fix offsets in categorical merge (#3242)

polars/polars-core/src/chunked_array/logical/categorical/ops/zip.rs

@@ -7,15 +7,14 @@ impl CategoricalChunked {
         other: &CategoricalChunked,
     ) -> Result<Self> {
         let cats = match &**self.get_rev_map() {
-            RevMapping::Local(_) => {
+            RevMapping::Local(rev_map) => {
                 // the logic for merging the rev maps will concatenate utf8 arrays
                 // to make sure the indexes still make sense we need to offset the right hand side
                 self.logical()
-                    .zip_with(mask, &(other.logical() + self.logical.len() as u32))?
+                    .zip_with(mask, &(other.logical() + rev_map.len() as u32))?
             }
             _ => self.logical().zip_with(mask, other.logical())?,
         };
-
         let new_state = self.merge_categorical_map(other);
         Ok(CategoricalChunked::from_cats_and_rev_map(cats, new_state))
     }

polars/tests/it/lazy/expressions/arity.rs

@@ -112,3 +112,39 @@ fn includes_null_predicate_3038() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_when_then_otherwise_cats() -> Result<()> {
+    let lf = df!["book" => [Some("bookA"),
+        None,
+        Some("bookB"),
+        None,
+        Some("bookA"),
+        Some("bookC"),
+        Some("bookC"),
+        Some("bookC")],
+        "user" => [Some("bob"), Some("bob"), Some("bob"), Some("tim"), Some("lucy"), Some("lucy"), None, None]
+    ]?.lazy();
+
+    let out = lf
+        .with_column(col("book").cast(DataType::Categorical(None)))
+        .with_column(col("user").cast(DataType::Categorical(None)))
+        .with_column(
+            when(col("book").eq(Null {}.lit()))
+                .then(col("user"))
+                .otherwise(col("book"))
+                .alias("a"),
+        )
+        .collect()?;
+
+    assert_eq!(
+        out.column("a")?
+            .categorical()?
+            .iter_str()
+            .flatten()
+            .collect::<Vec<_>>(),
+        &["bookA", "bob", "bookB", "tim", "bookA", "bookC", "bookC", "bookC"]
+    );
+
+    Ok(())
+}