tolower() in bulk at speed

https://dotat.at/@/2022-06-27-tolower-swar.html

Here's a fun bit of Monday optimization.

DNS servers often have to convert DNS names to their canonical lower case form. BIND has to do so a bit more than most because it tries to preserve the case of DNS names rather than canonicalizing them.

I was recently distracted by a cleanup oportunity: there were multiple maptolower lookup tables in BIND, so I decided to make a home for them so that we would only need one copy. Then I thought, surely there are faster ways to implement tolower() than a lookup table.

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