[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #6936: feat: reading array elements

["alamb (via GitHub)" <gi...@apache.org>]

alamb commented on code in PR #6936:
URL: https://github.com/apache/arrow-datafusion/pull/6936#discussion_r1280562577


##########
datafusion/physical-expr/src/array_expressions.rs:
##########
@@ -363,6 +363,177 @@ pub fn make_array(arrays: &[ArrayRef]) -> Result<ArrayRef> {
     }
 }
 
+fn return_empty(return_null: bool, data_type: DataType) -> Arc<dyn Array> {
+    if return_null {
+        new_null_array(&data_type, 1)
+    } else {
+        new_empty_array(&data_type)
+    }
+}
+
+macro_rules! list_slice {
+    ($ARRAY:expr, $I:expr, $J:expr, $RETURN_ELEMENT:expr, $ARRAY_TYPE:ident) => {{
+        let array = $ARRAY.as_any().downcast_ref::<$ARRAY_TYPE>().unwrap();
+        if $I == 0 && $J == 0 || $ARRAY.is_empty() {
+            return return_empty($RETURN_ELEMENT, $ARRAY.data_type().clone());
+        }
+
+        let i = if $I < 0 {
+            if $I.abs() as usize > array.len() {
+                return return_empty(true, $ARRAY.data_type().clone());
+            }
+
+            (array.len() as i64 + $I + 1) as usize
+        } else {
+            if $I == 0 {
+                1
+            } else {
+                $I as usize
+            }
+        };
+        let j = if $J < 0 {
+            if $J.abs() as usize > array.len() {
+                return return_empty(true, $ARRAY.data_type().clone());
+            }
+
+            if $RETURN_ELEMENT {
+                (array.len() as i64 + $J + 1) as usize
+            } else {
+                (array.len() as i64 + $J) as usize
+            }
+        } else {
+            if $J == 0 {
+                1
+            } else {
+                if $J as usize > array.len() {
+                    array.len()
+                } else {
+                    $J as usize
+                }
+            }
+        };
+
+        if i > j || i as usize > $ARRAY.len() {
+            return_empty($RETURN_ELEMENT, $ARRAY.data_type().clone())
+        } else {
+            Arc::new(array.slice((i - 1), (j + 1 - i)))
+        }
+    }};
+}
+
+macro_rules! slice {

Review Comment:
   > I don't quite comprehend your comment. If I use take kernel with ListArray, the program returns me the elements of ListArray (i. e. Array instances). So if it's really worth it can you give me more information how should I take data?
   
   I was thinking we call `take()` with the values array (like `take(list_array.values(), indices`)



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