Check out the fancy SSE4/BMI2 single instruction bit interleaving for geohash creation:
Lines 73 to 91 in 599c753
Also includes other refactorings against the original geohash-int
release from 2014, but code has diverged somewhat since then.
mkdir build
cd build
cmake ..
make -j12
A fast C99 geohash library which only provide int64 hash result. GeoHash.
Just copy all files into your project.