Add multi-channel audio support (#11)

Preserve channel layout during quantization for audio with more than 2 channels.
Previously, the quantization step would lose channel layout information, causing
issues with multi-channel audio comparison.

Also adds a 4-channel audio test and refactors buffer creation helpers to support
arbitrary channel counts.

Author: jcavar

Sources/AudioSnapshotTesting/Internal/AudioDataComparator.swift

@@ -59,23 +59,25 @@ enum AudioDataComparator {
     /// This matches the quantization that ALAC applies during encoding.
     private static func quantize(_ buffer: AVAudioPCMBuffer, bitDepth: AudioBitDepth) throws -> AVAudioPCMBuffer {
         let intFormat: AVAudioFormat?
-        switch bitDepth {
-        case .bits16:
+        let commonFormat: AVAudioCommonFormat = bitDepth == .bits16 ? .pcmFormatInt16 : .pcmFormatInt32
+
+        // For multi-channel audio (>2 channels), we need to preserve the channel layout
+        if let channelLayout = buffer.format.channelLayout {
             intFormat = AVAudioFormat(
-                commonFormat: .pcmFormatInt16,
+                commonFormat: commonFormat,
                 sampleRate: buffer.format.sampleRate,
-                channels: buffer.format.channelCount,
-                interleaved: false
+                interleaved: false,
+                channelLayout: channelLayout
             )
-        case .bits32:
+        } else {
             intFormat = AVAudioFormat(
-                commonFormat: .pcmFormatInt32,
+                commonFormat: commonFormat,
                 sampleRate: buffer.format.sampleRate,
                 channels: buffer.format.channelCount,
                 interleaved: false
             )
         }
-        
+
         guard let intFormat else {
             throw AudioComparisonError.formatCreationFailed
         }

Tests/AudioSnapshotTestingTests/AudioSnapshotTestingTests.swift

@@ -154,14 +154,45 @@ func audioSnapshotSynthesized() async throws {
     )
 }
 
-private func createBuffer(from samples: [Float]) -> AVAudioPCMBuffer {
-    let format = AVAudioFormat(standardFormatWithSampleRate: 32768, channels: 1)!
-    let buffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: AVAudioFrameCount(samples.count))!
-    let channelData = buffer.floatChannelData![0]
-    samples.enumerated().forEach { index, sample in
-        channelData[index] = sample
+@available(iOS 16, macOS 13, *)
+@Test(
+    "Multi-channel (4ch) audio comparison",
+    .audioSnapshot(record: false, strategy: .spectrogram(hopSize: 4096, frequencyCount: 2048))
+)
+func multiChannelComparison() async throws {
+    let frameCount = 44100
+    let frequencies: [Float] = [440, 880, 1320, 1760]
+    let channels = frequencies.map { frequency in
+        synthesizeSignal(frequencyAmplitudePairs: [(frequency, 0.5)], count: frameCount)
+    }
+    let buffer = createBuffer(channels: channels, sampleRate: 44100)
+    await assertAudioSnapshot(of: buffer, named: "multiChannelComparison.4ch")
+}
+
+private func createBuffer(from samples: [Float], sampleRate: Double = 32768) -> AVAudioPCMBuffer {
+    createBuffer(channels: [samples], sampleRate: sampleRate)
+}
+
+private func createBuffer(channels: [[Float]], sampleRate: Double = 32768) -> AVAudioPCMBuffer {
+    let channelCount = channels.count
+    let frameCount = channels.first?.count ?? 0
+
+    let format: AVAudioFormat
+    if channelCount <= 2 {
+        format = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: AVAudioChannelCount(channelCount))!
+    } else {
+        let layout = AVAudioChannelLayout(layoutTag: kAudioChannelLayoutTag_DiscreteInOrder | UInt32(channelCount))!
+        format = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channelLayout: layout)
+    }
+
+    let buffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: AVAudioFrameCount(frameCount))!
+    for (channelIndex, samples) in channels.enumerated() {
+        let channelData = buffer.floatChannelData![channelIndex]
+        for (frameIndex, sample) in samples.enumerated() {
+            channelData[frameIndex] = sample
+        }
     }
-    buffer.frameLength = AVAudioFrameCount(samples.count)
+    buffer.frameLength = AVAudioFrameCount(frameCount)
     return buffer
 }