Handle passing deflation scheme encoding between R and C++

[Banana1530]

Hmmm.... it looks like the answer is "sort of." The following works, but isn't totally type-safe. We could probably do a more general solution in the future. Issue?

#include "Rcpp.h"

enum DeflationMethod {
  PCA = 0, 
  PLS = 1,  
  CCA = 2
};

namespace Rcpp {
  SEXP wrap(DeflationMethod dm){
    Rcpp::IntegerVector dm_int = Rcpp::wrap(static_cast<int>(dm));
    dm_int.attr("class") = "DeflationMethod"; 
    return dm_int;
  }

  template <> DeflationMethod as(SEXP dm_sexp){
    Rcpp::IntegerVector dm_iv(dm_sexp);
    int dm_int = dm_iv(0); 
    DeflationMethod dm = static_cast<DeflationMethod>(dm_int);
    return dm; 
  }
}

// [[Rcpp::plugins(cpp11)]]
// [[Rcpp::export]]
DeflationMethod make_pls(){
  DeflationMethod pls = DeflationMethod::PLS;
  return pls;
}

// [[Rcpp::export]]
void take_pls(DeflationMethod x){
  Rcpp::Rcout << " x = " << x << std::endl; 
}

Originally posted by @michaelweylandt in https://github.com/DataSlingers/MoMA/pull/54

[Banana1530]

In the code C++ and R have their own enum-type structure to encode selection schemes and deflation schemes (DeflationScheme and SelectionScheme in moma_base.h, and DEFLATION_SCHEME and SELECTION_SCHEME in util.R). However communication between them is essentially passing integers.

To ensure consistency in encoding between R and C++ we need to manually maintain moma_base.h and util.R.