var map
  , you
  , pos
  , t_0
  , log = ''
  , url = 'data:image/png;base64,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'
  , head = '["latitude","longitude","precision","time"]'
  , zoom
  , time
  , from
  ;

google.maps.event.addDomListener(window, 'load', init);

function init() {
  map = new google.maps.Map( document.getElementById('map')
                           , { zoom: zoom
                             , mapTypeId: google.maps.MapTypeId.ROADMAP
                             });

  if (navigator.geolocation)
    navigator.geolocation.watchPosition(gotPosition, function() {
      noGeolocation('Error: The Geolocation service failed.');
    }, { enableHighAccuracy: true, maximumAge: 10e3, timeout: 20e3 });
  else
    noGeolocation('Error: Your browser doesn\'t support geolocation.');
}

function gotPosition(position) {
  var at  = position.coords
    , off = at.accuracy
    , z
    ;

  pos = ll(at.latitude, at.longitude);
  if (you) you.setPosition(pos); else {
    t_0 = Math.round(+new Date / 1000);
    you = new google.maps.Marker({ map: map
                                 , position: pos
                                 , icon: marker(url, s(20, 17), p(10, 8))
                                 });
    google.maps.event.addListener(you, 'click', function() {
      location = 'mailto:?subject=GPS%20Track&body='
               + encodeURIComponent(log + ' ]\n}\n');
    });
  }
  if (!zoom) map.setCenter(pos);

  // zoom in, as precision improves (or out again)
  if (off > 2e3) z = 15;
  if (off < 2e3) z = 16;
  if (off < 900) z = 17;
  if (off < 100) z = 18;
  if (z !== zoom) map.setZoom(zoom = z);

  map.panTo(pos);
  save(at);
}

function noGeolocation(message) {
  var opts = { map: map
             , position: ll(60, 105)
             , content: message
             }
    , info = new google.maps.InfoWindow(opts);
  map.setCenter(opts.position);
}

function s(w, h) { return new google.maps.Size(w, h); }
function p(x, y) { return new google.maps.Point(x, y); }
function ll(y, x) { return new google.maps.LatLng(y, x); }
function marker(url, size, hotspot, origin) {
  return new google.maps.MarkerImage(url, size, origin || p(0, 0), hotspot);
}

function save(at) {
  var lat = at.latitude.toFixed(6) // decimeter precision should be quite enough
    , lng = at.longitude.toFixed(6)
    , pre = at.accuracy.toFixed(0)
    ;
  time = Math.round(new Date / 1000) - t_0;
  log += (log ? ' ,' : '{"time":'+ t_0 +'\n,"head":\n '+ head +'\n,"data":\n [')
       + '[' + lat +','+ lng +','+ pre +','+ time +']\n';
}