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

2026-08-02 03:21:06 -04:00
parent 334022c0f1
commit 9228e93750
6 changed files with 299 additions and 22 deletions
@@ -10,15 +10,19 @@
 //   3. graceful degradation — noise, silence, and a source whose period changes mid-sample all
 //      return NONE. That is the contract the shifter's fixed-window fallback rests on: an
 //      estimate that is merely wrong would misalign every splice, which is worse than none.
-//   4. the band edges and the short-sample path.
-//   5. what the load pays, and that it does not grow with the sample length.
+//   4. the band edges and the short-sample path, including the lone-probe accept.
+//   5. the analysis span: a sustain loop stands in for the whole source, but never at the cost
+//      of search-band width.
+//   6. what the load pays, and that it does not grow with the sample length.

 #include "../src/core/instrument/engine/period_detect.h"

 #include <chrono>
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <cstdio>
+#include <tuple>
 #include <vector>

 using namespace reasampler;
@@ -217,7 +221,144 @@ static void testAShortSourceShortensTheSearchRatherThanRefusing() {
    CHECK(!detectPeriod(sineOfPeriod(40, 20.0), 44100).valid());
 }

-// --- 5. What the load pays ------------------------------------------------------------------
+// A lone probe is the one case the strict-majority rule cannot rule on, so pin BOTH halves of
+// the carve-out: which sources land in it, and that they are accepted rather than refused.
+// 30 Hz is first-class material here, and a short low-frequency source is exactly where the
+// blunt "require two probes" fix would have silently stopped detecting.
+static void testASourceTooShortForASecondProbeIsStillDetectedOnItsOneProbe() {
+    const int rate = 44100;
+    const std::size_t lagHi = static_cast<std::size_t>(rate / kPeriodDetectMinHz);
+    const std::size_t block = 2 * lagHi;
+    // Derive the frame count from the public constants rather than hardcoding one, so this test
+    // keeps naming the lone-probe case if the geometry ever moves. One probe fits while the
+    // span leaves less than lagHi of room after the first block.
+    const std::size_t frames = block + lagHi - 1;   // 8819 at 44.1k -> exactly one probe
+    CHECK(1 + (frames - block) / lagHi == 1);
+
+    const double p = static_cast<double>(rate) / 30.0;  // 1470 frames
+    const PeriodEstimate est = detectPeriod(sineOfPeriod(frames, p), rate);
+    std::printf("  lone probe, %zu frames @ 30 Hz -> %s (%.3f, want %.3f)\n", frames,
+                est.valid() ? "detected" : "NONE", est.frames, p);
+    CHECK(est.valid());
+    if (est.valid()) CHECK(std::fabs(est.frames - p) < 1.0);
+
+    // One frame more buys a second probe position; the answer must not change character.
+    const PeriodEstimate two = detectPeriod(sineOfPeriod(frames + 1, p), rate);
+    CHECK(1 + (frames + 1 - block) / lagHi == 2);
+    CHECK(two.valid());
+    if (two.valid()) CHECK(std::fabs(two.frames - p) < 1.0);
+}
+
+static void testTheTwoLowFrequenciesTheShifterWasBuiltForAreDetected() {
+    // 30 Hz and 29 Hz — the pair the Preserve geometry work is measured against. 29 Hz is the
+    // sharper case: its period does not divide the splice window, so the shifter needs the
+    // detected value to be right rather than merely present.
+    for (double hz : {30.0, 29.0}) {
+        const double p = 44100.0 / hz;
+        const PeriodEstimate est = detectPeriod(sineOfPeriod(160000, p), 44100);
+        std::printf("  %.0f Hz -> %s (%.3f, want %.3f)\n", hz, est.valid() ? "detected" : "NONE",
+                    est.frames, p);
+        CHECK(est.valid());
+        if (est.valid()) CHECK(std::fabs(est.frames - p) < 0.5);
+    }
+}
+
+// --- 5. The analysis span -------------------------------------------------------------------
+
+static void testTheLoopStandsInForTheSourceOnlyWhenItCostsNoSearchBand() {
+    const int rate = 44100;
+    const std::size_t frames = 120000;
+    // One full probe block — the span below which detectPeriod starts shortening its own
+    // longest lag, which is the only thing the narrower span may never cost.
+    const std::size_t minimum = 2 * static_cast<std::size_t>(rate / kPeriodDetectMinHz);
+
+    // No loop, an inverted span, and a span reaching past the PCM all yield the whole source.
+    for (const auto& [lo, hi, has] : {std::tuple<std::int64_t, std::int64_t, bool>{0, 0, false},
+                                      {60000, 120000, false},
+                                      {90000, 90000, true},
+                                      {90000, 80000, true},
+                                      {-1, 90000, true},
+                                      {60000, 130000, true}}) {
+        const AnalysisSpan s = periodAnalysisSpan(frames, lo, hi, has, rate);
+        CHECK(s.from == 0 && s.count == frames);
+    }
+
+    // A loop one frame under the minimum falls back to the WIDER span, not to none.
+    const AnalysisSpan shortLoop =
+        periodAnalysisSpan(frames, 60000, 60000 + static_cast<std::int64_t>(minimum) - 1, true,
+                           rate);
+    CHECK(shortLoop.from == 0 && shortLoop.count == frames);
+
+    // At the minimum exactly, the loop is taken.
+    const AnalysisSpan atMinimum =
+        periodAnalysisSpan(frames, 60000, 60000 + static_cast<std::int64_t>(minimum), true, rate);
+    CHECK(atMinimum.from == 60000 && atMinimum.count == minimum);
+
+    // And the too-short loop still DETECTS through the wider span — refusing there would be a
+    // regression against analysing the whole source, and a short sustain loop is common.
+    const double p = static_cast<double>(rate) / 30.0;
+    const std::vector<AudioSample> src = sineOfPeriod(frames, p);
+    const PeriodEstimate est = detectPeriod(src, rate, shortLoop.from, shortLoop.count);
+    std::printf("  short loop -> whole source: %s (%.3f)\n", est.valid() ? "detected" : "NONE",
+                est.frames);
+    CHECK(est.valid());
+    if (est.valid()) CHECK(std::fabs(est.frames - p) < 0.5);
+}
+
+static void testAPhraseWhoseLoopIsPitchedDifferentlyFromItsHeadDetectsOverTheLoop() {
+    // The case the whole-source analysis cannot answer: the head sustains one pitch, the looped
+    // tail another. Analysed whole, two probes land each side and the strict-majority rule
+    // correctly refuses — there is no ONE period over the whole source. But under Gate the
+    // splicer lives in the loop, whose period is perfectly well defined.
+    const int rate = 44100;
+    const std::size_t frames = 120000;
+    const std::int64_t loopStart = 60000;
+    const double headPeriod = 300.0;
+    const double loopPeriod = static_cast<double>(rate) / 30.0;  // 1470 frames
+
+    std::vector<AudioSample> src(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) ? headPeriod : loopPeriod);
+        src[i] = static_cast<AudioSample>(std::sin(phase));
+    }
+
+    // BEFORE this rule: the whole source is what was analysed, and it reports none.
+    const PeriodEstimate whole = detectPeriod(src, rate);
+    std::printf("  phrase analysed whole -> %s (%.3f)\n", whole.valid() ? "DETECTED" : "none",
+                whole.frames);
+    CHECK(!whole.valid());
+
+    // AFTER: the loop is long enough to host the full band, so it is the analysed span.
+    const AnalysisSpan span = periodAnalysisSpan(frames, loopStart,
+                                                 static_cast<std::int64_t>(frames), true, rate);
+    CHECK(span.from == static_cast<std::size_t>(loopStart));
+    const PeriodEstimate looped = detectPeriod(src, rate, span.from, span.count);
+    std::printf("  phrase analysed over its loop -> %s (%.3f, want %.3f)\n",
+                looped.valid() ? "detected" : "NONE", looped.frames, loopPeriod);
+    CHECK(looped.valid());
+    if (looped.valid()) CHECK(std::fabs(looped.frames - loopPeriod) < 2.0);
+
+    // The narrowed span must not turn a genuinely aperiodic loop into a period: same geometry,
+    // noise in the loop region.
+    std::vector<AudioSample> noisyLoop = src;
+    std::uint32_t rng = 777u;
+    for (std::size_t i = static_cast<std::size_t>(loopStart); i < frames; ++i) {
+        rng = rng * 1664525u + 1013904223u;
+        noisyLoop[i] = static_cast<AudioSample>((static_cast<double>(rng >> 8) / 8388608.0) - 1.0);
+    }
+    CHECK(!detectPeriod(noisyLoop, rate, span.from, span.count).valid());
+}
+
+static void testAnOutOfRangeSpanEstimatesNothing() {
+    const std::vector<AudioSample> src = sineOfPeriod(120000, 441.0);
+    CHECK(!detectPeriod(src, 44100, 120001, 10).valid());
+    CHECK(!detectPeriod(src, 44100, 119000, 5000).valid());
+    CHECK(!detectPeriod(src, 44100, 0, 0).valid());
+}
+
+// --- 6. What the load pays ------------------------------------------------------------------

 // The whole reason a detector is affordable in a sampler is that it runs ONCE, off the audio
 // thread, on a source that is already fully known. This prints what that once costs, and
@@ -255,6 +396,11 @@ int main() {
    testAPercussiveDecayIsNotForcedIntoAPeriod();
    testBelowTheBandReportsNoneAndAboveItReportsAWholeMultiple();
    testAShortSourceShortensTheSearchRatherThanRefusing();
+    testASourceTooShortForASecondProbeIsStillDetectedOnItsOneProbe();
+    testTheTwoLowFrequenciesTheShifterWasBuiltForAreDetected();
+    testTheLoopStandsInForTheSourceOnlyWhenItCostsNoSearchBand();
+    testAPhraseWhoseLoopIsPitchedDifferentlyFromItsHeadDetectsOverTheLoop();
+    testAnOutOfRangeSpanEstimatesNothing();
    testDetectionCostIsBoundedRegardlessOfSampleLength();

    if (g_fail == 0) {