fix bug with nulls at start/end

crates/polars-core/src/chunked_array/ops/fill_null.rs

@@ -299,7 +299,7 @@ where
         .collect_trusted();
 
     // Compute bitmask.
-    let num_start_nulls = ca.first_non_null().unwrap_or(0);
+    let num_start_nulls = ca.first_non_null().unwrap_or(ca.len());
     let mut bm = MutableBitmap::with_capacity(ca.len());
     bm.extend_constant(num_start_nulls, false);
     bm.extend_constant(ca.len() - num_start_nulls, true);
@@ -330,8 +330,10 @@ where
         .collect_reversed();
 
     // Compute bitmask.
-    let last_idx = ca.len().saturating_sub(1);
-    let num_end_nulls = last_idx - ca.last_non_null().unwrap_or(last_idx);
+    let num_end_nulls = ca
+        .last_non_null()
+        .map(|i| ca.len() - 1 - i)
+        .unwrap_or(ca.len());
     let mut bm = MutableBitmap::with_capacity(ca.len());
     bm.extend_constant(ca.len() - num_end_nulls, true);
     bm.extend_constant(num_end_nulls, false);

py-polars/tests/unit/dataframe/test_df.py

@@ -1866,10 +1866,24 @@ def test_fill_nan() -> None:
     assert df.fill_nan(2.0).dtypes == [pl.Float64, pl.Datetime]
 
 
+def test_forward_fill() -> None:
+    df = pl.DataFrame({"a": [1.0, None, 3.0]})
+    fill = df.select(pl.col("a").forward_fill())["a"]
+    assert_series_equal(fill, pl.Series("a", [1, 1, 3]).cast(pl.Float64))
+
+    df = pl.DataFrame({"a": [None, 1, None]})
+    fill = df.select(pl.col("a").forward_fill())["a"]
+    assert_series_equal(fill, pl.Series("a", [None, 1, 1]).cast(pl.Int64))
+
+
 def test_backward_fill() -> None:
     df = pl.DataFrame({"a": [1.0, None, 3.0]})
-    col_a_backward_fill = df.select([pl.col("a").backward_fill()])["a"]
-    assert_series_equal(col_a_backward_fill, pl.Series("a", [1, 3, 3]).cast(pl.Float64))
+    fill = df.select(pl.col("a").backward_fill())["a"]
+    assert_series_equal(fill, pl.Series("a", [1, 3, 3]).cast(pl.Float64))
+
+    df = pl.DataFrame({"a": [None, 1, None]})
+    fill = df.select(pl.col("a").backward_fill())["a"]
+    assert_series_equal(fill, pl.Series("a", [1, 1, None]).cast(pl.Int64))
 
 
 def test_shrink_to_fit() -> None: