In r-gris/torpor: Lazy Topology

knitr::opts_chunk$set(echo = TRUE)

What is TopoJSON?

url <- "https://raw.githubusercontent.com/shawnbot/d3-cartogram/master/data/us-states.topojson"


library(leaflet)
leaflet() %>% addGeoJSONv2(readLines(url))
## jsonlite does a weird simplify thing where a could-be-dimensioned array
## might be [1, 5] or [5, 1, 1] but for TopoJSON we can just treat all
## child-of-geometry arc arrays as 1D
## but then why is the coordinates also called "arcs"
x <- jsonlite::fromJSON(url, simplifyVector = FALSE)
names(x)
x$type
x$transform
names(x$objects)
lapply(x$objects, "[", "type")

lapply(x$objects, function(x) unique(x$geometries$type))

lapply(x$objects, function(x) length(x$geometries$arcs))

unlist(range(lapply(x$arcs, nrow)))
unique(range(lapply(x$arcs, ncol)))

str(x)

So the x$arcs are the coordinates, and must be qualified by transform.

coords_integer <- do.call(rbind, x$arcs) 
vec2mat <- function(x, n) matrix(rep(x, each = n), ncol = length(x))
coords <- vec2mat(x$transform$scale, nrow(coords_integer)) * coords_integer + vec2mat(x$transform$translate, nrow(coords_integer))
library(dplyr)
arcs_obj1 <- bind_rows(lapply(x$objects[[1]]$geometries$arcs, 
                              function(x) {
                                if (is.recursive(x)) x <- x[[1]]
                                tibble(a = as.vector(x))})
                       , .id = "geometry")

plot(coords)
lapply(split(arcs_obj1, arcs_obj1$geometry), function(x) lines(coords[x$a + 1, ]))