Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
Port of Firebase Push ID generator in Java (ref: 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).
*/
public class FirebasePushIDGenerator {
// Modeled after base64 web-safe chars, but ordered by ASCII.
private final static String PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
private static long lastPushTime = 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 static int[] lastRandChars = new int[72];
public static String generatePushId() {
long now = System.currentTimeMillis();
boolean duplicateTime = (now == lastPushTime);
lastPushTime = now;
char[] timeStampChars = new char[8];
for (int i = 7; i >= 0; i--) {
final Long module = now % 64;
timeStampChars[i] = PUSH_CHARS.charAt(module.intValue());
now = (long) Math.floor(now / 64);
}
if (now != 0) {
throw new AssertionError("We should have converted the entire timestamp.");
}
String id = new String(timeStampChars);
if (!duplicateTime) {
for (int i = 0; i < 12; i++) {
final Double times = Math.random() * 64;
lastRandChars[i] = (int) Math.floor(times.intValue());
}
} else {
// If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
int i = 0;
for (i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
lastRandChars[i] = 0;
}
lastRandChars[i]++;
}
for (int i = 0; i < 12; i++) {
id += PUSH_CHARS.charAt(lastRandChars[i]);
}
if (id.length() != 20) {
throw new AssertionError("Length should be 20.");
}
return id;
};
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.