Preserve's period detection: probes are placed by position, and a sustain loop is the span analysed

tests/test_sample_map.cpp

@@ -926,6 +926,45 @@ static void testBuildSampleDataDetectsThirtyHertzSourcePeriod() {
     CHECK(std::fabs(sd.sourcePeriodFrames - 1470.0) < 2.0);  // 44100 / 30 Hz
 }
 
+// The span half of the same wire: buildSampleData must hand detection the LOOP region when the
+// capture carries one, not the whole decoded PCM. Asserted through the real build for the same
+// reason as the test above — period_detect's own coverage cannot see which span the loader picks.
+static void testBuildSampleDataDetectsOverTheSustainLoopNotTheWholeSource() {
+    const int rate = 44100;
+    const std::size_t frames = 120000;
+    const std::int64_t loopStart = 60000;
+    const double kPi = 3.14159265358979323846;
+    const double loopPeriod = static_cast<double>(rate) / 30.0;  // 1470 frames
+
+    // Head at 147 Hz, looped tail at 30 Hz: analysed whole, the probes split two-and-two and
+    // detection correctly refuses. Analysed over the loop, the 30 Hz sustain is unambiguous.
+    std::vector<AudioSample> pcm(frames);
+    double phase = 0.0;
+    for (std::size_t i = 0; i < frames; ++i) {
+        phase += 2.0 * kPi / (i < static_cast<std::size_t>(loopStart) ? 300.0 : loopPeriod);
+        pcm[i] = static_cast<float>(std::sin(phase));
+    }
+
+    InstrumentParams noLoop;
+    const SampleData bare = buildSampleData(resolveCapture(ref("b/a.wav", 60), noLoop),
+                                            DecodedPcm{pcm, rate, {}});
+    CHECK(bare.sourcePeriodFrames == 0.0);  // no loop -> whole source -> no ONE period
+
+    InstrumentParams looped;
+    looped.loopOverride = SampleLoop{true, loopStart, static_cast<std::int64_t>(frames)};
+    const SampleData sd = buildSampleData(resolveCapture(ref("b/a.wav", 60), looped),
+                                          DecodedPcm{pcm, rate, {}});
+    CHECK(std::fabs(sd.sourcePeriodFrames - loopPeriod) < 2.0);
+
+    // A loop too short to host the full search band falls back to the whole source rather than
+    // to none — here that whole source has no one period, so the answer is the bare one above.
+    InstrumentParams shortLoop;
+    shortLoop.loopOverride = SampleLoop{true, 118000, static_cast<std::int64_t>(frames)};
+    const SampleData shortSd = buildSampleData(resolveCapture(ref("b/a.wav", 60), shortLoop),
+                                               DecodedPcm{pcm, rate, {}});
+    CHECK(shortSd.sourcePeriodFrames == bare.sourcePeriodFrames);
+}
+
 static void testBuildSampleDataCarriesTheVelocityCurve() {
     InstrumentParams p;
     p.velocityCurve = VelocityCurve::linear();
@@ -994,6 +1033,7 @@ int main() {
     testBuildSampleDataEmptyPcmIsUnplayable();
     testBuildSampleDataCarriesTheVelocityCurve();
     testBuildSampleDataDetectsThirtyHertzSourcePeriod();
+    testBuildSampleDataDetectsOverTheSustainLoopNotTheWholeSource();
 
     if (g_fail == 0) std::printf("sample_map: all tests passed\n");
     return g_fail != 0;