fix(musicxml gp-exporter): optimized handling of more than 4 voices

packages/alphatab/src/exporter/GpifWriter.ts

@@ -1,5 +1,6 @@
 import { GpifSoundMapper } from '@coderline/alphatab/exporter/GpifSoundMapper';
 import { VersionInfo } from '@coderline/alphatab/generated/VersionInfo';
+import { Logger } from '@coderline/alphatab/Logger';
 import { GeneralMidi } from '@coderline/alphatab/midi/GeneralMidi';
 import { MidiFileGenerator } from '@coderline/alphatab/midi/MidiFileGenerator';
 import { MidiUtils } from '@coderline/alphatab/midi/MidiUtils';
@@ -107,9 +108,9 @@ export class GpifWriter {
         for (const tracks of score.tracks) {
             for (const staff of tracks.staves) {
                 for (const bar of staff.bars) {
-                    this._writeBarNode(bars, bar);
+                    const activeVoices = this._writeBarNode(bars, bar);
 
-                    for (const voice of bar.voices) {
+                    for (const voice of activeVoices) {
                         this._writeVoiceNode(voices, voice);
 
                         for (const beat of voice.beats) {
@@ -1831,18 +1832,54 @@ export class GpifWriter {
         fermataNode.addElement('Offset').innerText = `${numerator}/${denominator}`;
     }
 
-    private _writeBarNode(parent: XmlNode, bar: Bar) {
+    private _writeBarNode(parent: XmlNode, bar: Bar): Voice[] {
         const barNode = parent.addElement('Bar');
         barNode.attributes.set('id', bar.id.toString());
 
-        barNode.addElement('Voices').innerText = bar.voices.map(v => (v.isEmpty ? '-1' : v.id.toString())).join(' ');
+        // Guitar Pro's format requires exactly 4 voice slots per bar. Preserve
+        // voice positions when they fit (index < 4); non-empty voices at index
+        // >= 4 (produced e.g. by MusicXML with sparse voice numbers) get placed
+        // into the first free slot. Empty voices at index < 4 keep their slot
+        // as '-1' so their beats are never written (avoids orphan <Beat> nodes).
+        const activeVoices: Voice[] = [];
+        const slots: string[] = ['-1', '-1', '-1', '-1'];
+        const overflowVoices: Voice[] = [];
+        for (let i = 0; i < bar.voices.length; i++) {
+            const v = bar.voices[i];
+            if (i < 4) {
+                if (!v.isEmpty) {
+                    slots[i] = v.id.toString();
+                    activeVoices.push(v);
+                }
+            } else if (!v.isEmpty) {
+                overflowVoices.push(v);
+            }
+        }
+        let dropped = 0;
+        for (const v of overflowVoices) {
+            const freeSlot = slots.indexOf('-1');
+            if (freeSlot >= 0) {
+                slots[freeSlot] = v.id.toString();
+                activeVoices.push(v);
+            } else {
+                dropped++;
+            }
+        }
+        if (dropped > 0) {
+            Logger.warning(
+                'GpifWriter',
+                `Bar ${bar.id} has ${activeVoices.length + dropped} non-empty voices; Guitar Pro supports max 4. Dropping ${dropped}.`
+            );
+        }
+        barNode.addElement('Voices').innerText = slots.join(' ');
         barNode.addElement('Clef').innerText = Clef[bar.clef];
         if (bar.clefOttava !== Ottavia.Regular) {
             barNode.addElement('Ottavia').innerText = Ottavia[bar.clefOttava].substr(1);
         }
         if (bar.simileMark !== SimileMark.None) {
             barNode.addElement('SimileMark').innerText = SimileMark[bar.simileMark];
         }
+        return activeVoices;
     }
 
     private _writeVoiceNode(parent: XmlNode, voice: Voice) {

packages/alphatab/src/importer/Gp3To5Importer.ts

@@ -1,11 +1,12 @@
-import { GeneralMidi } from '@coderline/alphatab/midi/GeneralMidi';
-
 import { ScoreImporter } from '@coderline/alphatab/importer/ScoreImporter';
 import { UnsupportedFormatError } from '@coderline/alphatab/importer/UnsupportedFormatError';
-
 import { IOHelper } from '@coderline/alphatab/io/IOHelper';
-import { OverflowError, type IReadable } from '@coderline/alphatab/io/IReadable';
+import { type IReadable, OverflowError } from '@coderline/alphatab/io/IReadable';
+import type { IWriteable } from '@coderline/alphatab/io/IWriteable';
+import { Logger } from '@coderline/alphatab/Logger';
+import { GeneralMidi } from '@coderline/alphatab/midi/GeneralMidi';
 import { AccentuationType } from '@coderline/alphatab/model/AccentuationType';
+import { AccidentalType } from '@coderline/alphatab/model/AccidentalType';
 import { Automation, AutomationType } from '@coderline/alphatab/model/Automation';
 import { Bar, BarLineStyle } from '@coderline/alphatab/model/Bar';
 import { Beat, BeatBeamingMode } from '@coderline/alphatab/model/Beat';
@@ -14,38 +15,34 @@ import { BrushType } from '@coderline/alphatab/model/BrushType';
 import { Chord } from '@coderline/alphatab/model/Chord';
 import { Clef } from '@coderline/alphatab/model/Clef';
 import { Color } from '@coderline/alphatab/model/Color';
+import { Direction } from '@coderline/alphatab/model/Direction';
 import { Duration } from '@coderline/alphatab/model/Duration';
 import { DynamicValue } from '@coderline/alphatab/model/DynamicValue';
+import { FadeType } from '@coderline/alphatab/model/FadeType';
 import type { Fingers } from '@coderline/alphatab/model/Fingers';
 import { GraceType } from '@coderline/alphatab/model/GraceType';
 import { HarmonicType } from '@coderline/alphatab/model/HarmonicType';
 import type { KeySignature } from '@coderline/alphatab/model/KeySignature';
 import type { KeySignatureType } from '@coderline/alphatab/model/KeySignatureType';
 import { Lyrics } from '@coderline/alphatab/model/Lyrics';
 import { MasterBar } from '@coderline/alphatab/model/MasterBar';
+import { ModelUtils } from '@coderline/alphatab/model/ModelUtils';
 import { Note } from '@coderline/alphatab/model/Note';
 import { NoteAccidentalMode } from '@coderline/alphatab/model/NoteAccidentalMode';
+import { Ottavia } from '@coderline/alphatab/model/Ottavia';
 import { PickStroke } from '@coderline/alphatab/model/PickStroke';
 import { PlaybackInformation } from '@coderline/alphatab/model/PlaybackInformation';
+import { Rasgueado } from '@coderline/alphatab/model/Rasgueado';
 import { Score, ScoreSubElement } from '@coderline/alphatab/model/Score';
 import { Section } from '@coderline/alphatab/model/Section';
 import { SlideInType } from '@coderline/alphatab/model/SlideInType';
 import { SlideOutType } from '@coderline/alphatab/model/SlideOutType';
 import type { Staff } from '@coderline/alphatab/model/Staff';
 import { Track } from '@coderline/alphatab/model/Track';
+import { TremoloPickingEffect } from '@coderline/alphatab/model/TremoloPickingEffect';
 import { TripletFeel } from '@coderline/alphatab/model/TripletFeel';
 import { VibratoType } from '@coderline/alphatab/model/VibratoType';
 import { Voice } from '@coderline/alphatab/model/Voice';
-
-import type { IWriteable } from '@coderline/alphatab/io/IWriteable';
-import { Logger } from '@coderline/alphatab/Logger';
-import { AccidentalType } from '@coderline/alphatab/model/AccidentalType';
-import { Direction } from '@coderline/alphatab/model/Direction';
-import { FadeType } from '@coderline/alphatab/model/FadeType';
-import { ModelUtils } from '@coderline/alphatab/model/ModelUtils';
-import { Ottavia } from '@coderline/alphatab/model/Ottavia';
-import { Rasgueado } from '@coderline/alphatab/model/Rasgueado';
-import { TremoloPickingEffect } from '@coderline/alphatab/model/TremoloPickingEffect';
 import { WahPedal } from '@coderline/alphatab/model/WahPedal';
 import { BeamDirection } from '@coderline/alphatab/rendering/utils/BeamDirection';
 
@@ -807,6 +804,33 @@ export class Gp3To5Importer extends ScoreImporter {
         if (beatCount === 0) {
             return;
         }
+
+        //
+        // fast path where we know we read the full voice
+        // - its the first voice to read
+        // - we are multivoice and know we can read all voices in full extend
+        const currentVoiceCount = bar.index === 0 ? 1 : bar.previousBar!.voices.length;
+        if (bar.voices.length === 0 || currentVoiceCount === 2) {
+            this._readAndChainVoice(track, bar, beatCount);
+            return;
+        }
+
+        //
+        // fallback to detection of empty voices we can skip.
+        // GP5 empty second voices are always a single beat with the isEmpty flag,
+        // so a beat count above 1 is guaranteed to be real content.
+        if (beatCount === 1 && this._skipEmptyVoice()) {
+            return;
+        }
+
+        //
+        // secondary fallback: the first filled second voice for this staff.
+        // we have to backfill and read fully.
+        ModelUtils.backfillStaffVoices(bar.staff, 2);
+        this._readAndChainVoice(track, bar, beatCount);
+    }
+
+    private _readAndChainVoice(track: Track, bar: Bar, beatCount: number) {
         const newVoice: Voice = new Voice();
         bar.addVoice(newVoice);
 
@@ -816,6 +840,38 @@ export class Gp3To5Importer extends ScoreImporter {
         }
     }
 
+    /**
+     * Attempts to skip a fully empty voice.
+     * @returns true if we detected an empty voice, false if the beat was not empty and a full voice has to be read.
+     */
+    private _skipEmptyVoice(): boolean {
+        const startOfVoice = this.data.position;
+
+        // aligned with readBeat, find out if we are having an empty beat.
+        const flags = this.data.readByte();
+        let isEmpty = false;
+        if ((flags & 0x40) !== 0) {
+            const type = this.data.readByte();
+            isEmpty = (type & 0x02) === 0;
+        }
+
+        // NOTE: we don't check any of the other flags on an empty voice, we assume no effects are set to be read.
+
+        // seek back to start of voice for a full read; costs a duplicate read of these bytes.
+        if (!isEmpty) {
+            this.data.position = startOfVoice;
+            return false;
+        }
+
+        // read the remaining bytes of the empty beat to avoid input stream desync (aligned with readBeat).
+        this.data.skip(2); // duration + stringFlags
+        if (this._versionNumber >= 500) {
+            this.data.skip(2); // flags2
+        }
+
+        return true;
+    }
+
     public readBeat(track: Track, bar: Bar, voice: Voice): void {
         const newBeat: Beat = new Beat();
         const flags: number = this.data.readByte();