calculate_tt

# converted from JavaScript; source: http://blog.jgc.org/2010/06/1010-code.html

def calculate_tt(lat, lon)
  alphabet = "ABCDEFGHJKMNPQRVWXY0123456789".split(//)
  base = alphabet.length 

  lat += 90.0
  lon += 180.0

  lat *= 10000.0
  lon *= 10000.0

  lat = lat.floor
  lon = lon.floor

  p = lat * 3600000.0 + lon

  tt_num = p * base
  c = 0

  (1..9).each do |i|
    c += ( p % base ) * i
    p = ( p / base ).floor
  end

  c %= 29

  tt_num += c
  tt_num = tt_num.floor

  tt = ""

  (0..9).each do |i|
    d = tt_num % base
    if ( ( i == 4 ) || ( i == 7 ) ) then
      tt = " " + tt
    end
    tt = alphabet.at(d.to_i).to_s + tt.to_s
    tt_num = ( tt_num / base ).floor
  end

  return tt
end
calculate_tt 51.09559, 1.12207

#=> "QPP YMD MGQA"