In a [[ https://gerrit.wikimedia.org/r/#/c/243383/9/includes/Sanitizer.php | recent code review ]] which modified Sanitizer::validateAttributes, [[ https://phabricator.wikimedia.org/p/tstarling/ | tstarling ]] noticed the following line, and asked an excellent question.
```
$whitelist = array_flip( $whitelist );
```
> This looks slow. Why aren't they both flipped before caching?
Beats me! The $whitelist in validateAttributes is passed from an array of arrays cached as a static in setupAttributeWhitelist. validateAttributes flips $whitelist to take advantage of isset's speed over in_array. (See [[ http://blog.straylightrun.net/2008/12/03/tip-of-the-day-codeissetcode-vs-codein_arraycode/ | this blog post ]] for discussion on isset performance.) There is only one isset statement in validateAttributes, however, so the flip would appear to undo any speed gain.
If the arrays were either flipped at the end of the initialization in setupAttributeWhitelist, or declared in flip order to begin with, that would apparently save flipping one of the arrays each time validateAttributes is called.
It would, however, change the output of public functions setupAttributeWhitelist and attributeWhitelist. Would the speed gain be worth it?