Use Int32 for offset arrays

src/nested_pandas/series/ext_array.py

@@ -621,12 +621,11 @@ def list_offsets(self) -> pa.Array:
 
         chunks = []
         # The offset of the current chunk in the flat array.
-        # It is 0 for the first chunk, and the last offset of the previous chunk for the next chunks,
-        # as a pa.Scalar.
-        chunk_offset: pa.Scalar | int = 0
+        # Offset arrays use int32 type, so we cast to it
+        chunk_offset = pa.scalar(0, type=pa.int32())
         for chunk in self._chunked_array.iterchunks():
             list_array = cast(pa.ListArray, chunk.field(0))
-            if chunk_offset == 0:
+            if chunk_offset.equals(pa.scalar(0, type=pa.int32())):
                 offsets = list_array.offsets
             else:
                 offsets = pa.compute.add(list_array.offsets[1:], chunk_offset)

src/nested_pandas/series/packer.py

@@ -297,4 +297,7 @@ def calculate_sorted_index_offsets(index: pd.Index) -> np.ndarray:
     offset_but_last = np.nonzero(~index.duplicated(keep="first"))[0]
     offset = np.append(offset_but_last, len(index))
 
+    # Arrow uses int32 for offsets
+    offset = offset.astype(np.int32)
+
     return offset

tests/nested_pandas/series/test_ext_array.py

@@ -614,8 +614,9 @@ def test_list_offsets_single_chunk():
     )
     ext_array = NestedExtensionArray(struct_array)
 
-    desired = pa.chunked_array([pa.array([0, 3, 6])])
-    assert_array_equal(ext_array.list_offsets, desired)
+    desired = pa.array([0, 3, 6], type=pa.int32())
+    # pyarrow returns a single bool for ==
+    assert ext_array.list_offsets == desired
 
 
 def test_list_offsets_multiple_chunks():
@@ -631,7 +632,8 @@ def test_list_offsets_multiple_chunks():
     ext_array = NestedExtensionArray(chunked_arrray)
 
     desired = chunked_arrray.combine_chunks().field("a").offsets
-    assert_array_equal(ext_array.list_offsets, desired)
+    # pyarrow returns a single bool for ==
+    assert ext_array.list_offsets == desired
 
 
 def test___getitem___with_integer():

tests/nested_pandas/series/test_packer.py

@@ -8,6 +8,17 @@
 from pandas.testing import assert_frame_equal, assert_series_equal
 
 
+def offsets_reused(nested_series):
+    """Check if the offset buffers are reused for all columns of the nested series"""
+    lists_df = nested_series.nest.to_lists()
+    first_offset_buffers = None
+    for column in lists_df.columns:
+        offset_buffers = pa.array(lists_df[column]).offsets.buffers()
+        if first_offset_buffers is None:
+            first_offset_buffers = offset_buffers
+        assert offset_buffers == first_offset_buffers
+
+
 def test_pack_with_flat_df():
     """Test pack(pd.DataFrame)."""
     df = pd.DataFrame(
@@ -28,6 +39,7 @@ def test_pack_with_flat_df():
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
         name="series",
     )
+    offsets_reused(series)
     assert_series_equal(series, desired)
 
 
@@ -51,6 +63,7 @@ def test_pack_with_flat_df_and_index():
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
         name="series",
     )
+    offsets_reused(series)
     assert_series_equal(series, desired)
 
 
@@ -85,6 +98,7 @@ def test_pack_with_series_of_dfs():
         name="nested",
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
     )
+    offsets_reused(series)
     assert_series_equal(series, desired)
 
 
@@ -109,7 +123,7 @@ def test_pack_flat():
         index=[1, 2, 3, 4],
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
     )
-
+    offsets_reused(actual)
     assert_series_equal(actual, desired)
 
 
@@ -134,7 +148,7 @@ def test_pack_sorted_df_into_struct():
         index=[1, 2, 3, 4],
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
     )
-
+    offsets_reused(actual)
     assert_series_equal(actual, desired)
 
 
@@ -172,6 +186,7 @@ def test_pack_lists():
         dtype=pd.ArrowDtype(pa.list_(pa.int64())),
     )
     series = packer.pack_lists(packed_df)
+    offsets_reused(series)
 
     for field_name in packed_df.columns:
         assert_series_equal(series.nest.get_list_series(field_name), packed_df[field_name])
@@ -221,6 +236,7 @@ def test_pack_seq_with_dfs_and_index():
         index=[100, 101, 102, 103],
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
     )
+    offsets_reused(series)
     assert_series_equal(series, desired)
 
 
@@ -249,6 +265,7 @@ def test_pack_seq_with_different_elements_and_index():
         index=[100, 101, 102, 103],
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
     )
+    offsets_reused(series)
     assert_series_equal(series, desired)
 
 
@@ -290,6 +307,7 @@ def test_pack_seq_with_series_of_dfs():
         dtype=NestedDtype.from_fields(dict(a=pa.int64(), b=pa.int64())),
         name="series",
     )
+    offsets_reused(series)
     assert_series_equal(series, desired)
 
 
@@ -402,7 +420,9 @@ def test_view_sorted_series_as_list_array_raises_when_not_sorted():
 )
 def test_calculate_sorted_index_offsets(index, offsets):
     """Test calculate_sorted_index_offsets()."""
-    assert_array_equal(packer.calculate_sorted_index_offsets(index), offsets)
+    actual = packer.calculate_sorted_index_offsets(index)
+    assert actual.dtype == np.int32
+    assert_array_equal(actual, offsets)
 
 
 def test_calculate_sorted_index_offsets_raises_when_not_sorted():