feat: implement jetstream memstore core behavior

src/NATS.Server/JetStream/Storage/IStreamStore.cs

@@ -5,5 +5,7 @@ namespace NATS.Server.JetStream.Storage;
 public interface IStreamStore
 {
     ValueTask<ulong> AppendAsync(string subject, ReadOnlyMemory<byte> payload, CancellationToken ct);
+    ValueTask<StoredMessage?> LoadAsync(ulong sequence, CancellationToken ct);
+    ValueTask PurgeAsync(CancellationToken ct);
     ValueTask<StreamState> GetStateAsync(CancellationToken ct);
 }

src/NATS.Server/JetStream/Storage/MemStore.cs

@@ -0,0 +1,57 @@
+using NATS.Server.JetStream.Models;
+
+namespace NATS.Server.JetStream.Storage;
+
+public sealed class MemStore : IStreamStore
+{
+    private readonly object _gate = new();
+    private ulong _last;
+    private readonly Dictionary<ulong, StoredMessage> _messages = new();
+
+    public ValueTask<ulong> AppendAsync(string subject, ReadOnlyMemory<byte> payload, CancellationToken ct)
+    {
+        lock (_gate)
+        {
+            _last++;
+            _messages[_last] = new StoredMessage
+            {
+                Sequence = _last,
+                Subject = subject,
+                Payload = payload,
+            };
+            return ValueTask.FromResult(_last);
+        }
+    }
+
+    public ValueTask<StoredMessage?> LoadAsync(ulong sequence, CancellationToken ct)
+    {
+        lock (_gate)
+        {
+            _messages.TryGetValue(sequence, out var msg);
+            return ValueTask.FromResult(msg);
+        }
+    }
+
+    public ValueTask PurgeAsync(CancellationToken ct)
+    {
+        lock (_gate)
+        {
+            _messages.Clear();
+            _last = 0;
+            return ValueTask.CompletedTask;
+        }
+    }
+
+    public ValueTask<StreamState> GetStateAsync(CancellationToken ct)
+    {
+        lock (_gate)
+        {
+            return ValueTask.FromResult(new StreamState
+            {
+                Messages = (ulong)_messages.Count,
+                FirstSeq = _messages.Count == 0 ? 0UL : _messages.Keys.Min(),
+                LastSeq = _last,
+            });
+        }
+    }
+}