Improve performance of toUpperCase and toLowerCase#194
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Fast-path unchanged ASCII runs inside translateCodePoints so case conversion no longer decodes every ASCII byte after crossing into the generic non-ASCII path. This specifically improves mixed ASCII/non-ASCII inputs such as öhello/ÖHELLO, where a non-ASCII code point is followed by a long ASCII tail that is already in the target case. In the targeted JMH benchmark with repeatCount=1024, mixed_non_ascii_ascii_noop improved from 13.9 us/op to 6.4 us/op for both toLowerCase and toUpperCase, about a 53% speedup, while mixed_non_ascii_ascii_change stayed roughly flat at around 20.6 us/op.
electrum
approved these changes
Apr 3, 2026
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The prior ascii fast path optimization was only applied to
toLowerCase, and this PR extends this totoUpperCase. Additionally, when there is mixed ascii and non-ascii, we can add a similar optimization where we skip the expensive per character decode for the ascii sequences. For inputs like öhello this results in a ~50% speed up.