speed up TRI0 for a path-type

[mdsumner]

This is a draft replacement for the triangulate_0 core:

x <- sf::read_sf(system.file("gpkg/nc.gpkg", package = "sf", mustWork = TRUE))
#x <- sf::st_sf(a = 1, geom = sf::st_sfc(sfzoo[[6]]))
library(silicate)
library(dplyr)
tri_faster <- function(x) {
  p <- PATH0(x)
  #pp <- tidyr::unnest(p$object["path_"], cols = c(path_)) %>% 
  pp <-  
    dplyr::transmute(dplyr::bind_rows(p[["object"]][["path_"]]), 
                     vertex_, object_, path_, tri_object = paste(object_, subobject, sep = "-"))
  path <- gibble::gibble(x)
  path$tri_object <- paste(path$object, path$subobject, sep = "-")
  ## have object
  ## we are subobject
  usub <- unique(pp$tri_object)
  coord <- p$vertex[pp$vertex_, ]
  trilist <- vector("list", length(usub)) 
  vertsbefore <- 0
  
  for (i in seq_along(usub)) {
    vertsnow <- path$nrow[path$tri_object == usub[i]]
    rowindex <- (vertsbefore + 1):(vertsbefore + sum(vertsnow))
    verts <- coord[rowindex, ]
    holes <- 0
    if (length(vertsnow) > 1) holes <- head(cumsum(vertsnow), -1L) + 1
    
    trindex <- decido::earcut(cbind(verts[["x_"]], verts[["y_"]]), holes)
    trimat <- matrix(trindex, ncol = 3L, byrow = TRUE)
    windex <- pp$vertex_[rowindex]
    #browser()
    trilist[[i]] <- tibble::tibble(.vx0 = pp$vertex_[windex[trimat[,1L]]],
                                   .vx1 = pp$vertex_[windex[trimat[,2L]]],
                                   .vx2 = pp$vertex_[windex[trimat[,3L]]],
                                   #path_ = pp$path_[rowindex],
                                   object_ = pp$object_[rowindex][1])
    
    
    vertsbefore <- vertsbefore + sum(vertsnow)
  }
  bind_rows(trilist)
  
}


system.time({
  tri <- tri_faster(x)
coord <- sc_coord(x)
plot(coord, pch = ".")
polygon(coord[rbind(tri$.vx0, tri$.vx1, tri$.vx2, NA), ])
})

 user  system elapsed 
  0.250   0.004   0.254 

library(quadmesh)
## the thing we want sped up
system.time(mesh_plot(TRI0(x)))
 user  system elapsed 
  0.970   0.016   0.986 

The key parts are maintaining the link between vertex and coordinate index (done with tortuous joining previously, and direct bind_rows on the object$path_ rather than unnesting).

Looks about 10x faster - motivation is to make mesh_plot(TRI0(x)) much faster.