Google plus code decoder/encoder

pluscode.js

'use strct'
const _ = require('lodash')

const LETTERS = Array.from("23456789CFGHJMPQRVWX")
const R_LETTERS = _.invert(LETTERS)

const ELEVENTH = (function() {
  const dy = 0.000025
  const dx = 0.00003125

  let res = {}

  for(let y = 0; y <= 4; y++) {
    for(let x = 0; x <= 3; x++) {
      res[LETTERS[y * 4 + x]] = [y * dy + (dy / 2), x * dx + (dx / 2)]
    }
  }

  return res
})()

function plus2coords(code) {
  code = code.replace("+", "")
  let eleventh = [0, 0]
  if(code.length == 11) {
    eleventh = ELEVENTH[code[10]]
    code = code.slice(0, 10)
  }
  let chunks = _.chunk(code, 2)
  let [ys, xs] = _.zip(...chunks)

  let x = -180.0
  let y = -90.0

  let dx = 20
  let dy = 20

  for(let letterPos in xs) {
    let digit = R_LETTERS[xs[letterPos]] - 0
    dx = Math.pow(20, 1 - letterPos)
    x = x + dx * digit
  }

  for(let letterPos in ys) {
    let digit = R_LETTERS[ys[letterPos]] - 0
    dy = Math.pow(20, 1 - letterPos)
    y = y + dy * digit
  }

  if(eleventh[0] != 0.0) {
    y = y + eleventh[0]
    x = x + eleventh[1]
  } else {
    y = y + dy / 2
    x = x + dx / 2
  }

  return [y, x]
}

function divmod(n, d) {
  return [Math.floor(n / d), n % d]
}

function coords2pluscode(lat, lon) {
  if(Math.abs(lat) > 90) { return "" }
  lat = lat + 90

  let rad = lon * Math.PI / 180
  norm_r = rad - (2 * Math.PI) * Math.floor((rad + Math.PI) / (2 * Math.PI))
  lon = norm_r * 180 / Math.PI + 180

  let q, r
  let dy = 20
  let dx = 20

  let code = Array.from("0000000000")

  ry = lat
  for(let i = 0; i < 5; i++) {
    [q, r] = divmod(ry, dy)
    dy = dy / 20
    ry = r
    code[i * 2] = LETTERS[q]
  }

  rx = lon
  for(let i = 0; i < 5; i++) {
    [q, r] = divmod(rx, dx)
    dx = dx / 20
    rx = r
    code[i * 2 + 1] = LETTERS[q]
  }

  code.splice(8, 0, '+')

  // Calculate eleventh letter
  qy = Math.floor(ry / 0.000025)
  qx = Math.floor(rx / 0.00003125)

  code.push(LETTERS[qy * 4 + qx])

  return code.join("")
}

console.log(plus2coords("8Q98HX6J+9F2")) // 37.560887,126.981141
console.log(plus2coords("8Q98HX6J+C58")) // 37.561037,126.980453
console.log(plus2coords("8Q98HX6J+9F")) // 37.560937,126.981187

console.log(coords2pluscode(37.560937,126.981187)) // 8Q98HX6J+9FF
console.log(coords2pluscode(37.560937,126.981203)) // 8Q98HX6J+9FG