perf: coalesce superseded queued playback seeks

Sources/App/CastSession.swift

@@ -255,6 +255,11 @@ final class CastSession {
         let deadline: Task<Void, Never>
     }
 
+    private struct Write {
+        let data: Data
+        let seek: Int?
+    }
+
     private let makeConnection: (CastDevice) -> NWConnection
     private let requestTimeout: Duration
     private let loadTimeout: Duration
@@ -270,7 +275,7 @@ final class CastSession {
     private var awaitingPong = false
     private var nextRequest = 0
     private var pending: [Int: Pending] = [:]
-    private var writes: [Data] = []
+    private var writes: [Write] = []
     private var writing = false
     private var launchRequest: Int?
     private var appTransport: String?
@@ -525,7 +530,17 @@ final class CastSession {
                          operation: String, generation: UUID? = nil, timeout: Duration? = nil,
                          id suppliedID: Int? = nil,
                          completion: @escaping ([String: Any]?, CastError?) -> Void) throws -> Int {
-        guard pending.count < 32 else { throw CastError.queueFull }
+        // Only the tail is replaceable: it has not reached the socket, and every
+        // intervening control or protocol frame remains an ordering barrier.
+        let superseded: Int?
+        if command["type"] as? String == "SEEK", namespace == CastWire.media,
+           let id = writes.last?.seek, let previous = pending[id],
+           previous.generation == generation, previous.destination == destination {
+            superseded = id
+        } else {
+            superseded = nil
+        }
+        guard pending.count < 32 || superseded != nil else { throw CastError.queueFull }
         let id = suppliedID ?? allocateRequest()
         var command = command
         command["requestId"] = id
@@ -537,8 +552,11 @@ final class CastSession {
         }
         pending[id] = Pending(namespace: namespace, destination: destination, operation: operation,
                               generation: generation, completion: completion, deadline: deadline)
-        do { try send(command, namespace: namespace, destination: destination) }
+        do {
+            try send(command, namespace: namespace, destination: destination, replacing: superseded)
+        }
         catch { pending.removeValue(forKey: id)?.deadline.cancel(); throw error }
+        if let superseded { complete(superseded, json: nil, error: .cancelled) }
         return id
     }
 
@@ -553,12 +571,17 @@ final class CastSession {
         for id in ids { complete(id, json: nil, error: .cancelled) }
     }
 
-    private func send(_ json: [String: Any], namespace: String, destination: String) throws {
+    private func send(_ json: [String: Any], namespace: String, destination: String,
+                      replacing: Int? = nil) throws {
         guard connection != nil else { throw CastError.disconnected }
-        guard writes.count < 32 else { throw CastError.queueFull }
+        guard writes.count < 32 || replacing != nil else { throw CastError.queueFull }
         let payload = try JSONSerialization.data(withJSONObject: json)
-        writes.append(try CastWire.frame(.init(source: source, destination: destination,
-                                               namespace: namespace, payload: payload)))
+        let write = Write(data: try CastWire.frame(.init(source: source, destination: destination,
+                                                        namespace: namespace, payload: payload)),
+                          seek: namespace == CastWire.media && json["type"] as? String == "SEEK" ?
+                              json["requestId"] as? Int : nil)
+        if replacing != nil { writes[writes.count - 1] = write }
+        else { writes.append(write) }
         flush()
     }
 
@@ -567,7 +590,7 @@ final class CastSession {
         writing = true
         let data = writes.removeFirst()
         let id = connectionID
-        socket.send(content: data, completion: .contentProcessed { [weak self] error in
+        socket.send(content: data.data, completion: .contentProcessed { [weak self] error in
             Task { @MainActor in
                 guard let self, self.connectionID == id else { return }
                 self.writing = false