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JavaScript code for generating Firebase Push IDs
// https://gist.github.com/mikelehen/3596a30bd69384624c11
/**
* 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).
*/
export class UniqueIdGenerator {
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
private static lastPushTime = 0;
// Modeled after base64 web-safe chars, but ordered by ASCII.
private static PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';
// 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.
private static lastRandChars = [];
// Generates chronologically orderable unique string one by one
public static generate() {
var now = new Date().getTime();
var duplicateTime = (now === UniqueIdGenerator.lastPushTime);
UniqueIdGenerator.lastPushTime = now;
var timeStampChars = new Array(8);
for (var i = 7; i >= 0; i--) {
timeStampChars[i] = UniqueIdGenerator.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++) {
UniqueIdGenerator.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 && UniqueIdGenerator.lastRandChars[i] === 63; i--) {
UniqueIdGenerator.lastRandChars[i] = 0;
}
UniqueIdGenerator.lastRandChars[i]++;
}
for (i = 0; i < 12; i++) {
id += UniqueIdGenerator.PUSH_CHARS.charAt(UniqueIdGenerator.lastRandChars[i]);
}
if (id.length != 20) throw new Error('Length should be 20.');
return id;
}
}
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