helpers.js

//Copied from from https://github.com/nherment/node-nmea/blob/master/lib/Helper.js

var m_hex = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'];

exports.toHexString = function(v) {
  var lsn;
  var msn;

  msn = (v >> 4) & 0x0f;
  lsn = (v >> 0) & 0x0f;
  return m_hex[msn] + m_hex[lsn];
};

exports.padLeft = function(s, len, ch) {
  while(s.length < len) {
    s = ch + s;
  }
  return s;
};

// verify the checksum
exports.verifyChecksum = function(sentence, checksum) {
  var q;
  var c1;
  var c2;
  var i;

  // skip the $
  i = 1;

  // init to first character
  c1 = sentence.charCodeAt(i);

  // process rest of characters, zero delimited
  for( i = 2; i < sentence.length; ++i) {
    c1 = c1 ^ sentence.charCodeAt(i);
  }

  // checksum is a 2 digit hex value
  c2 = parseInt(checksum, 16);

  // should be equal
  return ((c1 & 0xff) === c2);
};

// generate a checksum for  a sentence (no trailing *xx)
exports.computeChecksum = function(sentence) {
  var c1;
  var i;

  // skip the $
  i = 1;

  // init to first character    var count;

  c1 = sentence.charCodeAt(i);

  // process rest of characters, zero delimited
  for( i = 2; i < sentence.length; ++i) {
    c1 = c1 ^ sentence.charCodeAt(i);
  }

  return '*' + exports.toHexString(c1);
};

// =========================================
// field encoders
// =========================================

// encode latitude
// input: latitude in decimal degrees
// output: latitude in nmea format
// ddmm.mmm
exports.encodeLatitude = function(lat) {
  var d;
  var m;
  var f;
  var h;
  var s;
  var t;
  if(lat === undefined) {
    return '';
  }

  if(lat < 0) {
    h = 'S';
    lat = -lat;
  } else {
    h = 'N';
  }
  // get integer degrees
  d = Math.floor(lat);
  // degrees are always 2 digits
  s = d.toString();
  if(s.length < 2) {
    s = '0' + s;
  }
  // get fractional degrees
  f = lat - d;
  // convert to fractional minutes
  m = (f * 60.0);
  // format the fixed point fractional minutes
  t = m.toFixed(3);
  if(m < 10) {
    // add leading 0
    t = '0' + t;
  }

  s = s + t + ',' + h;
  return s;
};

// encode longitude
// input: longitude in decimal degrees
// output: longitude in nmea format
// dddmm.mmm
exports.encodeLongitude = function(lon) {
  var d;
  var m;
  var f;
  var h;
  var s;
  var t;

  if(lon === undefined) {
    return '';
  }

  if(lon < 0) {
    h = 'W';
    lon = -lon;
  } else {
    h = 'E';
  }

  // get integer degrees
  d = Math.floor(lon);
  // degrees are always 3 digits
  s = d.toString();
  while(s.length < 3) {
    s = '0' + s;
  }

  // get fractional degrees
  f = lon - d;
  // convert to fractional minutes and round up to the specified precision
  m = (f * 60.0);
  // minutes are always 6 characters = mm.mmm
  t = m.toFixed(3);
  if(m < 10) {
    // add leading 0
    t = '0' + t;
  }
  s = s + t + ',' + h;
  return s;
};

// 1 decimal, always meters
exports.encodeAltitude = function(alt) {
  if(alt === undefined) {
    return ',';
  }
  return alt.toFixed(1) + ',M';
};

// 1 decimal, always meters
exports.encodeGeoidalSeperation = function(geoidalSep) {
  if(geoidalSep === undefined) {
    return ',';
  }
  return geoidalSep.toFixed(1) + ',M';
};

// magnetic variation
exports.encodeMagVar = function(v) {
  var a;
  var s;
  if(v === undefined) {
    return ',';
  }
  a = Math.abs(v);
  s = (v < 0) ? (a.toFixed(1) + ',E') : (a.toFixed(1) + ',W');
  return exports.padLeft(s, 7, '0');
};

// degrees
exports.encodeDegrees = function(d) {
  if(d === undefined) {
    return '';
  }
  return exports.padLeft(d.toFixed(2), 6, '0');
};

exports.encodeDate = function(d) {
  var yr;
  var mn;
  var dy;

  if(d === undefined) {
    return '';
  }
  yr = d.getUTCFullYear();
  mn = d.getUTCMonth() + 1;
  dy = d.getUTCDate();
  return exports.padLeft(dy.toString(), 2, '0') + exports.padLeft(mn.toString(), 2, '0') + yr.toString().substr(2);
};

exports.encodeTime = function(d) {
  var h;
  var m;
  var s;

  if(d === undefined) {
    return '';
  }
  h = d.getUTCHours();
  m = d.getUTCMinutes();
  s = d.getUTCSeconds();
  return exports.padLeft(h.toString(), 2, '0') + exports.padLeft(m.toString(), 2, '0') + exports.padLeft(s.toString(), 2, '0');
};

exports.encodeKnots = function(k) {
  if(k === undefined) {
    return '';
  }
  return exports.padLeft(k.toFixed(1), 5, '0');
};

exports.encodeValue = function(v) {
  if(v === undefined) {
    return '';
  }
  return v.toString();
};

exports.encodeFixed = function(v, f) {
  if(v === undefined) {
    return '';
  }
  return v.toFixed(f);
};

// =========================================
// field traditionalDecoders
// =========================================

// separate number and units
exports.parseAltitude = function(alt, units) {
  var scale = 1.0;
  if(units === 'F') {
    scale = 0.3048;
  }
  return parseFloat(alt) * scale;
};

// separate degrees value and quadrant (E/W)
exports.parseDegrees = function(deg, quadrant) {
  var q = (quadrant === 'E') ? -1.0 : 1.0;

  return parseFloat(deg) * q;
};

// fields can be empty so have to wrap the global parseFloat
exports.parseFloatX = function(f) {
  if(f === '') {
    return 0.0;
  }
  return parseFloat(f);
};

// decode latitude
// input : latitude in nmea format
//      first two digits are degress
//      rest of digits are decimal minutes
// output : latitude in decimal degrees
exports.parseLatitude = function(lat, hemi) {
  var h = (hemi === 'N') ? 1.0 : -1.0;
  var a;
  var dg;
  var mn;
  var l;
  a = lat.split('.');
  if(a[0].length === 4) {
    // two digits of degrees
    dg = lat.substring(0, 2);
    mn = lat.substring(2);
  } else if(a[0].length === 3) {
    // 1 digit of degrees (in case no leading zero)
    dg = lat.substring(0, 1);
    mn = lat.substring(1);
  } else {
    // no degrees, just minutes (nonstandard but a buggy unit might do this)
    dg = '0';
    mn = lat;
  }
  // latitude is usually precise to 5-8 digits
  return ((parseFloat(dg) + (parseFloat(mn) / 60.0)) * h).toFixed(8);
};

// decode longitude
// first three digits are degress
// rest of digits are decimal minutes
exports.parseLongitude = function(lon, hemi) {
  var h;
  var a;
  var dg;
  var mn;
  h = (hemi === 'E') ? 1.0 : -1.0;
  a = lon.split('.');
  if(a[0].length === 5) {
    // three digits of degrees
    dg = lon.substring(0, 3);
    mn = lon.substring(3);
  } else if(a[0].length === 4) {
    // 2 digits of degrees (in case no leading zero)
    dg = lon.substring(0, 2);
    mn = lon.substring(2);
  } else if(a[0].length === 3) {
    // 1 digit of degrees (in case no leading zero)
    dg = lon.substring(0, 1);
    mn = lon.substring(1);
  } else {
    // no degrees, just minutes (nonstandard but a buggy unit might do this)
    dg = '0';
    mn = lon;
  }
  // longitude is usually precise to 5-8 digits
  return ((parseFloat(dg) + (parseFloat(mn) / 60.0)) * h).toFixed(8);
};

// fields can be empty so have to wrap the global parseInt
exports.parseIntX = function(i) {
  if(i === '') {
    return 0;
  }
  return parseInt(i, 10);
};

exports.parseDateTime = function(date, time) {
  var h = parseInt(time.slice(0, 2), 10);
  var m = parseInt(time.slice(2, 4), 10);
  var s = parseInt(time.slice(4, 6), 10);
  var D = parseInt(date.slice(0, 2), 10);
  var M = parseInt(date.slice(2, 4), 10) - 1;	 // UTC = 0..11
  var Y = parseInt(date.slice(4, 6), 10);
  // hack : GPRMC date doesn't specify century. GPS came out in 1973
  // so if year is less than 73 its 2000, otherwise 1900
  if (Y < 73) {
    Y = Y + 2000;
  }
  else {
    Y = Y + 1900;
  }

  return new Date(Date.UTC(Y, M, D, h, m, s));
};