Generate Twitter Snowflake ID's

SnowflakeIdGenerator.cs

public class SnowflakeIdGenerator
{
    public const byte DefaultMachineIdBits = 6;
    public const byte DefaultSequenceBits = 12;
    private readonly long _machineId = 0;
    private readonly byte _machineIdBits = 0;
    private readonly byte _sequenceBits = 0;
    private readonly long _maxSequence = 0;
    private readonly Stopwatch _stopwatch = Stopwatch.StartNew();
    private readonly object _lockObject = new object();
    private long _sequence = 0;
    private long _lastTimestamp = 0;
    private static SnowflakeIdGenerator _instance;

    public static void Init(ushort machineId, byte machineIdBits = DefaultMachineIdBits, byte sequenceBits = DefaultSequenceBits)
    {
        if (_instance != null)
            throw new InvalidOperationException($"Instance has already been initialized.");
        _instance = new SnowflakeIdGenerator(machineId, machineIdBits, sequenceBits);
    }

    public static long NewId()
    {
        if (_instance == null)
            throw new Exception("Instance has not been initialized.");
        return _instance.NewSequenceId();
    }

    private static long GetMaxOfBits(byte bits)
    {
        return (1L << bits) - 1;
    }

    private static int GetBitsLength(long number)
    {
        return (int)Math.Log(number, 2) + 1;
    }

    // 637134336000000000 = new DateTime(2020, 1, 1, 0, 0, 0, DateTimeKind.Utc).Ticks
    private static readonly long _offsetTicks = DateTime.UtcNow.Ticks - 637134336000000000;

    public SnowflakeIdGenerator(ushort machineId) : this(machineId, DefaultMachineIdBits, DefaultSequenceBits)
    {
    }

    public SnowflakeIdGenerator(ushort machineId, byte machineIdBits, byte sequenceBits)
    {
        if (sequenceBits > 20)
            throw new ArgumentOutOfRangeException(nameof(sequenceBits), "sequenceBits > 20");
        if (machineIdBits > 10)
            throw new ArgumentOutOfRangeException(nameof(machineIdBits), "machineIdBits > 10");
        _machineIdBits = machineIdBits;
        _sequenceBits = sequenceBits;
        _maxSequence = GetMaxOfBits(_sequenceBits);
        var maxMachineId = GetMaxOfBits(machineIdBits);
        if (machineId > maxMachineId)
            throw new ArgumentOutOfRangeException(nameof(machineId), $"machineId > {maxMachineId}");
        _machineId = machineId;
    }

    private long GetTimestampNow()
    {
        // 10000000 = TimeSpan.FromSeconds(1).Ticks
        return (_offsetTicks + _stopwatch.Elapsed.Ticks) / 10000000L;
    }

    private long GetNextTimestamp()
    {
        long timestamp = GetTimestampNow();
        if (timestamp < _lastTimestamp)
            throw new Exception("");
        while (timestamp == _lastTimestamp)
        {
            if (_sequence < _maxSequence)
            {
                _sequence++;
                return timestamp;
            }

            Thread.Sleep(0);
            timestamp = GetTimestampNow();
        }

        _sequence = 0;
        return timestamp;
    }

    public long NewSequenceId()
    {
        lock (_lockObject)
        {
            _lastTimestamp = GetNextTimestamp();
            //int bitsLength = GetBitsLength(_lastTimestamp);
            //Console.WriteLine($"Timestamp bits: {bitsLength}");
            int timestampShift = _machineIdBits + _sequenceBits;
            int machineIdShift = _sequenceBits;
            return (_lastTimestamp << timestampShift) | (_machineId << machineIdShift) | _sequence;
        }
    }
}