class OSGB_WGS84

# File lib/osgb_wgs84.rb, line 28
def self.OSGB36_to_OSNG(lat, long)
  lat = toRad(lat);
  lon = toRad(long);

  a = 6377563.396; b = 6356256.910       # Airy 1830 major & minor semi-axes
  f0 = 0.9996012717                 # NatGrid scale factor on central meridian
  lat0 = toRad(49); lon0 = toRad(-2)      # NatGrid true origin
  n0 = -100000; e0 = 400000;           # northing & easting of true origin, metres
  e2 = 1 - (b*b) / (a*a);             # eccentricity squared
  n = (a-b) / (a+b); n2 = n*n; n3 = n*n*n;

  cosLat = Math.cos(lat); sinLat = Math.sin(lat);
  nu = a*f0/Math.sqrt(1-e2*sinLat*sinLat);          # transverse radius of curvature
  rho = a*f0*(1-e2) / ( (1-e2*sinLat*sinLat) ** 1.5);  # meridional radius of curvature
  eta2 = nu/rho-1;

  ma = (1 + n + (5/4)*n2 + (5/4)*n3) * (lat-lat0);
  mb = (3*n + 3*n*n + (21/8)*n3) * Math.sin(lat-lat0) * Math.cos(lat+lat0);
  mc = ((15/8)*n2 + (15/8)*n3) * Math.sin(2*(lat-lat0)) * Math.cos(2*(lat+lat0));
  md = (35/24)*n3 * Math.sin(3*(lat-lat0)) * Math.cos(3*(lat+lat0));
  m = b * f0 * (ma - mb + mc - md);          # meridional arc

  cos3lat = cosLat*cosLat*cosLat;
  cos5lat = cos3lat*cosLat*cosLat;
  tan2lat = Math.tan(lat)*Math.tan(lat);
  tan4lat = tan2lat*tan2lat;

  i = m + n0;
  ii = (nu/2)*sinLat*cosLat;
  iii = (nu/24)*sinLat*cos3lat*(5-tan2lat+9*eta2);
  iiiA = (nu/720)*sinLat*cos5lat*(61-58*tan2lat+tan4lat);
  iv = nu*cosLat;
  v = (nu/6)*cos3lat*(nu/rho-tan2lat);
  vi = (nu/120) * cos5lat * (5 - 18*tan2lat + tan4lat + 14*eta2 - 58*tan2lat*eta2);

  dLon = lon-lon0;
  dLon2 = dLon*dLon
  dLon3 = dLon2*dLon
  dLon4 = dLon3*dLon
  dLon5 = dLon4*dLon
  dLon6 = dLon5*dLon

  n = i + ii*dLon2 + iii*dLon4 + iiiA*dLon6;
  e = e0 + iv*dLon + v*dLon3 + vi*dLon5;

  return [ e, n ]  #return raw easting and northings instead

  #return gridrefNumToLet( e, n, 8)
end

# File lib/osgb_wgs84.rb, line 15
def self.OSGB36_to_WGS84(p1lat, p1lon, p1height)
  p2 = convert(p1lat, p1lon, p1height, @e[:airy1830], @h[:osgb36toWGS84], @e[:wgs84]);
  return p2;
end

# File lib/osgb_wgs84.rb, line 79
def self.OSNG_numbers_to_letters(e, n, digits)
  #get the 100km-grid indices
  e100k = (e / 100000).floor; n100k = (n / 100000).floor;

  return '' if (e100k<0 or e100k>6 or n100k<0 or n100k>12)

  #translate those into numeric equivalents of the grid letters
  l1 = (19-n100k) - (19-n100k) % 5 + ((e100k+10) / 5).floor;
  l2 = (19-n100k) * 5 % 25 + e100k % 5;

  # compensate for skipped 'I' and build grid letter-pairs
  l1=l1+1 if (l1 > 7)
  l2=l2+1 if (l2 > 7)
  # letPair =  (l1 +'A'[0]).chr +  (l2 +'A'[0]).chr ;  #(Old code for Ruby 1.8 replaced with following line for ruby1.9)
  letPair = (l1 +'A'.unpack('C')[0]).chr + (l2 +'A'.unpack('C')[0]).chr ;

  # strip 100km-grid indices from easting & northing, and reduce precision
  e = ( (e % 100000) / (10 ** (5 - digits / 2)) ).floor;
  n = ( (n % 100000) / (10 ** (5 - digits / 2)) ).floor;

  gridRef = letPair + e.to_s.rjust(digits / 2) + n.to_s.rjust(digits / 2)

  return gridRef;
end

# File lib/osgb_wgs84.rb, line 21
def self.WGS84_to_OSGB36(p1lat, p1lon, p1height)
  p2 = convert(p1lat, p1lon, p1height, @e[:wgs84], @h[:wgs84toOSGB36], @e[:airy1830]);
  return p2;
end

# File lib/osgb_wgs84.rb, line 114
def self.convert(p1lat, p1lon, p1height, e1, t, e2)
  # -- convert polar to cartesian coordinates (using ellipse 1)

  p1lat = toRad(p1lat); p1lon = toRad(p1lon);

  a = e1[:a]; b = e1[:b];

  sinPhi = Math.sin(p1lat); cosPhi = Math.cos(p1lat);
  sinLambda = Math.sin(p1lon); cosLambda = Math.cos(p1lon);
  h = p1height;

  eSq = (a*a - b*b) / (a*a);
  nu = a / Math.sqrt(1 - eSq*sinPhi*sinPhi);

  x1 = (nu+h) * cosPhi * cosLambda;
  y1 = (nu+h) * cosPhi * sinLambda;
  z1 = ((1-eSq)*nu + h) * sinPhi;


  # -- apply helmert transform using appropriate params

  tx = t[:tx]; ty = t[:ty]; tz = t[:tz];
  rx = t[:rx] / 3600 * Math::PI/180;  #normalise seconds to radians
  ry = t[:ry] / 3600 * Math::PI/180;
  rz = t[:rz] / 3600 * Math::PI/180;
  s1 = t[:s] / 1e6 + 1;          #normalise ppm to (s+1)

  #apply transform
  x2 = tx + x1*s1 - y1*rz + z1*ry;
  y2 = ty + x1*rz + y1*s1 - z1*rx;
  z2 = tz - x1*ry + y1*rx + z1*s1;


  # -- convert cartesian to polar coordinates (using ellipse 2)

  a = e2[:a]; b = e2[:b];
  precision = 4 / a;  # results accurate to around 4 metres

  eSq = (a*a - b*b) / (a*a);
  p = Math.sqrt(x2*x2 + y2*y2);
  phi = Math.atan2(z2, p*(1-eSq)); phiP = 2 * Math::PI;
  while ( (phi-phiP).abs > precision) do
    nu = a / Math.sqrt(1 - eSq*Math.sin(phi)*Math.sin(phi));
    phiP = phi;
    phi = Math.atan2(z2 + eSq*nu*Math.sin(phi), p);
  end
  lambda = Math.atan2(y2, x2);
  h = p/Math.cos(phi) - nu;

  #return array [lat,lon,height]
  return [ toDeg(phi), toDeg(lambda), h ];
end

# File lib/osgb_wgs84.rb, line 110
def self.toDeg(rads)
  return rads * 180 / Math::PI
end

# File lib/osgb_wgs84.rb, line 106
def self.toRad(degrees)
  return degrees * Math::PI / 180;
end