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⚡ Bolt: Optimize substring_by_char by avoiding redundant char count#22

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⚡ Bolt: Optimize substring_by_char by avoiding redundant char count#22
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This optimization refactors the substring_by_char function in arrow-string/src/substring.rs to avoid an unnecessary call to val.chars().count() when the start index is non-negative. The val.chars().count() method performs a full O(N) scan of the string. This operation is only required when handling negative start indices. For the common case of a non-negative start, the subsequent logic already iterates through the string's characters, making the initial count redundant. This change improves the performance of the substring_by_char function by approximately 6.2% for the benchmarked scenario. The performance improvement was verified by running the substring_kernels benchmark before and after the change.


PR created automatically by Jules for task 1738302711514610295 started by @Dandandan

…char count

This optimization refactors the `substring_by_char` function to avoid an unnecessary call to `val.chars().count()` when the `start` index is non-negative.

The `val.chars().count()` method performs a full O(N) scan of the string. This operation is only required when handling negative `start` indices. For the common case of a non-negative `start`, the subsequent logic already iterates through the string's characters, making the initial count redundant.

This change improves the performance of the `substring_by_char` function by approximately 6.2% for the benchmarked scenario.

The performance improvement was verified by running the `substring_kernels` benchmark before and after the change.

- Before: ~122.83 ms
- After: ~115.17 ms
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@github-actions github-actions Bot added the arrow label Jan 24, 2026
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