R/gepaf.R

In mthh/gepaf: Google Encoded Polyline Algorithm Format

#' @name encodePolyline
#' @title Encode Coordinates to Google Polylines
#' @description Encode a data.frame of coordinates to a Google polyline.
#' @param df_coords a data frame of coordinates with two columns: latitudes and
#' longitudes. Coordinates must be in decimal degrees (WGS84).
#' @param factor number of decimal digits to be used.
#' @return An encoded polyline is returned.
#' @examples
#' coords <- data.frame(lat = c(38.5, 40.7, 43.252),
#'                      lon = c(-120.2, -120.95, -126.453))
#' encpoly <- encodePolyline(coords)
#' encpoly
#' @export
encodePolyline <- function(df_coords, factor = 5){
  factor <- 10^factor
  dims <- dim(df_coords)[1]
  output_string <- paste0(enc_coord(df_coords[1, 1], factor = factor),
                          enc_coord(df_coords[1, 2], factor = factor))
  for(i in seq(2, dims)){
    output_string <- paste0(output_string,
                            enc_coord(df_coords[i, 1] - df_coords[i-1, 1], factor = factor),
                            enc_coord(df_coords[i, 2] - df_coords[i-1, 2], factor = factor))
  }
  return(output_string)
}



## Encoding Utils
enc_coord <- function(coord, factor){
  output <- ""
  pt <- round(coord * factor, digits = 0) * 2
  if(pt < 0) pt <- bitops::bitFlip(pt)
  while(pt >= 0x20){
    output <- paste0(output,
                     intToUtf8(bitops::bitOr(0x20, bitops::bitAnd(pt, 0x1f)) + 63))
    pt <- bitops::bitShiftR(pt, 5)
  }
  return(paste0(output, intToUtf8(pt + 63)))
}

#' @name decodePolyline
#' @title Decode a Google Polyline to a Data Frame
#' @description Decode a Google polyline to a data frame of coordinates.
#' @param enc_polyline a Google polyline.
#' @param factor number of decimal digits to be used.
#' @return A data frame of latitudes and longitudes is returned.
#' @examples
#' coords <- decodePolyline(enc_polyline = "_p~iF~ps|U_ulLnnqC_mqNvxq`@")
#' coords
#' @export
decodePolyline <- function(enc_polyline, factor = 5) {
  factor <- 10^factor
  if(class(enc_polyline) != "character") stop("Wrong encoded string format")

  len <- nchar(enc_polyline)
  enc_polyline <- strsplit(enc_polyline, '')[[1]]

  idx <- 1
  res_idx <- 0
  lat = lon = 0
  changes <- list(NULL, NULL)
  pairs_points <- matrix(nrow = 0, ncol = 2)

  while(idx <= len) {
    for(u in c(1, 2)){
      shift = result = 0
      while(TRUE){
        byte <- as.integer(charToRaw(enc_polyline[idx])) - 63
        result <- bitops::bitOr(result,
                                bitops::bitShiftL(bitops::bitAnd(byte, 0x1f), shift))
        idx <- idx + 1
        shift <- shift + 5
        if(byte < 0x20) break
      }
      changes[u] <- ifelse(bitops::bitAnd(result, 1),
                           -(result - (bitops::bitShiftR(result, 1))),
                           bitops::bitShiftR(result, 1))
    }
    lat <- lat + changes[[1]]
    lon <- lon + changes[[2]]
    res_idx <- res_idx + 1
    pairs_points <- rbind(pairs_points, c(lat, lon)/factor)
  }
  coords <- data.frame(pairs_points)
  names(coords) <- c('lat', 'lon')
  return(coords)
}