Add prefetch queue dampening on audio, better handle segments with empty audio data

apple/OmnivoreKit/Sources/Services/AudioSession/AudioController.swift

@@ -1,5 +1,4 @@
 // swiftlint:disable file_length type_body_length
-
 #if os(iOS)
 
   import AVFoundation
@@ -39,23 +38,23 @@ public struct DigestAudioItem: AudioItemProperties {
   public var language: String?
   public var startIndex: Int = 0
   public var startOffset: Double = 0.0
-  
+
   public init(digest: DigestResult) {
     self.digest = digest
     self.itemID = digest.id
     self.title = digest.title
     self.startIndex = 0
     self.startOffset = 0
-    
+
     self.imageURL = nil
-    
+
     if let first = digest.speechFiles.first {
       self.language = first.language
       self.byline  = digest.byline
     }
   }
 }
-  
+
   // swiftlint:disable all
   @MainActor
   public class AudioController: NSObject, ObservableObject, AVAudioPlayerDelegate {
@@ -108,7 +107,7 @@ public struct DigestAudioItem: AudioItemProperties {
       playbackError = false
       self.itemAudioProperties = itemAudioProperties
       startAudio(atIndex: itemAudioProperties.startIndex, andOffset: itemAudioProperties.startOffset)
-      
+
       EventTracker.track(
         .audioSessionStart(
           linkID: itemAudioProperties.itemID,
@@ -319,7 +318,7 @@ public struct DigestAudioItem: AudioItemProperties {
     public func seek(toIdx: Int) {
       let before = durationBefore(playerIndex: toIdx)
       let remainder = 0.0
-      
+
       // if the foundIdx happens to be the current item, we just set the position
       if let playerItem = player?.currentItem as? SpeechPlayerItem {
         if playerItem.speechItem.audioIdx == toIdx {
@@ -666,8 +665,10 @@ public struct DigestAudioItem: AudioItemProperties {
       player = AVQueuePlayer(items: [])
       if let player = player {
         observer = player.observe(\.currentItem, options: [.new]) { _, _ in
-          self.currentAudioIndex = (player.currentItem as? SpeechPlayerItem)?.speechItem.audioIdx ?? 0
-          self.updateReadText()
+          DispatchQueue.main.async {
+            self.currentAudioIndex = (player.currentItem as? SpeechPlayerItem)?.speechItem.audioIdx ?? 0
+            self.updateReadText()
+          }
         }
       }
       
@@ -683,8 +684,8 @@ public struct DigestAudioItem: AudioItemProperties {
     func synthesizeFrom(start: Int, playWhenReady: Bool, atOffset: Double = 0.0) {
       if let synthesizer = self.synthesizer, let items = self.synthesizer?.createPlayerItems(from: start) {
         let prefetchQueue = OperationQueue()
-        prefetchQueue.maxConcurrentOperationCount = 5
-        
+        prefetchQueue.maxConcurrentOperationCount = 1
+
         for speechItem in items {
           let isLast = speechItem.audioIdx == synthesizer.document.utterances.count - 1
           let playerItem = SpeechPlayerItem(session: self, prefetchQueue: prefetchQueue, speechItem: speechItem) {

apple/OmnivoreKit/Sources/Services/AudioSession/SpeechPlayerItem.swift

@@ -40,12 +40,11 @@ class SpeechPlayerItem: AVPlayerItem {
     resourceLoaderDelegate.owner = self
 
     self.observer = observe(\.status, options: [.new]) { item, _ in
-      if item.status == .readyToPlay {
-        let duration = CMTimeGetSeconds(item.duration)
-        item.session.updateDuration(forItem: item.speechItem, newDuration: duration)
-      }
-      if item.status == .failed {
-        item.session.stopWithError()
+      DispatchQueue.main.async {
+        if item.status == .readyToPlay {
+          let duration = CMTimeGetSeconds(item.duration)
+          item.session.updateDuration(forItem: item.speechItem, newDuration: duration)
+        }
       }
     }
 
@@ -55,11 +54,29 @@ class SpeechPlayerItem: AVPlayerItem {
     ) { [weak self] _ in
       guard let self = self else { return }
       self.completed()
+      self.checkPrefetchQueue(prefetchQueue: prefetchQueue)
     }
 
     self.prefetchOperation = PrefetchSpeechItemOperation(speechItem: speechItem)
     if let prefetchOperation = self.prefetchOperation {
       prefetchQueue.addOperation(prefetchOperation)
+      prefetchOperation.completionBlock = {
+        self.checkPrefetchQueue(prefetchQueue: prefetchQueue)
+      }
+    }
+  }
+
+  func checkPrefetchQueue(prefetchQueue: OperationQueue) {
+    DispatchQueue.main.async {
+      if self.speechItem.audioIdx > self.session.currentAudioIndex + 5 {
+        // prefetch has gotten too far ahead of the audio. Pause the prefetch queue
+        print("PAUSING PREFETCH QUEUE", self.speechItem.audioIdx, self.session.currentAudioIndex + 10, self.speechItem.text)
+        prefetchQueue.isSuspended = true
+      }
+      if self.speechItem.audioIdx < self.session.currentAudioIndex + 5 {
+        print("RESUMING PREFETCH QUEUE", self.speechItem.audioIdx, self.session.currentAudioIndex + 5)
+        prefetchQueue.isSuspended = false
+      }
     }
   }
 

apple/OmnivoreKit/Sources/Services/AudioSession/SpeechSynthesizer.swift

@@ -222,9 +222,13 @@ struct SpeechSynthesizer {
 
     do {
       let jsonData = try decoder.decode(SynthesizeResult.self, from: data) as SynthesizeResult
-      let audioData = Data(fromHexEncodedString: jsonData.audioData)!
+      var audioData = Data(fromHexEncodedString: jsonData.audioData)!
       if audioData.count < 1 {
-        throw BasicError.message(messageText: "Audio data is empty")
+        if let silence = generateSilentAudioBuffer() {
+          audioData = silence
+        } else {
+          throw BasicError.message(messageText: "Audio data is empty")
+        }
       }
 
       try audioData.write(to: tempPath)
@@ -244,6 +248,34 @@ struct SpeechSynthesizer {
       throw BasicError.message(messageText: errorMessage)
     }
   }
+
+  static func generateSilentAudioBuffer() -> Data? {
+    let audioFormat = AVAudioFormat(standardFormatWithSampleRate: 44100, channels: 1)!
+    let frameCount = UInt32(audioFormat.sampleRate * 0.001)  // 1 millisecond of frames
+    guard let buffer = AVAudioPCMBuffer(pcmFormat: audioFormat, frameCapacity: frameCount) else {
+        return nil
+    }
+    buffer.frameLength = buffer.frameCapacity
+    return bufferToData(buffer: buffer)
+  }
+
+  static func bufferToData(buffer: AVAudioPCMBuffer) -> Data {
+    let channelCount = Int(buffer.format.channelCount)
+    let frames = Int(buffer.frameLength)
+    let channels = UnsafeBufferPointer(start: buffer.floatChannelData, count: channelCount)
+
+    var data = Data()
+
+    for frame in 0..<frames {
+        for channel in 0..<channelCount {
+            let value = channels[channel][frame]
+            var temp = value
+            data.append(UnsafeBufferPointer(start: &temp, count: 1))
+        }
+    }
+
+    return data
+  }
 }
 
 struct SynthesizeResult: Decodable {

apple/OmnivoreKit/Sources/Services/DataService/AI/AITasks.swift

@@ -172,7 +172,7 @@ extension DataService {
       try? data.write(to: localPath)
     }
   }
-  
+
   public func explain(text: String, libraryItemId: String) async throws -> String {
     let encoder = JSONEncoder()
     let explainRequest = ExplainRequest(text: text, libraryItemId: libraryItemId)