Adopted Span and MutableSpan for buffers across the library to adapt to Swift 6.4.

Sources/PlaydateKit/Sound/Effect/Aliases/Effect.Processor.swift

@@ -1,8 +1,9 @@
 extension Sound.Effect {
     /// Processes up to `AUDIO_FRAMES_PER_CYCLE` sample frames in signed
-    /// Q8.24 format. `bufferActive` is `false` when the input buffer is
-    /// silent. Returns `true` if the effect produced output.
-    public typealias Processor = (_ left: UnsafeMutableBufferPointer<Int32>,
-                                  _ right: UnsafeMutableBufferPointer<Int32>?,
+    /// Q8.24 format. `right` is empty when the channel is mono.
+    /// `bufferActive` is `false` when the input buffer is silent. Returns
+    /// `true` if the effect produced output.
+    public typealias Processor = (_ left: inout MutableSpan<Int32>,
+                                  _ right: inout MutableSpan<Int32>,
                                   _ bufferActive: Bool) -> Bool
 }

Sources/PlaydateKit/Sound/Effect/Classes/Effect.swift

@@ -30,9 +30,9 @@ extension Sound {
                 guard let effect, let left,
                       let userdata = effectAPI.pointee.getUserdata.unsafelyUnwrapped(effect) else { return 0 }
                 let box = Unmanaged<ProcessorBox>.fromOpaque(userdata).takeUnretainedValue()
-                let leftBuffer = UnsafeMutableBufferPointer(start: left, count: Int(nsamples))
-                let rightBuffer = right.map { UnsafeMutableBufferPointer(start: $0, count: Int(nsamples)) }
-                return box.processor(leftBuffer, rightBuffer, bufactive != 0) ? 1 : 0
+                var leftSpan = UnsafeMutableBufferPointer(start: left, count: Int(nsamples)).mutableSpan
+                var rightSpan = UnsafeMutableBufferPointer(start: right, count: right == nil ? 0 : Int(nsamples)).mutableSpan
+                return box.processor(&leftSpan, &rightSpan, bufactive != 0) ? 1 : 0
             }, box.toOpaque()).unsafelyUnwrapped
             isOwned = true
         }

Sources/PlaydateKit/Sound/Sound.swift

@@ -51,14 +51,14 @@ extension Sound {
 
     /// Sets a callback that records microphone input. Return `false` from the
     /// callback to stop recording. Pass `nil` to stop recording immediately.
-    /// The buffer contains mono 16-bit samples.
+    /// The span contains mono 16-bit samples.
     @discardableResult
     public static func setMicCallback(source: MicSource = .autodetect,
-                                      _ callback: ((UnsafeMutableBufferPointer<Int16>) -> Bool)?) -> Bool {
+                                      _ callback: ((Span<Int16>) -> Bool)?) -> Bool {
         micCallback = callback
         if callback != nil {
             return snd.pointee.setMicCallback.unsafelyUnwrapped({ _, buffer, length in
-                let samples = UnsafeMutableBufferPointer(start: buffer, count: Int(length))
+                let samples = UnsafeBufferPointer(start: buffer, count: Int(length)).span
                 return Sound.micCallback?(samples) == true ? 1 : 0
             }, nil, CPlaydate.MicSource(CPlaydate.MicSource.RawValue(source.rawValue))) != 0
         } else {
@@ -66,7 +66,7 @@ extension Sound {
         }
     }
 
-    nonisolated(unsafe) private static var micCallback: ((UnsafeMutableBufferPointer<Int16>) -> Bool)?
+    nonisolated(unsafe) private static var micCallback: ((Span<Int16>) -> Bool)?
 
     /// Asks the user for permission to record from the microphone. `purpose`
     /// is shown in the permission prompt. The completion receives whether

Sources/PlaydateKit/Sound/Source/Aliases/SoundCallbackSource.swift

@@ -1,6 +1,6 @@
 extension Sound.CallbackSource {
     /// Fills the sample buffers and returns `true` if output was
-    /// produced. `right` is non-nil only for stereo sources.
-    public typealias Callback = (_ left: UnsafeMutableBufferPointer<Int16>,
-                                 _ right: UnsafeMutableBufferPointer<Int16>?) -> Bool
+    /// produced. `right` is empty for mono sources.
+    public typealias Callback = (_ left: inout MutableSpan<Int16>,
+                                 _ right: inout MutableSpan<Int16>) -> Bool
 }

Sources/PlaydateKit/Sound/Source/Classes/CallbackSource.swift

@@ -27,9 +27,9 @@ extension Sound {
                                                UnsafeMutablePointer<Int16>?, Int32) -> Int32 = { context, left, right, length in
             guard let context, let left else { return 0 }
             let source = Unmanaged<CallbackSource>.fromOpaque(context).takeUnretainedValue()
-            let leftBuffer = UnsafeMutableBufferPointer(start: left, count: Int(length))
-            let rightBuffer = right.map { UnsafeMutableBufferPointer(start: $0, count: Int(length)) }
-            return source.callback(leftBuffer, rightBuffer) ? 1 : 0
+            var leftSpan = UnsafeMutableBufferPointer(start: left, count: Int(length)).mutableSpan
+            var rightSpan = UnsafeMutableBufferPointer(start: right, count: right == nil ? 0 : Int(length)).mutableSpan
+            return source.callback(&leftSpan, &rightSpan) ? 1 : 0
         }
 
         /// Attaches the C object created for this source.

Sources/PlaydateKit/Sound/Source/Classes/FilePlayer.swift

@@ -7,7 +7,7 @@ extension Sound {
 
         var loopCallback: ((FilePlayer) -> Void)?
         var fadeCallback: ((FilePlayer) -> Void)?
-        var mp3DataSource: ((UnsafeMutableBufferPointer<UInt8>) -> Int)?
+        var mp3DataSource: ((inout MutableSpan<UInt8>) -> Int)?
         private var retainedRateModulator: SignalValue?
 
         override init(pointer: OpaquePointer?, isOwned: Bool) {
@@ -131,13 +131,13 @@ extension Sound {
         /// fills the buffer and returns the number of bytes written; return 0
         /// to signal the end of the stream.
         public func setMP3StreamSource(bufferLength: Float,
-                                       _ dataSource: @escaping (UnsafeMutableBufferPointer<UInt8>) -> Int) {
+                                       _ dataSource: @escaping (inout MutableSpan<UInt8>) -> Int) {
             mp3DataSource = dataSource
             FilePlayer.api.pointee.setMP3StreamSource.unsafelyUnwrapped(pointer, { data, bytes, userdata in
                 guard let userdata, let data else { return 0 }
                 let player = Unmanaged<FilePlayer>.fromOpaque(userdata).takeUnretainedValue()
-                let buffer = UnsafeMutableBufferPointer(start: data, count: Int(bytes))
-                return Int32(player.mp3DataSource?(buffer) ?? 0)
+                var buffer = UnsafeMutableBufferPointer(start: data, count: Int(bytes)).mutableSpan
+                return Int32(player.mp3DataSource?(&buffer) ?? 0)
             }, Unmanaged.passUnretained(self).toOpaque(), bufferLength)
         }
 

Sources/PlaydateKit/Sound/Synth/Classes/Synth.swift

@@ -75,9 +75,9 @@ extension Sound {
                 { userdata, left, right, nsamples, rate, drate in
                     guard let userdata, let left else { return 0 }
                     let box = Unmanaged<GeneratorBox>.fromOpaque(userdata).takeUnretainedValue()
-                    let leftBuffer = UnsafeMutableBufferPointer(start: left, count: Int(nsamples))
-                    let rightBuffer = right.map { UnsafeMutableBufferPointer(start: $0, count: Int(nsamples)) }
-                    return Int32(box.generator.render(leftBuffer, rightBuffer, rate, drate))
+                    var leftSpan = UnsafeMutableBufferPointer(start: left, count: Int(nsamples)).mutableSpan
+                    var rightSpan = UnsafeMutableBufferPointer(start: right, count: right == nil ? 0 : Int(nsamples)).mutableSpan
+                    return Int32(box.generator.render(&leftSpan, &rightSpan, rate, drate))
                 },
                 { userdata, note, velocity, length in
                     guard let userdata else { return }

Sources/PlaydateKit/Sound/Synth/Structures/Synth.Generator.swift

@@ -2,11 +2,11 @@ extension Sound.Synth {
     /// Custom generator callbacks. Samples are in signed Q8.24 format.
     public struct Generator {
         /// Renders up to 256 sample frames into `left` (and `right` for
-        /// stereo generators). `rate` is the per-frame phase step in
+        /// stereo generators; it is empty for mono ones). `rate` is the per-frame phase step in
         /// Q0.32 format and `drate` its per-frame change. Returns the
         /// number of frames rendered.
-        public var render: (_ left: UnsafeMutableBufferPointer<Int32>,
-                            _ right: UnsafeMutableBufferPointer<Int32>?,
+        public var render: (_ left: inout MutableSpan<Int32>,
+                            _ right: inout MutableSpan<Int32>,
                             _ rate: UInt32, _ drate: Int32) -> Int
         /// Called when a note starts. `length` is -1 for indefinite notes.
         public var noteOn: ((_ note: Sound.MIDINote, _ velocity: Float, _ length: Float) -> Void)?
@@ -17,8 +17,8 @@ extension Sound.Synth {
         /// valid.
         public var setParameter: ((_ parameter: Int, _ value: Float) -> Bool)?
 
-        public init(render: @escaping (_ left: UnsafeMutableBufferPointer<Int32>,
-                                       _ right: UnsafeMutableBufferPointer<Int32>?,
+        public init(render: @escaping (_ left: inout MutableSpan<Int32>,
+                                       _ right: inout MutableSpan<Int32>,
                                        _ rate: UInt32, _ drate: Int32) -> Int,
                     noteOn: ((_ note: Sound.MIDINote, _ velocity: Float, _ length: Float) -> Void)? = nil,
                     release: ((_ stop: Bool) -> Void)? = nil,