Fix take on sliced RunEnd array (#859)

seems to be a regression from #840

encodings/runend/src/compute.rs

@@ -50,7 +50,7 @@ impl TakeFn for RunEndArray {
                         vortex_error::vortex_bail!(OutOfBounds: usize_idx, 0, self.len());
                     }
 
-                    Ok(idx as u64)
+                    Ok((usize_idx + self.offset()) as u64)
                 })
                 .collect::<VortexResult<Vec<u64>>>()?
         });
@@ -336,4 +336,19 @@ mod test {
             vec![Some(1), None, None, Some(3),]
         );
     }
+
+    #[test]
+    fn sliced_take() {
+        let sliced = slice(ree_array().as_ref(), 4, 9).unwrap();
+        let taken = take(
+            sliced.as_ref(),
+            PrimitiveArray::from(vec![1, 3, 4]).as_ref(),
+        )
+        .unwrap();
+
+        assert_eq!(taken.len(), 3);
+        assert_eq!(scalar_at(taken.as_ref(), 0).unwrap(), 4.into());
+        assert_eq!(scalar_at(taken.as_ref(), 1).unwrap(), 2.into());
+        assert_eq!(scalar_at(taken.as_ref(), 2).unwrap(), 5.into());
+    }
 }

encodings/runend/src/runend.rs

@@ -84,7 +84,8 @@ impl RunEndArray {
             .map(|s| s.to_ends_index(self.ends().len()))
     }
 
-    /// Convert a batch of logical indices into an index for the values.
+    /// Convert a batch of logical indices into an index for the values. Expects indices to be adjusted by offset unlike
+    /// [Self::find_physical_index]
     ///
     /// See: [find_physical_index][Self::find_physical_index].
     pub fn find_physical_indices(&self, indices: &[u64]) -> VortexResult<Vec<usize>> {