JavaScript code for generating Firebase Push IDs

## generate-pushid.js
/**
 * Fancy ID generator that creates 20-character string identifiers with the following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
 * 3. They sort *lexicographically* (so the timestamp is converted to characters that will sort properly).
 * 4. They're monotonically increasing.  Even if you generate more than one in the same timestamp, the
 *    latter ones will sort after the former ones.  We do this by using the previous random bits
 *    but "incrementing" them by 1 (only in the case of a timestamp collision).
 */
generatePushID = (function() {
  // Modeled after base64 web-safe chars, but ordered by ASCII.
  var PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';

  // Timestamp of last push, used to prevent local collisions if you push twice in one ms.
  var lastPushTime = 0;

  // We generate 72-bits of randomness which get turned into 12 characters and appended to the
  // timestamp to prevent collisions with other clients.  We store the last characters we
  // generated because in the event of a collision, we'll use those same characters except
  // "incremented" by one.
  var lastRandChars = [];

  return function() {
    var now = new Date().getTime();
    var duplicateTime = (now === lastPushTime);
    lastPushTime = now;

    var timeStampChars = new Array(8);
    for (var i = 7; i >= 0; i--) {
      timeStampChars[i] = PUSH_CHARS.charAt(now % 64);
      // NOTE: Can't use << here because javascript will convert to int and lose the upper bits.
      now = Math.floor(now / 64);
    }
    if (now !== 0) throw new Error('We should have converted the entire timestamp.');

    var id = timeStampChars.join('');

    if (!duplicateTime) {
      for (i = 0; i < 12; i++) {
        lastRandChars[i] = Math.floor(Math.random() * 64);
      }
    } else {
      // If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
      for (i = 11; i >= 0 && lastRandChars[i] === 63; i--) {
        lastRandChars[i] = 0;
      }
      lastRandChars[i]++;
    }
    for (i = 0; i < 12; i++) {
      id += PUSH_CHARS.charAt(lastRandChars[i]);
    }
    if(id.length != 20) throw new Error('Length should be 20.');

    return id;
  };
})();
	/**
	* Fancy ID generator that creates 20-character string identifiers with the following properties:
	*
	* 1. They're based on timestamp so that they sort after any existing ids.
	* 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
	* 3. They sort lexicographically (so the timestamp is converted to characters that will sort properly).
	* 4. They're monotonically increasing. Even if you generate more than one in the same timestamp, the
	* latter ones will sort after the former ones. We do this by using the previous random bits
	* but "incrementing" them by 1 (only in the case of a timestamp collision).
	*/
	generatePushID = (function() {
	// Modeled after base64 web-safe chars, but ordered by ASCII.
	var PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';

	// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
	var lastPushTime = 0;

	// We generate 72-bits of randomness which get turned into 12 characters and appended to the
	// timestamp to prevent collisions with other clients. We store the last characters we
	// generated because in the event of a collision, we'll use those same characters except
	// "incremented" by one.
	var lastRandChars = [];

	return function() {
	var now = new Date().getTime();
	var duplicateTime = (now === lastPushTime);
	lastPushTime = now;

	var timeStampChars = new Array(8);
	for (var i = 7; i >= 0; i--) {
	timeStampChars[i] = PUSH_CHARS.charAt(now % 64);
	// NOTE: Can't use << here because javascript will convert to int and lose the upper bits.
	now = Math.floor(now / 64);
	}
	if (now !== 0) throw new Error('We should have converted the entire timestamp.');

	var id = timeStampChars.join('');

	if (!duplicateTime) {
	for (i = 0; i < 12; i++) {
	lastRandChars[i] = Math.floor(Math.random() * 64);
	}
	} else {
	// If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
	for (i = 11; i >= 0 && lastRandChars[i] === 63; i--) {
	lastRandChars[i] = 0;
	}
	lastRandChars[i]++;
	}
	for (i = 0; i < 12; i++) {
	id += PUSH_CHARS.charAt(lastRandChars[i]);
	}
	if(id.length != 20) throw new Error('Length should be 20.');

	return id;
	};
	})();