fix(rust, python): fix invalid access when groupby rolling produces empty sets

polars/polars-time/src/groupby/dynamic.rs

@@ -7,7 +7,7 @@ use polars_core::series::IsSorted;
 use polars_core::utils::ensure_sorted_arg;
 use polars_core::utils::flatten::flatten_par;
 use polars_core::POOL;
-use polars_utils::slice::SortedSlice;
+use polars_utils::slice::{GetSaferUnchecked, SortedSlice};
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 use smartstring::alias::String as SmartString;
@@ -638,15 +638,19 @@ fn update_subgroups_idx(
     sub_groups
         .iter()
         .map(|&[first, len]| {
-            let new_first = unsafe { *base_g.1.get_unchecked(first as usize) };
+            let new_first = if len == 0 {
+                // in case the group is empty
+                // keep the original first so that the
+                // groupby keys still point to the original group
+                base_g.0
+            } else {
+                unsafe { *base_g.1.get_unchecked_release(first as usize) }
+            };
 
             let first = first as usize;
             let len = len as usize;
             let idx = (first..first + len)
-                .map(|i| {
-                    debug_assert!(i < base_g.1.len());
-                    unsafe { *base_g.1.get_unchecked(i) }
-                })
+                .map(|i| unsafe { *base_g.1.get_unchecked_release(i) })
                 .collect_trusted::<Vec<_>>();
             (new_first, idx)
         })

py-polars/tests/unit/operations/test_groupby_rolling.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from datetime import datetime
+from datetime import date, datetime
 from typing import TYPE_CHECKING, Any
 
 import pytest
@@ -258,3 +258,42 @@ def test_groupby_rolling_dynamic_sortedness_check() -> None:
         match=r"argument in operation 'groupby_rolling' is not explicitly sorted",
     ):
         df.groupby_rolling("idx", period="2i").agg(pl.col("idx").alias("idx1"))
+
+
+def test_groupby_rolling_empty_groups_9973() -> None:
+    dt1 = date(2001, 1, 1)
+    dt2 = date(2001, 1, 2)
+
+    data = pl.DataFrame(
+        {
+            "id": ["A", "A", "B", "B", "C", "C"],
+            "date": [dt1, dt2, dt1, dt2, dt1, dt2],
+            "value": [1.0, 2.0, 3.0, 4.0, 5.0, 6.0],
+        }
+    ).sort(by=["id", "date"])
+
+    expected = pl.DataFrame(
+        {
+            "id": ["A", "A", "B", "B", "C", "C"],
+            "date": [
+                date(2001, 1, 1),
+                date(2001, 1, 2),
+                date(2001, 1, 1),
+                date(2001, 1, 2),
+                date(2001, 1, 1),
+                date(2001, 1, 2),
+            ],
+            "value": [[2.0], [], [4.0], [], [6.0], []],
+        }
+    )
+
+    out = data.groupby_rolling(
+        index_column="date",
+        by="id",
+        period="2d",
+        offset="1d",
+        closed="left",
+        check_sorted=True,
+    ).agg(pl.col("value"))
+
+    assert_frame_equal(out, expected)