Mono-per-track: a new note releases the track's previous one (v0.10.0)

A sustaining note held with no explicit note-off used to pile up until
voice-stealing killed it. Now, when a note appears under the playhead on a
track, that track's still-held note is released automatically, so a line moves
note-to-note like a real monophonic voice.

Each sequencer voice carries an ownerTrack tag (the lane+track that started it),
and triggerCell calls releaseTrackVoices(trackKey) before playing a new note. It
releases only voices this track owns and only gated (still-held) ones, so chords
built across several tracks on the same bus keep sounding and release tails ring
out. Uses note-off (not kill) for a click-free legato hand-off. Arp voices are
unowned and keep self-managing as mono-per-bus.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/build.gradle.kts

@@ -22,8 +22,8 @@ android {
         // (android.media.midi) and AAudio low-latency audio we rely on.
         minSdk = 26
         targetSdk = 34
-        versionCode = 26
-        versionName = "0.9.1"
+        versionCode = 27
+        versionName = "0.10.0"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"

app/src/main/java/space/rcmd/android/sizzle/audio/AudioEngine.kt

@@ -858,6 +858,11 @@ class AudioEngine(
         if (!cell.isPlayable) return
         // Remember the channel so a later note-off on this track releases it.
         trackLastChannel[trackKey] = cell.channel
+        // Mono-per-track: a new note releases whatever this track was still holding,
+        // so a held note is stopped when the next one appears under the playhead — no
+        // explicit note-off cell needed. Only this track's own voices are released, so
+        // notes other tracks hold on the same bus (chords across tracks) keep sounding.
+        releaseTrackVoices(trackKey)
         for (bus in 0 until Pattern.TRACK_COUNT) {
             if (proj.mixer.channels[bus].midiChannel != cell.channel) continue
             val arp = arps[lane * Pattern.TRACK_COUNT + bus]
@@ -888,19 +893,33 @@ class AudioEngine(
                 }
             } else {
                 arp.stop()
-                playNote(proj, bus, note, velocity)
+                playNote(proj, bus, note, velocity, ownerTrack = trackKey)
             }
         }
     }
 
+    /** Release (fade out) every still-*held* sequencer voice started by tracker
+     *  [trackKey] — used to make a track monophonic: its previous note is released when
+     *  it plays a new one. Only gated voices are touched (release tails ring out), and
+     *  matching by owner means sibling tracks' notes on a shared bus are left alone.
+     *  Array loops allocate no iterator — safe on the audio thread. */
+    private fun releaseTrackVoices(trackKey: Int) {
+        for (v in seqVoices) if (v.ownerTrack == trackKey && v.isGated) v.noteOff()
+        for (v in sampleVoices) if (v.ownerTrack == trackKey && v.isGated) v.noteOff()
+    }
+
     /**
      * Sound a note on a mixer [bus]: allocate a [SampleVoice] from the bus pool when
      * the channel's instrument is a Sampler/SoundFont with a loaded sample, otherwise
      * a [SynthVoice] for the NES synth. Pooled allocation (up to [POLYPHONY] per bus,
      * stealing the oldest when full) is what makes the instrument polyphonic. Shared
      * by the sequencer and the arpeggiator.
+     *
+     * [ownerTrack] tags the allocated voice with the tracker (lane,track) that started
+     * it (or -1 for arp/unowned notes), so [releaseTrackVoices] can later release just
+     * this track's note — see [triggerCell].
      */
-    private fun playNote(proj: Project, bus: Int, note: Int, velocity: Int) {
+    private fun playNote(proj: Project, bus: Int, note: Int, velocity: Int, ownerTrack: Int = -1) {
         // The allocators ([allocSynthVoice]/[allocSampleVoice]) queue the chosen voice
         // into this block's active list, so a note started mid-block sounds sample-
         // accurately within the same block.
@@ -908,14 +927,16 @@ class AudioEngine(
         val vs = voiceSample(slot, note)
         when {
             vs != null ->
-                allocSampleVoice(bus).noteOn(vs.sample, note, velocity, vs.root, vs.sliceStart, vs.sliceEnd, vs.volume,
+                allocSampleVoice(bus).also { it.ownerTrack = ownerTrack }.noteOn(
+                    vs.sample, note, velocity, vs.root, vs.sliceStart, vs.sliceEnd, vs.volume,
                     vs.attack, vs.decay, vs.sustain, vs.release, vs.loop, vs.crossfadeMs)
             // Empty bus (no instrument — e.g. the default channel 0) or a Sampler
             // with an empty pad → silent, matching the live audition path.
             slot?.type == null || slot.type == ToolboxType.SAMPLER -> {}
             // NES synth, or a SoundFont with nothing loaded → the tone generator.
             else -> {
-                allocSynthVoice(bus).noteOn(note, velocity, synthPatch[bus] ?: patchForSlot(slot, proj.tempoBpm))
+                allocSynthVoice(bus).also { it.ownerTrack = ownerTrack }
+                    .noteOn(note, velocity, synthPatch[bus] ?: patchForSlot(slot, proj.tempoBpm))
             }
         }
     }

app/src/main/java/space/rcmd/android/sizzle/audio/SampleVoice.kt

@@ -28,6 +28,10 @@ class SampleVoice(private val engineSampleRate: Int) {
     private var endIndex = 0
     private var amp = 0f
     var pitch = -1; private set
+    /** Which tracker (lane,track) started this voice, or -1 if unowned. See
+     *  [SynthVoice.ownerTrack]: lets the sequencer make a track mono without cutting
+     *  other tracks' notes on the same bus. */
+    var ownerTrack = -1
 
     // ---- sustain loop (SoundFont): wrap [loopStartIdx, loopEndIdx) while sounding,
     // until a note-off's release envelope ends the voice. Uses the file's loop points
@@ -143,7 +147,7 @@ class SampleVoice(private val engineSampleRate: Int) {
         phase = Phase.RELEASE
     }
 
-    fun kill() { sample = null; pitch = -1; env = 0f; endGain = 1f; lastY = 0f; declick = 0f; loopActive = false }
+    fun kill() { sample = null; pitch = -1; ownerTrack = -1; env = 0f; endGain = 1f; lastY = 0f; declick = 0f; loopActive = false }
 
     /** One sample of the decaying declick offset (used on the paths that stop the
      *  voice, so any carried level rings out smoothly instead of snapping to 0). */

app/src/main/java/space/rcmd/android/sizzle/audio/SynthVoice.kt

@@ -26,6 +26,10 @@ class SynthVoice(private val sampleRate: Int) {
 
     private var patch: SynthPatch = SynthPatch.DEFAULT
     var pitch = -1; private set
+    /** Which tracker (lane,track) started this voice, or -1 if unowned (live/arp).
+     *  Lets the sequencer release a track's previous note when it plays a new one
+     *  (mono-per-track) without touching notes other tracks hold on the same bus. */
+    var ownerTrack = -1
     private var velocity = 0f
     private var baseFreq = 440.0
     private var keyNorm = 0f // note position relative to C-4 (60), -1..1
@@ -84,6 +88,7 @@ class SynthVoice(private val sampleRate: Int) {
     fun kill() {
         ampEnv.kill(); filtEnv.kill()
         pitch = -1
+        ownerTrack = -1
         icB = 0f; icL = 0f
     }