// Unique ID creation requires a high quality random # generator.  We feature
  // detect to determine the best RNG source, normalizing to a function that
  // returns 128-bits of randomness, since that's what's usually required
  var rng = require('./lib/rng');
  var bytesToUuid = require('./lib/bytesToUuid');
  
  // **`v1()` - Generate time-based UUID**
  //
  // Inspired by https://github.com/LiosK/UUID.js
  // and http://docs.python.org/library/uuid.html
  
  // random #'s we need to init node and clockseq
  var _seedBytes = rng();
  
  // Per 4.5, create and 48-bit node id, (47 random bits + multicast bit = 1)
  var _nodeId = [
    _seedBytes[0] | 0x01,
    _seedBytes[1], _seedBytes[2], _seedBytes[3], _seedBytes[4], _seedBytes[5]
  ];
  
  // Per 4.2.2, randomize (14 bit) clockseq
  var _clockseq = (_seedBytes[6] << 8 | _seedBytes[7]) & 0x3fff;
  
  // Previous uuid creation time
  var _lastMSecs = 0, _lastNSecs = 0;
  
  // See https://github.com/broofa/node-uuid for API details
  function v1(options, buf, offset) {
    var i = buf && offset || 0;
    var b = buf || [];
  
    options = options || {};
  
    var clockseq = options.clockseq !== undefined ? options.clockseq : _clockseq;
  
    // UUID timestamps are 100 nano-second units since the Gregorian epoch,
    // (1582-10-15 00:00).  JSNumbers aren't precise enough for this, so
    // time is handled internally as 'msecs' (integer milliseconds) and 'nsecs'
    // (100-nanoseconds offset from msecs) since unix epoch, 1970-01-01 00:00.
    var msecs = options.msecs !== undefined ? options.msecs : new Date().getTime();
  
    // Per 4.2.1.2, use count of uuid's generated during the current clock
    // cycle to simulate higher resolution clock
    var nsecs = options.nsecs !== undefined ? options.nsecs : _lastNSecs + 1;
  
    // Time since last uuid creation (in msecs)
    var dt = (msecs - _lastMSecs) + (nsecs - _lastNSecs)/10000;
  
    // Per 4.2.1.2, Bump clockseq on clock regression
    if (dt < 0 && options.clockseq === undefined) {
      clockseq = clockseq + 1 & 0x3fff;
    }
  
    // Reset nsecs if clock regresses (new clockseq) or we've moved onto a new
    // time interval
    if ((dt < 0 || msecs > _lastMSecs) && options.nsecs === undefined) {
      nsecs = 0;
    }
  
    // Per 4.2.1.2 Throw error if too many uuids are requested
    if (nsecs >= 10000) {
      throw new Error('uuid.v1(): Can\'t create more than 10M uuids/sec');
    }
  
    _lastMSecs = msecs;
    _lastNSecs = nsecs;
    _clockseq = clockseq;
  
    // Per 4.1.4 - Convert from unix epoch to Gregorian epoch
    msecs += 12219292800000;
  
    // `time_low`
    var tl = ((msecs & 0xfffffff) * 10000 + nsecs) % 0x100000000;
    b[i++] = tl >>> 24 & 0xff;
    b[i++] = tl >>> 16 & 0xff;
    b[i++] = tl >>> 8 & 0xff;
    b[i++] = tl & 0xff;
  
    // `time_mid`
    var tmh = (msecs / 0x100000000 * 10000) & 0xfffffff;
    b[i++] = tmh >>> 8 & 0xff;
    b[i++] = tmh & 0xff;
  
    // `time_high_and_version`
    b[i++] = tmh >>> 24 & 0xf | 0x10; // include version
    b[i++] = tmh >>> 16 & 0xff;
  
    // `clock_seq_hi_and_reserved` (Per 4.2.2 - include variant)
    b[i++] = clockseq >>> 8 | 0x80;
  
    // `clock_seq_low`
    b[i++] = clockseq & 0xff;
  
    // `node`
    var node = options.node || _nodeId;
    for (var n = 0; n < 6; ++n) {
      b[i + n] = node[n];
    }
  
    return buf ? buf : bytesToUuid(b);
  }
  
  module.exports = v1;