Don’t stop early: Case-folding source code at memory speed

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Suppose a user searches for café and your corpus contains CAFÉ, or they type straße and you’ve stored STRASSE. To make these count as matches, you need a canonical form that erases case distinctions, so that two strings which differ only in case compare equal. That form is case folding, and it shows up wherever text is matched rather than displayed: search engines, regex (?i) flags, case-insensitive usernames and hostnames.

It’s a basic operation, but at GitHub we run it a lot. Blackbird, GitHub’s code search engine, indexes over 180 million repositories—more than 480TB of source code. Every byte is case-folded before we extract ngrams and build the index, and for every potential query result, another (implicit or explicit) case folding operation is needed to locate matches. At that scale, the speed of even a basic operation starts to matter.

This post is about how we made it fast, and it starts somewhere counterintuitive: the biggest win in the ASCII fast path came from removing an optimization, not adding one. It turns out to be faster to sweep the whole buffer with no branches than to stop early at the first non-ASCII byte. We open-sourced the result as a Rust crate called casefold.

Folding is not lowercasing

It is tempting to reach for str::to_lowercase, but lowercasing and folding are different operations with different goals:

Lowercasing is for display, and it’s locale- and context-sensitive: Greek final sigma lowercases to ς at the end of a word and σ elsewhere, and Turkish I lowercases differently than English I. Case folding is for comparison, and it’s …

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