feat: AudioLevel helper and AudioRecorder.onLevel

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Sources/SstCore/AudioLevel.swift

@@ -0,0 +1,24 @@
+import Foundation
+
+public enum AudioLevel {
+    /// Reported for empty or silent buffers (log of zero would be -infinity).
+    public static let silenceFloorDbfs: Float = -120
+
+    public static func rmsDbfs(_ samples: [Float]) -> Float {
+        samples.withUnsafeBufferPointer { rmsDbfs($0) }
+    }
+
+    public static func rmsDbfs(_ samples: UnsafeBufferPointer<Float>) -> Float {
+        guard !samples.isEmpty else { return silenceFloorDbfs }
+        var sum: Float = 0
+        for sample in samples { sum += sample * sample }
+        let rms = (sum / Float(samples.count)).squareRoot()
+        guard rms > 0 else { return silenceFloorDbfs }
+        return max(silenceFloorDbfs, 20 * log10(rms))
+    }
+
+    /// VU reading for a dBFS level, where `referenceDbfs` is 0 VU.
+    public static func vu(fromDbfs dbfs: Float, referenceDbfs: Float = -20) -> Float {
+        dbfs - referenceDbfs
+    }
+}

Sources/SstCore/AudioRecorder.swift

@@ -12,6 +12,10 @@ public final class AudioRecorder: AudioRecording {
     private let engine = AVAudioEngine()
     private let lock = NSLock()
     private var buffer: [Float] = []
+
+    /// RMS level (dBFS) of every delivered input buffer. Called on the audio thread; set before `start()`.
+    public var onLevel: ((Float) -> Void)?
+
     private let targetFormat = AVAudioFormat(
         commonFormat: .pcmFormatFloat32,
         sampleRate: 16_000,
@@ -32,8 +36,11 @@ public final class AudioRecorder: AudioRecording {
         let ratio = targetFormat.sampleRate / inputFormat.sampleRate
 
         input.removeTap(onBus: 0)
-        input.installTap(onBus: 0, bufferSize: 4096, format: inputFormat) { [weak self] pcm, _ in
+        input.installTap(onBus: 0, bufferSize: 1024, format: inputFormat) { [weak self] pcm, _ in
             guard let self else { return }
+            if let onLevel = self.onLevel, let channel = pcm.floatChannelData?[0] {
+                onLevel(AudioLevel.rmsDbfs(UnsafeBufferPointer(start: channel, count: Int(pcm.frameLength))))
+            }
             let capacity = AVAudioFrameCount(Double(pcm.frameLength) * ratio) + 1
             guard let out = AVAudioPCMBuffer(pcmFormat: self.targetFormat, frameCapacity: capacity) else { return }