feat: execute full-repo remaining parity closure plan

src/NATS.Server/LeafNodes/LeafHubSpokeMapper.cs

@@ -0,0 +1,30 @@
+namespace NATS.Server.LeafNodes;
+
+public enum LeafMapDirection
+{
+    Inbound,
+    Outbound,
+}
+
+public sealed record LeafMappingResult(string Account, string Subject);
+
+public sealed class LeafHubSpokeMapper
+{
+    private readonly IReadOnlyDictionary<string, string> _hubToSpoke;
+    private readonly IReadOnlyDictionary<string, string> _spokeToHub;
+
+    public LeafHubSpokeMapper(IReadOnlyDictionary<string, string> hubToSpoke)
+    {
+        _hubToSpoke = hubToSpoke;
+        _spokeToHub = hubToSpoke.ToDictionary(static p => p.Value, static p => p.Key, StringComparer.Ordinal);
+    }
+
+    public LeafMappingResult Map(string account, string subject, LeafMapDirection direction)
+    {
+        if (direction == LeafMapDirection.Outbound && _hubToSpoke.TryGetValue(account, out var spoke))
+            return new LeafMappingResult(spoke, subject);
+        if (direction == LeafMapDirection.Inbound && _spokeToHub.TryGetValue(account, out var hub))
+            return new LeafMappingResult(hub, subject);
+        return new LeafMappingResult(account, subject);
+    }
+}