// Standalone tests for reasampler::sampler_core — no VST3, no REAPER, no test
// framework. Same fast build/run loop as bank_model_tests / peaks_tests: feed known
// inputs, assert the engine's behavior.
//
// Covers (PLAN.md S3 / CONTEXT.md §Phase S pure core):
//   1. polyphonic allocation — N notes -> N voices; note-off releases the right voice.
//   2. voice stealing at the bound — deterministic policy (release-first, then oldest).
//   3. ADSR envelope shape vs a known signal, incl. release-before-sustain.
//   4. repitch ratio correctness across +/-1 octave from root incl. unity, asserted on
//      the observed period of a synthesized sine.
//   5. loop-point sustain — held note past sample end loops [start,end) seamlessly;
//      zero-length loop and absent-loop behavior.
//   6. keymap: chromatic-from-single-root; zoned ranges with boundary notes; velocity
//      -> volume; out-of-zone note -> defined no-play.
//
// The plain-data boundary (no VST3/REAPER types in the core) is enforced STRUCTURALLY
// by the CMake target linking neither SDK — this file includes only sampler_core.h +
// the standard library, which is itself the compile-time proof.

#include "../src/vst/sampler_core.h"

#include <cmath>
#include <cstdio>
#include <vector>

using namespace reasampler;

static int g_fail = 0;
#define CHECK(cond) do { if(!(cond)) { \
    std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)

static bool approx(double a, double b, double tol) { return std::fabs(a - b) <= tol; }

constexpr double kPi = 3.14159265358979323846;

// A silent (all-1.0) sample so a rendered voice's output tracks the envelope * velocity
// directly (DC of amplitude 1). Root at note 60 by default.
static SampleData dcSample(std::size_t frames, int rootNote = 60) {
    SampleData s;
    s.frames.assign(frames, 1.0f);
    s.rootNote = rootNote;
    return s;
}

// A mono sine of `cycles` periods over `frames` frames — used to observe repitch by
// measuring the played-back period.
static SampleData sineSample(std::size_t frames, double cycles, int rootNote = 60) {
    SampleData s;
    s.frames.resize(frames);
    for (std::size_t i = 0; i < frames; ++i) {
        s.frames[i] = static_cast<float>(std::sin(2.0 * kPi * cycles *
                      static_cast<double>(i) / static_cast<double>(frames)));
    }
    s.rootNote = rootNote;
    return s;
}

// An ADSR that stays fully open (level 1) forever while held, so voice output equals
// velocity gain — isolates allocation/repitch/loop tests from envelope shaping.
static AdsrParams flatAdsr() {
    AdsrParams a;
    a.attackFrames = 0;
    a.decayFrames = 0;
    a.sustainLevel = 1.0;
    a.releaseFrames = 0;  // note-off -> instant silence.
    return a;
}

// ---------------------------------------------------------------------------
// 6. Keymap resolution.
// ---------------------------------------------------------------------------

static void testChromaticSingleRoot() {
    Keymap km = Keymap::singleSampleChromatic(dcSample(100, 60));
    CHECK(km.zones.size() == 1);
    // Every note in 0..127 resolves to the single zone.
    for (int n = 0; n <= 127; ++n) {
        ZoneResolution r = km.resolve(n, 100);
        CHECK(r.matched);
        CHECK(r.zoneIndex == 0);
    }
}

static void testZonedRangesBoundaries() {
    Keymap km;
    km.samples.push_back(dcSample(100, 48));  // low sample
    km.samples.push_back(dcSample(100, 72));  // high sample
    // Two adjacent zones: [36,59] and [60,83]. Boundary notes 59/60 must land in the
    // correct zone; a first-match order test would catch an off-by-one.
    km.zones.push_back(KeyZone{36, 59, 48, 0});
    km.zones.push_back(KeyZone{60, 83, 72, 1});

    CHECK(km.resolve(36, 100).matched);
    CHECK(km.resolve(36, 100).zoneIndex == 0);
    CHECK(km.resolve(59, 100).zoneIndex == 0);   // last note of zone 0
    CHECK(km.resolve(60, 100).zoneIndex == 1);   // first note of zone 1
    CHECK(km.resolve(83, 100).zoneIndex == 1);   // last note of zone 1

    // Out of every zone -> defined no-play (not a match, not zone 0).
    CHECK(!km.resolve(35, 100).matched);
    CHECK(!km.resolve(84, 100).matched);
    CHECK(!km.resolve(127, 100).matched);
}

static void testFirstMatchOnOverlap() {
    // Overlapping zones: the earlier zone wins (documented deterministic rule).
    Keymap km;
    km.samples.push_back(dcSample(10, 60));
    km.samples.push_back(dcSample(10, 60));
    km.zones.push_back(KeyZone{0, 127, 60, 0});   // catch-all first
    km.zones.push_back(KeyZone{60, 60, 60, 1});   // shadowed by the catch-all
    CHECK(km.resolve(60, 100).zoneIndex == 0);
}

// ---------------------------------------------------------------------------
// 4. Repitch ratio correctness.
// ---------------------------------------------------------------------------

static void testPitchRatioMath() {
    CHECK(approx(pitchRatio(60, 60), 1.0, 1e-9));   // unity at root
    CHECK(approx(pitchRatio(72, 60), 2.0, 1e-9));   // +1 octave
    CHECK(approx(pitchRatio(48, 60), 0.5, 1e-9));   // -1 octave
    CHECK(approx(pitchRatio(61, 60), std::pow(2.0, 1.0 / 12.0), 1e-9));  // +1 semitone
}

// Observe repitch on the rendered signal: a voice played an octave above root should
// advance through the sample twice as fast, so a sine's observed period halves. We
// measure the period by counting the interval between positive-going zero crossings.
static double observedPeriodFrames(const std::vector<AudioSample>& out) {
    std::vector<std::size_t> upCrossings;
    for (std::size_t i = 1; i < out.size(); ++i) {
        if (out[i - 1] <= 0.0f && out[i] > 0.0f) upCrossings.push_back(i);
    }
    if (upCrossings.size() < 2) return 0.0;
    // Average spacing between crossings.
    double sum = 0.0;
    for (std::size_t i = 1; i < upCrossings.size(); ++i) {
        sum += static_cast<double>(upCrossings[i] - upCrossings[i - 1]);
    }
    return sum / static_cast<double>(upCrossings.size() - 1);
}

static void testRepitchObservedPeriod() {
    // A sine of 20 cycles over 8000 frames -> native period 400 frames at unity.
    const std::size_t frames = 8000;
    const double cycles = 20.0;
    const double nativePeriod = static_cast<double>(frames) / cycles;  // 400

    // Unity: played at root, observed period ~= native.
    {
        Keymap km = Keymap::singleSampleChromatic(sineSample(frames, cycles, 60));
        VoiceEngine eng(4, km, flatAdsr());
        eng.noteOn(60, 127);
        std::vector<AudioSample> out;
        eng.render(out, frames);
        double p = observedPeriodFrames(out);
        CHECK(approx(p, nativePeriod, 2.0));
    }
    // +1 octave: advances 2x, observed period halves.
    {
        Keymap km = Keymap::singleSampleChromatic(sineSample(frames, cycles, 60));
        VoiceEngine eng(4, km, flatAdsr());
        eng.noteOn(72, 127);
        std::vector<AudioSample> out;
        eng.render(out, frames / 2);  // half as many frames covers the whole sample
        double p = observedPeriodFrames(out);
        CHECK(approx(p, nativePeriod / 2.0, 2.0));
    }
    // -1 octave: advances 0.5x, observed period doubles.
    {
        Keymap km = Keymap::singleSampleChromatic(sineSample(frames, cycles, 60));
        VoiceEngine eng(4, km, flatAdsr());
        eng.noteOn(48, 127);
        std::vector<AudioSample> out;
        eng.render(out, frames);
        double p = observedPeriodFrames(out);
        CHECK(approx(p, nativePeriod * 2.0, 4.0));
    }
}

// ---------------------------------------------------------------------------
// 3. ADSR envelope shape vs a known signal.
// ---------------------------------------------------------------------------

static void testAdsrShape() {
    AdsrParams p;
    p.attackFrames = 10;
    p.decayFrames = 10;
    p.sustainLevel = 0.5;
    p.releaseFrames = 10;
    AdsrEnvelope env;
    env.configure(p);
    env.noteOn();

    // Attack: 0 -> ramps up. Frame 0 == 0, rising each frame.
    double prev = -1.0;
    for (int i = 0; i < 10; ++i) {
        double v = env.tick();
        CHECK(v >= prev);          // monotonic non-decreasing through attack
        CHECK(v >= 0.0 && v <= 1.0);
        prev = v;
    }
    // Decay: from 1.0 down toward sustain 0.5, monotonic non-increasing.
    prev = 2.0;
    for (int i = 0; i < 10; ++i) {
        double v = env.tick();
        CHECK(v <= prev + 1e-9);   // non-increasing through decay
        CHECK(v >= 0.5 - 1e-9);    // never below sustain during decay
        prev = v;
    }
    // Sustain: holds 0.5 indefinitely.
    for (int i = 0; i < 100; ++i) {
        CHECK(approx(env.tick(), 0.5, 1e-9));
    }
    CHECK(env.stage() == AdsrEnvelope::Stage::Sustain);

    // Release: 0.5 -> 0 over 10 frames, then Finished + latched at 0.
    env.noteOff();
    prev = 1.0;
    for (int i = 0; i < 10; ++i) {
        double v = env.tick();
        CHECK(v <= prev + 1e-9);   // non-increasing through release
        prev = v;
    }
    CHECK(env.finished());
    for (int i = 0; i < 10; ++i) CHECK(approx(env.tick(), 0.0, 1e-12));
}

static void testAdsrReleaseBeforeSustain() {
    // noteOff during the attack ramp releases from the PARTIAL level, not sustain.
    AdsrParams p;
    p.attackFrames = 100;
    p.decayFrames = 10;
    p.sustainLevel = 0.8;
    p.releaseFrames = 20;
    AdsrEnvelope env;
    env.configure(p);
    env.noteOn();

    // Advance 50 frames into a 100-frame attack -> partial level ~0.5.
    double last = 0.0;
    for (int i = 0; i < 50; ++i) last = env.tick();
    CHECK(last > 0.3 && last < 0.7);        // partway up the attack ramp
    CHECK(env.stage() == AdsrEnvelope::Stage::Attack);

    env.noteOff();
    CHECK(env.stage() == AdsrEnvelope::Stage::Release);
    // First release frame must be at or below the partial level we left off at —
    // NOT jump up to sustain 0.8. This is the release-before-sustain guarantee.
    double firstRelease = env.tick();
    CHECK(firstRelease <= last + 1e-9);
    CHECK(firstRelease < p.sustainLevel);   // proves it did not snap to sustain
    // Decays to zero.
    double prev = firstRelease;
    for (int i = 0; i < 20; ++i) {
        double v = env.tick();
        CHECK(v <= prev + 1e-9);
        prev = v;
    }
    CHECK(env.finished());
}

static void testAdsrZeroAttackDecay() {
    // Zero attack + zero decay -> jumps straight to sustain on the first ticks.
    AdsrParams p;
    p.attackFrames = 0;
    p.decayFrames = 0;
    p.sustainLevel = 0.7;
    p.releaseFrames = 5;
    AdsrEnvelope env;
    env.configure(p);
    env.noteOn();
    // Zero attack emits the attack peak (1.0) on frame 0 and immediately transitions
    // through the (also zero-length) decay, so by frame 1 the envelope is holding
    // sustain. The peak-at-boundary is the documented single-frame edge, not a bug.
    CHECK(approx(env.tick(), 1.0, 1e-9));   // frame 0: attack peak
    CHECK(approx(env.tick(), 0.7, 1e-9));   // frame 1: sustain
    CHECK(approx(env.tick(), 0.7, 1e-9));
    CHECK(env.stage() == AdsrEnvelope::Stage::Sustain);
}

// ---------------------------------------------------------------------------
// 1. Polyphonic allocation + note-off routing.
// ---------------------------------------------------------------------------

static void testPolyphonicAllocation() {
    Keymap km = Keymap::singleSampleChromatic(dcSample(1000, 60));
    VoiceEngine eng(8, km, flatAdsr());

    // Four simultaneous notes -> four active voices, each on a distinct voice.
    std::size_t v60 = eng.noteOn(60, 100);
    std::size_t v64 = eng.noteOn(64, 100);
    std::size_t v67 = eng.noteOn(67, 100);
    std::size_t v72 = eng.noteOn(72, 100);
    CHECK(v60 != VoiceEngine::kNoVoice);
    CHECK(eng.activeVoiceCount() == 4);
    CHECK(v60 != v64 && v64 != v67 && v67 != v72 && v60 != v72);

    // Note-off on 64 releases exactly one voice; with instant release it goes idle
    // after the next render frame.
    eng.noteOff(64);
    std::vector<AudioSample> out;
    eng.render(out, 1);
    CHECK(eng.activeVoiceCount() == 3);

    // The still-held notes keep sounding.
    eng.render(out, 1);
    CHECK(eng.activeVoiceCount() == 3);
}

static void testNoteOffReleasesNewestSameNote() {
    // Prove that noteOff releases the NEWEST (highest startOrder) instance of a
    // 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 = 10;   // short but non-zero so voice stays active through release
    VoiceEngine eng(8, km, a);

    std::size_t first  = eng.noteOn(60, velOld);  // older voice, lower gain
    std::size_t second = eng.noteOn(60, velNew);  // newer voice, higher gain
    CHECK(first != second);
    CHECK(eng.activeVoiceCount() == 2);

    // While both are held, combined output equals gainOld + gainNew.
    {
        std::vector<AudioSample> out;
        eng.render(out, 1);
        CHECK(approx(out[0], gainOld + gainNew, 1e-4));
    }

    // Release once — must target the NEWEST voice (second).
    eng.noteOff(60);

    // 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() {
    Keymap km;
    km.samples.push_back(dcSample(100, 60));
    km.zones.push_back(KeyZone{60, 72, 60, 0});
    VoiceEngine eng(4, km, flatAdsr());

    std::size_t v = eng.noteOn(30, 100);  // below the only zone
    CHECK(v == VoiceEngine::kNoVoice);
    CHECK(eng.activeVoiceCount() == 0);   // no voice consumed
}

// ---------------------------------------------------------------------------
// 2. Voice stealing at the bound.
// ---------------------------------------------------------------------------

static void testStealsReleasingVoiceFirst() {
    Keymap km = Keymap::singleSampleChromatic(dcSample(100000, 60));
    AdsrParams a = flatAdsr();
    a.releaseFrames = 100000;  // long release so a released voice stays "active".
    VoiceEngine eng(2, km, a);

    std::size_t vA = eng.noteOn(60, 100);  // startOrder 1
    std::size_t vB = eng.noteOn(62, 100);  // startOrder 2
    CHECK(eng.activeVoiceCount() == 2);

    // Release the NEWER voice (62) — it becomes the only releasing voice.
    eng.noteOff(62);
    std::vector<AudioSample> out;
    eng.render(out, 1);
    CHECK(eng.activeVoiceCount() == 2);  // both still ringing (long release)

    // A new note with the pool full must steal the RELEASING voice (vB), not the
    // older held voice (vA) — release-first policy.
    std::size_t vC = eng.noteOn(64, 100);
    CHECK(vC == vB);
    CHECK(eng.activeVoiceCount() == 2);
}

static void testStealsOldestWhenNoneReleasing() {
    Keymap km = Keymap::singleSampleChromatic(dcSample(100000, 60));
    AdsrParams a = flatAdsr();
    a.releaseFrames = 100000;
    VoiceEngine eng(2, km, a);

    std::size_t vA = eng.noteOn(60, 100);  // startOrder 1 (oldest)
    std::size_t vB = eng.noteOn(62, 100);  // startOrder 2
    CHECK(vA != vB);

    // No voice released; both held. A new note steals the OLDEST (vA).
    std::size_t vC = eng.noteOn(64, 100);
    CHECK(vC == vA);
    CHECK(eng.activeVoiceCount() == 2);

    // The stolen voice now carries note 64; a note-off on 60 (the stolen-away note)
    // finds nothing to release.
    std::size_t before = eng.activeVoiceCount();
    eng.noteOff(60);
    std::vector<AudioSample> out;
    eng.render(out, 1);
    CHECK(eng.activeVoiceCount() == before);  // 60 no longer exists; no-op
}

// ---------------------------------------------------------------------------
// 5. Loop-point-aware sustain.
// ---------------------------------------------------------------------------

static void testLoopSustainSeamless() {
    // A sample whose [0,20) frames are a distinctive ramp and [20,40) is a flat loop
    // region of value 0.5. Held far past the sample end, the voice must keep emitting
    // the loop region (0.5) rather than going silent.
    SampleData s;
    s.frames.resize(40);
    for (int i = 0; i < 20; ++i) s.frames[i] = static_cast<float>(i) / 20.0f;  // attack
    for (int i = 20; i < 40; ++i) s.frames[i] = 0.5f;                          // loop body
    s.rootNote = 60;
    s.loop.hasLoop = true;
    s.loop.start = 20;
    s.loop.end = 40;

    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, 200);  // 5x the sample length

    // Voice is still active (looping), not exhausted.
    CHECK(eng.activeVoiceCount() == 1);
    // Frames well past the loop start must sit at the loop body value 0.5.
    for (std::size_t i = 60; i < out.size(); ++i) {
        CHECK(approx(out[i], 0.5, 1e-4));
    }
}

static void testZeroLengthLoopGoesSilent() {
    // A zero-length loop (start == end) is the "no sustain" marker: the note runs off
    // the sample end and the voice goes idle, rather than spinning on an empty span.
    SampleData s = dcSample(50, 60);   // 50 frames of 1.0
    s.loop.hasLoop = true;
    s.loop.start = 25;
    s.loop.end = 25;                   // zero length
    Keymap km = Keymap::singleSampleChromatic(std::move(s));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);

    std::vector<AudioSample> out;
    eng.render(out, 100);              // past the 50-frame end
    // After frame ~50 the voice should have gone idle (no loop to sustain it).
    CHECK(eng.activeVoiceCount() == 0);
    // Tail frames are silent.
    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);
    // s.loop.hasLoop stays false.
    Keymap km = Keymap::singleSampleChromatic(std::move(s));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);
    std::vector<AudioSample> out;
    eng.render(out, 100);
    CHECK(eng.activeVoiceCount() == 0);
    for (std::size_t i = 60; i < out.size(); ++i) CHECK(approx(out[i], 0.0, 1e-6));
}

// ---------------------------------------------------------------------------
// start point (S11): the voice's initial read position is SampleData::startFrame.
// ---------------------------------------------------------------------------

static void testStartFrameOffsetsInitialRead() {
    // A per-frame ramp (frame i holds i*0.01) so the first rendered value pinpoints the
    // read position. startFrame = 30 -> the first output frame reads frame 30 (0.30).
    SampleData s;
    s.frames.resize(100);
    for (int i = 0; i < 100; ++i) s.frames[i] = static_cast<float>(i) * 0.01f;
    s.rootNote = 60;
    s.startFrame = 30;
    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> out;
    eng.render(out, 3);
    CHECK(approx(out[0], 0.30, 1e-4));  // starts at frame 30, not 0
    CHECK(approx(out[1], 0.31, 1e-4));  // advances by unity ratio
    CHECK(approx(out[2], 0.32, 1e-4));
}

static void testStartFrameZeroIsUnchanged() {
    // startFrame default 0 is exactly the pre-S11 behavior: read begins at frame 0.
    SampleData s;
    s.frames.resize(20);
    for (int i = 0; i < 20; ++i) s.frames[i] = static_cast<float>(i) * 0.05f;
    s.rootNote = 60;  // startFrame stays 0
    Keymap km = Keymap::singleSampleChromatic(std::move(s));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);
    std::vector<AudioSample> out;
    eng.render(out, 1);
    CHECK(approx(out[0], 0.0, 1e-6));  // frame 0
}

static void testStartFrameOutOfRangeClampsToZero() {
    // A start point at/past the sample end degrades to frame 0 (play from the top), never an
    // out-of-bounds read that would start the voice already exhausted.
    SampleData s = dcSample(10, 60);          // 10 frames of 1.0
    s.startFrame = 10;                         // == frameCount: out of range
    Keymap km = Keymap::singleSampleChromatic(std::move(s));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);
    std::vector<AudioSample> out;
    eng.render(out, 5);
    // Reads from frame 0: the DC sample plays its 1.0 body rather than an immediate idle.
    CHECK(eng.activeVoiceCount() == 1);
    CHECK(approx(out[0], 1.0, 1e-4));
}

static void testStartFrameWithLoop() {
    // Start point and loop compose: begin reading mid-sample, then sustain the loop region.
    SampleData s;
    s.frames.resize(40);
    for (int i = 0; i < 40; ++i) s.frames[i] = static_cast<float>(i) * 0.01f;
    for (int i = 20; i < 40; ++i) s.frames[i] = 0.5f;   // loop body
    s.rootNote = 60;
    s.startFrame = 10;                                   // begin at frame 10
    s.loop.hasLoop = true;
    s.loop.start = 20;
    s.loop.end = 40;
    Keymap km = Keymap::singleSampleChromatic(std::move(s));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);
    std::vector<AudioSample> out;
    eng.render(out, 200);
    CHECK(approx(out[0], 0.10, 1e-4));       // started at frame 10
    CHECK(eng.activeVoiceCount() == 1);      // loop sustains it
    for (std::size_t i = 60; i < out.size(); ++i) CHECK(approx(out[i], 0.5, 1e-4));
}

static void testStartAfterLoopEndWrapsIntoLoop() {
    // Regression (S11 reviewer finding): if startFrame > loop.end (but still < frameCount),
    // the voice's initial read head is past the loop end. The wrap-while in renderFrame must
    // pull it back into [loopStart, loopEnd) on the very first frame, so the note sounds from
    // somewhere inside the loop rather than running off the sample end silently.
    //
    // Setup: 100-frame sample; loop is [20, 40); startFrame = 60 (past loop.end = 40).
    // loop body is a constant 0.5 so every frame inside it reads 0.5.
    // After wrap: readPos starts inside [20, 40), first output frame == 0.5.
    // Voice must stay active (loop sustains it) and emit the loop value, NOT go silent.
    SampleData s;
    s.frames.resize(100, 0.0f);
    for (int i = 20; i < 40; ++i) s.frames[i] = 0.5f;  // loop body
    s.rootNote = 60;
    s.startFrame = 60;  // > loop.end (40), < frameCount (100)
    s.loop.hasLoop = true;
    s.loop.start = 20;
    s.loop.end   = 40;

    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);

    // Voice must still be active — the usable loop keeps it alive indefinitely.
    CHECK(eng.activeVoiceCount() == 1);
    // Every frame after the first wrap must read 0.5 (the loop body). We skip the very
    // first frame because the fractional-position wrap lands somewhere in [20,40) and the
    // exact offset depends on how many loop lengths fit into 60; what matters is that the
    // voice is alive and emitting the loop value, not 0.0 (pre-loop region).
    for (std::size_t i = 5; i < out.size(); ++i) {
        CHECK(approx(out[i], 0.5, 1e-4));
    }
}

// ---------------------------------------------------------------------------
// velocity -> volume.
// ---------------------------------------------------------------------------

static void testVelocityToVolume() {
    Keymap km = Keymap::singleSampleChromatic(dcSample(100, 60));  // DC 1.0
    // Full velocity -> full gain; half velocity -> ~half gain (flat envelope so the
    // rendered value is exactly velocity/127 on a DC-1 sample).
    {
        VoiceEngine eng(1, km, flatAdsr());
        eng.noteOn(60, 127);
        std::vector<AudioSample> out;
        eng.render(out, 1);
        CHECK(approx(out[0], 1.0, 1e-4));
    }
    {
        VoiceEngine eng(1, km, flatAdsr());
        eng.noteOn(60, 64);
        std::vector<AudioSample> out;
        eng.render(out, 1);
        CHECK(approx(out[0], 64.0 / 127.0, 1e-4));
    }
    {
        VoiceEngine eng(1, km, flatAdsr());
        eng.noteOn(60, 1);
        std::vector<AudioSample> out;
        eng.render(out, 1);
        CHECK(approx(out[0], 1.0 / 127.0, 1e-4));
    }
}

// Two voices summed: polyphony mixes additively.
static void testPolyphonyMixesAdditively() {
    Keymap km = Keymap::singleSampleChromatic(dcSample(100, 60));  // DC 1.0
    VoiceEngine eng(4, km, flatAdsr());
    eng.noteOn(60, 127);   // gain 1.0
    eng.noteOn(60, 127);   // gain 1.0 (second voice, same note)
    std::vector<AudioSample> out;
    eng.render(out, 1);
    CHECK(approx(out[0], 2.0, 1e-4));  // both voices sum
}

// ---------------------------------------------------------------------------
// 7. Stereo channel dimension (S7).
// ---------------------------------------------------------------------------

// A distinct-per-channel stereo DC sample: L = `l`, R = `r` everywhere. A stereo render
// must keep them distinct; a mono render (channel 0 only) sees L.
static SampleData stereoDcSample(std::size_t frames, float l, float r, int rootNote = 60) {
    SampleData s;
    s.frames.assign(frames, l);
    s.framesR.assign(frames, r);
    s.rootNote = rootNote;
    return s;
}

static void testChannelCount() {
    // Mono: framesR empty -> 1 channel. Stereo: matching-length framesR -> 2.
    CHECK(dcSample(10, 60).channelCount() == 1);
    CHECK(stereoDcSample(10, 1.0f, -1.0f).channelCount() == 2);
    // A mismatched framesR length is treated as mono (a bad pair never half-plays).
    SampleData bad = dcSample(10, 60);
    bad.framesR.assign(5, 0.5f);  // wrong length
    CHECK(bad.channelCount() == 1);
}

static void testStereoRenderKeepsChannelsDistinct() {
    // A stereo sample (L=1.0, R=-1.0) rendered stereo must emit L and R distinctly, each
    // scaled by velocity (full here). If the engine copied L to both channels the R check fails.
    Keymap km = Keymap::singleSampleChromatic(stereoDcSample(100, 1.0f, -1.0f, 60));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);

    std::vector<AudioSample> left(8, 0.f), right(8, 0.f);
    eng.render(left.data(), right.data(), 8);
    for (std::size_t i = 0; i < 8; ++i) {
        CHECK(approx(left[i], 1.0, 1e-4));    // channel 0
        CHECK(approx(right[i], -1.0, 1e-4));  // channel 1 — distinct, NOT a copy of L
    }
}

static void testMonoSamplePlaysDualMonoInStereo() {
    // A MONO sample rendered through the stereo path plays dual-mono: both channels equal
    // (centered), not silent on the right. The cross-mode "mono source in stereo mode" case.
    Keymap km = Keymap::singleSampleChromatic(dcSample(100, 60));  // mono, DC 1.0
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);
    std::vector<AudioSample> left(8, 0.f), right(8, 0.f);
    eng.render(left.data(), right.data(), 8);
    for (std::size_t i = 0; i < 8; ++i) {
        CHECK(approx(left[i], 1.0, 1e-4));
        CHECK(approx(right[i], 1.0, 1e-4));  // R == L (dual-mono), not 0
    }
}

static void testMonoRenderUnchangedByStereoData() {
    // Regression: the mono render path (renderFrame) reads channel 0 ONLY and is byte-identical
    // whether or not a second channel is present. A stereo sample rendered mono == its L channel.
    Keymap kmS = Keymap::singleSampleChromatic(stereoDcSample(100, 0.75f, -0.25f, 60));
    VoiceEngine engS(1, kmS, flatAdsr());
    engS.noteOn(60, 127);
    std::vector<AudioSample> mono;
    engS.render(mono, 8);  // the mono overload
    for (std::size_t i = 0; i < 8; ++i) CHECK(approx(mono[i], 0.75, 1e-4));  // == L, ignores R
}

static void testStereoRenderAdvancesLikeMonoRepitch() {
    // The stereo path must advance the read head by the SAME per-frame ratio as the mono path,
    // so repitch is identical. Play a stereo sine (both channels the same signal) an octave up
    // and confirm the observed period halves — the mono repitch assertion, on the stereo path.
    const std::size_t frames = 8000;
    const double cycles = 20.0;
    const double nativePeriod = static_cast<double>(frames) / cycles;  // 400
    SampleData s;
    s.frames.resize(frames);
    s.framesR.resize(frames);
    for (std::size_t i = 0; i < frames; ++i) {
        const float v = static_cast<float>(std::sin(2.0 * kPi * cycles *
                        static_cast<double>(i) / static_cast<double>(frames)));
        s.frames[i] = v;
        s.framesR[i] = v;
    }
    s.rootNote = 60;
    Keymap km = Keymap::singleSampleChromatic(std::move(s));
    VoiceEngine eng(4, km, flatAdsr());
    eng.noteOn(72, 127);  // +1 octave
    std::vector<AudioSample> left(frames / 2, 0.f), right(frames / 2, 0.f);
    eng.render(left.data(), right.data(), frames / 2);
    CHECK(approx(observedPeriodFrames(left), nativePeriod / 2.0, 2.0));
    CHECK(approx(observedPeriodFrames(right), nativePeriod / 2.0, 2.0));  // R repitches identically
}

static void testStereoRenderSumsVoicesPerChannel() {
    // Two voices on a stereo sample sum PER CHANNEL (additive polyphony holds in stereo).
    Keymap km = Keymap::singleSampleChromatic(stereoDcSample(100, 0.5f, -0.5f, 60));
    VoiceEngine eng(4, km, flatAdsr());
    eng.noteOn(60, 127);
    eng.noteOn(60, 127);  // second voice, same note
    std::vector<AudioSample> left(1, 0.f), right(1, 0.f);
    eng.render(left.data(), right.data(), 1);
    CHECK(approx(left[0], 1.0, 1e-4));    // 0.5 + 0.5
    CHECK(approx(right[0], -1.0, 1e-4));  // -0.5 + -0.5
}

static void testStereoRenderNullBufferIsNoOp() {
    Keymap km = Keymap::singleSampleChromatic(stereoDcSample(100, 1.0f, -1.0f, 60));
    VoiceEngine eng(1, km, flatAdsr());
    eng.noteOn(60, 127);
    std::vector<AudioSample> buf(4, 0.f);
    eng.render(nullptr, buf.data(), 4);   // null left -> no-op, no crash
    eng.render(buf.data(), nullptr, 4);   // null right -> no-op
    for (float v : buf) CHECK(approx(v, 0.0, 1e-9));  // untouched
}

int main() {
    testChromaticSingleRoot();
    testZonedRangesBoundaries();
    testFirstMatchOnOverlap();
    testPitchRatioMath();
    testRepitchObservedPeriod();
    testAdsrShape();
    testAdsrReleaseBeforeSustain();
    testAdsrZeroAttackDecay();
    testPolyphonicAllocation();
    testNoteOffReleasesNewestSameNote();
    testOutOfZoneNoteConsumesNoVoice();
    testStealsReleasingVoiceFirst();
    testStealsOldestWhenNoneReleasing();
    testLoopSustainSeamless();
    testZeroLengthLoopGoesSilent();
    testSingleFrameLoop();
    testAbsentLoopGoesSilent();
    testStartFrameOffsetsInitialRead();
    testStartFrameZeroIsUnchanged();
    testStartFrameOutOfRangeClampsToZero();
    testStartFrameWithLoop();
    testStartAfterLoopEndWrapsIntoLoop();
    testVelocityToVolume();
    testPolyphonyMixesAdditively();
    testChannelCount();
    testStereoRenderKeepsChannelsDistinct();
    testMonoSamplePlaysDualMonoInStereo();
    testMonoRenderUnchangedByStereoData();
    testStereoRenderAdvancesLikeMonoRepitch();
    testStereoRenderSumsVoicesPerChannel();
    testStereoRenderNullBufferIsNoOp();

    if (g_fail == 0) {
        std::printf("all sampler_core tests passed\n");
        return 0;
    }
    std::printf("%d sampler_core check(s) failed\n", g_fail);
    return 1;
}