Fix mode for multiple modes (#3566)

polars/polars-core/src/chunked_array/ops/unique/mod.rs

@@ -142,21 +142,16 @@ where
         .into_iter()
         .collect_trusted::<Vec<_>>();
     groups.sort_unstable_by_key(|k| k.1.len());
-    let first = &groups[0];
+    let last = &groups.last().unwrap();
 
-    let max_occur = first.1.len();
+    let max_occur = last.1.len();
 
     // collect until we don't take with trusted len anymore
     // TODO! take directly from iter, but first remove standard trusted-length collect.
-    let mut was_equal = true;
     let idx = groups
         .iter()
         .rev()
-        .take_while(|v| {
-            let current = was_equal;
-            was_equal = v.1.len() == max_occur;
-            current
-        })
+        .take_while(|v| v.1.len() == max_occur)
         .map(|v| v.0)
         .collect::<Vec<_>>();
     // Safety:
@@ -457,4 +452,22 @@ mod test {
             ]
         );
     }
+
+    #[test]
+    #[cfg(feature = "mode")]
+    fn mode() {
+        let ca = Int32Chunked::from_slice("a", &[0, 1, 2, 3, 4, 4, 5, 6, 5, 0]);
+        let mut result = Vec::from(&ca.mode().unwrap());
+        result.sort_by(|a, b| a.unwrap().cmp(&b.unwrap()));
+        assert_eq!(&result, &[Some(0), Some(4), Some(5)]);
+
+        let ca2 = Int32Chunked::from_slice("b", &[1, 1]);
+        let mut result2 = Vec::from(&ca2.mode().unwrap());
+        result2.sort_by(|a, b| a.unwrap().cmp(&b.unwrap()));
+        assert_eq!(&result2, &[Some(1)]);
+
+        let ca3 = Int32Chunked::from_slice("c", &[]);
+        let result3 = Vec::from(&ca3.mode().unwrap());
+        assert_eq!(result3, &[]);
+    }
 }