Firebase Java Push Id Generator

FirebasePushIdGenerator.java

package firebase;

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 LAST_PUSH_TIME = 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[] LAST_RAND_CHARS = new int[72];
	
	public static synchronized String generate() {

		long now = System.currentTimeMillis();
		boolean duplicateTime = now == LAST_PUSH_TIME;
		LAST_PUSH_TIME = now;

		char[] timeStampChars = new char[8];
		for (int i = 7; i >= 0; i--) {
			timeStampChars[i] = PUSH_CHARS.charAt((int)(now % 64));
			now = (long) Math.floor(now / 64);
		}
		
		if (now != 0) {
			throw new AssertionError("We should have converted the entire timestamp.");
		}
		
		StringBuilder id = new StringBuilder(20);
		for (char c : timeStampChars) {
			id.append(c);
		}
		
		if (!duplicateTime) {
			for (int i = 0; i < 12; i++) {
				LAST_RAND_CHARS[i] = (int) Math.floor(Double.valueOf(Math.random() * 64).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 && LAST_RAND_CHARS[i] == 63; i--) {
				LAST_RAND_CHARS[i] = 0;
			}
			LAST_RAND_CHARS[i]++;
		}
		
		for (int i = 0; i < 12; i++) {
			id.append(PUSH_CHARS.charAt(LAST_RAND_CHARS[i]));
		}
		
		if (id.length() != 20) {
			throw new AssertionError("Length should be 20.");
		}

		return id.toString();
	}
	
}

FirebasePushIdGeneratorTest.java

package firebase;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;

import org.junit.Assert;
import org.junit.Test;

import firebase.FirebasePushIdGenerator;

public class FirebasePushIdGeneratorTest {

	@Test
	public void testGeneration() throws InterruptedException, ExecutionException {
		
		final ExecutorService es = new ThreadPoolExecutor(20, 20, 10000, TimeUnit.DAYS, new ArrayBlockingQueue<>(10000));
		final ConcurrentMap<String, Boolean> set = new ConcurrentHashMap<>(1000000);
		final AtomicBoolean hasDuplicates = new AtomicBoolean(false);
		
		final List<Future<?>> futures = new ArrayList<>();
		
		for (int i=0; i<100; i++) {
			Future<?> f = es.submit(new Runnable() {
				@Override
				public void run() {
					for (int i=0; i<10000; i++) {
						final String id = FirebasePushIdGenerator.generate();
						if (set.putIfAbsent(id, Boolean.TRUE) != null) {
							hasDuplicates.set(true);
							System.out.println("Duplicate detected! " + id);
						}
					}
				}
			});
			futures.add(f);
		}
		
		for (Future<?> f : futures) {
			f.get();
		}
		
		es.shutdown();
		es.awaitTermination(20L, TimeUnit.MINUTES);
		
		Assert.assertFalse("There are duplicate keys!", hasDuplicates.get());
		
	}
	
}

Port of the JavaScript version: https://gist.github.com/mikelehen/3596a30bd69384624c11

LAST_RAND_CHARS ought to have length 12. @aokholm mentioned this in the comments for the original gist.