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Foxnouns.Backend/Database/SnowflakeGenerator.cs

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+using NodaTime;
+
+namespace Foxnouns.Backend.Database;
+
+public class SnowflakeGenerator : ISnowflakeGenerator
+{
+    /// <summary>
+    /// Singleton instance of SnowflakeGenerator. Where possible, use an injected ISnowflakeGenerator instead
+    /// (see IServiceCollection.AddSnowflakeGenerator).
+    /// </summary>
+    public static SnowflakeGenerator Instance { get; } = new();
+
+    private readonly byte _processId;
+    private long _increment;
+
+    public SnowflakeGenerator(int? processId = null)
+    {
+        processId ??= Environment.ProcessId;
+        _processId = (byte)(processId % 32);
+        _increment = Random.Shared.NextInt64() % 4096;
+    }
+
+    /// <summary>
+    /// Generate a snowflake ID for the given timestamp.
+    /// </summary>
+    /// <param name="time">An optional timestamp. If null, use the current timestamp.</param>
+    /// <returns>A new snowflake ID.</returns>
+    public Snowflake GenerateSnowflake(Instant? time = null)
+    {
+        time ??= SystemClock.Instance.GetCurrentInstant();
+        var increment = Interlocked.Increment(ref _increment);
+        var threadId = Environment.CurrentManagedThreadId % 32;
+        var timestamp = time.Value.ToUnixTimeMilliseconds() - Snowflake.Epoch;
+
+        return (timestamp << 22) | (uint)(_processId << 17) | (uint)(threadId << 12) | (increment % 4096);
+    }
+}
+
+public static class SnowflakeGeneratorServiceExtensions
+{
+    public static IServiceCollection AddSnowflakeGenerator(this IServiceCollection services, int? processId = null)
+    {
+        return services.AddSingleton<ISnowflakeGenerator>(new SnowflakeGenerator(processId));
+    }
+}