feat: add AudioCapture with AVAudioEngine mic tap and PCM conversion

MyVoxtral/MyVoxtral/Audio/AudioCapture.swift

@@ -0,0 +1,92 @@
+import AVFoundation
+
+final class AudioCapture {
+    private let engine = AVAudioEngine()
+    private let targetSampleRate: Double = 16000
+    private let chunkDurationMs: Double = 480
+
+    var onChunk: ((Data) -> Void)?
+
+    private var buffer = Data()
+    private let bytesPerChunk: Int
+
+    init() {
+        // 16kHz * 2 bytes (16-bit) * 1 channel * 0.48s = 15360 bytes
+        bytesPerChunk = Int(targetSampleRate * 2 * chunkDurationMs / 1000)
+    }
+
+    func start() throws {
+        let inputNode = engine.inputNode
+        let inputFormat = inputNode.outputFormat(forBus: 0)
+
+        let targetFormat = AVAudioFormat(
+            commonFormat: .pcmFormatInt16,
+            sampleRate: targetSampleRate,
+            channels: 1,
+            interleaved: true
+        )!
+
+        guard let converter = AVAudioConverter(from: inputFormat, to: targetFormat) else {
+            throw AudioCaptureError.converterCreationFailed
+        }
+
+        let bufferSize = AVAudioFrameCount(inputFormat.sampleRate * chunkDurationMs / 1000)
+
+        inputNode.installTap(onBus: 0, bufferSize: bufferSize, format: inputFormat) { [weak self] pcmBuffer, _ in
+            guard let self else { return }
+
+            let frameCount = AVAudioFrameCount(
+                Double(pcmBuffer.frameLength) * self.targetSampleRate / inputFormat.sampleRate
+            )
+            guard let convertedBuffer = AVAudioPCMBuffer(
+                pcmFormat: targetFormat,
+                frameCapacity: frameCount
+            ) else { return }
+
+            var error: NSError?
+            let status = converter.convert(to: convertedBuffer, error: &error) { _, outStatus in
+                outStatus.pointee = .haveData
+                return pcmBuffer
+            }
+
+            guard status != .error, error == nil else { return }
+
+            let byteCount = Int(convertedBuffer.frameLength) * 2 // 16-bit = 2 bytes
+            guard let int16Ptr = convertedBuffer.int16ChannelData?[0] else { return }
+            let data = Data(bytes: int16Ptr, count: byteCount)
+
+            self.buffer.append(data)
+
+            while self.buffer.count >= self.bytesPerChunk {
+                let chunk = self.buffer.prefix(self.bytesPerChunk)
+                self.buffer = Data(self.buffer.dropFirst(self.bytesPerChunk))
+                self.onChunk?(Data(chunk))
+            }
+        }
+
+        engine.prepare()
+        try engine.start()
+    }
+
+    func stop() {
+        engine.inputNode.removeTap(onBus: 0)
+        engine.stop()
+
+        // Flush remaining buffer
+        if !buffer.isEmpty {
+            onChunk?(buffer)
+            buffer = Data()
+        }
+    }
+}
+
+enum AudioCaptureError: Error, LocalizedError {
+    case converterCreationFailed
+
+    var errorDescription: String? {
+        switch self {
+        case .converterCreationFailed:
+            return "Failed to create audio format converter"
+        }
+    }
+}