Swift MMDB native library - IP to country lookup.
If you want to look up countries for an IP address, because say all your comment spam in your blog comes from the same country, then the nice folks at MaxMind will hook you up with their GeoLite2 databases for no cost, though there is a license to pay attention to.
These files are in MMDB format which excels at encoding partitions of natural numbers. The format is documented and there are a number of libraries to access the files, but I wanted native Swift to keep my cross platform building nightmares in check.
You can check out swift-mmdb which I keep on github. I haven't made a change in 4 years, but that's just because it keeps working.
The performance cost is about 9MB for the data and 150µS per lookup.