HaoYang670 commented on code in PR #1832:
URL: https://github.com/apache/arrow-rs/pull/1832#discussion_r894301166
##########
arrow/src/compute/kernels/substring.rs:
##########
@@ -182,24 +182,66 @@ pub fn substring_by_char<OffsetSize: OffsetSizeTrait>(
start: i64,
length: Option<u64>,
) -> Result<GenericStringArray<OffsetSize>> {
- Ok(array
- .iter()
- .map(|val| {
- val.map(|val| {
- let char_count = val.chars().count();
- let start = if start >= 0 {
- start.to_usize().unwrap().min(char_count)
- } else {
- char_count - (-start).to_usize().unwrap().min(char_count)
- };
- let length = length.map_or(char_count - start, |length| {
- length.to_usize().unwrap().min(char_count - start)
- });
+ let mut vals = BufferBuilder::<u8>::new(array.value_data().len());
+ let mut offsets = BufferBuilder::<OffsetSize>::new(array.len() + 1);
+ offsets.append(OffsetSize::zero());
+ let length = length.map(|len| len.to_usize().unwrap());
+
+ array.iter().for_each(|val| {
Review Comment:
> https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html is free and
safe in this context FWIW
Our `GenericStringIter` is implemented based on this. That's why I guess
`array.iter()` is optimal enough.
BTW, skipping nulls could not be the hotspot in this function, because it is
just a bitwise operation.
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