From 36b124458beefd0d2d9cfebb61294674b44128b5 Mon Sep 17 00:00:00 2001
From: Gijs Burghoorn <me@gburghoorn.com>
Date: Fri, 6 Dec 2024 08:14:16 +0100
Subject: [PATCH] fix: Invalid `bitwise_xor` for ScalarColumn (#20140)

---
 crates/polars-core/src/frame/column/mod.rs    | 19 ++++++++-----------
 crates/polars-core/src/frame/column/scalar.rs | 16 ++++++++++------
 .../tests/unit/operations/test_bitwise.py     | 13 +++++++++++++
 3 files changed, 31 insertions(+), 17 deletions(-)

diff --git a/crates/polars-core/src/frame/column/mod.rs b/crates/polars-core/src/frame/column/mod.rs
index 39f363f6463d..6a94750e425c 100644
--- a/crates/polars-core/src/frame/column/mod.rs
+++ b/crates/polars-core/src/frame/column/mod.rs
@@ -1196,19 +1196,13 @@ impl Column {
     }
 
     pub fn bitand(&self, rhs: &Self) -> PolarsResult<Self> {
-        // @partition-opt
-        // @scalar-opt
-        (self.as_materialized_series() & rhs.as_materialized_series()).map(Column::from)
+        self.try_apply_broadcasting_binary_elementwise(rhs, |l, r| l & r)
     }
     pub fn bitor(&self, rhs: &Self) -> PolarsResult<Self> {
-        // @partition-opt
-        // @scalar-opt
-        (self.as_materialized_series() | rhs.as_materialized_series()).map(Column::from)
+        self.try_apply_broadcasting_binary_elementwise(rhs, |l, r| l | r)
     }
     pub fn bitxor(&self, rhs: &Self) -> PolarsResult<Self> {
-        // @partition-opt
-        // @scalar-opt
-        (self.as_materialized_series() ^ rhs.as_materialized_series()).map(Column::from)
+        self.try_apply_broadcasting_binary_elementwise(rhs, |l, r| l ^ r)
     }
 
     pub fn try_add_owned(self, other: Self) -> PolarsResult<Self> {
@@ -1341,7 +1335,11 @@ impl Column {
             Column::Scalar(s) => {
                 // We don't really want to deal with handling the full semantics here so we just
                 // cast to a single value series. This is a tiny bit wasteful, but probably fine.
-                s.as_single_value_series().xor_reduce()
+                //
+                // We have to deal with the fact that xor is 0 if there is an even number of
+                // elements and the value if there is an odd number of elements. If there are zero
+                // elements the result should be `null`.
+                s.as_n_values_series(2 - s.len() % 2).xor_reduce()
             },
         }
     }
@@ -1349,7 +1347,6 @@ impl Column {
         match self {
             Column::Series(s) => s.n_unique(),
             Column::Partitioned(s) => s.partitions().n_unique(),
-            // @scalar-opt
             Column::Scalar(s) => s.as_single_value_series().n_unique(),
         }
     }
diff --git a/crates/polars-core/src/frame/column/scalar.rs b/crates/polars-core/src/frame/column/scalar.rs
index aabadf5ab933..cc33697a42ff 100644
--- a/crates/polars-core/src/frame/column/scalar.rs
+++ b/crates/polars-core/src/frame/column/scalar.rs
@@ -108,13 +108,17 @@ impl ScalarColumn {
     ///
     /// If the [`ScalarColumn`] has `length=0` the resulting `Series` will also have `length=0`.
     pub fn as_single_value_series(&self) -> Series {
+        self.as_n_values_series(1)
+    }
+
+    /// Take the [`ScalarColumn`] as a series with a `n` values.
+    ///
+    /// If the [`ScalarColumn`] has `length=0` the resulting `Series` will also have `length=0`.
+    pub fn as_n_values_series(&self, n: usize) -> Series {
+        let length = usize::min(n, self.length);
         match self.materialized.get() {
-            Some(s) => s.head(Some(1)),
-            None => Self::_to_series(
-                self.name.clone(),
-                self.scalar.clone(),
-                usize::min(1, self.length),
-            ),
+            Some(s) => s.head(Some(length)),
+            None => Self::_to_series(self.name.clone(), self.scalar.clone(), length),
         }
     }
 
diff --git a/py-polars/tests/unit/operations/test_bitwise.py b/py-polars/tests/unit/operations/test_bitwise.py
index f701e64b1c70..ae56c44d8b12 100644
--- a/py-polars/tests/unit/operations/test_bitwise.py
+++ b/py-polars/tests/unit/operations/test_bitwise.py
@@ -205,3 +205,16 @@ def test_bit_group_by(dtype: pl.DataType) -> None:
         ),
         check_row_order=False,
     )
+
+
+def test_scalar_bitwise_xor() -> None:
+    df = pl.select(
+        pl.repeat(pl.lit(0x80, pl.UInt8), i).bitwise_xor().alias(f"l{i}")
+        for i in range(5)
+    ).transpose()
+
+    assert_series_equal(
+        df.to_series(),
+        pl.Series("x", [None, 0x80, 0x00, 0x80, 0x00], pl.UInt8),
+        check_names=False,
+    )