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src/StellaOps.Scanner.WebService/Services/ScanProgressStream.cs

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+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Threading.Channels;
+using StellaOps.Scanner.WebService.Domain;
+
+namespace StellaOps.Scanner.WebService.Services;
+
+public interface IScanProgressPublisher
+{
+    ScanProgressEvent Publish(
+        ScanId scanId,
+        string state,
+        string? message = null,
+        IReadOnlyDictionary<string, object?>? data = null,
+        string? correlationId = null);
+}
+
+public interface IScanProgressReader
+{
+    bool Exists(ScanId scanId);
+
+    IAsyncEnumerable<ScanProgressEvent> SubscribeAsync(ScanId scanId, CancellationToken cancellationToken);
+}
+
+public sealed class ScanProgressStream : IScanProgressPublisher, IScanProgressReader
+{
+    private sealed class ProgressChannel
+    {
+        private readonly List<ScanProgressEvent> history = new();
+        private readonly Channel<ScanProgressEvent> channel = Channel.CreateUnbounded<ScanProgressEvent>(new UnboundedChannelOptions
+        {
+            AllowSynchronousContinuations = true,
+            SingleReader = false,
+            SingleWriter = false
+        });
+
+        public int Sequence { get; private set; }
+
+        public ScanProgressEvent Append(ScanProgressEvent progressEvent)
+        {
+            history.Add(progressEvent);
+            channel.Writer.TryWrite(progressEvent);
+            return progressEvent;
+        }
+
+        public IReadOnlyList<ScanProgressEvent> Snapshot()
+        {
+            return history.Count == 0
+                ? Array.Empty<ScanProgressEvent>()
+                : history.ToArray();
+        }
+
+        public ChannelReader<ScanProgressEvent> Reader => channel.Reader;
+
+        public int NextSequence() => ++Sequence;
+    }
+
+    private static readonly IReadOnlyDictionary<string, object?> EmptyData = new ReadOnlyDictionary<string, object?>(new Dictionary<string, object?>(StringComparer.OrdinalIgnoreCase));
+
+    private readonly ConcurrentDictionary<string, ProgressChannel> channels = new(StringComparer.OrdinalIgnoreCase);
+    private readonly TimeProvider timeProvider;
+
+    public ScanProgressStream(TimeProvider timeProvider)
+    {
+        this.timeProvider = timeProvider ?? throw new ArgumentNullException(nameof(timeProvider));
+    }
+
+    public bool Exists(ScanId scanId)
+        => channels.ContainsKey(scanId.Value);
+
+    public ScanProgressEvent Publish(
+        ScanId scanId,
+        string state,
+        string? message = null,
+        IReadOnlyDictionary<string, object?>? data = null,
+        string? correlationId = null)
+    {
+        var channel = channels.GetOrAdd(scanId.Value, _ => new ProgressChannel());
+
+        ScanProgressEvent progressEvent;
+        lock (channel)
+        {
+            var sequence = channel.NextSequence();
+            var correlation = correlationId ?? $"{scanId.Value}:{sequence:D4}";
+            var payload = data is null || data.Count == 0
+                ? EmptyData
+                : new ReadOnlyDictionary<string, object?>(new Dictionary<string, object?>(data, StringComparer.OrdinalIgnoreCase));
+
+            progressEvent = new ScanProgressEvent(
+                scanId,
+                sequence,
+                timeProvider.GetUtcNow(),
+                state,
+                message,
+                correlation,
+                payload);
+
+            channel.Append(progressEvent);
+        }
+
+        return progressEvent;
+    }
+
+    public async IAsyncEnumerable<ScanProgressEvent> SubscribeAsync(
+        ScanId scanId,
+        [EnumeratorCancellation] CancellationToken cancellationToken)
+    {
+        if (!channels.TryGetValue(scanId.Value, out var channel))
+        {
+            yield break;
+        }
+
+        IReadOnlyList<ScanProgressEvent> snapshot;
+        lock (channel)
+        {
+            snapshot = channel.Snapshot();
+        }
+
+        foreach (var progressEvent in snapshot)
+        {
+            yield return progressEvent;
+        }
+
+        var reader = channel.Reader;
+        while (await reader.WaitToReadAsync(cancellationToken).ConfigureAwait(false))
+        {
+            while (reader.TryRead(out var progressEvent))
+            {
+                yield return progressEvent;
+            }
+        }
+    }
+}