module Twd97

# File lib/twd97.rb, line 3
def convert(x, y)
  x = x.to_f
  y = y.to_f
  a = 6378137.0
  b = 6356752.314245
  lng0 = 121 * Math::PI / 180
  k0 = 0.9999
  dx = 250000
  dy = 0
  e = ((1 - (b ** 2) / (a ** 2)) ** 0.5)
  x -= dx
  y -= dy
  mm = y / k0
  mu = mm / (a * (1.0 - (e ** 2) / 4.0 - 3 * (e ** 4) / 64.0 - 5 * (e ** 6) / 256.0))
  e1 = (1.0 - ((1.0 - (e ** 2)) ** 0.5)) / (1.0 + ((1.0 - (e ** 2)) ** 0.5))
  j1 = (3 * e1 / 2 - 27 * (e1 ** 3) / 32.0)
  j2 = (21 * (e1 ** 2) / 16 - 55 * (e1 ** 4) / 32.0)
  j3 = (151 * (e1 ** 3) / 96.0)
  j4 = (1097 * (e1 ** 4) / 512.0)
  fp = mu + j1 * Math.sin(2 * mu) + j2 * Math.sin(4 * mu) + j3 * Math.sin(6 * mu) + j4 * Math.sin(8 * mu)
  e2 = ((e * a / b) ** 2)
  c1 = (e2 * Math.cos(fp) ** 2)
  t1 = (Math.tan(fp) ** 2)
  r1 = a * (1 - (e ** 2)) / ((1 - (e ** 2) * (Math.sin(fp) ** 2)) ** (3.0 / 2.0))
  n1 = a / ((1 - (e ** 2) * (Math.sin(fp) ** 2)) ** 0.5)

  dd = x / (n1 * k0)
  q1 = n1 * Math.tan(fp) / r1
  q2 = ((dd ** 2) / 2.0)
  q3 = (5 + 3 * t1 + 10 * c1 - 4 * (c1 ** 2) - 9 * e2) * (dd ** 4) / 24.0
  q4 = (61 + 90 * t1 + 298 * c1 + 45 * (t1 ** 2) - 3 * (c1 ** 2) - 252 * e2) * (dd ** 6) / 720.0
  lat = fp - q1 * (q2 - q3 + q4)
  q5 = dd
  q6 = (1 + 2 * t1 + c1) * (dd ** 3) / 6
  q7 = (5 - 2 * c1 + 28 * t1 - 3 * (c1 ** 2) + 8 * e2 + 24 * (t1 ** 2)) * (dd ** 5) / 120.0
  lng = lng0 + (q5 - q6 + q7) / Math.cos(fp)
  lat = (lat * 180) / Math::PI
  lng = (lng * 180) / Math::PI
  { lat: lat, lng: lng }
end