Fix slicing already sliced RunEndArray (#781)

encodings/runend/src/compute.rs

@@ -98,7 +98,7 @@ impl SliceFn for RunEndArray {
             slice(&self.values(), slice_begin, slice_end + 1)?,
             self.validity().slice(start, stop)?,
             stop - start,
-            start,
+            start + self.offset(),
         )?
         .into_array())
     }
@@ -225,6 +225,33 @@ mod test {
         );
     }
 
+    #[test]
+    fn double_slice() {
+        let arr = slice(
+            RunEndArray::try_new(
+                vec![2u32, 5, 10].into_array(),
+                vec![1i32, 2, 3].into_array(),
+                Validity::NonNullable,
+            )
+            .unwrap()
+            .array(),
+            3,
+            8,
+        )
+        .unwrap();
+        assert_eq!(arr.len(), 5);
+
+        let doubly_sliced = slice(&arr, 0, 3).unwrap();
+
+        assert_eq!(
+            doubly_sliced
+                .into_primitive()
+                .unwrap()
+                .maybe_null_slice::<i32>(),
+            vec![2, 2, 3]
+        );
+    }
+
     #[test]
     fn slice_end_inclusive() {
         let arr = slice(