(() => {
  
  function getFace(){
    var nofix = 0;
    
    function formatTime(now) {
      var fd = now.toUTCString().split(" ");
      return fd[4];
    }
    
    function timeSince(t) {
      var hms = t.split(":");
      var now = new Date();
      
      var sn = 3600*(now.getHours()) + 60*(now.getMinutes()) + 1*(now.getSeconds());
      var st = 3600*(hms[0]) + 60*(hms[1]) + 1*(hms[2]);
      
      return (sn - st);
    }


    Number.prototype.toRad = function() { return this*Math.PI/180; };
    /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
    /*  Ordnance Survey Grid Reference functions  (c) Chris Veness 2005-2014                          */
    /*   - www.movable-type.co.uk/scripts/gridref.js                                                  */
    /*   - www.movable-type.co.uk/scripts/latlon-gridref.html                                         */
    /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
    function OsGridRef(easting, northing) {
      this.easting = 0|easting;
      this.northing = 0|northing;
    }
    OsGridRef.latLongToOsGrid = function(point) {
      var lat = point.lat.toRad();
      var lon = point.lon.toRad();

      var a = 6377563.396, b = 6356256.909;          // Airy 1830 major & minor semi-axes
      var F0 = 0.9996012717;                         // NatGrid scale factor on central meridian
      var lat0 = (49).toRad(), lon0 = (-2).toRad();  // NatGrid true origin is 49�N,2�W
      var N0 = -100000, E0 = 400000;                 // northing & easting of true origin, metres
      var e2 = 1 - (b*b)/(a*a);                      // eccentricity squared
      var n = (a-b)/(a+b), n2 = n*n, n3 = n*n*n;

      var cosLat = Math.cos(lat), sinLat = Math.sin(lat);
      var nu = a*F0/Math.sqrt(1-e2*sinLat*sinLat);              // transverse radius of curvature
      var rho = a*F0*(1-e2)/Math.pow(1-e2*sinLat*sinLat, 1.5);  // meridional radius of curvature
      var eta2 = nu/rho-1;

      var Ma = (1 + n + (5/4)*n2 + (5/4)*n3) * (lat-lat0);
      var Mb = (3*n + 3*n*n + (21/8)*n3) * Math.sin(lat-lat0) * Math.cos(lat+lat0);
      var Mc = ((15/8)*n2 + (15/8)*n3) * Math.sin(2*(lat-lat0)) * Math.cos(2*(lat+lat0));
      var Md = (35/24)*n3 * Math.sin(3*(lat-lat0)) * Math.cos(3*(lat+lat0));
      var M = b * F0 * (Ma - Mb + Mc - Md);              // meridional arc

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

      var I = M + N0;
      var II = (nu/2)*sinLat*cosLat;
      var III = (nu/24)*sinLat*cos3lat*(5-tan2lat+9*eta2);
      var IIIA = (nu/720)*sinLat*cos5lat*(61-58*tan2lat+tan4lat);
      var IV = nu*cosLat;
      var V = (nu/6)*cos3lat*(nu/rho-tan2lat);
      var VI = (nu/120) * cos5lat * (5 - 18*tan2lat + tan4lat + 14*eta2 - 58*tan2lat*eta2);

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

      var N = I + II*dLon2 + III*dLon4 + IIIA*dLon6;
      var E = E0 + IV*dLon + V*dLon3 + VI*dLon5;

      return new OsGridRef(E, N);
    };


    /*
     * converts northing, easting to standard OS grid reference.
     *
     * [digits=10] - precision (10 digits = metres)
     *   to_map_ref(8, 651409, 313177); => 'TG 5140 1317'
     *   to_map_ref(0, 651409, 313177); => '651409,313177'
     *
     */
    function to_map_ref(digits, easting, northing) {
      if (![ 0,2,4,6,8,10,12,14,16 ].includes(Number(digits))) throw new RangeError(`invalid precision '${digits}'`); // eslint-disable-line comma-spacing

      let e = easting;
      let n = northing;

      // use digits = 0 to return numeric format (in metres) - note northing may be >= 1e7
      if (digits == 0) {
        const format = { useGrouping: false,  minimumIntegerDigits: 6, maximumFractionDigits: 3 };
        const ePad = e.toLocaleString('en', format);
        const nPad = n.toLocaleString('en', format);
        return `${ePad},${nPad}`;
      }

      // get the 100km-grid indices
      const e100km = Math.floor(e / 100000), n100km = Math.floor(n / 100000);

      // translate those into numeric equivalents of the grid letters
      let l1 = (19 - n100km) - (19 - n100km) % 5 + Math.floor((e100km + 10) / 5);
      let l2 = (19 - n100km) * 5 % 25 + e100km % 5;

      // compensate for skipped 'I' and build grid letter-pairs
      if (l1 > 7) l1++;
      if (l2 > 7) l2++;
      const letterPair = String.fromCharCode(l1 + 'A'.charCodeAt(0), l2 + 'A'.charCodeAt(0));

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

      // pad eastings & northings with leading zeros
      e = e.toString().padStart(digits/2, '0');
      n = n.toString().padStart(digits/2, '0');

      return `${letterPair} ${e} ${n}`;
    }
    
    function draw() {
      var gps_on = false;

      var fix = {
        fix: 0,
        alt: 0,
        lat: 0,
	lon: 0,
	speed: 0,
	time: 0,
	satellites: 0
      };

      var y_line = 26;
      var y_start = 46;
      var x_start = 10;
      
      // only attempt to get gps fix if gpsservuce is loaded
      if (WIDGETS.gpsservice !== undefined) {
        fix = WIDGETS.gpsservice.gps_get_fix();
	gps_on = WIDGETS.gpsservice.gps_get_status();
      }

      g.reset();
      g.clearRect(0,24,239,239);

      if (fix.fix) {
	var time = formatTime(fix.time);
	var age = timeSince(time);
	var os = OsGridRef.latLongToOsGrid(fix);
	var ref = to_map_ref(6, os.easting, os.northing);
	age = age >=0 ? age : 0;  // avoid -1 etc

	g.reset();
	g.clearRect(0,24,239,239);
	g.setFontAlign(0,0);
	g.setFont("Vector");  
	g.setColor(1,1,1); 
	g.setFontVector(40);
	g.drawString(time, 120, 80);
  
	g.setFontVector(40);
	g.setColor(0xFFC0); 
	g.drawString(ref, 120,140, true);
  
	g.setFont("6x8",2);
	g.setColor(1,1,1); 
	g.drawString(age, 120, 194);

      } else if (gps_on) {

	g.setFontAlign(0, 1);
	g.setFont("6x8", 2);
	g.drawString("Gridref Watch", 120, 60);
	g.drawString("Waiting for GPS", 120, 80);
	nofix = (nofix+1) % 4;
	g.drawString(".".repeat(nofix) + " ".repeat(4-nofix), 120, 120);
	g.setFontAlign(0,0);
	g.drawString(fix.satellites + " satellites", 120, 100);
	
      } else if (!gps_on) {
	
	g.setFontAlign(0, 0);
	g.setFont("6x8", 3);
	g.drawString("Gridref Watch", 120, 80);
	g.drawString("GPS is off", 120, 160);
	
      }
    }

    function onSecond(){
      var t = new Date();
      if ((t.getSeconds() % 5) === 0) draw();
    }
    
    return {init:draw, tick:onSecond};
  }

  return getFace;

})();