The writers had a presentation of the techniques found here: http://www.interfacesymposia.org/I04/I2004Proceedings/ChenChao/ChenChao.presentation.pdf
According to the authors, there’s an add-on package to R that implements their original Fortran:
Here are the working links to the R package:
- https://cran.r-project.org/web/packages/randomForest/index.html
- https://CRAN.R-project.org/package=randomForest
This can save you from having to implement this manually.