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August 24, 2017 09:18
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Java code for generating Firebase Push IDs
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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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