[GitHub] [arrow-rs] HaoYang670 commented on a diff in pull request #1665: Add dictionary array support for substring function

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

HaoYang670 commented on code in PR #1665:
URL: https://github.com/apache/arrow-rs/pull/1665#discussion_r867324877


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arrow/src/compute/kernels/substring.rs:
##########
@@ -18,13 +18,136 @@
 //! Defines kernel to extract a substring of an Array
 //! Supported array types: \[Large\]StringArray, \[Large\]BinaryArray
 
+use crate::array::DictionaryArray;
 use crate::buffer::MutableBuffer;
+use crate::datatypes::*;
 use crate::{array::*, buffer::Buffer};
 use crate::{
     datatypes::DataType,
     error::{ArrowError, Result},
 };
 use std::cmp::Ordering;
+use std::sync::Arc;
+
+/// Returns an ArrayRef with substrings of all the elements in `array`.
+///
+/// # Arguments
+///
+/// * `start` - The start index of all substrings.
+/// If `start >= 0`, then count from the start of the string,
+/// otherwise count from the end of the string.
+///
+/// * `length`(option) - The length of all substrings.
+/// If `length` is `None`, then the substring is from `start` to the end of the string.
+///
+/// Attention: Both `start` and `length` are counted by byte, not by char.
+///
+/// # Basic usage
+/// ```
+/// # use arrow::array::StringArray;
+/// # use arrow::compute::kernels::substring::substring;
+/// let array = StringArray::from(vec![Some("arrow"), None, Some("rust")]);
+/// let result = substring(&array, 1, Some(4)).unwrap();
+/// let result = result.as_any().downcast_ref::<StringArray>().unwrap();
+/// assert_eq!(result, &StringArray::from(vec![Some("rrow"), None, Some("ust")]));
+/// ```
+///
+/// # Error
+/// - The function errors when the passed array is not a \[Large\]String array or \[Large\]Binary array.
+/// - The function errors if the offset of a substring in the input array is at invalid char boundary (only for \[Large\]String array).
+///
+/// ## Example of trying to get an invalid utf-8 format substring
+/// ```
+/// # use arrow::array::StringArray;
+/// # use arrow::compute::kernels::substring::substring;
+/// let array = StringArray::from(vec![Some("E=mc²")]);
+/// let error = substring(&array, 0, Some(5)).unwrap_err().to_string();
+/// assert!(error.contains("invalid utf-8 boundary"));
+/// ```
+pub fn substring(array: &dyn Array, start: i64, length: Option<u64>) -> Result<ArrayRef> {

Review Comment:
   Just a nit: Maybe we could let `length` be `Option<u32>`. Because the longest length will not exceed `1<<32 - 1` (for `LargeBinaryArray` and `LargeStringArray`)



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