Develop a custom Rcpp function to be used with terra::focalCpp to calculate the percent of a specific value within a moving window

I'm trying to speed up some raster processing I'm doing using terra::focal by using focalCpp.

Here is some example data with 1s and NAs included to replicate an actual dataset

nr <- nc <- 50
r <- rast(ncols=nc, nrows=nr, ext= c(0, nc, 0, nr))
values(r) <- rep(c(rep(NA, 10), rep(1, 10), seq(1,8), rep(1,12), rep(NA,5), rep(1,15),seq(1,8), rep(NA,12), seq(1,20)), 50)
 

This is the original function used with focal that I'm trying to duplicate in Rcpp

fxnpercent = function(x) {
  q=x # make a copy of x
  q[q!=1] = NA # q becomes just 1s
  length(q[!is.na(q)])/length(x[!is.na(x)]) * 100 # gets percentage of 1s
}

This is the original focal function with a window approximated by a 200m buffer

# moving window matrix 
mat = matrix(1,15,15) # create matrix of 1's that envelopes the extent of the buffer
gr = expand.grid(1:nrow(mat), 1:nrow(mat)) # df of all pairwise values based on row/col index
center = 8 # centroid index of the square grid
gr$dist = sqrt((gr$Var1-center)^2 + (gr$Var2-center)^2) # euclidean distance calucation 
threshold = 200/30 # 200m threshold is converted into number of pixels from center
gr$inside = ifelse(gr$dist < threshold, 1, NA) # if distance is less than threshold, grid value is one, otherwise NA
w = matrix(gr$inside, 15,15)  # Using gr$inside, place indexed values into matrix of original dimensions

#output percent from moving window
percent = terra::focal(x=r, w=w, fun=fxnpercent, na.policy="all")

And this is my attempt to duplicate the fxnpercent function in Rcpp

cppFunction( 
    'NumericVector fxnpercent(NumericVector x, size_t ni, size_t nw) {
		NumericVector out(ni);

		// loop over cells
		size_t start = 0;
		for (size_t i=0; i<ni; i++) {
			size_t end = start + nw;
			// compute something for a window
			double v = 0;
			// loop over the values of a window
			for (size_t j=start; j<end; j++) {
				if (x[j] != 1) {
				  v += std::nan("");
				}
				else {
				  v += x[j];
				}
				
			}
			
			NumericVector x1 = x[!is_na(x)];
			NumericVector v1 = v;
			NumericVector v2 = v1[!is_nan(x)];
			
			size_t v2size = v2.size();
			size_t x1size = x1.size();
			
			out[i] = (v2size / x1size) * 100;
			
			start = end;
		}
		return out;
	}'
  )

After lots of troubleshooting to get the syntax correct in Rcpp, I try to run this function with focalCpp and I have an error.

percent = focalCpp(r, w=w, fun=fxnpercent, na.policy="all")

#my current error

Error: [focalCpp] test failed

I think I need to do some calculations within the window loop in the Rcpp function but I'm having trouble with understanding how to set it up to work correctly.

NaN checking is not done via is_nan() but rather using std::isnan()

#include<Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
NumericVector fxnpercent(NumericVector x, size_t ni, size_t nw) {
  NumericVector out(ni);
  
  // loop over cells
  size_t start = 0;
  for (size_t i=0; i<ni; i++) {
    size_t end = start + nw;
    // compute something for a window
    double v = 0;
    double totalCount = 0;
    double countOfOnes = 0;

    // loop over the values of a window
    for (size_t j=start; j<end; j++) {
      if (!std::isnan(x[j])) {
        totalCount++;
        if (x[j] == 1) {
          countOfOnes++;
        }
      }
    }

    if (totalCount > 0) {
      out[i] = (countOfOnes / totalCount) * 100;
    } else {
      out[i] = std::nan(""); // it is NaN if all are NaN
    }
    
    start = end;
  }
  return out;
}