Created
February 11, 2015 17:34
-
-
Save mikelehen/3596a30bd69384624c11 to your computer and use it in GitHub Desktop.
JavaScript code for generating Firebase Push IDs
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
/** | |
* 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; | |
}; | |
})(); |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
web sdk source
push
repoServerTime offsets now, nextPushId is the same as generatePushId except now is passed as argument.
nextPushId