Additions to the pr_geohash gem for distance calculation

pr_geohash_ext.rb

require "pr_geohash"

module GeoHash

  RADIUS_OF_THE_EARTH = 6371
  RADIANS_CONVERSION_FACTOR = 180 / Math::PI

  # decode geohash to a latitude/longitude point in the centre of the
  # bounding box described by the geohash
  def decode_point(geohash)
    lat_min, lng_min, lat_max, lng_max = decode(geohash).flatten
    lat = round_between((lat_min + lat_max) / 2, lat_min, lat_max)
    lng = round_between((lng_min + lng_max) / 2, lng_min, lng_max)
    [lat, lng]
  end
  module_function :decode_point

  # return distance between the centre point of each geohash in kilometres
  def distance(origin_geohash, destination_geohash)
    origin_lat, origin_long = GeoHash.decode_point(origin_geohash)
    dest_lat, dest_long = GeoHash.decode_point(destination_geohash)

    sin_lats = Math.sin(rad(origin_lat)) * Math.sin(rad(dest_lat))
    cos_lats = Math.cos(rad(origin_lat)) * Math.cos(rad(dest_lat))
    cos_longs = Math.cos(rad(dest_long) - rad(origin_long))

    x = sin_lats + (cos_lats * cos_longs)
    x = [x, 1.0].min
    x = [x, -1.0].max

    Math.acos(x) * RADIUS_OF_THE_EARTH
  end
  module_function :distance

  # :stopdoc:
  # nominally private, but not possible as a module_function
  # (which they have to be available for the other module_functions)

  def round_between(value, min, max, max_length=10)
    rounded = value
    while max_length > 0 && rounded > min && rounded < max
      rounded = value.round(max_length)
      max_length -= 1
    end
    value.round(max_length + 1)
  end
  module_function :round_between

  def rad(degree)
    degree / RADIANS_CONVERSION_FACTOR
  end
  module_function :rad

end