[GitHub] [arrow-datafusion] alamb commented on a change in pull request #1660: fix: substr - correct behaivour with negative start pos

[GitBox <gi...@apache.org>]

alamb commented on a change in pull request #1660:
URL: https://github.com/apache/arrow-datafusion/pull/1660#discussion_r796026830



##########
File path: datafusion/src/physical_plan/unicode_expressions.rs
##########
@@ -457,21 +457,29 @@ pub fn substr<T: StringOffsetSizeTrait>(args: &[ArrayRef]) -> Result<ArrayRef> {
                                 start,
                                 count
                             )))
-                        } else if start <= 0 {
-                            Ok(Some(string.to_string()))
                         } else {
                             let graphemes = string.graphemes(true).collect::<Vec<&str>>();
-                            let start_pos = start as usize - 1;
-                            let count_usize = count as usize;
-                            if graphemes.len() < start_pos {
+                            let (start_pos, end_pos) = if start <= 0 {
+                                let end_pos = start + count - 1;
+                                (
+                                    0_usize,
+                                    if end_pos < 0 {
+                                        // we use 0 as workaround for usize to return empty string
+                                        0
+                                    } else {
+                                        end_pos as usize
+                                    },

Review comment:
       @ovr  would you like to make this change so we can merge the PR? I can find some time to do so too if that would help.




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