test: S7xS11 compose — stereo+startFrame+loop share one read head; v4 stereo envelope + v2 zone overrides round-trip together

tests/test_sampler_core.cpp

@@ -783,6 +783,52 @@ static void testStereoRenderNullBufferIsNoOp() {
     for (float v : buf) CHECK(approx(v, 0.0, 1e-9));  // untouched
 }
 
+static void testStereoStartFrameLoopShareOneReadHead() {
+    // S7 x S11 compose: a STEREO sample with a startFrame AND a sustain loop must read BOTH
+    // channels from the SAME single read head — one offset, one loop wrap, applied to L and R
+    // identically (only the sampled value differs). A per-frame L/R ramp that is a fixed offset
+    // apart (R = L + 0.5) pins the read position on both channels: if the stereo path ever gave
+    // L and R independent heads, the constant L->R offset would break at the start jump or the
+    // loop seam.
+    SampleData s;
+    s.frames.resize(40);
+    s.framesR.resize(40);
+    for (int i = 0; i < 40; ++i) {
+        s.frames[i]  = static_cast<float>(i) * 0.01f;         // L: 0.00 .. 0.39
+        s.framesR[i] = static_cast<float>(i) * 0.01f + 0.5f;  // R: L + 0.5, everywhere
+    }
+    s.rootNote = 60;
+    s.startFrame = 10;          // begin BOTH channels at frame 10
+    s.loop.hasLoop = true;
+    s.loop.start = 20;
+    s.loop.end   = 30;          // loop [20,30): frames 20..29
+    CHECK(s.channelCount() == 2);
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, flatAdsr());
+    eng.noteOn(60, 127);        // unity ratio, full velocity, flat gain
+
+    std::vector<AudioSample> left(200, 0.f), right(200, 0.f);
+    eng.render(left.data(), right.data(), 200);
+
+    // First frame: both channels start at frame 10 (L=0.10, R=0.60) — the shared start offset.
+    CHECK(approx(left[0], 0.10, 1e-4));
+    CHECK(approx(right[0], 0.60, 1e-4));
+    // The loop sustains the voice indefinitely.
+    CHECK(eng.activeVoiceCount() == 1);
+    // At every rendered frame R - L == 0.5 exactly: both channels read the SAME frame index
+    // (one read head) through the start jump and every loop wrap. A per-channel head drift would
+    // break this invariant at the seam.
+    for (std::size_t i = 0; i < left.size(); ++i) {
+        CHECK(approx(right[i] - left[i], 0.5, 1e-4));
+    }
+    // Once fully inside the loop (start=10 -> reaches loop.start=20 within a handful of unity-ratio
+    // frames), every L value sits in the loop band [0.20, 0.30): the shared head is sustaining the
+    // loop region on both channels, never running off the sample end.
+    for (std::size_t i = 15; i < left.size(); ++i) {
+        CHECK(left[i] >= 0.20 - 1e-4 && left[i] < 0.30 + 1e-4);
+    }
+}
+
 int main() {
     testChromaticSingleRoot();
     testZonedRangesBoundaries();
@@ -815,6 +861,7 @@ int main() {
     testStereoRenderAdvancesLikeMonoRepitch();
     testStereoRenderSumsVoicesPerChannel();
     testStereoRenderNullBufferIsNoOp();
+    testStereoStartFrameLoopShareOneReadHead();
 
     if (g_fail == 0) {
         std::printf("all sampler_core tests passed\n");