Add SnowflakeId generator class.

src/DotNetCore.CAP/Infrastructure/SnowflakeId.cs

+// Copyright 2010-2012 Twitter, Inc.
+// An object that generates IDs. This is broken into a separate class in case we ever want to support multiple worker threads per process
+
+using System;
+
+namespace DotNetCore.CAP.Infrastructure
+{
+    public class SnowflakeId
+    {
+        public const long Twepoch = 1288834974657L;
+
+        private const int WorkerIdBits = 5;
+        private const int DatacenterIdBits = 5;
+        private const int SequenceBits = 12;
+        private const long MaxWorkerId = -1L ^ (-1L << WorkerIdBits);
+        private const long MaxDatacenterId = -1L ^ (-1L << DatacenterIdBits);
+
+        private const int WorkerIdShift = SequenceBits;
+        private const int DatacenterIdShift = SequenceBits + WorkerIdBits;
+        public const int TimestampLeftShift = SequenceBits + WorkerIdBits + DatacenterIdBits;
+        private const long SequenceMask = -1L ^ (-1L << SequenceBits);
+
+        private static SnowflakeId _snowflakeId;
+
+        private readonly object _lock = new object();
+        private long _lastTimestamp = -1L;
+
+        private SnowflakeId(long workerId, long datacenterId, long sequence = 0L)
+        {
+            WorkerId = workerId;
+            DatacenterId = datacenterId;
+            Sequence = sequence;
+
+            // sanity check for workerId
+            if (workerId > MaxWorkerId || workerId < 0)
+                throw new ArgumentException($"worker Id can't be greater than {MaxWorkerId} or less than 0");
+
+            if (datacenterId > MaxDatacenterId || datacenterId < 0)
+                throw new ArgumentException($"datacenter Id can't be greater than {MaxDatacenterId} or less than 0");
+        }
+
+        public long WorkerId { get; protected set; }
+        public long DatacenterId { get; protected set; }
+
+        public long Sequence { get; internal set; }
+
+        public static SnowflakeId Default(long datacenterId = 0)
+        {
+            return _snowflakeId ?? (_snowflakeId = new SnowflakeId(AppDomain.CurrentDomain.Id, datacenterId));
+        }
+
+        public virtual long NextId()
+        {
+            lock (_lock)
+            {
+                var timestamp = TimeGen();
+
+                if (timestamp < _lastTimestamp)
+                    throw new Exception(
+                        $"InvalidSystemClock: Clock moved backwards, Refusing to generate id for {_lastTimestamp - timestamp} milliseconds");
+
+                if (_lastTimestamp == timestamp)
+                {
+                    Sequence = (Sequence + 1) & SequenceMask;
+                    if (Sequence == 0) timestamp = TilNextMillis(_lastTimestamp);
+                }
+                else
+                {
+                    Sequence = 0;
+                }
+
+                _lastTimestamp = timestamp;
+                var id = ((timestamp - Twepoch) << TimestampLeftShift) |
+                         (DatacenterId << DatacenterIdShift) |
+                         (WorkerId << WorkerIdShift) | Sequence;
+
+                return id;
+            }
+        }
+
+        protected virtual long TilNextMillis(long lastTimestamp)
+        {
+            var timestamp = TimeGen();
+            while (timestamp <= lastTimestamp) timestamp = TimeGen();
+            return timestamp;
+        }
+
+        protected virtual long TimeGen()
+        {
+            return DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
+        }
+    }
+}
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