Firebase unique ID generator

nextPushId.js

function nextPushId () {
  // 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 (now) {
    var duplicateTime = now === lastPushTime
    lastPushTime = now
    var i
    var timeStampChars = new Array(8)
    for (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)
    }
    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])
    }
    return id
  }
}