#12 Clipping prevention - a start

Source/Core/AudioGraph/EffectsUnits/ReplayGain/LoudnessScan/FFmpegReplayGainScanner.swift

@@ -69,7 +69,7 @@ class FFmpegReplayGainScanner: ReplayGainScanner {
 
             let ctx = try FFmpegFileContext(for: file)
 
-            guard let stream = ctx.bestAudioStream else {return .noResult}
+            guard let stream = ctx.bestAudioStream else {return nil}
             let codec = try FFmpegAudioCodec(fromParameters: stream.avStream.codecpar)
 
             channelCount = Int(codec.channelCount)

Source/Core/EBUR128/EBUR128State.swift

@@ -24,6 +24,9 @@ class EBUR128State {
     let sampleRate: Int
     let mode: EBUR128Mode
 
+    static let targetLoudness: Double = -18
+    static let maxPeak: Double = 1
+
     init(channelCount: Int, sampleRate: Int, mode: EBUR128Mode) throws {
 
         self.channelCount = channelCount
@@ -50,8 +53,18 @@ class EBUR128State {
 
         let loudness = try computeLoudness()
         let peak = try computePeak()
+        let replayGain = Self.targetLoudness - loudness
+        var replayGainToPreventClipping: Double = replayGain
+
+        let newPeak = pow(10.0, replayGain / 20) * peak
+
+        if newPeak > Self.maxPeak {
+
+            let adjustment = 20 * log10(newPeak / Self.maxPeak)
+            replayGainToPreventClipping -= adjustment
+        }
 
-        return EBUR128AnalysisResult(loudness: loudness, peak: peak)
+        return EBUR128AnalysisResult(loudness: loudness, peak: peak, replayGain: replayGain, replayGainToPreventClipping: replayGainToPreventClipping)
     }
 
     func computeLoudness() throws -> Double {
@@ -112,14 +125,9 @@ enum EBUR128Mode {
 
 struct EBUR128AnalysisResult {
 
-    static let targetLoudness: Double = -18
-
     let loudness: Double
     let peak: Double
 
-    var replayGain: Double {
-        Self.targetLoudness - loudness
-    }
-
-    static let noResult: EBUR128AnalysisResult = .init(loudness: 0, peak: 0)
+    let replayGain: Double
+    let replayGainToPreventClipping: Double
 }

Source/Core/TrackIO/Model/ReplayGain.swift

@@ -14,16 +14,49 @@ struct ReplayGain {
 
     let trackGain: Float?
     let trackPeak: Float?
+    let trackGainToPreventClipping: Float?
+
     let albumGain: Float?
     let albumPeak: Float?
 
+    private static let maxPeak: Float = 1
+
     init?(trackGain: Float? = nil, trackPeak: Float? = nil, albumGain: Float? = nil, albumPeak: Float? = nil) {
 
         guard trackGain != nil || albumGain != nil else {return nil}
 
         self.trackGain = trackGain
         self.trackPeak = trackPeak
+
+        var trackGainToPreventClipping: Float
+
+        if let theTrackGain = trackGain, let theTrackPeak = trackPeak {
+
+            trackGainToPreventClipping = theTrackGain
+
+            let newPeak = pow(10.0, theTrackGain / 20) * theTrackPeak
+
+            if newPeak > Self.maxPeak {
+                trackGainToPreventClipping -= (20 * log10(newPeak / Self.maxPeak))
+            }
+
+            self.trackGainToPreventClipping = trackGainToPreventClipping
+
+        } else {
+            self.trackGainToPreventClipping = nil
+        }
+
         self.albumGain = albumGain
         self.albumPeak = albumPeak
     }
+
+    init(ebur128AnalysisResult: EBUR128AnalysisResult) {
+
+        self.trackGain = Float(ebur128AnalysisResult.replayGain)
+        self.trackPeak = Float(ebur128AnalysisResult.peak)
+        self.trackGainToPreventClipping = Float(ebur128AnalysisResult.replayGainToPreventClipping)
+
+        self.albumGain = nil
+        self.albumPeak = nil
+    }
 }