Add support for multichannel waveforms (#4)

So far, we've only supported showing mixed down track. This PR adds a
support for showing multichannel waveforms one below the other (similar
to Audacity).

Author: jcavar

Sources/AudioSnapshotTesting/AVAudioPCMBufferExtensions.swift

@@ -7,35 +7,46 @@ extension AVAudioPCMBuffer {
     /// this function reduces this by choosing absolute max sample value
     /// in a bucket of samples.
     /// Returns a pair of buckets and max bucket value
-    func reduce(bucketCount: Int) -> ([Float], Float) {
+    func reduce(bucketCount: Int, mono: Bool = true) -> [DownsampledChannelData] {
         let frameCount = Int(self.frameLength)
-        guard frameCount > 0 else { return ([], 0) }
-        let mono = mixToMono()
-        guard let mono = mono.floatChannelData else {
+        guard frameCount > 0 else { return [DownsampledChannelData(channel: 0, samples: [], max: 0)] }
+        let buffer = mono ? mixToMono() : self
+        guard let data = buffer.floatChannelData else {
             fatalError("Not a float audio format")
         }
-        let samples = Array(UnsafeBufferPointer(start: mono[0], count: frameCount))
-        let samplesPerBucket = max(1, Double(frameCount) / Double(bucketCount))
-
-        var buckets = [Float](repeating: 0, count: bucketCount)
-        var maxBucket: Float = 0
-        for i in 0..<bucketCount {
-            let bucketStart = Int(Double(i) * samplesPerBucket)
-            let bucketEnd = min(bucketStart + Int(samplesPerBucket), frameCount)
-            guard bucketStart < bucketEnd else { break }
-            let bucketSamples = samples[bucketStart..<bucketEnd]
-            let avgSample = bucketSamples.reduce(into: Float(0)) { currentMax, value in
-                if abs(value) > abs(currentMax) {
-                    currentMax = value
-                }
-            }
-            buckets[i] = avgSample
-            if abs(avgSample) > maxBucket {
-                maxBucket = abs(avgSample)
+        return (0..<Int(buffer.format.channelCount)).map {
+            process(channel: $0, data: data[$0], frameCount: frameCount, bucketCount: bucketCount)
+        }
+    }
+}
+
+private func process(
+    channel: Int,
+    data: UnsafeMutablePointer<Float>,
+    frameCount: Int,
+    bucketCount: Int
+) -> DownsampledChannelData {
+    let samples = Array(UnsafeBufferPointer(start: data, count: frameCount))
+    let samplesPerBucket = max(1, Double(frameCount) / Double(bucketCount))
+
+    var buckets = [Float](repeating: 0, count: bucketCount)
+    var maxBucket: Float = 0
+    for i in 0..<bucketCount {
+        let bucketStart = Int(Double(i) * samplesPerBucket)
+        let bucketEnd = min(bucketStart + Int(samplesPerBucket), frameCount)
+        guard bucketStart < bucketEnd else { break }
+        let bucketSamples = samples[bucketStart..<bucketEnd]
+        let avgSample = bucketSamples.reduce(into: Float(0)) { currentMax, value in
+            if abs(value) > abs(currentMax) {
+                currentMax = value
             }
         }
-        return (buckets, maxBucket)
+        buckets[i] = avgSample
+        if abs(avgSample) > maxBucket {
+            maxBucket = abs(avgSample)
+        }
     }
+    return DownsampledChannelData(channel: channel, samples: buckets, max: maxBucket)
 }
 
 private extension AVAudioPCMBuffer {

Sources/AudioSnapshotTesting/AudioSnapshotTesting.swift

@@ -21,25 +21,20 @@ public extension Snapshotting where Format == PlatformImage, Value == (AVAudioPC
     ///    - width: The width of the resulting image.
     ///    - height: The height of the resulting image.
     ///    - strategy: The strategy to use when generating the waveform. Defaults to `.joinedLines`.
-    static func waveform(width: Int, height: Int, strategy: WaveformStrategy = .joinedLines) -> Snapshotting {
+    ///    - mono: A boolean indicating whether to mix down to a mono signal before generating the waveform. Defaults to `true`.
+    static func waveform(width: Int, height: Int, strategy: WaveformStrategy = .joinedLines, mono: Bool = true) -> Snapshotting {
         Snapshotting<PlatformView, PlatformImage>.image(size: .init(width: width, height: height))
             .pullback { buffer1, buffer2 in
-                let (buckets1, max1) = buffer1.reduce(bucketCount: width)
-                let (buckets2, max2) = buffer2.reduce(bucketCount: width)
-                let data1 = buckets1.enumerated().map(Bucket.init)
-                let data2 = buckets2.enumerated().map(Bucket.init)
                 let verticalPadding: CGFloat = 4
                 let waveformHeight = CGFloat(height) - (verticalPadding * 2)
-                let waveform1 = WaveformView(
-                    buckets: data1,
-                    absMax: max1,
+                let waveform1 = MultiChannelWaveformView(
+                    data: buffer1.reduce(bucketCount: width, mono: mono),
                     height: waveformHeight,
                     color: .red,
                     strategy: strategy
                 )
-                let waveform2 = WaveformView(
-                    buckets: data2,
-                    absMax: max2,
+                let waveform2 = MultiChannelWaveformView(
+                    data: buffer2.reduce(bucketCount: width, mono: mono),
                     height: waveformHeight,
                     color: .green,
                     strategy: strategy
@@ -62,16 +57,14 @@ public extension Snapshotting where Format == PlatformImage, Value == AVAudioPCM
     ///    - width: The width of the resulting image.
     ///    - height: The height of the resulting image.
     ///    - strategy: The strategy to use when generating the waveform. Defaults to `.joinedLines`.
-    static func waveform(width: Int, height: Int, strategy: WaveformStrategy = .joinedLines) -> Snapshotting {
+    ///    - mono: A boolean indicating whether to mix down to a mono signal before generating the waveform. Defaults to `true`.
+    static func waveform(width: Int, height: Int, strategy: WaveformStrategy = .joinedLines, mono: Bool = true) -> Snapshotting {
         Snapshotting<PlatformView, PlatformImage>.image(size: .init(width: width, height: height))
             .pullback { buffer in
                 let verticalPadding: CGFloat = 4
                 let waveformHeight = CGFloat(height) - (verticalPadding * 2)
-                let (buckets, max) = buffer.reduce(bucketCount: width)
-                let data = buckets.enumerated().map(Bucket.init)
-                let waveform = WaveformView(
-                    buckets: data,
-                    absMax: max,
+                let waveform = MultiChannelWaveformView(
+                    data: buffer.reduce(bucketCount: width, mono: mono),
                     height: waveformHeight,
                     color: .red,
                     strategy: strategy

Sources/AudioSnapshotTesting/DownsampledChannelData.swift

@@ -0,0 +1,7 @@
+struct DownsampledChannelData: Identifiable {
+    let channel: Int
+    let samples: [Float]
+    let max: Float
+
+    var id: Int { channel }
+}

Sources/AudioSnapshotTesting/MultiChannelWaveformView.swift

@@ -0,0 +1,23 @@
+import SwiftUI
+
+struct MultiChannelWaveformView: View {
+    let data: [DownsampledChannelData]
+    let height: CGFloat
+    let color: Color
+    var strategy: WaveformStrategy = .joinedLines
+
+    var body: some View {
+        VStack(spacing: 4) {
+            let waveformHeight = height / CGFloat(data.count)
+            ForEach(data) {
+                WaveformView(
+                    buckets: $0.samples.enumerated().map(Bucket.init),
+                    absMax: $0.max,
+                    height: waveformHeight,
+                    color: color,
+                    strategy: strategy
+                )
+            }
+        }
+    }
+}

Tests/AudioSnapshotTestingTests/AudioSnapshotTestingTests.swift

@@ -27,6 +27,15 @@ func fileWaveformMetronome() async throws {
     )
 }
 
+@Test(.snapshots(record: false, diffTool: .ksdiff))
+@MainActor
+func stereoFileWaveform() async throws {
+    assertSnapshot(
+        of: try AVAudioPCMBuffer.read(wave: "left-right-metronome"),
+        as: .waveform(width: 4000, height: 1000, strategy: .individualLines, mono: false)
+    )
+}
+
 @Test(.snapshots(record: false, diffTool: .ksdiff))
 @MainActor
 func fileWaveformOverlay() async throws {