R - Grow data patches in a raster by one cell at a time

grow_by_one.R

# after https://twitter.com/ColinJCarlson/status/1142202184324530176
# kind of riffs of the game of life demo in ?raster::focal

grow_by_one <- function(x = NULL) {
  focal(x, w = matrix(1, ncol = 3, nrow = 3),
        fun = function(win) {
          if(!is.na(win[5])) {
            return(win[5])
          }
          
          if(any(!is.na(win[c(2,4,6,8)]))) {
            mean(win, na.rm = TRUE)
            } else {
              NA
            }
          }
        )
  }