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/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */

package org.elasticsearch.common;

import java.util.Base64;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * These are essentially flake ids, but we use 6 (not 8) bytes for timestamp, and use 3 (not 2) bytes for sequence number.
 * For more information about flake ids, check out
 * https://archive.fo/2015.07.08-082503/http://www.boundary.com/blog/2012/01/flake-a-decentralized-k-ordered-unique-id-generator-in-erlang/
 */

class LegacyTimeBasedUUIDGenerator implements UUIDGenerator {

    // We only use bottom 3 bytes for the sequence number.  Paranoia: init with random int so that if JVM/OS/machine goes down, clock slips
    // backwards, and JVM comes back up, we are less likely to be on the same sequenceNumber at the same time:
    private final AtomicInteger sequenceNumber = new AtomicInteger(SecureRandomHolder.INSTANCE.nextInt());

    // Used to ensure clock moves forward:
    private long lastTimestamp;

    private static final byte[] SECURE_MUNGED_ADDRESS = MacAddressProvider.getSecureMungedAddress();

    static {
        assert SECURE_MUNGED_ADDRESS.length == 6;
    }

    /** Puts the lower numberOfLongBytes from l into the array, starting index pos. */
    private static void putLong(byte[] array, long l, int pos, int numberOfLongBytes) {
        for (int i=0; i>> (i*8));
        }
    }

    @Override
    public String getBase64UUID()  {
        final int sequenceId = sequenceNumber.incrementAndGet() & 0xffffff;
        long timestamp = System.currentTimeMillis();

        synchronized (this) {
            // Don't let timestamp go backwards, at least "on our watch" (while this JVM is running).  We are still vulnerable if we are
            // shut down, clock goes backwards, and we restart... for this we randomize the sequenceNumber on init to decrease chance of
            // collision:
            timestamp = Math.max(lastTimestamp, timestamp);

            if (sequenceId == 0) {
                // Always force the clock to increment whenever sequence number is 0, in case we have a long time-slip backwards:
                timestamp++;
            }

            lastTimestamp = timestamp;
        }

        final byte[] uuidBytes = new byte[15];

        // Only use lower 6 bytes of the timestamp (this will suffice beyond the year 10000):
        putLong(uuidBytes, timestamp, 0, 6);

        // MAC address adds 6 bytes:
        System.arraycopy(SECURE_MUNGED_ADDRESS, 0, uuidBytes, 6, SECURE_MUNGED_ADDRESS.length);

        // Sequence number adds 3 bytes:
        putLong(uuidBytes, sequenceId, 12, 3);

        assert 9 + SECURE_MUNGED_ADDRESS.length == uuidBytes.length;

        return Base64.getUrlEncoder().withoutPadding().encodeToString(uuidBytes);
    }
}