap fallback ignoring range boundaries

sorts/intro_sort.py

@@ -187,7 +187,8 @@ def intro_sort[T: Comparable](
     """
     while end - start > size_threshold:
         if max_depth == 0:
-            return heap_sort(array)
+            array[start:end] = heap_sort(array[start:end])
+            return array
         max_depth -= 1
         pivot = median_of_3(array, start, start + ((end - start) // 2) + 1, end - 1)
         p = partition(array, start, end, pivot)

tests/test_sorts.py

@@ -34,6 +34,7 @@
 from sorts.gnome_sort import gnome_sort
 from sorts.heap_sort import heap_sort
 from sorts.insertion_sort import insertion_sort
+from sorts.intro_sort import intro_sort as intro_sort_range
 from sorts.intro_sort import sort as intro_sort
 from sorts.iterative_merge_sort import iter_merge_sort
 from sorts.merge_insertion_sort import merge_insertion_sort
@@ -63,6 +64,17 @@ def test_heap_sort() -> None:
     assert heap_sort([5, 4, 3, 2, 1]) == [1, 2, 3, 4, 5]
 
 
+@pytest.mark.parametrize("max_depth", [0, 1])
+def test_intro_sort_heap_fallback_preserves_surrounding_items(max_depth: int) -> None:
+    collection = [100, *range(40, 0, -1), -100]
+    expected = [100, *range(1, 41), -100]
+
+    result = intro_sort_range(collection, 1, 41, 16, max_depth)
+
+    assert result is collection
+    assert collection == expected
+
+
 SORTS = (
     binary_insertion_sort,
     bubble_sort_iterative,