Conversions between latitude, longitude and Maidenhead Locator System

mls.rb

module MLS
  extend self

  GRID = ("A".."R").to_a
  SQUARE = ("0".."9").to_a
  SUB = ("a".."x").to_a

  deg = 180.0
  PARTS = [[18, GRID], [10, SQUARE], [24, SUB], [10, SQUARE]]
  PARTS.map! { |div, part| [deg = deg / div, part] }

  def from_latlng(lat, lng, precision = 4)
    lat += 90
    lng += 180

    PARTS[0...precision].inject("") do |res, (div, part)|
      part_lat, lat = lat.divmod(div)
      part_lng, lng = lng.divmod(div * 2)
      res << part[part_lng] << part[part_lat]
    end
  end
  
  def to_latlng(mls)
    mls = mls.split('').reverse
    
    lat = lng = m_index = 0
    PARTS[0...mls.length / 2].reverse.each do |value, part|
      lat += value * part.find_index(mls[m_index])
      lng += value * part.find_index(mls[m_index + 1]) * 2
      m_index += 2
    end
    [lat - 90, lng - 180]
  end
end