[petterparker]

this one might fit the description as well:

Rax is a radix tree implementation initially written to be used in a specific place of Redis in order to solve a performance problem, but immediately converted into a stand alone project to make it reusable for Redis itself, outside the initial intended application, and for other projects as well.

https://github.com/antirez/rax

[jb1991]

This is not a single file library so it doesn’t quite fall under this category of source code. Most or perhaps all of the nothings files are considered “header only“, something that can be very convenient for adding libraries.