tileXYToQuadKey.R

# converts slippy map XYZ tile to quadkey
# https://docs.microsoft.com/en-us/bingmaps/articles/bing-maps-tile-system

as_binary = function(x){
  tmp = rev(as.integer(intToBits(x)))
  id = seq_len(match(1, tmp, length(tmp)) - 1)
  tmp[-id]
}

deg2num = function(lat_deg, lon_deg, zoom) {
  lat_rad = lat_deg * pi /180
  n = 2.0 ^ zoom
  xtile = floor((lon_deg + 180.0) / 360.0 * n)
  ytile = floor((1.0 - log(tan(lat_rad) + (1 / cos(lat_rad))) / pi) / 2.0 * n)
  c(xtile, ytile)
}

# reference JavaScript implementations
# https://developer.here.com/documentation/traffic/dev_guide/common/map_tile/topics/quadkeys.html
tileXYToQuadKey = function(xTile, yTile, z) {
  quadKey = ""
  for (i in z:1) {
    digit = 0
    mask = bitwShiftL(1, i - 1)
    xtest = as_binary(bitwAnd(xTile, mask))
    if(any(xtest)) {
      digit = digit + 1
    }
    
    ytest = as_binary(bitwAnd(yTile, mask))
    if(any(ytest)) {
      digit = digit + 2
    }
    quadKey = paste0(quadKey, digit)
  }
  quadKey
}


stopifnot(tileXYToQuadKey(35210, 21493, 16) == "1202102332221212")