zero-alloc audio pipeline: pointer-based IO from ring buffer to stdout

Replace Data/AudioPacket allocations with raw pointer callbacks through
the entire audio pipeline. Ring buffer hands out direct pointers (or
linearizes into a pre-allocated scratch buffer on wrap-around), converter
accepts/emits pointers via its cached buffers, and output handler writes
to stdout via write(2) with EINTR handling.

Remove AudioPacket (dead code), --flush flag (no-op with raw write(2)).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Sources/AudioTeeCLI/AudioTee.swift

@@ -9,7 +9,6 @@ struct AudioTee {
   var stereo: Bool = false
   var sampleRate: Double?
   var chunkDuration: Double = 0.2
-  var flush: Bool = false
 
   init() {}
 
@@ -33,7 +32,6 @@ struct AudioTee {
           audiotee --include-processes 1234 5678 9012  # Tap only these processes
           audiotee --exclude-processes 1234 5678       # Tap everything except these
           audiotee --mute                       # Mute processes being tapped
-          audiotee --flush                      # Flush stdout after each chunk
         """
     )
 
@@ -45,7 +43,6 @@ struct AudioTee {
       name: "exclude-processes", help: "Process IDs to exclude (space-separated)")
     parser.addFlag(name: "mute", help: "Mute processes being tapped")
     parser.addFlag(name: "stereo", help: "Records in stereo")
-    parser.addFlag(name: "flush", help: "Flush stdout after each audio chunk (reduces latency when piping)")
     parser.addOption(
       name: "sample-rate",
       help: "Target sample rate (8000, 16000, 22050, 24000, 32000, 44100, 48000)")
@@ -63,7 +60,6 @@ struct AudioTee {
       audioTee.excludeProcesses = try parser.getArrayValue("exclude-processes", as: Int32.self)
       audioTee.mute = parser.getFlag("mute")
       audioTee.stereo = parser.getFlag("stereo")
-      audioTee.flush = parser.getFlag("flush")
       audioTee.sampleRate = try parser.getOptionalValue("sample-rate", as: Double.self)
       audioTee.chunkDuration = try parser.getValue("chunk-duration", as: Double.self)
 
@@ -99,11 +95,11 @@ struct AudioTee {
   func run() throws {
     setupSignalHandlers()
 
-    Logger.info("Starting AudioTee...")
+    AudioTeeLogging.logger.info("Starting AudioTee...")
 
     // Validate chunk duration
     guard chunkDuration > 0 && chunkDuration <= 5.0 else {
-      Logger.error(
+      AudioTeeLogging.logger.error(
         "Invalid chunk duration",
         context: ["chunk_duration": String(chunkDuration), "valid_range": "0.0 < duration <= 5.0"])
       throw ExitCode.failure
@@ -123,7 +119,7 @@ struct AudioTee {
     do {
       try audioTapManager.setupAudioTap(with: tapConfig)
     } catch AudioTeeError.pidTranslationFailed(let failedPIDs) {
-      Logger.error(
+      AudioTeeLogging.logger.error(
         "Failed to translate process IDs to audio objects",
         context: [
           "failed_pids": failedPIDs.map(String.init).joined(separator: ", "),
@@ -131,21 +127,21 @@ struct AudioTee {
         ])
       throw ExitCode.failure
     } catch {
-      Logger.error(
+      AudioTeeLogging.logger.error(
         "Failed to setup audio tap", context: ["error": String(describing: error)])
       throw ExitCode.failure
     }
 
     guard let deviceID = audioTapManager.getDeviceID() else {
-      Logger.error("Failed to get device ID from audio tap manager")
+      AudioTeeLogging.logger.error("Failed to get device ID from audio tap manager")
       throw ExitCode.failure
     }
 
-    let outputHandler = BinaryAudioOutputHandler(flushAfterWrite: flush)
-    let recorder = AudioRecorder(
+    let outputHandler = BinaryAudioOutputHandler()
+    let recorder = try AudioRecorder(
       deviceID: deviceID, outputHandler: outputHandler, convertToSampleRate: sampleRate,
       chunkDuration: chunkDuration)
-    recorder.startRecording()
+    try recorder.startRecording()
 
     // Run until the run loop is stopped (by signal handler)
     while true {
@@ -155,17 +151,17 @@ struct AudioTee {
       }
     }
 
-    Logger.info("Shutting down...")
+    AudioTeeLogging.logger.info("Shutting down...")
     recorder.stopRecording()
   }
 
   private func setupSignalHandlers() {
     signal(SIGINT) { _ in
-      Logger.info("Received SIGINT, initiating graceful shutdown...")
+      AudioTeeLogging.logger.info("Received SIGINT, initiating graceful shutdown...")
       CFRunLoopStop(CFRunLoopGetMain())
     }
     signal(SIGTERM) { _ in
-      Logger.info("Received SIGTERM, initiating graceful shutdown...")
+      AudioTeeLogging.logger.info("Received SIGTERM, initiating graceful shutdown...")
       CFRunLoopStop(CFRunLoopGetMain())
     }
   }

Sources/AudioTeeCLI/BinaryOutputHandler.swift

@@ -0,0 +1,34 @@
+import AudioTeeCore
+import Foundation
+
+/// CLI-specific output handler that writes raw PCM audio to stdout
+/// and lifecycle messages to stderr via the logger.
+class BinaryAudioOutputHandler: AudioOutputHandler {
+  private let fd = STDOUT_FILENO
+
+  func handleAudioData(_ pointer: UnsafeRawPointer, count: Int) {
+    var written = 0
+    while written < count {
+      let result = write(fd, pointer.advanced(by: written), count - written)
+      if result >= 0 {
+        written += result
+      } else if errno == EINTR {
+        continue
+      } else {
+        break  // EPIPE, EIO, etc — consumer gone or real error
+      }
+    }
+  }
+
+  func handleMetadata(_ metadata: AudioStreamMetadata) {
+    AudioTeeLogging.logger.writeMessage(.metadata, data: metadata)
+  }
+
+  func handleStreamStart() {
+    AudioTeeLogging.logger.writeMessage(.streamStart, data: Optional<String>.none)
+  }
+
+  func handleStreamStop() {
+    AudioTeeLogging.logger.writeMessage(.streamStop, data: Optional<String>.none)
+  }
+}

Sources/AudioTeeCore/Core/AudioBuffer.swift

@@ -1,105 +1,121 @@
 import CoreAudio
 import Foundation
 
+/// Ring buffer for accumulating raw audio data and extracting fixed-size chunks.
+///
+/// Uses a raw heap-allocated pointer rather than Swift Array to avoid
+/// copy-on-write reference-count checks on every mutation. This buffer
+/// lives on the real-time audio IO thread and is never shared, so COW
+/// semantics are pure overhead.
 public class AudioBuffer {
-  private var buffer: [UInt8]
+  /// Raw heap-allocated ring buffer backing store.
+  private let buffer: UnsafeMutableRawPointer
+  /// Pre-allocated buffer for linearizing chunks that straddle the ring
+  /// buffer boundary. Avoids a heap allocation on the wrap-around path.
+  private let linearizationBuffer: UnsafeMutableRawPointer
   private var writeIndex: Int = 0
   private var readIndex: Int = 0
   private var availableBytes: Int = 0
   private let maxBufferSize: Int
 
-  private let bytesPerChunk: Int
-  private let chunkDuration: Double
+  public let bytesPerChunk: Int
 
   public init(format: AudioStreamBasicDescription, chunkDuration: Double = 0.2) {
-
     // Pre-calculate chunk parameters
     let bytesPerFrame = Int(format.mBytesPerFrame)
     let samplesPerChunk = Int(format.mSampleRate * chunkDuration)
     self.bytesPerChunk = samplesPerChunk * bytesPerFrame
-    self.chunkDuration = Double(samplesPerChunk) / format.mSampleRate
 
-    // Calculate max buffer size to hold ~10 seconds of audio, way more than the maximum we allow
+    // Calculate max buffer size to hold ~10 seconds of audio (safety limit)
     let bytesPerSecond = Int(format.mSampleRate) * bytesPerFrame
     self.maxBufferSize = bytesPerSecond * 10
 
-    // Pre-allocated ring buffer
-    self.buffer = Array(repeating: 0, count: maxBufferSize)
+    // Allocate raw memory. We use UnsafeMutableRawPointer instead of [UInt8]
+    // to eliminate Swift Array's COW ref-count check on every write/read.
+    self.buffer = UnsafeMutableRawPointer.allocate(
+      byteCount: maxBufferSize,
+      alignment: MemoryLayout<UInt8>.alignment
+    )
+    buffer.initializeMemory(as: UInt8.self, repeating: 0, count: maxBufferSize)
+
+    self.linearizationBuffer = UnsafeMutableRawPointer.allocate(
+      byteCount: bytesPerChunk,
+      alignment: MemoryLayout<UInt8>.alignment
+    )
   }
 
-  public func append(_ data: Data) {
-    guard availableBytes + data.count <= maxBufferSize else {
-      Logger.error(
+  deinit {
+    buffer.deallocate()
+    linearizationBuffer.deallocate()
+  }
+
+  /// Appends audio data directly from a raw pointer into the ring buffer.
+  /// This is the fast path used by the IO proc callback: one memcpy from
+  /// the Core Audio buffer into our ring buffer, with no intermediate
+  /// Data allocation.
+  public func append(from source: UnsafeRawPointer, count: Int) {
+    guard count >= 0 else {
+      AudioTeeLogging.logger.error(
+        "Audio buffer append called with negative count",
+        context: ["count": String(count)])
+      return
+    }
+
+    guard availableBytes + count <= maxBufferSize else {
+      AudioTeeLogging.logger.error(
         "Audio buffer overflow",
         context: [
-          "requested": String(data.count),
+          "requested": String(count),
           "available": String(maxBufferSize - availableBytes),
         ])
       return
     }
 
-    data.withUnsafeBytes { bytes in
-      let sourceBytes = bytes.bindMemory(to: UInt8.self)
-      let dataSize = sourceBytes.count
-
-      // Check if we can copy in one block (no wrap-around)
-      if writeIndex + dataSize <= maxBufferSize {
-        // only one write needed
-        buffer.replaceSubrange(writeIndex..<writeIndex + dataSize, with: sourceBytes)
-        writeIndex = (writeIndex + dataSize) % maxBufferSize
+    if writeIndex + count <= maxBufferSize {
+      // Single contiguous write — no wrap-around needed
+      buffer.advanced(by: writeIndex).copyMemory(from: source, byteCount: count)
+      writeIndex = (writeIndex + count) % maxBufferSize
     } else {
-        // two writes needed due to wrap-around
+      // Two writes needed due to wrap-around at the end of the ring buffer
       let firstChunkSize = maxBufferSize - writeIndex
-        let secondChunkSize = dataSize - firstChunkSize
+      let secondChunkSize = count - firstChunkSize
 
-        buffer.replaceSubrange(writeIndex..<maxBufferSize, with: sourceBytes.prefix(firstChunkSize))
-        buffer.replaceSubrange(0..<secondChunkSize, with: sourceBytes.suffix(secondChunkSize))
+      buffer.advanced(by: writeIndex).copyMemory(from: source, byteCount: firstChunkSize)
+      buffer.copyMemory(from: source.advanced(by: firstChunkSize), byteCount: secondChunkSize)
 
       writeIndex = secondChunkSize
     }
+
+    availableBytes += count
   }
 
-    availableBytes += data.count
-  }
-
-  public func processChunks() -> [AudioPacket] {
-    var packets: [AudioPacket] = []
-
-    while let packet = nextChunk() {
-      packets.append(packet)
-    }
-
-    return packets
-  }
-
-  private func nextChunk() -> AudioPacket? {
-    // Check if we have enough data for a complete chunk
-    guard availableBytes >= bytesPerChunk else { return nil }
-
-    var chunkData = Data(capacity: bytesPerChunk)
-
-    // Check if we can copy in one block (no wrap-around)
+  /// Calls `handler` once for each complete chunk available in the buffer.
+  /// The pointer passed to the handler is valid only for the duration of
+  /// that call. In the common (contiguous) case this points directly into
+  /// the ring buffer — zero copies. In the wrap-around case the chunk is
+  /// linearized into a pre-allocated scratch buffer — one memcpy, zero
+  /// heap allocations.
+  public func processChunks(_ handler: (UnsafeRawPointer, Int) -> Void) {
+    while availableBytes >= bytesPerChunk {
       if readIndex + bytesPerChunk <= maxBufferSize {
-      // one copy needed
-      chunkData.append(contentsOf: buffer[readIndex..<readIndex + bytesPerChunk])
+        // Contiguous: point directly into the ring buffer
+        handler(buffer.advanced(by: readIndex), bytesPerChunk)
         readIndex = (readIndex + bytesPerChunk) % maxBufferSize
       } else {
-      // two copies needed due to wrap-around
+        // Wrap-around: linearize into the pre-allocated scratch buffer
         let firstChunkSize = maxBufferSize - readIndex
         let secondChunkSize = bytesPerChunk - firstChunkSize
 
-      chunkData.append(contentsOf: buffer[readIndex..<maxBufferSize])
-      chunkData.append(contentsOf: buffer[0..<secondChunkSize])
+        linearizationBuffer.copyMemory(
+          from: buffer.advanced(by: readIndex), byteCount: firstChunkSize)
+        linearizationBuffer.advanced(by: firstChunkSize).copyMemory(
+          from: buffer, byteCount: secondChunkSize)
 
+        handler(linearizationBuffer, bytesPerChunk)
         readIndex = secondChunkSize
       }
 
       availableBytes -= bytesPerChunk
-
-    return AudioPacket(
-      timestamp: Date(),
-      duration: chunkDuration,
-      data: chunkData
-    )
+    }
   }
 }

Sources/AudioTeeCore/Core/AudioFormatConverter.swift

@@ -2,12 +2,22 @@ import AVFoundation
 import CoreAudio
 import Foundation
 
-/// Simple audio format converter using AVFoundation
+/// Audio format converter using AVFoundation's AVAudioConverter.
+///
+/// Pre-allocates input/output buffers on first use and reuses them across
+/// transform() calls. This eliminates two AVAudioPCMBuffer heap allocations
+/// per chunk — significant when chunks are small (50ms = 20 calls/sec).
 public class AudioFormatConverter {
   private let avConverter: AVAudioConverter
   private let sourceFormat: AVAudioFormat
   private let targetFormat: AVAudioFormat
 
+  /// Pre-allocated buffers reused across transform() calls. Lazily created
+  /// on first transform() since we need the actual input frame count to
+  /// size them correctly.
+  private var cachedInputBuffer: AVAudioPCMBuffer?
+  private var cachedOutputBuffer: AVAudioPCMBuffer?
+
   public init(sourceFormat: AudioStreamBasicDescription, targetFormat: AudioStreamBasicDescription)
     throws
   {
@@ -28,7 +38,7 @@ public class AudioFormatConverter {
     self.targetFormat = targetAVFormat
     self.avConverter = converter
 
-    Logger.debug(
+    AudioTeeLogging.logger.debug(
       "Audio converter created",
       context: [
         "source_sample_rate": String(sourceAVFormat.sampleRate),
@@ -39,7 +49,7 @@ public class AudioFormatConverter {
 
     // Warn about upsampling once during initialization
     if targetAVFormat.sampleRate > sourceAVFormat.sampleRate {
-      Logger.info(
+      AudioTeeLogging.logger.info(
         "Upsampling audio - this doesn't add frequency content above the original Nyquist limit",
         context: [
           "source_rate": String(sourceAVFormat.sampleRate),
@@ -48,58 +58,108 @@ public class AudioFormatConverter {
     }
   }
 
-  /// Get the target format as AudioStreamBasicDescription
+  /// The source format this converter reads from.
+  public var sourceFormatDescription: AudioStreamBasicDescription {
+    return sourceFormat.streamDescription.pointee
+  }
+
+  /// The target format this converter produces.
   public var targetFormatDescription: AudioStreamBasicDescription {
     return targetFormat.streamDescription.pointee
   }
 
-  public func transform(_ packet: AudioPacket) -> AudioPacket {
-    let inputData = packet.data
+  /// Returns pre-allocated input and output buffers sized for the given
+  /// input frame count. Allocates once on first call; reuses on subsequent
+  /// calls when capacity is sufficient. Re-allocates if a larger frame
+  /// count arrives (shouldn't happen with fixed chunk sizes, but handled
+  /// gracefully).
+  private func getBuffers(inputFrameCount: AVAudioFrameCount)
+    -> (input: AVAudioPCMBuffer, output: AVAudioPCMBuffer)?
+  {
+    // ceil() prevents float-to-int truncation from undersizing the buffer
+    // by one frame (e.g. 3199.9999 → 3199 instead of 3200).
+    let outputFrameCount = AVAudioFrameCount(
+      ceil(Double(inputFrameCount) * (targetFormat.sampleRate / sourceFormat.sampleRate))
+    )
 
-    // Calculate frame counts
-    let inputFrameCount =
-      inputData.count / Int(sourceFormat.streamDescription.pointee.mBytesPerFrame)
-    let outputFrameCount = Int(
-      Double(inputFrameCount) * (targetFormat.sampleRate / sourceFormat.sampleRate))
-
-    // Create input buffer
-    guard
-      let inputBuffer = AVAudioPCMBuffer(
-        pcmFormat: sourceFormat, frameCapacity: AVAudioFrameCount(inputFrameCount))
-    else {
-      Logger.error("Failed to create input buffer")
-      return packet
+    // Reuse cached buffers if they have sufficient capacity
+    if let inputBuf = cachedInputBuffer,
+      let outputBuf = cachedOutputBuffer,
+      inputBuf.frameCapacity >= inputFrameCount,
+      outputBuf.frameCapacity >= outputFrameCount
+    {
+      // Reset frame lengths for reuse — the underlying memory is retained,
+      // we just tell AVAudioPCMBuffer how many frames are valid this time.
+      inputBuf.frameLength = 0
+      outputBuf.frameLength = 0
+      return (inputBuf, outputBuf)
     }
 
-    // Copy input data to buffer
-    inputData.withUnsafeBytes { bytes in
+    // Allocate new buffers (first call, or unexpected capacity increase)
+    guard
+      let inputBuf = AVAudioPCMBuffer(
+        pcmFormat: sourceFormat, frameCapacity: inputFrameCount)
+    else {
+      AudioTeeLogging.logger.error("Failed to create input buffer")
+      return nil
+    }
+
+    guard
+      let outputBuf = AVAudioPCMBuffer(
+        pcmFormat: targetFormat, frameCapacity: outputFrameCount)
+    else {
+      AudioTeeLogging.logger.error("Failed to create output buffer")
+      return nil
+    }
+
+    // Cache for reuse on subsequent calls
+    cachedInputBuffer = inputBuf
+    cachedOutputBuffer = outputBuf
+
+    AudioTeeLogging.logger.debug(
+      "Allocated converter buffers",
+      context: [
+        "input_frame_capacity": String(inputFrameCount),
+        "output_frame_capacity": String(outputFrameCount),
+      ])
+
+    return (inputBuf, outputBuf)
+  }
+
+  /// Converts audio data in-place through the pre-allocated converter buffers.
+  /// Calls `handler` with a pointer to the converted output, valid only for
+  /// the duration of that call. Returns false on failure (caller should
+  /// pass through the original data or drop it).
+  @discardableResult
+  public func transform(
+    from source: UnsafeRawPointer, count: Int,
+    handler: (UnsafeRawPointer, Int) -> Void
+  ) -> Bool {
+    let bytesPerFrame = Int(sourceFormat.streamDescription.pointee.mBytesPerFrame)
+    let inputFrameCount = AVAudioFrameCount(count / bytesPerFrame)
+
+    guard let (inputBuffer, outputBuffer) = getBuffers(inputFrameCount: inputFrameCount) else {
+      return false
+    }
+
+    // Copy source data into the reusable input buffer
     let dest = inputBuffer.audioBufferList.pointee.mBuffers.mData!
-      dest.copyMemory(from: bytes.baseAddress!, byteCount: inputData.count)
-    }
-    inputBuffer.frameLength = AVAudioFrameCount(inputFrameCount)
+    dest.copyMemory(from: source, byteCount: count)
+    inputBuffer.frameLength = inputFrameCount
 
-    // Create output buffer
-    guard
-      let outputBuffer = AVAudioPCMBuffer(
-        pcmFormat: targetFormat, frameCapacity: AVAudioFrameCount(outputFrameCount))
-    else {
-      Logger.error("Failed to create output buffer")
-      return packet
-    }
-
-    // Perform conversion - simpler approach
+    // Perform conversion — we do NOT call avConverter.reset() between
+    // calls because the resampler maintains internal state for continuity
+    // across chunks (avoiding discontinuity artifacts).
     var error: NSError?
 
     let status = avConverter.convert(to: outputBuffer, error: &error) {
       requestedPackets, outStatus in
-      // Always provide our input buffer and let converter manage it
       outStatus.pointee = .haveData
       return inputBuffer
     }
 
-    // Check if conversion produced output (regardless of status code)
     guard outputBuffer.frameLength > 0 else {
-      Logger.error(
+      AudioTeeLogging.logger.error(
         "Audio conversion produced no output",
         context: [
           "status": String(describing: status),
@@ -107,20 +167,13 @@ public class AudioFormatConverter {
           "input_frames": String(inputBuffer.frameLength),
           "output_capacity": String(outputBuffer.frameCapacity),
         ])
-      return packet
+      return false
     }
 
-    // Extract converted data
-    let outputData = Data(
-      bytes: outputBuffer.audioBufferList.pointee.mBuffers.mData!,
-      count: Int(outputBuffer.frameLength * targetFormat.streamDescription.pointee.mBytesPerFrame))
-
-    // Return new packet with converted audio (keeping original metadata for simplicity)
-    return AudioPacket(
-      timestamp: packet.timestamp,
-      duration: packet.duration,
-      data: outputData
-    )
+    let outputCount = Int(
+      outputBuffer.frameLength * targetFormat.streamDescription.pointee.mBytesPerFrame)
+    handler(outputBuffer.audioBufferList.pointee.mBuffers.mData!, outputCount)
+    return true
   }
 
   public static func toSampleRate(

Sources/AudioTeeCore/Core/AudioFormatManager.swift

@@ -3,25 +3,26 @@ import CoreAudio
 import Foundation
 
 public class AudioFormatManager {
-  public static func getDeviceFormat(deviceID: AudioObjectID) -> AudioStreamBasicDescription {
+  public static func getDeviceFormat(deviceID: AudioObjectID) throws -> AudioStreamBasicDescription
+  {
     // First, wait for the device to become alive/ready
     let deviceReadyTimeout = 2.0  // 2 seconds max wait
     let pollInterval = 0.1  // 100ms poll interval
     let maxPolls = Int(deviceReadyTimeout / pollInterval)
 
-    Logger.debug(
+    AudioTeeLogging.logger.debug(
       "Waiting for audio device to become ready", context: ["device_id": String(deviceID)])
 
     // Poll device readiness
     for poll in 1...maxPolls {
       if isAudioDeviceValid(deviceID) {
-        Logger.debug(
+        AudioTeeLogging.logger.debug(
           "Audio device is ready", context: ["device_id": String(deviceID), "polls": String(poll)])
         break
       }
 
       if poll == maxPolls {
-        Logger.info(
+        AudioTeeLogging.logger.info(
           "Device did not become ready within timeout, proceeding anyway",
           context: [
             "device_id": String(deviceID),
@@ -30,7 +31,7 @@ public class AudioFormatManager {
         break
       }
 
-      Logger.info("------- not ready; retrying...")
+      AudioTeeLogging.logger.info("------- not ready; retrying...")
 
       Thread.sleep(forTimeInterval: pollInterval)
     }
@@ -49,11 +50,12 @@ public class AudioFormatManager {
         deviceID, &propertyAddress, 0, nil, &propertySize, &streamFormat)
 
       if status == noErr {
-        Logger.debug("Successfully retrieved device format", context: ["attempt": String(attempt)])
+        AudioTeeLogging.logger.debug(
+          "Successfully retrieved device format", context: ["attempt": String(attempt)])
         return streamFormat
       }
 
-      Logger.info(
+      AudioTeeLogging.logger.info(
         "------- Failed to get stream format after device ready check, retrying...",
         context: [
           "attempt": String(attempt),
@@ -69,16 +71,14 @@ public class AudioFormatManager {
     }
 
     // If all attempts failed after device readiness confirmation, this is a genuine error
-    Logger.error(
+    AudioTeeLogging.logger.error(
       "Failed to get device format after device readiness check and retries",
       context: [
         "device_id": String(deviceID),
         "device_was_ready": "true",
       ])
 
-    fatalError(
-      "Failed to get stream format from ready device: \(deviceID). This indicates a Core Audio subsystem error."
-    )
+    throw AudioTeeError.deviceFormatUnavailable(deviceID)
   }
 
   static func createMetadata(for format: AudioStreamBasicDescription) -> AudioStreamMetadata {
@@ -94,14 +94,8 @@ public class AudioFormatManager {
     )
   }
 
-  public static func writeMetadata(for format: AudioStreamBasicDescription) {
-    let metadata = createMetadata(for: format)
-    Logger.writeMessage(.metadata, data: metadata)
-    Logger.writeMessage(.streamStart, data: Optional<String>.none)
-  }
-
   public static func logFormatInfo(_ format: AudioStreamBasicDescription) {
-    Logger.debug(
+    AudioTeeLogging.logger.debug(
       "Using device's native format",
       context: [
         "channels": String(format.mChannelsPerFrame),

Sources/AudioTeeCore/Core/AudioPacket.swift

@@ -1,17 +0,0 @@
-import Foundation
-
-public struct AudioPacket {
-  public let timestamp: Date
-  public let duration: Double
-  public let data: Data
-
-  public init(
-    timestamp: Date,
-    duration: Double,
-    data: Data
-  ) {
-    self.timestamp = timestamp
-    self.duration = duration
-    self.data = data
-  }
-}

Sources/AudioTeeCore/Core/AudioRecorder.swift

@@ -10,15 +10,25 @@ public class AudioRecorder {
   private var outputHandler: AudioOutputHandler
   private var converter: AudioFormatConverter?
 
+  /// The audio format this recorder produces (after any conversion).
+  public var outputFormat: AudioStreamBasicDescription {
+    return finalFormat
+  }
+
+  /// Whether this recorder is performing sample rate conversion.
+  public var isConverting: Bool {
+    return converter != nil
+  }
+
   public init(
     deviceID: AudioObjectID, outputHandler: AudioOutputHandler, convertToSampleRate: Double? = nil,
     chunkDuration: Double = 0.2
-  ) {
+  ) throws {
     self.deviceID = deviceID
     self.outputHandler = outputHandler
 
     // Get source format and set up conversion if requested
-    let sourceFormat = AudioFormatManager.getDeviceFormat(deviceID: deviceID)
+    let sourceFormat = try AudioFormatManager.getDeviceFormat(deviceID: deviceID)
 
     // Set up the audio buffer using source format and configurable chunk duration
     self.audioBuffer = AudioBuffer(format: sourceFormat, chunkDuration: chunkDuration)
@@ -26,7 +36,8 @@ public class AudioRecorder {
     if let targetSampleRate = convertToSampleRate {
       // Validate sample rate
       guard AudioFormatConverter.isValidSampleRate(targetSampleRate) else {
-        Logger.error("Invalid sample rate", context: ["sample_rate": String(targetSampleRate)])
+        AudioTeeLogging.logger.error(
+          "Invalid sample rate", context: ["sample_rate": String(targetSampleRate)])
         self.converter = nil
         self.finalFormat = sourceFormat
         return
@@ -36,10 +47,10 @@ public class AudioRecorder {
         let converter = try AudioFormatConverter.toSampleRate(targetSampleRate, from: sourceFormat)
         self.converter = converter
         self.finalFormat = converter.targetFormatDescription
-        Logger.info(
+        AudioTeeLogging.logger.info(
           "Audio conversion enabled", context: ["target_sample_rate": String(targetSampleRate)])
       } catch {
-        Logger.error(
+        AudioTeeLogging.logger.error(
           "Failed to create audio converter, using original format",
           context: ["error": String(describing: error)])
         self.converter = nil
@@ -51,8 +62,8 @@ public class AudioRecorder {
     }
   }
 
-  public func startRecording() {
-    Logger.debug("Starting audio recording")
+  public func startRecording() throws {
+    AudioTeeLogging.logger.debug("Starting audio recording")
 
     // Log format info and send metadata for final format
     AudioFormatManager.logFormatInfo(finalFormat)
@@ -60,15 +71,15 @@ public class AudioRecorder {
     outputHandler.handleMetadata(metadata)
     outputHandler.handleStreamStart()
 
-    setupAndStartIOProc()
+    try setupAndStartIOProc()
 
-    Logger.info("Audio device started successfully")
+    AudioTeeLogging.logger.info("Audio device started successfully")
   }
 
   // Note to self, what about installTap? Would require audio engine and a node?
   // No; AudioEngine.installTap() can only fire as often as 100ms. too slow for us
-  private func setupAndStartIOProc() {
-    Logger.debug("Creating IO proc")
+  private func setupAndStartIOProc() throws {
+    AudioTeeLogging.logger.debug("Creating IO proc")
     var status = AudioDeviceCreateIOProcID(
       deviceID,
       {
@@ -82,15 +93,15 @@ public class AudioRecorder {
     )
 
     guard status == noErr else {
-      fatalError("Failed to create IO proc: \(status)")
+      throw AudioTeeError.ioProcCreationFailed(status)
     }
 
-    Logger.debug("Starting audio device")
+    AudioTeeLogging.logger.debug("Starting audio device")
     status = AudioDeviceStart(deviceID, ioProcID)
 
     if status != noErr {
       cleanupIOProc()
-      fatalError("Failed to start audio device: \(status). Device ID: \(deviceID)")
+      throw AudioTeeError.deviceStartFailed(status)
     }
   }
 
@@ -98,14 +109,16 @@ public class AudioRecorder {
     let bufferList = inputData.pointee
     let firstBuffer = bufferList.mBuffers
 
-    guard firstBuffer.mData != nil && firstBuffer.mDataByteSize > 0 else {
-      "Warning: Received empty audio buffer".print(to: .standardError)
+    guard let sourcePointer = firstBuffer.mData, firstBuffer.mDataByteSize > 0 else {
+      AudioTeeLogging.logger.error("Received empty audio buffer")
       return noErr
     }
 
-    // Append raw audio data to buffer
-    let audioData = Data(bytes: firstBuffer.mData!, count: Int(firstBuffer.mDataByteSize))
-    audioBuffer?.append(audioData)
+    // Copy directly from the Core Audio buffer into our ring buffer.
+    // This avoids creating an intermediate Data object (heap alloc + memcpy)
+    // on every IO callback (~10ms). The pointer is valid for the duration
+    // of this callback, so this is safe.
+    audioBuffer?.append(from: sourcePointer, count: Int(firstBuffer.mDataByteSize))
 
     processAudioBuffer()
 
@@ -119,10 +132,17 @@ public class AudioRecorder {
   }
 
   private func processAudioBuffer() {
-    // Process and send complete chunks, applying conversion if needed
-    audioBuffer?.processChunks().forEach { packet in
-      let processedPacket = converter?.transform(packet) ?? packet
-      outputHandler.handleAudioPacket(processedPacket)
+    audioBuffer?.processChunks { pointer, count in
+      if let converter = self.converter {
+        if !converter.transform(from: pointer, count: count, handler: { outPtr, outCount in
+          self.outputHandler.handleAudioData(outPtr, count: outCount)
+        }) {
+          // Conversion failed — pass through unconverted audio
+          self.outputHandler.handleAudioData(pointer, count: count)
+        }
+      } else {
+        self.outputHandler.handleAudioData(pointer, count: count)
+      }
     }
   }
 

Sources/AudioTeeCore/Core/AudioTapManager.swift

@@ -10,7 +10,7 @@ public class AudioTapManager {
   public init() {}
 
   deinit {
-    Logger.debug("Cleaning up audio tap manager")
+    AudioTeeLogging.logger.debug("Cleaning up audio tap manager")
 
     if let tapID = tapID {
       AudioHardwareDestroyProcessTap(tapID)
@@ -25,7 +25,7 @@ public class AudioTapManager {
 
   /// Sets up the audio tap and aggregate device
   public func setupAudioTap(with config: TapConfiguration) throws {
-    Logger.debug("Setting up audio tap manager")
+    AudioTeeLogging.logger.debug("Setting up audio tap manager")
 
     tapID = try createSystemAudioTap(with: config)
     deviceID = try createAggregateDevice()
@@ -36,7 +36,7 @@ public class AudioTapManager {
 
     try addTapToAggregateDevice(tapID: tapID, deviceID: deviceID)
 
-    Logger.debug("Audio tap manager setup complete")
+    AudioTeeLogging.logger.debug("Audio tap manager setup complete")
   }
 
   /// Returns the aggregate device ID for recording
@@ -45,7 +45,7 @@ public class AudioTapManager {
   }
 
   private func createSystemAudioTap(with config: TapConfiguration) throws -> AudioObjectID {
-    Logger.debug("Creating tap description")
+    AudioTeeLogging.logger.debug("Creating tap description")
     let description = CATapDescription()
 
     description.name = "audiotee-tap"
@@ -58,7 +58,7 @@ public class AudioTapManager {
     description.deviceUID = nil  // system default
     description.stream = 0  // first stream of output device
 
-    Logger.debug(
+    AudioTeeLogging.logger.debug(
       "Tap description configured",
       context: [
         "name": description.name,
@@ -69,14 +69,15 @@ public class AudioTapManager {
       ])
 
     // Create the tap
-    Logger.debug("Creating tap")
+    AudioTeeLogging.logger.debug("Creating tap")
     var tapID = AudioObjectID(kAudioObjectUnknown)
     let status = AudioHardwareCreateProcessTap(description, &tapID)
 
-    Logger.debug(
+    AudioTeeLogging.logger.debug(
       "AudioHardwareCreateProcessTap completed", context: ["status": String(status)])
     guard status == kAudioHardwareNoError else {
-      Logger.error("Failed to create audio tap", context: ["status": String(status)])
+      AudioTeeLogging.logger.error(
+        "Failed to create audio tap", context: ["status": String(status)])
       throw AudioTeeError.tapCreationFailed(status)
     }
 
@@ -88,7 +89,7 @@ public class AudioTapManager {
       tapID, &propertyAddress, 0, nil, &propertySize, &streamDescription)
 
     if formatStatus == noErr {
-      Logger.debug(
+      AudioTeeLogging.logger.debug(
         "Tap format retrieved",
         context: [
           "channels": String(streamDescription.mChannelsPerFrame),
@@ -115,7 +116,8 @@ public class AudioTapManager {
     let status = AudioHardwareCreateAggregateDevice(description as CFDictionary, &deviceID)
 
     guard status == kAudioHardwareNoError else {
-      Logger.error("Failed to create aggregate device", context: ["status": String(status)])
+      AudioTeeLogging.logger.error(
+        "Failed to create aggregate device", context: ["status": String(status)])
       throw AudioTeeError.aggregateDeviceCreationFailed(status)
     }
 
@@ -142,7 +144,7 @@ public class AudioTapManager {
     }
 
     guard status == kAudioHardwareNoError else {
-      Logger.error(
+      AudioTeeLogging.logger.error(
         "Failed to add tap to aggregate device", context: ["status": String(status)])
       throw AudioTeeError.tapAssignmentFailed(status)
     }

Sources/AudioTeeCore/Core/AudioTeeErrors.swift

@@ -1,3 +1,4 @@
+import CoreAudio
 import Foundation
 
 // MARK: - Core AudioTee Errors
@@ -8,6 +9,9 @@ public enum AudioTeeError: Error {
   case aggregateDeviceCreationFailed(OSStatus)
   case tapAssignmentFailed(OSStatus)
   case pidTranslationFailed([Int32])
+  case deviceFormatUnavailable(AudioObjectID)
+  case ioProcCreationFailed(OSStatus)
+  case deviceStartFailed(OSStatus)
 }
 
 // MARK: - Audio Format Conversion Errors

Sources/AudioTeeCore/Output/AudioOutputProtocol.swift

@@ -2,7 +2,9 @@ import Foundation
 
 /// Protocol for handling audio output in different formats
 public protocol AudioOutputHandler {
-  func handleAudioPacket(_ packet: AudioPacket)
+  /// Called with a pointer to raw PCM audio data. The pointer is only
+  /// valid for the duration of this call.
+  func handleAudioData(_ pointer: UnsafeRawPointer, count: Int)
   func handleMetadata(_ metadata: AudioStreamMetadata)
   func handleStreamStart()
   func handleStreamStop()

Sources/AudioTeeCore/Output/BinaryOutputHandler.swift

@@ -1,28 +0,0 @@
-import Foundation
-
-public class BinaryAudioOutputHandler: AudioOutputHandler {
-  private let flushAfterWrite: Bool
-
-  public init(flushAfterWrite: Bool = false) {
-    self.flushAfterWrite = flushAfterWrite
-  }
-  public func handleAudioPacket(_ packet: AudioPacket) {
-    // Write raw binary audio data directly to stdout
-    FileHandle.standardOutput.write(packet.data)
-    if flushAfterWrite {
-      fflush(stdout)
-    }
-  }
-
-  public func handleMetadata(_ metadata: AudioStreamMetadata) {
-    Logger.writeMessage(.metadata, data: metadata)
-  }
-
-  public func handleStreamStart() {
-    Logger.writeMessage(.streamStart, data: Optional<String>.none)
-  }
-
-  public func handleStreamStop() {
-    Logger.writeMessage(.streamStop, data: Optional<String>.none)
-  }
-}

Sources/AudioTeeCore/Utils/AudioTeeLogger.swift

@@ -0,0 +1,44 @@
+import Foundation
+
+// MARK: - Logging protocol
+
+/// Protocol that library consumers implement to receive log output.
+/// The library never writes to stderr directly — it calls through this.
+public protocol AudioTeeLogger {
+  func debug(_ message: String, context: [String: String]?)
+  func info(_ message: String, context: [String: String]?)
+  func error(_ message: String, context: [String: String]?)
+
+  /// Called for structured lifecycle messages (metadata, stream_start, stream_stop).
+  /// Default implementation is a no-op — pure library consumers get metadata
+  /// via AudioOutputHandler instead.
+  func writeMessage<T: Codable>(_ type: MessageType, data: T?)
+}
+
+// MARK: - Defaults
+
+extension AudioTeeLogger {
+  /// Library consumers typically don't need structured message output;
+  /// they receive metadata via the AudioOutputHandler protocol instead.
+  public func writeMessage<T: Codable>(_ type: MessageType, data: T?) {}
+
+  /// Convenience overloads so callers can omit context when it's nil.
+  public func debug(_ message: String) { debug(message, context: nil) }
+  public func info(_ message: String) { info(message, context: nil) }
+  public func error(_ message: String) { error(message, context: nil) }
+}
+
+// MARK: - Global logging configuration
+
+/// Global logger instance. Defaults to StderrJSONLogger (CLI behavior).
+/// Library consumers can replace this before calling any AudioTeeCore API.
+///
+///     // Silence all logging:
+///     AudioTeeLogging.logger = NullLogger()
+///
+///     // Custom logging:
+///     AudioTeeLogging.logger = MyOSLogLogger()
+///
+public enum AudioTeeLogging {
+  nonisolated(unsafe) public static var logger: AudioTeeLogger = StderrJSONLogger()
+}

Sources/AudioTeeCore/Utils/Logger.swift

@@ -1,7 +1,10 @@
 import Foundation
 
-public class Logger {
-  nonisolated(unsafe) private static let dateFormatter: ISO8601DateFormatter = {
+/// Default logger implementation that writes JSON messages to stderr.
+/// This is the CLI-appropriate logger; library consumers can replace it
+/// via AudioTeeLogging.logger.
+public class StderrJSONLogger: AudioTeeLogger {
+  private let dateFormatter: ISO8601DateFormatter = {
     let formatter = ISO8601DateFormatter()
     formatter.formatOptions = [
       .withInternetDateTime,
@@ -10,17 +13,22 @@ public class Logger {
     return formatter
   }()
 
-  private static let jsonEncoder: JSONEncoder = {
+  private let jsonEncoder: JSONEncoder = {
     let encoder = JSONEncoder()
-    encoder.dateEncodingStrategy = .custom { date, encoder in
-      var container = encoder.singleValueContainer()
-      try container.encode(dateFormatter.string(from: date))
-    }
     return encoder
   }()
 
-  // Write any message with the unified envelope
-  public static func writeMessage<T: Codable>(_ type: MessageType, data: T? = nil) {
+  public init() {
+    // Configured in init because stored property initializers can't
+    // reference other instance properties (self.dateFormatter).
+    jsonEncoder.dateEncodingStrategy = .custom { [dateFormatter] date, encoder in
+      var container = encoder.singleValueContainer()
+      try container.encode(dateFormatter.string(from: date))
+    }
+  }
+
+  // Write any message with the unified envelope to stderr
+  public func writeMessage<T: Codable>(_ type: MessageType, data: T?) {
     let message = Message(type: type, data: data)
     do {
       let jsonData = try jsonEncoder.encode(message)
@@ -32,17 +40,17 @@ public class Logger {
   }
 
   // Convenience methods for different message types
-  public static func info(_ message: String, context: [String: String]? = nil) {
+  public func info(_ message: String, context: [String: String]? = nil) {
     let logData = LogData(message: message, context: context)
     writeMessage(.info, data: logData)
   }
 
-  public static func error(_ message: String, context: [String: String]? = nil) {
+  public func error(_ message: String, context: [String: String]? = nil) {
     let logData = LogData(message: message, context: context)
     writeMessage(.error, data: logData)
   }
 
-  public static func debug(_ message: String, context: [String: String]? = nil) {
+  public func debug(_ message: String, context: [String: String]? = nil) {
     let logData = LogData(message: message, context: context)
     writeMessage(.debug, data: logData)
   }

Sources/AudioTeeCore/Utils/Utils.swift

@@ -15,7 +15,7 @@ func isAudioDeviceValid(_ deviceID: AudioObjectID) -> Bool {
 
   let valid = status == kAudioHardwareNoError && isAlive == 1
 
-  Logger.debug(
+  AudioTeeLogging.logger.debug(
     "Checked device validity",
     context: [
       "device_id": String(deviceID),
@@ -63,7 +63,7 @@ func translatePIDsToProcessObjects(_ pids: [Int32]) throws -> [AudioObjectID] {
 
     if status == kAudioHardwareNoError && processObject != kAudioObjectUnknown {
       processObjects.append(processObject)
-      Logger.debug(
+      AudioTeeLogging.logger.debug(
         "Translated PID to process object",
         context: [
           "pid": String(pid),
@@ -71,7 +71,7 @@ func translatePIDsToProcessObjects(_ pids: [Int32]) throws -> [AudioObjectID] {
         ])
     } else {
       failedPIDs.append(pid)
-      Logger.debug(
+      AudioTeeLogging.logger.debug(
         "Failed to translate PID to process object",
         context: [
           "pid": String(pid),
@@ -87,11 +87,3 @@ func translatePIDsToProcessObjects(_ pids: [Int32]) throws -> [AudioObjectID] {
 
   return processObjects
 }
-
-extension String {
-  func print(to fileHandle: FileHandle) {
-    if let data = (self + "\n").data(using: .utf8) {
-      fileHandle.write(data)
-    }
-  }
-}

Tests/AudioTeeCoreTests/AudioBufferTests.swift

@@ -0,0 +1,219 @@
+import CoreAudio
+import XCTest
+
+@testable import AudioTeeCore
+
+// CoreAudio defines its own AudioBuffer struct, which collides with ours.
+// Explicit module qualification avoids ambiguity in tests that import both.
+private typealias AudioBuffer = AudioTeeCore.AudioBuffer
+
+final class AudioBufferTests: XCTestCase {
+
+  // MARK: - Helpers
+
+  /// Creates a minimal AudioStreamBasicDescription for testing.
+  /// 16kHz, 16-bit, mono = 2 bytes per frame, 32000 bytes/sec.
+  private func makeFormat(
+    sampleRate: Double = 16000,
+    bytesPerFrame: UInt32 = 2,
+    bitsPerChannel: UInt32 = 16
+  ) -> AudioStreamBasicDescription {
+    return AudioStreamBasicDescription(
+      mSampleRate: sampleRate,
+      mFormatID: kAudioFormatLinearPCM,
+      mFormatFlags: kAudioFormatFlagIsPacked | kAudioFormatFlagIsSignedInteger,
+      mBytesPerPacket: bytesPerFrame,
+      mFramesPerPacket: 1,
+      mBytesPerFrame: bytesPerFrame,
+      mChannelsPerFrame: 1,
+      mBitsPerChannel: bitsPerChannel,
+      mReserved: 0
+    )
+  }
+
+  /// Creates a repeating byte pattern of the given length.
+  private func makeData(byte: UInt8, count: Int) -> Data {
+    return Data(repeating: byte, count: count)
+  }
+
+  /// Appends Data to an AudioBuffer via the raw pointer path,
+  /// matching how processAudio() calls append(from:count:).
+  private func appendData(_ data: Data, to buffer: AudioBuffer) {
+    data.withUnsafeBytes { bytes in
+      buffer.append(from: bytes.baseAddress!, count: bytes.count)
+    }
+  }
+
+  /// Collects chunks from the buffer as Data objects for test verification.
+  private func collectChunks(from buffer: AudioBuffer) -> [Data] {
+    var chunks: [Data] = []
+    buffer.processChunks { pointer, count in
+      chunks.append(Data(bytes: pointer, count: count))
+    }
+    return chunks
+  }
+
+  // MARK: - Basic append + processChunks
+
+  func testSingleChunkExtraction() {
+    // 16kHz, 2 bytes/frame, 0.1s chunk = 3200 bytes per chunk
+    let format = makeFormat()
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.1)
+    let chunkSize = 3200  // 16000 * 0.1 * 2
+
+    let data = makeData(byte: 0xAB, count: chunkSize)
+    appendData(data, to: buffer)
+
+    let chunks = collectChunks(from: buffer)
+    XCTAssertEqual(chunks.count, 1)
+    XCTAssertEqual(chunks[0].count, chunkSize)
+    XCTAssertEqual(chunks[0], data)
+  }
+
+  func testMultipleChunksExtracted() {
+    let format = makeFormat()
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.1)
+    let chunkSize = 3200
+
+    // Append 2.5 chunks worth
+    appendData(makeData(byte: 0x01, count: chunkSize * 2 + chunkSize / 2), to: buffer)
+
+    let chunks = collectChunks(from: buffer)
+    // Should get 2 complete chunks, remainder stays in buffer
+    XCTAssertEqual(chunks.count, 2)
+    XCTAssertEqual(chunks[0].count, chunkSize)
+    XCTAssertEqual(chunks[1].count, chunkSize)
+  }
+
+  func testInsufficientDataReturnsNoChunks() {
+    let format = makeFormat()
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.1)
+    let chunkSize = 3200
+
+    // Append less than one chunk
+    appendData(makeData(byte: 0xFF, count: chunkSize - 1), to: buffer)
+
+    let chunks = collectChunks(from: buffer)
+    XCTAssertEqual(chunks.count, 0)
+  }
+
+  // MARK: - Wrap-around
+
+  func testWrapAroundWrite() {
+    // 8kHz, 2 bytes/frame, 0.3s chunks → chunkSize = 4800, maxBuffer = 160000.
+    // 160000 / 4800 = 33.33 — chunks do NOT divide evenly into the buffer,
+    // so after enough writes the writeIndex will straddle the boundary.
+    let format = makeFormat(sampleRate: 8000)
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.3)
+    let chunkSize = 4800  // 8000 * 0.3 * 2
+
+    // Write 33 chunks (158400 bytes), drain them all.
+    // writeIndex = 158400, readIndex = 158400. 1600 bytes remain before boundary.
+    for _ in 0..<33 {
+      appendData(makeData(byte: 0x00, count: chunkSize), to: buffer)
+    }
+    let drained = collectChunks(from: buffer)
+    XCTAssertEqual(drained.count, 33)
+
+    // Next write of 4800 bytes starts at 158400. 158400 + 4800 = 163200 > 160000.
+    // This MUST take the wrap-around else branch in append():
+    //   firstChunkSize = 160000 - 158400 = 1600
+    //   secondChunkSize = 4800 - 1600 = 3200
+    // Verify by using distinct byte patterns for the portion before and after the boundary.
+    var wrappingData = Data()
+    wrappingData.append(makeData(byte: 0xAA, count: 1600))  // fills to boundary
+    wrappingData.append(makeData(byte: 0xBB, count: 3200))  // wraps to start
+    XCTAssertEqual(wrappingData.count, chunkSize)
+    appendData(wrappingData, to: buffer)
+
+    let chunks = collectChunks(from: buffer)
+    XCTAssertEqual(chunks.count, 1)
+    XCTAssertEqual(chunks[0], wrappingData)
+  }
+
+  func testWrapAroundRead() {
+    // Same setup as above: position readIndex so that a chunk extraction
+    // straddles the ring buffer boundary, exercising the else branch in nextChunk().
+    let format = makeFormat(sampleRate: 8000)
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.3)
+    let chunkSize = 4800
+
+    // Write and drain 33 chunks. Both indices land at 158400.
+    for _ in 0..<33 {
+      appendData(makeData(byte: 0x00, count: chunkSize), to: buffer)
+    }
+    _ = collectChunks(from: buffer)
+
+    // Write one chunk starting at 158400. The write itself wraps (tested above),
+    // but crucially the READ will also wrap: readIndex = 158400,
+    // 158400 + 4800 = 163200 > 160000 → else branch in nextChunk():
+    //   firstChunkSize = 160000 - 158400 = 1600 (read from end of buffer)
+    //   secondChunkSize = 4800 - 1600 = 3200 (read from start of buffer)
+    var crossBoundaryData = Data()
+    crossBoundaryData.append(makeData(byte: 0xCC, count: 1600))
+    crossBoundaryData.append(makeData(byte: 0xDD, count: 3200))
+    appendData(crossBoundaryData, to: buffer)
+
+    let chunks = collectChunks(from: buffer)
+    XCTAssertEqual(chunks.count, 1)
+    XCTAssertEqual(chunks[0], crossBoundaryData)
+  }
+
+  // MARK: - Overflow guard
+
+  func testOverflowPreventsWrite() {
+    let format = makeFormat(sampleRate: 8000)
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.1)
+    let maxBuffer = 160000
+
+    // Fill the buffer completely
+    appendData(makeData(byte: 0x01, count: maxBuffer), to: buffer)
+
+    // Try to append more — should be silently rejected (overflow guard)
+    appendData(makeData(byte: 0x02, count: 100), to: buffer)
+
+    // Drain and verify we only got the original data
+    let chunks = collectChunks(from: buffer)
+    let totalBytes = chunks.reduce(0) { $0 + $1.count }
+    XCTAssertEqual(totalBytes, maxBuffer)
+
+    // Every byte should be 0x01, not 0x02
+    for chunk in chunks {
+      XCTAssertTrue(chunk.allSatisfy { $0 == 0x01 })
+    }
+  }
+
+  // MARK: - Incremental appends accumulate correctly
+
+  func testIncrementalAppendsThenChunk() {
+    let format = makeFormat()
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.1)
+    let chunkSize = 3200
+
+    // Simulate many small IO callbacks building up to one chunk
+    let callbackSize = 320  // 10 callbacks to fill one chunk
+    for i in 0..<10 {
+      appendData(makeData(byte: UInt8(i), count: callbackSize), to: buffer)
+    }
+
+    let chunks = collectChunks(from: buffer)
+    XCTAssertEqual(chunks.count, 1)
+    XCTAssertEqual(chunks[0].count, chunkSize)
+
+    // Verify the data is in the correct order
+    for i in 0..<10 {
+      let slice = chunks[0].subdata(in: (i * callbackSize)..<((i + 1) * callbackSize))
+      XCTAssertTrue(slice.allSatisfy { $0 == UInt8(i) })
+    }
+  }
+
+  // MARK: - Chunk size
+
+  func testBytesPerChunkIsCorrect() {
+    let format = makeFormat()
+    let buffer = AudioBuffer(format: format, chunkDuration: 0.1)
+
+    // 16kHz * 0.1s * 2 bytes/frame = 3200
+    XCTAssertEqual(buffer.bytesPerChunk, 3200)
+  }
+}

Tests/AudioTeeCoreTests/AudioPacketTests.swift

@@ -1,30 +0,0 @@
-import XCTest
-@testable import AudioTeeCore
-
-final class AudioPacketTests: XCTestCase {
-  func testPacketCreation() {
-    let timestamp = Date()
-    let duration = 1.0
-    let data = Data([0x01, 0x02, 0x03, 0x04])
-    
-    let packet = AudioPacket(
-      timestamp: timestamp,
-      duration: duration,
-      data: data
-    )
-    
-    XCTAssertEqual(packet.timestamp, timestamp)
-    XCTAssertEqual(packet.duration, duration)
-    XCTAssertEqual(packet.data, data)
-  }
-  
-  func testPacketDataSize() {
-    let packet = AudioPacket(
-      timestamp: Date(),
-      duration: 0.5,
-      data: Data(repeating: 0xFF, count: 1024)
-    )
-    
-    XCTAssertEqual(packet.data.count, 1024)
-  }
-}