fix: Fix group first value after group-by slice (#18603)

crates/polars-expr/src/expressions/slice.rs

@@ -60,9 +60,16 @@ fn check_argument(arg: &Series, groups: &GroupsProxy, name: &str, expr: &Expr) -
     Ok(())
 }
 
-fn slice_groups_idx(offset: i64, length: usize, first: IdxSize, idx: &[IdxSize]) -> IdxItem {
+fn slice_groups_idx(offset: i64, length: usize, mut first: IdxSize, idx: &[IdxSize]) -> IdxItem {
     let (offset, len) = slice_offsets(offset, length, idx.len());
-    (first + offset as IdxSize, idx[offset..offset + len].into())
+
+    // If slice isn't out of bounds, we replace first.
+    // If slice is oob, the `idx` vec will be empty and `first` will be ignored
+    if let Some(f) = idx.get(offset) {
+        first = *f;
+    }
+    // This is a clone of the vec, which is unfortunate. Maybe we have a `sliceable` unitvec one day.
+    (first, idx[offset..offset + len].into())
 }
 
 fn slice_groups_slice(offset: i64, length: usize, first: IdxSize, len: IdxSize) -> [IdxSize; 2] {

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

@@ -273,3 +273,18 @@ def test_group_by_slice_all_keys() -> None:
 
     gb = df.group_by(["a", "b", "c"], maintain_order=True)
     assert_frame_equal(gb.tail(1), gb.head(1))
+
+
+def test_slice_first_in_agg_18551() -> None:
+    df = pl.DataFrame({"id": [1, 1, 2], "name": ["A", "B", "C"], "value": [31, 21, 32]})
+
+    assert df.group_by("id", maintain_order=True).agg(
+        sort_by=pl.col("name").sort_by("value"),
+        x=pl.col("name").sort_by("value").slice(0, 1).first(),
+        y=pl.col("name").sort_by("value").slice(1, 1).first(),
+    ).to_dict(as_series=False) == {
+        "id": [1, 2],
+        "sort_by": [["B", "A"], ["C"]],
+        "x": ["B", "C"],
+        "y": ["A", None],
+    }