Random location near source

random_close_location.rb

def random_close_location(max_dist = 0.5, km = true)
  # http://www.geomidpoint.com/random/calculation.html

  return unless source && source[:lat] != nil && source[:lng] != nil

  deg_to_rad = Math::PI / 180
  rad_to_deg = 180 / Math::PI
  radius_earth_M = 3960.056052
  radius_earth_Km = 6372.796924

  rand1 = rand
  rand2 = rand

  # Convert all latitudes and longitudes to radians
  start_lat = source[:lat] * deg_to_rad
  start_lng = source[:lng] * deg_to_rad

  # Convert maximum distance to radians.
  max_dist_rad = max_dist / (km ? radius_earth_Km : radius_earth_M)

  # Compute a random distance from 0 to maxdist scaled
  # so that points on larger circles have a greater probability
  # of being chosen than points on smaller circles as described earlier.
  dist = Math::acos( rand1 * (Math::cos(max_dist_rad) - 1) + 1 )

  # Compute a random bearing from 0 to 2*PI radians (0 to 360 degrees),
  # with all bearings having an equal probability of being chosen.
  brg = 2 * Math::PI * rand2

  # Use the starting point, random distance and random bearing to calculate the coordinates of the final random point.
  lat = Math::asin( Math::sin(start_lat) * Math::cos(dist) + Math::cos(start_lat) * Math::sin(dist) * Math::cos(brg) )
  lng = start_lng + Math::atan2( Math::sin(brg) * Math::sin(dist) * Math::cos(start_lat), Math::cos(dist) - Math::sin(start_lat) * Math.sin(lat) )

  if (lng < -1 * Math::PI)
    lng = lng + 2 * Math::PI
  elsif (lng > Math::PI)
    lng = lng - 2 * Math::PI
  end

  [lat * rad_to_deg, lng * rad_to_deg]
end

TODO: pass source as argument