PCG32, as transcribed in src/godot/rng.h from Godot 4.1.4's thirdparty/misc/pcg.cpp:

// *Really* minimal PCG32 code / (c) 2014 M.E. O'Neill / pcg-random.org
// Licensed under Apache License 2.0 (NO WARRANTY, etc. see website)

The Apache License 2.0 is in apache-2.0.txt.
