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Java code for generating Firebase Push IDs
package com.github.longkerdandy.matrix.backend.id;
import java.util.Random;
/**
* 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).
*
* Inspired by Firbase's PushId.
*
* Blog post: https://firebase.googleblog.com/2015/02/the-2120-ways-to-ensure-unique_68.html?m=1
* Github gist: https://gist.github.com/mikelehen/3596a30bd69384624c11
*/
public class IdGenerator {
// Modeled after base64 web-safe chars, but ordered by ASCII.
private final static String ID_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
// Pseudorandom number generator
private final Random random;
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
private long lastTimestamp = 0L;
// 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 int[] lastRandChars = new int[12];
/**
* Create a new IdGenerator with specific Random instance
*
* @param random Random, SecureRandom or ThreadLocalRandom
*/
public IdGenerator(Random random) {
this.random = random;
}
/**
* Generate next chronological 20-character unique id
*
* @return New ID
*/
public String nextId() {
long now = System.currentTimeMillis();
char[] result = new char[20];
// random chars
int[] randChars = compareAndUpdate(now);
for (int i = 0; i < 12; i++) {
result[i + 8] = ID_CHARS.charAt(randChars[i]);
}
// timestamp chars
for (int i = 7; i >= 0; i--) {
final Long module = now % 64;
result[i] = ID_CHARS.charAt(module.intValue());
now = (long) Math.floor(now / 64);
}
if (now != 0) {
throw new AssertionError("We should have converted the entire timestamp.");
}
return new String(result);
}
/**
* Compare the timestamp and update last record state. To be thread safe, all the compare and
* update operation are included in this method
*
* @param timestamp Timestamp
* @return Random Chars should be used
*/
private synchronized int[] compareAndUpdate(long timestamp) {
if (timestamp != this.lastTimestamp) {
// If the timestamp has changed since last request, create a new random number.
this.lastTimestamp = timestamp;
for (int i = 0; i < 12; i++) {
this.lastRandChars[i] = this.random.nextInt(64);
}
} else {
// If the timestamp hasn't changed since last request, use the same random number, except incremented by 1.
int i;
for (i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
this.lastRandChars[i] = 0;
}
if (i < 0) {
i = 11;
}
this.lastRandChars[i]++;
}
// return a new array
return this.lastRandChars.clone();
}
}
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longkerdandy commented Aug 24, 2017

Benchmark with JMH on my i7-7820K laptop:
Thoughput: 13300000 ops/s
Average time: 0.075 us/op

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longkerdandy commented Aug 24, 2017

Suggestions are welcome

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