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.