var _rng;

var globalVar = typeof window !== 'undefined'
    ? window
    : typeof global !== 'undefined' ? global : null;

if (globalVar && globalVar.crypto && crypto.getRandomValues) {
  // WHATWG crypto-based RNG - http://wiki.whatwg.org/wiki/Crypto
  // Moderately fast, high quality
  var _rnds8 = new Uint8Array(16);
  _rng = function whatwgRNG() {
    crypto.getRandomValues(_rnds8);
    return _rnds8;
  };
}

if (!_rng) {
  // Math.random()-based (RNG)
  //
  // If all else fails, use Math.random().  It's fast, but is of unspecified
  // quality.
  var _rnds = new Array(16);
  _rng = function () {
    for (var i = 0, r; i < 16; i++) {
      if ((i & 0x03) === 0) r = Math.random() * 0x100000000;
      _rnds[i] = r >>> ((i & 0x03) << 3) & 0xff;
    }

    return _rnds;
  };
}

//     uuid.js
//
//     Copyright (c) 2010-2012 Robert Kieffer
//     MIT License - http://opensource.org/licenses/mit-license.php

// 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('./rng');

// Maps for number <-> hex string conversion
var _byteToHex = [];
var _hexToByte = {};
for (var i = 0; i < 256; i++) {
  _byteToHex[i] = (i + 0x100).toString(16).substr(1);
  _hexToByte[_byteToHex[i]] = i;
}

// **`parse()` - Parse a UUID into it's component bytes**
function parse(s, buf, offset) {
  var i = (buf && offset) || 0, ii = 0;

  buf = buf || [];
  s.toLowerCase().replace(/[0-9a-f]{2}/g, function (oct) {
    if (ii < 16) { // Don't overflow!
      buf[i + ii++] = _hexToByte[oct];
    }
  });

  // Zero out remaining bytes if string was short
  while (ii < 16) {
    buf[i + ii++] = 0;
  }

  return buf;
}

// **`unparse()` - Convert UUID byte array (ala parse()) into a string**
function unparse(buf, offset) {
  var i = offset || 0, bth = _byteToHex;
  return bth[buf[i++]] + bth[buf[i++]] +
      bth[buf[i++]] + bth[buf[i++]] + '-' +
      bth[buf[i++]] + bth[buf[i++]] + '-' +
      bth[buf[i++]] + bth[buf[i++]] + '-' +
      bth[buf[i++]] + bth[buf[i++]] + '-' +
      bth[buf[i++]] + bth[buf[i++]] +
      bth[buf[i++]] + bth[buf[i++]] +
      bth[buf[i++]] + bth[buf[i++]];
}

// **`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 : unparse(b);
}

// **`v4()` - Generate random UUID**

// See https://github.com/broofa/node-uuid for API details
function v4(options, buf, offset) {
  // Deprecated - 'format' argument, as supported in v1.2
  var i = buf && offset || 0;

  if (typeof(options) == 'string') {
    buf = options == 'binary' ? new Array(16) : null;
    options = null;
  }
  options = options || {};

  var rnds = options.random || (options.rng || _rng)();

  // Per 4.4, set bits for version and `clock_seq_hi_and_reserved`
  rnds[6] = (rnds[6] & 0x0f) | 0x40;
  rnds[8] = (rnds[8] & 0x3f) | 0x80;

  // Copy bytes to buffer, if provided
  if (buf) {
    for (var ii = 0; ii < 16; ii++) {
      buf[i + ii] = rnds[ii];
    }
  }

  return buf || unparse(rnds);
}

// Export public API
var uuid = v4;
uuid.v1 = v1;
uuid.v4 = v4;
uuid.parse = parse;
uuid.unparse = unparse;

module.exports = uuid;
