Snowflake.java

package us.t4y.scratch;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Adapted from Twitter's Snowflake
 * http://github.com/twitter/snowflake
 */
public class Snowflake {

	private static final Logger log = LoggerFactory.getLogger(Snowflake.class);

	private static final long epoch = 1288834974657L;

	private static final long workerIdBits = 5L;
	private static final long datacenterIdBits = 5L;
	private static final long sequenceBits = 12L;

	private static final long workerIdShift = sequenceBits;
	private static final long datacenterIdShift = sequenceBits + workerIdBits;
	private static final long timestampShift = sequenceBits + workerIdBits + datacenterIdBits;

	private static final long sequenceMask = -1L ^ (-1L << sequenceBits);
	private static final long workerIdMask = -1L ^ (-1L << workerIdBits);
	private static final long datacenterIdMask = -1L ^ (-1L << datacenterIdBits);

	private final long workerId;
	private final long datacenterId;

	private long sequence = 0L;
	private long lastTimestamp = -1L;

	public Snowflake(final long datacenterId, final long workerId) {
		this.datacenterId = datacenterId & datacenterIdMask;
		this.workerId = workerId & workerIdMask;
	}

	public synchronized long nextId() {
		long timestamp = timestamp();

		if (lastTimestamp == timestamp) {
			sequence = (sequence + 1) & sequenceMask;
			if (sequence == 0) {
				System.out.println("rollover");
				timestamp = tilNextMillis(lastTimestamp);
			}
		} else {
			sequence = 0;
		}

		if (timestamp < lastTimestamp) {
			log.error("Clock is moving backwards. Rejecting requests until " + lastTimestamp + ".");
			throw new IllegalStateException("Clock moved backwards. Refusing to generate id for " + (lastTimestamp - timestamp) + " milliseconds");
		}

		lastTimestamp = timestamp;
		return ((timestamp - epoch) << timestampShift)
				| (datacenterId << datacenterIdShift)
				| (workerId << workerIdShift)
				| sequence;
	}

	protected long tilNextMillis(long lastTimestamp) {
		long timestamp = timestamp();
		while (timestamp <= lastTimestamp) {
			timestamp = timestamp();
		}
		return timestamp;
	}

	protected long timestamp() {
		return System.currentTimeMillis();
	}

}