fix: Revert categorical unique code

crates/polars-core/src/chunked_array/logical/categorical/ops/unique.rs

@@ -1,6 +1,4 @@
-use polars_compute::unique::{
-    DictionaryRangedUniqueState, PrimitiveRangedUniqueState, RangedUniqueKernel,
-};
+use polars_compute::unique::{DictionaryRangedUniqueState, RangedUniqueKernel};
 
 use super::*;
 
@@ -43,32 +41,11 @@ impl CategoricalChunked {
                 Ok(out)
             }
         } else {
-            let has_nulls = (self.null_count() > 0) as u32;
-            let mut state = match cat_map.as_ref() {
-                RevMapping::Global(map, values, _) => {
-                    if self.is_enum() {
-                        PrimitiveRangedUniqueState::new(0, values.len() as u32 + has_nulls)
-                    } else {
-                        let mut min = u32::MAX;
-                        let mut max = 0u32;
-
-                        for &v in map.keys() {
-                            min = min.min(v);
-                            max = max.max(v);
-                        }
-
-                        PrimitiveRangedUniqueState::new(min, max + has_nulls)
-                    }
-                },
-                RevMapping::Local(values, _) => {
-                    PrimitiveRangedUniqueState::new(0, values.len() as u32 + has_nulls)
-                },
-            };
-
+            let mut state = DictionaryRangedUniqueState::new(cat_map.get_categories().to_boxed());
             for chunk in self.physical().downcast_iter() {
-                state.append(chunk);
+                state.key_state().append(chunk);
             }
-            let unique = state.finalize_unique();
+            let (_, unique, _) = state.finalize_unique().take();
             let ca = unsafe {
                 UInt32Chunked::from_chunks_and_dtype_unchecked(
                     self.physical().name().clone(),

py-polars/polars/_utils/various.py

@@ -652,6 +652,7 @@ def re_escape(s: str) -> str:
     return re.sub(f"([{re_rust_metachars}])", r"\\\1", s)
 
 
+# Don't rename or move. This is used by polars cloud
 def display_dot_graph(
     *,
     dot: str,

py-polars/tests/unit/datatypes/test_categorical.py

@@ -905,3 +905,23 @@ def test_categorical_unique() -> None:
     s = pl.Series(["a", "b", None], dtype=pl.Categorical)
     assert s.n_unique() == 3
     assert s.unique().to_list() == ["a", "b", None]
+
+
+@StringCache()
+def test_categorical_unique_20539() -> None:
+    df = pl.DataFrame({"number": [1, 1, 2, 2, 3], "letter": ["a", "b", "b", "c", "c"]})
+
+    result = (
+        df.cast({"letter": pl.Categorical})
+        .group_by("number")
+        .agg(
+            unique=pl.col("letter").unique(),
+            unique_with_order=pl.col("letter").unique(maintain_order=True),
+        )
+    )
+
+    assert result.sort("number").to_dict(as_series=False) == {
+        "number": [1, 2, 3],
+        "unique": [["a", "b"], ["b", "c"], ["c"]],
+        "unique_with_order": [["a", "b"], ["b", "c"], ["c"]],
+    }