fix(tests): rework testNoteOffReleasesNewestSameNote to actually prove newest-first release; add single-frame-loop test; add two comments in sampler_core.cpp

src/vst/sampler_core.cpp

@@ -199,6 +199,9 @@ AudioSample Voice::renderFrame() {
     if (loopUsable && i1 >= loop.end) {
         i1 = loop.start;  // seamless wrap for the interpolation partner.
     }
+    // i0 is always in [0, frameCount) after the early-out above; the guard is purely
+    // defensive. i1 (the interpolation partner) can exceed frameCount when no loop
+    // wraps it — only that partner actually needs the clamp.
     const double s0 = (i0 >= 0 && i0 < frameCount) ? static_cast<double>(pcm[i0]) : 0.0;
     const double s1 = (i1 >= 0 && i1 < frameCount) ? static_cast<double>(pcm[i1]) : 0.0;
     const double interp = s0 + (s1 - s0) * frac;
@@ -288,7 +291,7 @@ void VoiceEngine::noteOff(int note) {
 
 void VoiceEngine::render(std::vector<AudioSample>& out, std::size_t frameCount) {
     const std::size_t base = out.size();
-    out.resize(base + frameCount, 0.0f);
+    out.resize(base + frameCount, 0.0f);  // S4: caller must pre-reserve — no allocation allowed under the VST3 process callback.
     for (Voice& voice : voices_) {
         if (!voice.active()) continue;
         for (std::size_t f = 0; f < frameCount; ++f) {

tests/test_sampler_core.cpp

@@ -309,26 +309,62 @@ static void testPolyphonicAllocation() {
 }
 
 static void testNoteOffReleasesNewestSameNote() {
-    Keymap km = Keymap::singleSampleChromatic(dcSample(1000, 60));
+    // Prove that noteOff releases the NEWEST (highest startOrder) instance of a
-    // Long release so we can observe which voice entered release.
+    // re-triggered note, leaving the older voice in sustain.
+    //
+    // Two voices at distinct velocities so their output is distinguishable:
+    //   "first"  (older)  -> velocity 64  -> gain ~0.504 (G_old)
+    //   "second" (newer)  -> velocity 127 -> gain  1.0   (G_new)
+    //
+    // With a DC-1 sample and sustain=1, while both are held:
+    //   render sum == G_old + G_new.
+    //
+    // After noteOff (must release newest), the newer voice enters a short release.
+    // Render past releaseFrames: newer voice finishes; only the older voice remains.
+    // Sum then equals G_old, and activeVoiceCount drops to 1. If the WRONG voice
+    // were released, the older would finish and the remaining sum would equal G_new
+    // (1.0 vs ~0.504) — the velocities make the error distinguishable.
+    const int velOld = 64;
+    const int velNew = 127;
+    const double gainOld = velOld / 127.0;   // ~0.504
+    const double gainNew = velNew / 127.0;   // 1.0
+
+    Keymap km = Keymap::singleSampleChromatic(dcSample(100000, 60));
     AdsrParams a = flatAdsr();
-    a.releaseFrames = 1000;
+    a.releaseFrames = 10;   // short but non-zero so voice stays active through release
     VoiceEngine eng(8, km, a);
 
-    std::size_t first = eng.noteOn(60, 100);
+    std::size_t first  = eng.noteOn(60, velOld);  // older voice, lower gain
-    std::size_t second = eng.noteOn(60, 100);  // same note re-triggered
+    std::size_t second = eng.noteOn(60, velNew);  // newer voice, higher gain
     CHECK(first != second);
     CHECK(eng.activeVoiceCount() == 2);
 
-    eng.noteOff(60);  // releases the NEWEST (second)
+    // While both are held, combined output equals gainOld + gainNew.
+    {
         std::vector<AudioSample> out;
         eng.render(out, 1);
-    // Both still active (long release), but only the newest is releasing.
+        CHECK(approx(out[0], gainOld + gainNew, 1e-4));
-    CHECK(eng.activeVoiceCount() == 2);
+    }
-    // A second note-off releases the older one too.
+
+    // Release once — must target the NEWEST voice (second).
     eng.noteOff(60);
-    eng.render(out, 1);
+
-    CHECK(eng.activeVoiceCount() == 2);  // both releasing, not yet finished
+    // Render past the release (releaseFrames == 10): newer voice goes Finished.
+    std::vector<AudioSample> out;
+    eng.render(out, 20);
+
+    // Newer voice must be done; only the older voice remains.
+    CHECK(eng.activeVoiceCount() == 1);
+    // Tail frames must equal gainOld (~0.504), NOT gainNew (1.0).
+    // If the older voice were released instead, the tail would be ~1.0 here.
+    for (std::size_t i = 15; i < out.size(); ++i) {
+        CHECK(approx(out[i], gainOld, 1e-4));
+    }
+
+    // A second note-off releases the remaining older voice.
+    eng.noteOff(60);
+    eng.render(out, 20);
+    CHECK(eng.activeVoiceCount() == 0);
 }
 
 static void testOutOfZoneNoteConsumesNoVoice() {
@@ -444,6 +480,33 @@ static void testZeroLengthLoopGoesSilent() {
     for (std::size_t i = 60; i < out.size(); ++i) CHECK(approx(out[i], 0.0, 1e-6));
 }
 
+static void testSingleFrameLoop() {
+    // A loop of exactly one frame [start, start+1) — the narrowest valid loop.
+    // The path is correct-by-luck (loopLen = 1.0 divides evenly into any integer
+    // readPos advance at unity ratio), but Tier-2 tight loops make it load-bearing.
+    SampleData s;
+    s.frames.resize(10);
+    for (int i = 0; i < 10; ++i) s.frames[i] = static_cast<float>(i) * 0.1f;
+    s.rootNote = 60;
+    s.loop.hasLoop = true;
+    s.loop.start = 5;
+    s.loop.end = 6;  // single-frame loop: [5, 6)
+
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, flatAdsr());
+    eng.noteOn(60, 127);  // unity ratio, full velocity
+
+    std::vector<AudioSample> out;
+    eng.render(out, 50);  // well past the sample end
+
+    // Voice must still be active — the single-frame loop keeps it alive.
+    CHECK(eng.activeVoiceCount() == 1);
+    // Every frame from the loop-start onward must be the value of frame 5 (0.5).
+    for (std::size_t i = 10; i < out.size(); ++i) {
+        CHECK(approx(out[i], 0.5, 1e-4));
+    }
+}
+
 static void testAbsentLoopGoesSilent() {
     // No loop at all: held note runs off the end and goes idle (same as zero-length).
     SampleData s = dcSample(50, 60);
@@ -515,6 +578,7 @@ int main() {
     testStealsOldestWhenNoneReleasing();
     testLoopSustainSeamless();
     testZeroLengthLoopGoesSilent();
+    testSingleFrameLoop();
     testAbsentLoopGoesSilent();
     testVelocityToVolume();
     testPolyphonyMixesAdditively();