debug/fix: try sanitizing loop_delay_us to avoid endless musicFeedThread hangs

app/build.gradle

@@ -12,8 +12,8 @@ android {
         applicationId "at.lockstep"
         minSdk 24
         targetSdk 34
-        versionCode 1
+        versionCode 10004
-        versionName "1.0"
+        versionName "1.0.4"
 
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         vectorDrawables {
@@ -24,6 +24,7 @@ android {
                 //path 'src/main/cpp/CMakeLists.txt'
                 //cppFlags ''
                 arguments "-DANDROID_STL=c++_shared"
+                cppFlags "-DLIBPASADA_VERSION=1.0.4"
 
                 //cppFlags "-std=c++14"
                 //arguments '-DANDROID_STL=c++_static'

app/src/main/cpp/CMakeLists.txt

@@ -35,12 +35,14 @@ add_library(${CMAKE_PROJECT_NAME} SHARED
     jni_mpg123.cpp
     jni_lockstep.cpp
     jni_stepdetector.cpp
-    StepDetector.cpp
+    jni_libpasada.cpp
+    LibPasada.cpp
+        #// jni_logging.cpp // same implemented in jni_libpasada.cpp // JNI_OnLoad and JNI_OnUnload
 )
 
 find_package (oboe REQUIRED CONFIG)
 
-add_library(ndk-logger SHARED logging.cpp jni_logging.cpp)
+add_library(ndk-logger SHARED logging.cpp)  # jni_logging.cpp
 target_link_libraries(ndk-logger log)
 
 # Add pre-built libmpg123 library

app/src/main/cpp/LibPasada.cpp

@@ -0,0 +1,199 @@
+#include "LibPasada.h"
+#include "PlaybackEngine.h"
+#include "step_detector.h"
+#include <string>
+
+/** Current track reached end; service should advance queue and call {@link LibPasada#play}. */
+void PasadaPlaybackListener::onTrackFinished() {
+    emitTrackFinished();
+}
+
+/** Decode / Oboe / pipeline failure; service should stop safely and surface error. */
+void PasadaPlaybackListener::onError(int errorCode, std::string message) {
+    emitError(errorCode, message);
+}
+
+void PasadaPlaybackListener::onTrackClosed(int fd) {
+    emitTrackClosed(fd);
+}
+
+/**
+ * Mirrors the libpasada state machine documented in DESIGN.md.
+ * Keep values in sync with PasadaState.java
+ */
+/*enum PasadaState {
+    LOADED = 0,
+    INITIALIZED = 1,
+    PLAYING = 2,
+    PAUSED = 3,
+    FINISHED = 4,
+    STOPPED = 5
+};*/
+
+/**
+ * JNI entry point for libpasada.
+ * <p>Native state machine: LOADED → INITIALIZED → PLAYING ↔ PAUSED → FINISHED → STOPPED
+ */
+
+LibPasada::LibPasada() : state(LOADED), engine(nullptr), detector(nullptr)
+{}
+
+/** We must clean up here whatever is left over when init() or stop() fails. */
+LibPasada::~LibPasada() {
+    // delete StepDetector first, lest it spuriously pushes StepListener events
+    std::lock_guard<std::mutex> lock(mtxDetector);
+    if (detector != nullptr) {
+        auto *stepDetector = reinterpret_cast<StepDetector *>(detector);
+        delete stepDetector;
+        detector = nullptr;
+    }
+    // init(): if new StepDetector() fails, we have a leftover 'engine'
+    if (engine != nullptr) {
+        auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+        delete playbackEngine;
+        engine = nullptr;
+    }
+}
+
+/** Called each time a run/playlist session starts (Oboe silent, buffers ready). */
+void LibPasada::init() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    std::lock_guard<std::mutex> lock(mtxDetector);
+    if(state == LOADED || state == STOPPED) {
+        // perform init
+        auto *playbackEngine = new PlaybackEngine("", 0, this);
+        engine = playbackEngine;
+
+        auto *stepListener = reinterpret_cast<StepListener *>(playbackEngine);
+        // the hardcoded 'fps' is only an initial value, which is re-computed in StepDetector
+        auto *stepDetector = new StepDetector(60.0 /* FPS */, stepListener);
+        detector = stepDetector;
+
+        state = INITIALIZED;
+    }
+}
+
+/** Submit one accelerometer sample (m/s²); may be called from a sensor thread. */
+void LibPasada::feedAccel(float x, float y, float z, long long timestamp_nanos) {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    std::lock_guard<std::mutex> lock(mtxDetector);
+    if(state == LOADED || state == STOPPED) {
+        // player is not in an initialized state - ignore accelerometer data
+        return;
+    }
+    auto *stepDetector = reinterpret_cast<StepDetector *>(detector);
+    stepDetector->filter(static_cast<double>(timestamp_nanos), std::vector<float> {x, y, z});
+}
+
+/**
+ * Open MP3 from an already-open file descriptor and begin adaptive playback.
+ *
+ * @param fd open read FD (Java retains ownership; do not close until track changes)
+ * @param offset start offset within the FD (0 for whole file)
+ * @param length byte length from offset ({@code -1} if unknown / to EOF)
+ */
+void LibPasada::play(int fd, long long offset, long long length) {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    state = PLAYING;
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    playbackEngine->playMusic(fd, offset, length);
+}
+
+/** PLAYING → PAUSED (silent output, graph kept alive). */
+void LibPasada::pause() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    if(state != PLAYING) return;
+    state = PAUSED;
+    // TODO: pause plays a rect noise in PlaybackEngine. debug it!
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    playbackEngine->pause();
+}
+
+/** PAUSED → PLAYING (same track, same decode position, same FD). */
+void LibPasada::resume() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    if(state != PAUSED) return;
+    state = PLAYING;
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    playbackEngine->resume();
+}
+
+/** Tear down Oboe for this run segment → STOPPED. */
+void LibPasada::stop() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    state = STOPPED;
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    playbackEngine->stop();
+}
+
+/**
+ * Milliseconds from the start of the current track — same timebase as ExoPlayer
+ * {@code getCurrentPosition()}. Safe to poll from background threads.
+ */
+long LibPasada::getCurrentPositionMs() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    if(state == LOADED)
+        return 0;
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    return static_cast<long>(playbackEngine->getCurrentPosition() * 1000);
+}
+
+/** Track duration in ms, or {@code 0} if not yet known. */
+long LibPasada::getDurationMs() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    if(state == LOADED)
+        return 0;
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    return static_cast<long>(playbackEngine->getDuration() * 1000);
+}
+
+// defined in jni_libpasada.cpp
+void emitTrackFinished();
+
+void LibPasada::onTrackFinished() {
+    {
+        std::lock_guard<std::mutex> lockState(mtxState);
+        if (state != PLAYING)
+            return; // this should never happen
+        state = FINISHED;
+    }
+    emitTrackFinished();
+}
+
+/** Whether adapted audio is actively being output (not paused, not finished). */
+bool LibPasada::isPlaying() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    return state == PLAYING;
+}
+
+/** Current native state; see {@link PasadaState}. Must not throw. */
+int LibPasada::getState() {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    return state;
+}
+
+/** Seek within the current track. */
+void LibPasada::seekTo(long positionMs) {
+    std::lock_guard<std::mutex> lockState(mtxState);
+    if(state != PLAYING && state != PAUSED)
+        return; // silently ignore seeks if we do not have a music file loaded
+
+    auto *playbackEngine = reinterpret_cast<PlaybackEngine *>(engine);
+    playbackEngine->seekTo(positionMs / 1000.0);
+}
+
+/** Runtime metrics / last error string for logging and debug UI. Must not throw. */
+std::string LibPasada::getDiagnostics() { return ""; }
+
+/** Register listener for async events raised from the audio/native thread. */
+//void LibPasada::setPlaybackListener(PasadaPlaybackListener listener) {} // JNI-side only
+
+#define STRINGIFY(s) #s
+
+std::string LibPasada::getVersion() {
+#ifdef LIBPASADA_VERSION
+    return STRINGIFY(LIBPASADA_VERSION);
+#else
+    return "0.0.1";
+#endif
+}

app/src/main/cpp/LibPasada.h

@@ -0,0 +1,98 @@
+//
+// Created by david on 24.05.2026.
+//
+
+#ifndef LOCKSTEP_LIBPASADA_H
+#define LOCKSTEP_LIBPASADA_H
+
+#include "PasadaPlaybackListener.h"
+#include <string>
+#include <mutex>
+
+// JNI helpers for calling into Java code (PasadaPlaybackListener)
+void emitTrackFinished();
+void emitError(int errorCode, const std::string& message);
+void emitTrackClosed(int fd);
+
+/**
+ * Mirrors the libpasada state machine documented in DESIGN.md.
+ * Keep values in sync with PasadaState.java
+ */
+enum PasadaState {
+    LOADED = 0,
+    INITIALIZED = 1,
+    PLAYING = 2,
+    PAUSED = 3,
+    FINISHED = 4,
+    STOPPED = 5
+};
+
+/**
+ * JNI entry point for libpasada.
+ * <p>Native state machine: LOADED → INITIALIZED → PLAYING ↔ PAUSED → FINISHED → STOPPED
+ */
+class LibPasada : public PasadaPlaybackListener {
+private:
+    PasadaState state;
+    std::mutex mtxState;
+    //PasadaPlaybackListener listener;
+    void *engine;
+    void *detector;
+    std::mutex mtxDetector;
+
+public:
+    LibPasada();
+    virtual ~LibPasada();
+
+    /** Called each time a run/playlist session starts (Oboe silent, buffers ready). */
+    void init();
+
+    /** Submit one accelerometer sample (m/s²); may be called from a sensor thread. */
+    void feedAccel(float x, float y, float z, long long timestamp_nanos);
+
+    /**
+     * Open MP3 from an already-open file descriptor and begin adaptive playback.
+     *
+     * @param fd open read FD (Java retains ownership; do not close until track changes)
+     * @param offset start offset within the FD (0 for whole file)
+     * @param length byte length from offset ({@code -1} if unknown / to EOF)
+     */
+    void play(int fd, long long offset, long long length);
+    /** PLAYING → PAUSED (silent output, graph kept alive). */
+    void pause();
+    /** PAUSED → PLAYING (same track, same decode position, same FD). */
+    void resume();
+    /** Tear down Oboe for this run segment → STOPPED. */
+    void stop();
+
+    /**
+     * Milliseconds from the start of the current track — same timebase as ExoPlayer
+     * {@code getCurrentPosition()}. Safe to poll from background threads.
+     */
+    long getCurrentPositionMs();
+
+    /** Track duration in ms, or {@code 0} if not yet known. */
+    long getDurationMs();
+
+    /** Whether adapted audio is actively being output (not paused, not finished). */
+    bool isPlaying();
+
+    /** Current native state; see {@link PasadaState}. Must not throw. */
+    int getState();
+
+    /** Seek within the current track. */
+    void seekTo(long positionMs);
+
+    /** Runtime metrics / last error string for logging and debug UI. Must not throw. */
+    std::string getDiagnostics();
+
+    /** Register listener for async events raised from the audio/native thread. */
+    //void setPlaybackListener(PasadaPlaybackListener listener); // JNI-side only
+
+    /** Current track reached end; service should advance queue and call {@link LibPasada#play}. */
+    void onTrackFinished() override;
+
+    std::string getVersion();
+};
+
+#endif //LOCKSTEP_LIBPASADA_H

app/src/main/cpp/PasadaPlaybackListener.h

@@ -0,0 +1,25 @@
+//
+// Created by david on 02.06.2026.
+//
+
+#ifndef LOCKSTEP_PASADAPLAYBACKLISTENER_H
+#define LOCKSTEP_PASADAPLAYBACKLISTENER_H
+
+#include <string>
+
+/**
+ * Callbacks invoked from native (Oboe or internal worker thread).
+ */
+class PasadaPlaybackListener {
+public:
+    PasadaPlaybackListener() = default;
+    virtual ~PasadaPlaybackListener() = default;
+    /** Current track reached end; service should advance queue and call {@link LibPasada#play}. */
+    virtual void onTrackFinished();
+    virtual void onTrackClosed(int fd);
+
+    /** Decode / Oboe / pipeline failure; service should stop safely and surface error. */
+    virtual void onError(int errorCode, std::string message);
+};
+
+#endif //LOCKSTEP_PASADAPLAYBACKLISTENER_H

app/src/main/cpp/PlaybackEngine.cpp

@@ -11,6 +11,9 @@
 #include <memory>
 #include <vector>
 #include <chrono>
+#include <sys/types.h>
+#include <unistd.h>
+#include <random>
 
 /**
  * Read samples from the next mp3-frame into the struct MP3File's buffer.
@@ -67,7 +70,8 @@ struct RbLogger : public RubberBand::RubberBandStretcher::Logger {
     virtual ~RbLogger() { }
 };
 
-PlaybackEngine::PlaybackEngine(std::string filesDir, int resid):
+PlaybackEngine::PlaybackEngine(std::string filesDir, int resid, PasadaPlaybackListener *listener):
+    listener(listener),
     stretcher(
             /* sampleRate: */ 48000,
             /* channels: */ 2,
@@ -80,6 +84,11 @@ PlaybackEngine::PlaybackEngine(std::string filesDir, int resid):
     haveMusicFile(false),
     exitMusicFeedThread(false),
     isSetMusic(false),
+    isPaused(false),
+    markerPos(0.0),
+    duration(0.0),
+    seekPos(0.0),
+    requestSeek(false),
     android_fd(0),
     haveTimeRatio(false),
     timeRatio(1.0),
@@ -96,8 +105,13 @@ PlaybackEngine::PlaybackEngine(std::string filesDir, int resid):
 
     // load "bump" sound effect
     std::vector<float> samples;
+    if (!filesDir.empty()) {
         bool is_ok = read_mp3(mFilesDir + "/" + std::to_string(resid) + ".mp3", samples);
         LOGI("read_mp3() for bump effect, is_ok=%d", is_ok);
+    } else {
+        samples.resize(1);
+        LOGI("keeping bump effect silent, since have no filesDir");
+    }
 
     mPlayer = new MixingPlayer(samples);
 
@@ -145,7 +159,9 @@ void PlaybackEngine::initRubberBand() {
 void PlaybackEngine::closeRubberBand() {
     if(musicFeed) {
         exitMusicFeedThread.store(true);
+        LOGI("PlaybackEngine: musicFeed->join() ...");
         musicFeed->join();
+        LOGI("PlaybackEngine: musicFeed->join() done.");
         musicFeed = nullptr;
     }
     closeMusicFile();
@@ -155,10 +171,14 @@ void PlaybackEngine::closeMusicFile() {
     haveMusicFile.store(false);
     musicFile = nullptr;
     if(android_fd) {
-        close(android_fd);
+        //close(android_fd);
+        LOGI("onTrackClosed(fd=%d) ...", android_fd);
+        if(listener) listener->onTrackClosed(android_fd);
+        LOGI("onTrackClosed(fd=%d) done.", android_fd);
         android_fd = 0;
     }
     isSetMusic.store(false);
+    if(mPlayer)
         mPlayer->setMusic(nullptr);
 }
 
@@ -185,6 +205,10 @@ void PlaybackEngine::mapChannels(int *channel_map, int num_ch_in, int num_ch_out
 }
 
 void PlaybackEngine::musicFeedThread() {
+    /*
+    std::mt19937 mt(42);
+    std::uniform_real_distribution<double> dist(0.98, 1.0);
+*/
     LOGI("starting musicFeedThread()");
 
     // strecher num channels: same as output num channels
@@ -211,7 +235,8 @@ void PlaybackEngine::musicFeedThread() {
     int* channel_map = (int*) malloc(num_ch_out * sizeof(int));
 
     // initial guess, as long as we do not have a music file (otherwise we should divide by mp3->rate)
-    size_t loop_delay_us = 1000000 * buf_size_samples / playbackRate.load();
+    size_t pr = std::max(1, playbackRate.load());
+    size_t loop_delay_us = std::min((size_t) 50000, (size_t) 1000000 * buf_size_samples / pr);
 
     int idebug = 0;
 
@@ -284,31 +309,87 @@ void PlaybackEngine::musicFeedThread() {
             num_samples = num_buf_samples;
         }
 
-        if (!haveMusicFile.load()) {
+        if (!haveMusicFile.load() || isPaused.load()) {
-            loop_delay_us = 1000000 * num_samples / playbackRate.load();
+            pr = std::max(1, playbackRate.load());
+            loop_delay_us = std::min((size_t) 50000, (size_t) 1000000 * num_samples / pr);
             if(idebug++ < 10) {
                 LOGI("feed %d silence samples", num_samples);
                 // 1024, 512, 512
                 // 7 x 512
             }
-            memset(buf, 0, num_samples*num_ch_out*sizeof(float));
+            //memset(buf, 0, num_samples*num_ch_out*sizeof(float)); // wrong, because buffer is not contiguous for channels
+            for(int i = 0; i < num_ch_out; i++) {
+                memset(buf_ptr[i], 0, num_samples * sizeof(float));
+            }
             stretcher.process(buf_ptr, num_samples, false);
             continue;
         }
 
         if(idebug++ < 10) {
-            loop_delay_us = 1000000 * num_samples / musicFile->rate;
+            pr = std::max(1L, musicFile->rate);
+            loop_delay_us = std::min((size_t) 50000, (size_t) 1000000 * num_samples / pr);
             LOGI("feed %d music samples", num_samples);
             // feed 1024 music samples
             // => stretcher is asking for 1024 = getSamplesRequired()
             // ca. 21 ms
+
+/*
+            int64_t a = mpg123_seek(musicFile->handle, 0, SEEK_CUR);
+            int64_t b = mpg123_tell(musicFile->handle);
+            LOGI("  checking mpg123_seek(): len=%lld seek=%lld tell=%lld",
+                 (long long)musicFile->num_samples, (long long)a, (long long)b);
+*/
+        }
+
+        markerPos.store((musicFile->num_samples - musicFile->remaining_samples) / musicFile->samples_per_frame * musicFile->secs_per_frame);
+        duration.store(musicFile->duration);
+
+        if(requestSeek.load()) {
+            double pos = seekPos.load();
+            // compute seek target in samples, clip to [0..num_samples]
+            auto seek_samples = static_cast<long>(pos * musicFile->samples_per_frame / musicFile->secs_per_frame);
+            LOGI("seek to %.3lf sec = %lld samples", pos, (long long) seek_samples);
+            seek_samples = std::max(seek_samples, 0L);
+            seek_samples = std::min(seek_samples, musicFile->num_samples);
+            // perform the seek
+            off_t seekResult = mp3file_seek(musicFile.get(), seek_samples);
+            idebug = 0;
+            /*if(seekResult > seek_samples) {
+                LOGE("error seeking in mp3 file: mpg123_seek(handle, %ld, SEEK_SET)=%lld", seek_samples, (long long) seekResult);
+                seekPos.store(pos * dist(mt)); // randomize seek target a bit
+            } else {*/
+                // update structure
+                // compute seek delta, to update byte 'offset'
+                //auto seek_samples_delta = seekResult - (musicFile->num_samples - musicFile->remaining_samples);
+                //auto seek_bytes_delta = seek_samples_delta * num_ch_in * sizeof(int16_t);
+                musicFile->remaining_samples = static_cast<int>(musicFile->num_samples - seekResult);
+                //musicFile->offset += seek_bytes_delta;
+                musicFile->offset = lseek(musicFile->android_fd, 0, SEEK_CUR);
+                // it is not possible to obtain bytes offset reliably??
+                //musicFile->num_bytes = -1; // pretend we can read until the end
+                requestSeek.store(false);
+            /*}*/
         }
 
         size_t done = 0; // bytes!
         size_t read_size_bytes = std::min(num_samples * num_ch_in * sizeof(int16_t), cbuf_size_bytes);
+        /*if (musicFile->offset != 0 && musicFile->num_bytes != -1) { read_size_bytes = std::min(read_size_bytes_calc, (size_t) musicFile->num_bytes - musicFile->offset); }*/
+        //if (idebug < 10) LOGI("mft rsbc=%lld rsb=%lld nb=%lld o=%lld", (long long) read_size_bytes_calc, (long long) read_size_bytes, (long long) musicFile->num_bytes, (long long) musicFile->offset);
         int err = mpg123_read(musicFile->handle, cbuf, read_size_bytes, &done);
-        musicFile->remaining_samples -= done / sizeof(int16_t);
+        //if (idebug < 10) LOGI("mft done=%lld err=%lld small=%d", (long long) done, (long long) err, read_size_bytes < read_size_bytes_calc);
-        musicFile->offset = 0; // unused here
+        /*if (musicFile->offset != 0 && musicFile->num_bytes != -1 && err == MPG123_OK && read_size_bytes < read_size_bytes_calc) {
+            LOGW("out of bytes, finished playing");
+            err = MPG123_DONE;
+        }*/
+        musicFile->remaining_samples -= done / num_ch_in / sizeof(int16_t);
+        //musicFile->offset += done;
+        musicFile->offset = lseek(musicFile->android_fd, 0, SEEK_CUR);
+        if(musicFile->offset >= musicFile->num_bytes) {
+            // TODO: this is inaccurate. either we do not decode everything, or we overrun past the end
+            // TODO: maybe decode as a "stream" instead.
+            LOGW("out of bytes, finished playing");
+            err = MPG123_DONE;
+        }
         if (err != MPG123_OK && err != MPG123_DONE) {
             // error!
             LOGE("error reading mp3 file: mpg123_read() err=%d done=%d", err, done);
@@ -322,12 +403,11 @@ void PlaybackEngine::musicFeedThread() {
         }
         if(err == MPG123_DONE) {
             // next iteration will play silence
+            // we keep Stretcher and Oboe alive
             LOGI("finished reading mp3 file (MPG123_DONE)");
+            idebug = 0;
             closeMusicFile();
-            stretcher.setTimeRatio(1.0); // buffer size for playing silence is computed from 'playbackRate', so reset timeRatio
+            if(listener) listener->onTrackFinished();
-            stretcher.setPitchScale(1.0);
-            stretcher.process(buf_ptr, 0, true); // set end of playback
-            mPlayer->stopAudio();
             continue;
         }
 
@@ -356,23 +436,67 @@ void PlaybackEngine::musicFeedThread() {
     LOGI("musicFeedThread() exited.");
 }
 
+void PlaybackEngine::pause() {
+    LOGI("PlaybackEngine::pause() set isPaused.");
+    // next iteration will play silence
+    isPaused.store(true);
+}
+
+void PlaybackEngine::resume() {
+    // next iteration will continue to decode
+    isPaused.store(false);
+}
+
+double PlaybackEngine::getCurrentPosition() {
+    return markerPos.load();
+}
+double PlaybackEngine::getDuration() {
+    return duration.load();
+}
+
+void PlaybackEngine::seekTo(double markerPosSec) {
+    seekPos.store(markerPosSec);
+    requestSeek.store(true);
+}
+
+void PlaybackEngine::stop() {
+    LOGI("PlaybackEngine::stop()");
+    closeRubberBand();
+    if (mPlayer) {
+        LOGI("PlaybackEngine: mPlayer->stopAudio() ...");
+        mPlayer->stopAudio();
+        LOGI("PlaybackEngine: mPlayer->stopAudio() done.");
+        delete mPlayer;
+        mPlayer = nullptr;
+    }
+    markerPos.store(0.0);
+    duration.store(0.0);
+}
+
 PlaybackEngine::~PlaybackEngine() {
     LOGI("~PlaybackEngine()");
     closeRubberBand();
+    if (mPlayer) {
         mPlayer->stopAudio();
         delete mPlayer;
         mPlayer = nullptr;
     }
+}
 
 void PlaybackEngine::playBeat() {
     if(mPlayer) mPlayer->setStartBeat();
 }
 
-void PlaybackEngine::playMusic(int fd) {
+void PlaybackEngine::playMusic(int fd, long long offset, long long length) {
     if(!mPlayer) return;
     LOGI("PlaybackEngine::playMusic(fd=%d)", fd);
+    if(musicFile) {
+        // when changing tracks in the UI, close the previous file
+        closeMusicFile();
+    }
     android_fd = fd;
-    musicFile.reset(mp3file_open_fd(android_fd, 0));
+    if(fd != 0)
+        musicFile.reset(mp3file_open_fd(android_fd, offset, length, 0));
     if(musicFile) {
         timeRatio.store(((double) playbackRate.load()) / ((double) musicFile->rate));
         haveTimeRatio.store(true);
@@ -382,6 +506,7 @@ void PlaybackEngine::playMusic(int fd) {
 
     bool is_finished = (stretcher.available() == -1);
     if(is_finished) {
+        LOGE("stretcher.available() == -1, this should not happen anymore with current wiring");
         // so that we may play again after "final chunk"
         closeRubberBand();
         initRubberBand();

app/src/main/cpp/PlaybackEngine.h

@@ -5,11 +5,12 @@
 #ifndef LOCKSTEP_PLAYBACKENGINE_H
 #define LOCKSTEP_PLAYBACKENGINE_H
 
-#include "StepListener.h"
+#include "step_detector.h"
 #include "MixingPlayer.h"
 #include "RubberBandStretcher.h"
 #include "mp3file.h"
 #include "AudioCallback.h"
+#include "PasadaPlaybackListener.h"
 #include <string>
 #include <thread>
 #include <memory>
@@ -36,12 +37,28 @@ private:
 
 class PlaybackEngine : public StepListener {
 public:
-    PlaybackEngine(std::string filesDir, int resid);
+    /**
+     * @param filesDir resources directory, used only to fetch the beat sound template
+     * @param resid beat sound template file
+     * @param listener contract: onTrackFinished() only fires during regular playback finished - not when stopped, when exiting, etc.
+     */
+    PlaybackEngine(std::string filesDir, int resid, PasadaPlaybackListener *listener = nullptr);
     virtual ~PlaybackEngine();
     /** Play a beat sound. */
     virtual void playBeat();
-    void playMusic(int fd);
+    /** pass -1 to length for read until eof. */
+    void playMusic(int fd, long long offset = 0, long long length = -1);
+    void pause();
+    void resume();
+    void stop();
+    /** read marker in the music file in sec */
+    double getCurrentPosition();
+    /** duration of the music file in sec */
+    double getDuration();
+    /** seek to a marker in the music file in sec */
+    void seekTo(double markerPos);
 private:
+    PasadaPlaybackListener *listener;
     RubberBand::RubberBandStretcher stretcher;
     MixingPlayer *mPlayer;
     std::string mFilesDir;
@@ -49,8 +66,17 @@ private:
     std::atomic<bool> haveMusicFile;
     std::unique_ptr<std::thread> musicFeed;
     std::atomic<bool> exitMusicFeedThread;
-    /** where musicFeedThread() keeps track of the fact that we have music set -- will start the audio cb */
+    /** where musicFeedThread() keeps track of the fact that we have music set -- will start the audio callback */
     std::atomic<bool> isSetMusic;
+    std::atomic<bool> isPaused;
+    /** read marker in the music file in sec */
+    std::atomic<double> markerPos;
+    /** duration of the music file in sec */
+    std::atomic<double> duration;
+    /** seek position request in the music file in sec, valid during seek requests */
+    std::atomic<double> seekPos;
+    /** whether a seek request is active */
+    std::atomic<bool> requestSeek;
     int android_fd;
     std::atomic<bool> haveTimeRatio;
     std::atomic<double> timeRatio;

app/src/main/cpp/StepDetector.cpp

@@ -1,49 +0,0 @@
-//
-// Created by david on 03.03.2026.
-//
-
-#include "StepDetector.h"
-
-// TODO: repeated code in libpasada. We could use the StepDetector class from there in jni_stepdetector.cpp
-
-// TODO: we are hardcoding filter coefficients for 60 Hz
-// TODO: this is tolerable for 50 Hz
-
-// TODO: check if we can do with floats instead of doubles
-// (check how much the [already bad] accuracy of filtering suffers)
-
-// TODO: in Java, check if delta timestamps effectively match FPS
-// TODO: FPS constant should be passed as argument to C++ (but we keep an FPS define to validate the coefficients)
-
-// Butterworth filter: order=5, fc=0.5, fs=60, btype='highpass'
-static std::vector hpf_taps_b {0.91875845, -4.59379227,  9.18758454, -9.18758454,  4.59379227, -0.91875845};
-static std::vector hpf_taps_a {1.        , -4.83056552,  9.33652742, -9.02545247,  4.36360803, -0.8441171};
-static size_t upslope_width = 4;
-const size_t len_refr = (size_t) (FPS / (MAX_BPM / 60));
-
-StepDetector::StepDetector(StepListener *listener) :
-    listener(listener),
-    f_highpass(hpf_taps_b, hpf_taps_a),
-    f_neg(1, 0, 0, std::vector {-1.0}),
-    f_ssf(upslope_width),
-    f_ssd(len_refr),
-    f_sqi(upslope_width)
-{}
-
-#if (FPS != 60)
-#error "FPS must currently be 60, as highpass taps are pre-computed for that value"
-#endif
-
-void StepDetector::filter(std::vector<float> values) {
-    // TODO: later on, we should use a vector projection towards gravity
-    auto s0 = (double) values[1]; // take y-axis value for now
-    auto s1 = f_highpass.filter(s0);
-    auto s2 = f_neg.filter(s1);
-    auto s3 = f_ssf.filter(s2);
-    auto s4 = f_ssd.filter(s3);
-    auto q5 = f_sqi.filter(s2, s3, s4);
-    // is step, step quality is OK, and we have a listener?
-    if(s4 > 0.0 && q5 > 0.0 && listener != nullptr) {
-        listener->playBeat();
-    }
-}

app/src/main/cpp/StepDetector.h

@@ -1,27 +0,0 @@
-//
-// Created by david on 03.03.2026.
-//
-
-#ifndef LOCKSTEP_STEPDETECTOR_H
-#define LOCKSTEP_STEPDETECTOR_H
-
-#include "StepListener.h"
-#include "iir_filter.h"
-#include "ssf_filter.h"
-#include <vector>
-
-class StepDetector {
-protected:
-    StepListener *listener;
-    IirFilter f_highpass;
-    Filt f_neg;
-    SsfFilter f_ssf;
-    SsfStepDetector f_ssd;
-    RunningQualityFilter f_sqi;
-
-public:
-    StepDetector(StepListener *listener);
-    void filter(std::vector<float> values);
-};
-
-#endif //LOCKSTEP_STEPDETECTOR_H

app/src/main/cpp/StepListener.h

@@ -1,14 +0,0 @@
-//
-// Created by david on 03.03.2026.
-//
-
-#ifndef LOCKSTEP_STEPLISTENER_H
-#define LOCKSTEP_STEPLISTENER_H
-
-class StepListener {
-public:
-    virtual ~StepListener() {}
-    virtual void playBeat() = 0;
-};
-
-#endif //LOCKSTEP_STEPLISTENER_H

app/src/main/cpp/jni_libpasada.cpp

@@ -0,0 +1,399 @@
+//
+// Created by david on 24.05.2026.
+//
+#include <jni.h>
+#include <string>
+#include "LibPasada.h"
+#include "logging.h"
+
+static JavaVM* g_vm = nullptr;
+static jobject g_listener = nullptr;      // global ref, or nullptr
+static jmethodID g_onTrackFinished = nullptr;
+static jmethodID g_onTrackClosed = nullptr;
+static jmethodID g_onError = nullptr;
+
+static LibPasada* g_libpasada = nullptr;
+
+void clearListener(JNIEnv* env);
+
+extern "C"
+JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void*) {
+    g_vm = vm;
+
+    JNIEnv *	env	= NULL;
+    //jclass	clazz;
+    //LOGV("JNI_Onload vm:%p reserved:%p", vm, reserved);
+
+    if (JNI_OK != vm->GetEnv((void**) &env, JNI_VERSION_1_6)) {
+        return JNI_VERSION_1_6;
+    }
+
+    logback_init(vm, env);
+    LOGD("liblockstep-native jni_libpasada.cpp JNI_OnLoad()");
+
+    return JNI_VERSION_1_6;
+}
+
+extern "C" void
+JNI_OnUnload(JavaVM * vm, void * reserved)
+{
+    JNIEnv *    env     = NULL;
+    //LOGV("JNI_OnUnload vm:%p reserved:%p", vm, reserved);
+
+    if (JNI_OK != vm->GetEnv((void**) &env, JNI_VERSION_1_6)) {
+        return;
+    }
+    logback_uninit(env);
+}
+
+/** Called each time a run/playlist session starts (Oboe silent, buffers ready). */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_init(
+        JNIEnv *env,
+        jclass /*unused*/) {
+    LOGD("liblockstep-native jni_libpasada.cpp LibPasada_init()");
+    if (g_libpasada) {
+        clearListener(env);
+        delete g_libpasada;
+    }
+    try {
+        g_libpasada = new LibPasada();
+        g_libpasada->init();
+        return;
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+    }
+    delete g_libpasada;
+    g_libpasada = nullptr;
+}
+
+/** Submit one accelerometer sample (m/s^2); may be called from a sensor thread. */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_feedAccel(JNIEnv *env, jclass clazz, jfloat x, jfloat y,
+                                                   jfloat z, jlong timestamp_nanos) {
+    if (g_libpasada == nullptr)
+        return;
+
+    try {
+        g_libpasada->feedAccel(x, y, z, timestamp_nanos);
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return;
+    }
+}
+
+/**
+ * Open MP3 from an already-open file descriptor and begin adaptive playback.
+ *
+ * @param fd open read FD (Java retains ownership; do not close until track changes)
+ * @param offset start offset within the FD (0 for whole file)
+ * @param length byte length from offset ({@code -1} if unknown / to EOF)
+ */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_play(JNIEnv *env, jclass clazz, jint fd, jlong offset,
+                                              jlong length) {
+    LOGD("liblockstep-native jni_libpasada.cpp LibPasada_play()");
+    if (g_libpasada == nullptr)
+        return;
+
+    try {
+        g_libpasada->play(fd, offset, length);
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return;
+    }
+}
+
+/** PLAYING → PAUSED (silent output, graph kept alive). */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_pause(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr)
+        return;
+    try {
+        g_libpasada->pause();
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return;
+    }
+}
+
+/** PAUSED → PLAYING (same track, same decode position, same FD). */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_resume(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr)
+        return;
+    try {
+        g_libpasada->resume();
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return;
+    }
+}
+
+/** Tear down Oboe for this run segment → STOPPED. */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_stop(JNIEnv *env, jclass clazz) {
+    clearListener(env);
+    if (g_libpasada == nullptr)
+        return;
+    try {
+        g_libpasada->stop();
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+    }
+    delete g_libpasada;
+    g_libpasada = nullptr;
+}
+
+/**
+ * Milliseconds from the start of the current track — same timebase as ExoPlayer
+ * {@code getCurrentPosition()}. Safe to poll from background threads.
+ */
+extern "C"
+JNIEXPORT jlong JNICALL
+Java_at_lockstep_player_pasada_LibPasada_getCurrentPositionMs(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr)
+        return 0;
+    try {
+        return g_libpasada->getCurrentPositionMs();
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return 0;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return 0;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return 0;
+    }
+}
+
+/** Track duration in ms, or {@code 0} if not yet known. */
+extern "C"
+JNIEXPORT jlong JNICALL
+Java_at_lockstep_player_pasada_LibPasada_getDurationMs(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr)
+        return 0;
+    try {
+        return g_libpasada->getDurationMs();
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return 0;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return 0;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return 0;
+    }
+}
+
+/** Whether adapted audio is actively being output (not paused, not finished). */
+extern "C"
+JNIEXPORT jboolean JNICALL
+Java_at_lockstep_player_pasada_LibPasada_isPlaying(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr)
+        return false;
+    try {
+        return g_libpasada->isPlaying();
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return false;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return false;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return false;
+    }
+}
+/** Current native state; see {@link PasadaState}. */
+extern "C"
+JNIEXPORT jint JNICALL
+Java_at_lockstep_player_pasada_LibPasada_getState(JNIEnv *env, jclass clazz) {
+    if(g_libpasada == nullptr)
+        return STOPPED;
+    // explicitly no C++/JNI exception translation - keep getState() simple!
+    // there is no reasonable state for us to return here otherwise.
+    return g_libpasada->getState();
+}
+
+/** Seek within the current track. */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_seekTo(JNIEnv *env, jclass clazz, jlong position_ms) {
+    if (g_libpasada == nullptr)
+        return;
+    try {
+        g_libpasada->seekTo((long) position_ms);
+    } catch (const std::bad_alloc&) {
+        jclass cls = env->FindClass("java/lang/OutOfMemoryError");
+        if (cls) env->ThrowNew(cls, "native allocation failed");
+        return;
+    } catch (const std::exception& e) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, e.what());
+        return;
+    } catch (...) {
+        jclass cls = env->FindClass("java/lang/RuntimeException");
+        if (cls) env->ThrowNew(cls, "unknown native exception");
+        return;
+    }
+}
+/** Runtime metrics / last error string for logging and debug UI. */
+extern "C"
+JNIEXPORT jstring JNICALL
+Java_at_lockstep_player_pasada_LibPasada_getDiagnostics(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr) {
+        return env->NewStringUTF("");
+    }
+    std::string message = g_libpasada->getDiagnostics();
+    return env->NewStringUTF(message.c_str());
+}
+
+
+/** Register listener for async events raised from the audio/native thread. */
+extern "C"
+JNIEXPORT void JNICALL
+Java_at_lockstep_player_pasada_LibPasada_setPlaybackListener(JNIEnv *env, jclass clazz,
+                                                             jobject listener) {
+    // 1. Drop previous listener
+    if (g_listener != nullptr) {
+        clearListener(env);
+    }
+    if (listener == nullptr) {
+        clearListener(env);
+        return;
+    }
+    // 2. Keep listener alive past this call
+    g_listener = env->NewGlobalRef(listener);
+    // 3. Resolve methods once (valid until class unload)
+    jclass cls = env->GetObjectClass(g_listener);
+    g_onTrackFinished = env->GetMethodID(cls, "onTrackFinished", "()V");
+    g_onTrackClosed = env->GetMethodID(cls, "onTrackClosed", "(I)V");
+    g_onError = env->GetMethodID(
+            cls, "onError", "(ILjava/lang/String;)V");
+    env->DeleteLocalRef(cls);
+    if (g_onTrackFinished == nullptr || g_onError == nullptr) {
+        env->DeleteGlobalRef(g_listener);
+        g_listener = nullptr;
+        // ExceptionPending if method missing
+    }
+}
+
+//
+// JNI helpers for calling into Java code (PasadaPlaybackListener)
+//
+void emitTrackFinished() {
+    if (!g_vm || !g_listener || !g_onTrackFinished) return;
+    JNIEnv* env = nullptr;
+    if (g_vm->AttachCurrentThread(&env, nullptr) != JNI_OK) return;
+    env->CallVoidMethod(g_listener, g_onTrackFinished);
+    if (env->ExceptionCheck()) env->ExceptionClear();
+    // DetachCurrentThread only if this thread won't call JNI again
+}
+void emitError(int errorCode, const char* message) {
+    if (!g_vm || !g_listener || !g_onError) return;
+    JNIEnv* env = nullptr;
+    if (g_vm->AttachCurrentThread(&env, nullptr) != JNI_OK) return;
+    jstring jMessage = env->NewStringUTF(message != nullptr ? message : "");
+    env->CallVoidMethod(g_listener, g_onError, static_cast<jint>(errorCode), jMessage);
+    env->DeleteLocalRef(jMessage);
+    if (env->ExceptionCheck()) {
+        env->ExceptionClear();
+    }
+}
+void emitTrackClosed(int fd) {
+    if (!g_vm || !g_listener || !g_onTrackClosed) return;
+    JNIEnv* env = nullptr;
+    if (g_vm->AttachCurrentThread(&env, nullptr) != JNI_OK) return;
+    env->CallVoidMethod(g_listener, g_onTrackClosed, static_cast<jint>(fd));
+    if (env->ExceptionCheck()) env->ExceptionClear();
+    // DetachCurrentThread only if this thread won't call JNI again
+}
+void emitError(int errorCode, const std::string& message) {
+    emitError(errorCode, message.c_str());
+}
+void clearListener(JNIEnv* env) {
+    if (g_listener) {
+        env->DeleteGlobalRef(g_listener);
+        g_listener = nullptr;
+    }
+    g_onTrackFinished = nullptr;
+    g_onError = nullptr;
+}
+
+extern "C"
+JNIEXPORT jstring JNICALL
+Java_at_lockstep_player_pasada_LibPasada_getVersion(JNIEnv *env, jclass clazz) {
+    if (g_libpasada == nullptr) {
+        return env->NewStringUTF("");
+    }
+    std::string version = g_libpasada->getVersion();
+    return env->NewStringUTF(version.c_str());
+}

app/src/main/cpp/jni_lockstep.cpp

@@ -22,6 +22,27 @@ Java_at_lockstep_pb_PlaybackEngine_native_1createEngine(
     return reinterpret_cast<jlong>(engine);
 }
 
+JNIEXPORT void JNICALL
+Java_at_lockstep_pb_PlaybackEngine_native_1stop(
+        JNIEnv *env,
+        jclass,
+        jlong engineHandle) {
+
+    auto *engine = reinterpret_cast<PlaybackEngine *>(engineHandle);
+    engine->stop();
+}
+
+JNIEXPORT void JNICALL
+Java_at_lockstep_pb_PlaybackEngine_native_1pause(
+        JNIEnv *env,
+        jclass,
+        jlong engineHandle) {
+
+    auto *engine = reinterpret_cast<PlaybackEngine *>(engineHandle);
+    // TODO: this is broken. there is a square wave noise played, instead of silence
+    engine->pause(); // do not tear down the Oboe pipeline, such that we can play again later (without having to re-construct PlaybackEngine)
+}
+
 JNIEXPORT void JNICALL
 Java_at_lockstep_pb_PlaybackEngine_native_1deleteEngine(
         JNIEnv *env,

app/src/main/cpp/jni_logging.cpp

@@ -66,14 +66,14 @@ JNI_OnLoad(JavaVM * vm, void * reserved)
     jclass	clazz;
     //LOGV("JNI_Onload vm:%p reserved:%p", vm, reserved);
 
-    if (JNI_OK != vm->GetEnv((void**) &env, JNI_VERSION_1_4)) {
+    if (JNI_OK != vm->GetEnv((void**) &env, JNI_VERSION_1_6)) {
-        //LOGE("JNI_OnUnload GetEnv JNI_VERSION_1_4 failed");
+        //LOGE("JNI_OnUnload GetEnv JNI_VERSION_1_6 failed");
-        return JNI_VERSION_1_4;
+        return JNI_VERSION_1_6;
     }
 
     logback_init(vm, env);
 
-    return JNI_VERSION_1_4;
+    return JNI_VERSION_1_6;
 }
 
 extern "C" void
@@ -82,8 +82,8 @@ JNI_OnUnload(JavaVM * vm, void * reserved)
     JNIEnv *    env     = NULL;
     //LOGV("JNI_OnUnload vm:%p reserved:%p", vm, reserved);
 
-    if (JNI_OK != vm->GetEnv((void**) &env, JNI_VERSION_1_4)) {
+    if (JNI_OK != vm->GetEnv((void**) &env, JNI_VERSION_1_6)) {
-        //LOGE("JNI_OnUnload GetEnv JNI_VERSION_1_4 failed");
+        //LOGE("JNI_OnUnload GetEnv JNI_VERSION_1_6 failed");
         return;
     }
 

app/src/main/cpp/jni_stepdetector.cpp

@@ -3,7 +3,7 @@
 //
 
 #include <jni.h>
-#include "StepDetector.h"
+#include "step_detector.h"
 #include <new>
 #include <vector>
 
@@ -26,7 +26,8 @@ Java_at_lockstep_filter_StepDetector_native_1create(
         jclass /*unused*/, jlong engineHandle) {
     auto *listener = reinterpret_cast<StepListener *>(engineHandle);
     // We use std::nothrow so `new` returns a nullptr if the engine creation fails
-    auto *detector = new(std::nothrow) StepDetector(listener);
+    // the hardcoded 'fps' is only an initial value, which is re-computed in StepDetector
+    auto *detector = new(std::nothrow) StepDetector(60.0 /* FPS */, listener);
     return reinterpret_cast<jlong>(detector);
 }
 
@@ -46,7 +47,7 @@ Java_at_lockstep_filter_StepDetector_native_1filter(
     jsize length = env->GetArrayLength(values);
     std::vector<float> vecValues(nativeValues, nativeValues + length);
     auto *detector = reinterpret_cast<StepDetector *>(handle);
-    detector->filter(vecValues);
+    detector->filter((double) timestamp, vecValues);
 }
 
 }

app/src/main/cpp/logging.cpp

@@ -91,7 +91,7 @@ __logback_print(const int level, const char * format, ...)
     JNIEnv *	env = NULL;
     jint	err = JNI_OK;
     if (! __logback_jvm) return;
-    if (JNI_EDETACHED == (err = __logback_jvm->GetEnv((void**) &env, JNI_VERSION_1_4))) {
+    if (JNI_EDETACHED == (err = __logback_jvm->GetEnv((void**) &env, JNI_VERSION_1_6))) {
         if (__logback_jvm->AttachCurrentThread(&env, NULL) < 0) {
             __android_log_write(LOG_LEVEL_WARN, LOG_TAG, "__logback_write failed to get env neither attach current thread");
         }
@@ -115,7 +115,7 @@ __logback_vprint(const int level, const char * format, va_list ap)
     JNIEnv *	env = NULL;
     jint	err = JNI_OK;
     if (! __logback_jvm) return;
-    if (JNI_EDETACHED == (err = __logback_jvm->GetEnv((void**) &env, JNI_VERSION_1_4))) {
+    if (JNI_EDETACHED == (err = __logback_jvm->GetEnv((void**) &env, JNI_VERSION_1_6))) {
         if (__logback_jvm->AttachCurrentThread(&env, NULL) < 0) {
             __android_log_write(LOG_LEVEL_WARN, LOG_TAG, "__logback_write failed to get env neither attach current thread");
         }
@@ -136,7 +136,7 @@ __logback_write(const int level, const char * msg)
     JNIEnv *	env = NULL;
     jint	err = JNI_OK;
     if (! __logback_jvm) return;
-    if (JNI_EDETACHED == (err = __logback_jvm->GetEnv((void**) &env, JNI_VERSION_1_4))) {
+    if (JNI_EDETACHED == (err = __logback_jvm->GetEnv((void**) &env, JNI_VERSION_1_6))) {
         if (__logback_jvm->AttachCurrentThread(&env, NULL) < 0) {
             __android_log_write(LOG_LEVEL_WARN, LOG_TAG, "__logback_write failed to get env neither attach current thread");
         }

app/src/main/cpp/mp3file.cpp

@@ -15,6 +15,7 @@ MP3File* mp3file_init(mpg123_handle *handle) {
     if(mp3file == NULL) return NULL;
     memset(mp3file, 0, sizeof(MP3File));
     mp3file->handle = handle;
+    mp3file->num_bytes = -1;
     return mp3file;
 }
 
@@ -96,7 +97,7 @@ on_error:
 #undef handleError
 }
 
-MP3File* mp3file_open_fd(int fd, int forceEncoding) {
+MP3File* mp3file_open_fd(int fd, long long offset, long long length, int forceEncoding) {
     const char *errorText = "";
 #define handleError(text) \
     do { \
@@ -104,6 +105,8 @@ MP3File* mp3file_open_fd(int fd, int forceEncoding) {
         goto on_error; \
     } while(0)
 
+    int64_t len, a, b;
+
     int err = MPG123_OK;
     mpg123_handle *mh = mpg123_new(NULL, &err);
     if(err != MPG123_OK || mh == NULL) {
@@ -113,9 +116,14 @@ MP3File* mp3file_open_fd(int fd, int forceEncoding) {
 
     MP3File* mp3 = mp3file_init(mh);
     if(mp3 == NULL) handleError("malloc() failed");
+    lseek(fd, static_cast<off_t>(offset), SEEK_SET);
+    mp3->num_bytes = length;
     err = mpg123_open_fd(mh, fd);
     if(err != MPG123_OK) handleError("mpg123_open()");
 
+    // hopefully the magic bullet for seeking in the file (otherwise, mpg123_seek() returns totally random offsets)
+    mpg123_scan(mh);
+
     int encoding;
     err = mpg123_getformat(mh, &mp3->rate, &mp3->channels, &encoding);
     if(err != MPG123_OK) handleError("mpg123_getformat()");
@@ -149,10 +157,21 @@ MP3File* mp3file_open_fd(int fd, int forceEncoding) {
     else
         mp3->duration = mp3->num_samples / mp3->samples_per_frame * mp3->secs_per_frame;
 
-    mp3->offset = 0;
+    mp3->offset = lseek(fd, 0, SEEK_CUR);
     mp3->remaining_samples = (int) mp3->num_samples;
     LOGI("channels: %d rate: %ld num_samples: %ld", mp3->channels, mp3->rate, mp3->num_samples);
     mp3->android_fd = fd;
+
+    // begin debug: seek returns wildly random offsets
+
+    len = mpg123_length(mh);
+    a = mpg123_seek(mh, 0, SEEK_SET);
+    b = mpg123_tell(mh);
+
+    LOGI("using mpg123_scan: len=%lld seek=%lld tell=%lld",
+           (long long)len, (long long)a, (long long)b);
+    // end debug
+
     return mp3;
 
 on_error:
@@ -162,3 +181,22 @@ on_error:
 
 #undef handleError
 }
+
+off_t mp3file_seek(MP3File *mp3file, off_t samples) {
+    LOGD("mp3file_seek(): seek to %ld samples", samples);
+    off_t seekResult = mpg123_seek(mp3file->handle, samples, SEEK_SET);
+    LOGD("  mp3file_seek(): %ld samples remaining", samples - seekResult);
+    if(seekResult > samples + 4800) {
+        LOGE("mp3file_seek(): error: landed beyond the seek position!");
+        return seekResult;
+    }
+    // try to refine the seek position
+    int max_iter = 6;
+    int cur_iter = 0;
+    for(; cur_iter < max_iter && seekResult < samples - 4800; cur_iter++) {
+        seekResult = mpg123_seek(mp3file->handle, samples - seekResult, SEEK_CUR);
+        LOGD("  mp3file_seek(): %ld samples remaining", samples - seekResult);
+    }
+    LOGD("mp3file_seek(): finished");
+    return seekResult;
+}

app/src/main/cpp/mp3file.h

@@ -22,13 +22,17 @@ struct MP3File
     size_t buffer_size;
     unsigned char* buffer;
     /** total samples (stereo of 10 frames remaining will have 20 'remaining_samples' here) */
-    int remaining_samples;
+    long remaining_samples;
+    /** zero if opened using a filename, otherwise bytes offset currently read into 'android_fd' */
     size_t offset;
+    /** number of bytes to read at most from 'android_fd' */
+    long long num_bytes;
 };
 
 MP3File* mp3file_init(mpg123_handle *handle);
 void mp3file_delete(MP3File *mp3file);
 MP3File* mp3file_open(const char *filename, int forceEncoding = 0);
-MP3File* mp3file_open_fd(int fd, int forceEncoding = 0);
+MP3File* mp3file_open_fd(int fd, long long offset = 0, long long length = -1, int forceEncoding = 0);
+off_t mp3file_seek(MP3File *mp3file, off_t samples);
 
 #endif //SAMPLES_MP3FILE_H

app/src/main/java/at/lockstep/app/LstForegroundService.java

@@ -6,6 +6,8 @@ import android.app.NotificationManager;
 import android.app.Service;
 import android.content.Context;
 import android.content.Intent;
+import android.content.pm.PackageInfo;
+import android.content.pm.PackageManager;
 import android.hardware.Sensor;
 import android.hardware.SensorEvent;
 import android.hardware.SensorEventListener;
@@ -38,6 +40,7 @@ public class LstForegroundService extends Service implements SensorEventListener
 
     public static final String ACTION_START = "at.lockstep.action.START";
     public static final String ACTION_STOP = "at.lockstep.action.STOP";
+    public static final String ACTION_PAUSE = "at.lockstep.action.PAUSE";
 
     private SensorManager sensorManager;
     private Sensor accelerometer;
@@ -58,6 +61,11 @@ public class LstForegroundService extends Service implements SensorEventListener
         intent.setAction(ACTION_STOP);
         return intent;
     }
+    public static Intent pauseIntent(Context context) {
+        Intent intent = new Intent(context, LstForegroundService.class);
+        intent.setAction(ACTION_PAUSE);
+        return intent;
+    }
 
     @Override
     public void onCreate() {
@@ -96,7 +104,9 @@ public class LstForegroundService extends Service implements SensorEventListener
                 String contentUri = intent.getStringExtra("content_uri");
                 try {
                     if(contentUri != null) {
-                        PlaybackEngine.playMusic(uriToFd(contentUri));
+                        PlaybackEngine.playMusic(uriToFd(contentUri), this, R.raw.track_beat);
+                    } else {
+                        PlaybackEngine.playMusic(0, this, R.raw.track_beat);
                     }
                 } catch (IOException e) {
                     // TODO proper error handling
@@ -107,6 +117,10 @@ public class LstForegroundService extends Service implements SensorEventListener
             } else if (ACTION_STOP.equals(action)) {
                 Log.i("LstForegroundService", "ACTION_STOP");
                 stopCollectionAndSelf();
+                PlaybackEngine.stop();
+            } else if (ACTION_PAUSE.equals(action)) {
+                Log.i("LstForegroundService", "ACTION_PAUSE");
+                PlaybackEngine.pause();
             }
         }
         return START_STICKY;
@@ -150,7 +164,8 @@ public class LstForegroundService extends Service implements SensorEventListener
             sensorManager.registerListener(
                     this,
                     accelerometer,
-                    SensorManager.SENSOR_DELAY_GAME
+                    SensorManager.SENSOR_DELAY_GAME,
+                    0 // maxReportLatencyUs (we want real-time delivery)
             );
             isCollecting = true;
             onStartRecording(meta);
@@ -225,7 +240,7 @@ public class LstForegroundService extends Service implements SensorEventListener
         }
         public SensorData(long timestamp, float[] values) {
             this.timestamp = timestamp;
-            this.values = values;
+            this.values = Arrays.copyOf(values, values.length);
         }
     }
 
@@ -233,10 +248,12 @@ public class LstForegroundService extends Service implements SensorEventListener
     public static class SensorDataArray {
         private ArrayList<SensorData> data = new ArrayList<SensorData>();
         private String meta;
+        private int versionCode;
         public void add(SensorEvent event) { data.add(new SensorData(event)); }
         public void add(SensorData d) { data.add(d); }
         public void clear() { data.clear(); }
         public void setMeta(String meta) { this.meta = meta; }
+        public void setVersionCode(int versionCode) { this.versionCode = versionCode; }
     }
 
     private final SensorDataArray recording = new SensorDataArray();
@@ -245,6 +262,11 @@ public class LstForegroundService extends Service implements SensorEventListener
     private void onStartRecording(String meta) {
         recordingStartTime = SystemClock.elapsedRealtimeNanos();
         recording.setMeta(meta);
+
+        try {
+            PackageInfo pInfo = this.getPackageManager().getPackageInfo(this.getPackageName(), 0);
+            recording.setVersionCode(pInfo.versionCode);
+        } catch (PackageManager.NameNotFoundException ignored) {}
     }
     private void onStopRecording() {
         if(listener != null) {

app/src/main/java/at/lockstep/app/MainActivity.java

@@ -32,6 +32,7 @@ import java.io.Writer;
 public class MainActivity extends AppCompatActivity implements LstForegroundService.OnResultListener {
     private Button btnStart;
     private Button btnStop;
+    private Button btnPause;
     private Button btnMediaStoreBenchmark;
     private Button btnPickSong;
     private final ActivityResultLauncher<Intent> launcher;
@@ -61,6 +62,7 @@ public class MainActivity extends AppCompatActivity implements LstForegroundServ
 
         btnStart = findViewById(R.id.btnStart);
         btnStop = findViewById(R.id.btnStop);
+        btnPause = findViewById(R.id.btnPause);
         btnMediaStoreBenchmark = findViewById(R.id.btnMediaStoreBenchmark);
         btnPickSong = findViewById(R.id.btnPickSong);
 
@@ -83,6 +85,10 @@ public class MainActivity extends AppCompatActivity implements LstForegroundServ
                 startService(LstForegroundService.stopIntent(MainActivity.this))
         );
 
+        btnPause.setOnClickListener(v ->
+                startService(LstForegroundService.pauseIntent(MainActivity.this))
+        );
+
         btnMediaStoreBenchmark.setOnClickListener(v -> {
             Intent intent = new Intent(MainActivity.this, MediaStoreBenchmarkActivity.class);
             startActivity(intent);

app/src/main/java/at/lockstep/app/MediaStoreBenchmarkActivity.java

@@ -106,15 +106,17 @@ public class MediaStoreBenchmarkActivity extends Activity {
                 MediaStore.Audio.Media.TITLE + " ASC"
         )) {
             if (cursor != null) {
+                int idColumn = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media._ID);
                 int titleColumn = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.TITLE);
                 int dataColumn = cursor.getColumnIndex(MediaStore.Audio.Media.DATA);
 
                 while (cursor.moveToNext()) {
+                    String contentUri = cursor.getString(idColumn); // the content:// Uri for the MediaStore item
                     String title = cursor.getString(titleColumn);
                     String path = dataColumn != -1 ? cursor.getString(dataColumn) : null;
 
                     if (path != null) {
-                        musicList.add(title + "\n" + path);
+                        musicList.add(title + "\n" + path + "\n" + contentUri);
                     } else {
                         musicList.add(title + "\n[path unavailable]");
                     }

app/src/main/java/at/lockstep/pb/PlaybackEngine.java

@@ -76,16 +76,38 @@ public class PlaybackEngine {
         return mEngineHandle;
     }
 
-    public static void playMusic(int fd) {
+    public static void playMusic(int fd, Context context, int beat_resid) {
+        if (mEngineHandle == 0) {
+            create(context, beat_resid);
+        }
         if (mEngineHandle != 0) {
             native_playMusic(mEngineHandle, fd);
         }
     }
 
+    public static void stop() {
+        if (mEngineHandle != 0) {
+            //native_stop(mEngineHandle);
+            // for now, (pause is bugged), deleting engine is the only clean way
+            native_deleteEngine(mEngineHandle);
+            mEngineHandle = 0;
+        }
+    }
+
+    public static void pause() {
+        if (mEngineHandle != 0) {
+            Log.i("PlaybackEngine", "native_pause() ...");
+            native_pause(mEngineHandle);
+            Log.i("PlaybackEngine", "native_pause() done.");
+        }
+    }
+
     private static native long native_createEngine(String filesDir, int resid);
     private static native void native_deleteEngine(long engineHandle);
     private static native void native_setDefaultStreamValues(int sampleRate, int framesPerBurst);
 
     private static native int native_mpg123_init();
     private static native void native_playMusic(long engineHandle, int fd);
+    private static native void native_stop(long engineHandle);
+    private static native void native_pause(long engineHandle);
 }

app/src/main/java/at/lockstep/player/pasada/LibPasada.java

@@ -0,0 +1,75 @@
+package at.lockstep.player.pasada;
+
+/**
+ * JNI entry point for libpasada.
+ *
+ * <p>Native state machine: LOADED → INITIALIZED → PLAYING ↔ PAUSED → FINISHED → STOPPED
+ *
+ * <p>Call {@link #loadNative()} once before any other method.
+ */
+public final class LibPasada {
+
+    private static boolean loaded;
+
+    private LibPasada() {}
+
+    /** Loads {@code libpasada.so}. Safe to call multiple times. */
+    public static synchronized void loadNative() {
+        if (loaded) {
+            return;
+        }
+        System.loadLibrary("lockstep-native");
+        loaded = true;
+    }
+
+    /** Called each time a run/playlist session starts (Oboe silent, buffers ready). */
+    public static native void init();
+
+    /** Submit one accelerometer sample (m/s²); may be called from a sensor thread. */
+    public static native void feedAccel(float x, float y, float z, long timestampNanos);
+
+    /**
+     * Open MP3 from an already-open file descriptor and begin adaptive playback.
+     *
+     * @param fd open read FD (Java retains ownership; do not close until track changes)
+     * @param offset start offset within the FD (0 for whole file)
+     * @param length byte length from offset ({@code -1} if unknown / to EOF)
+     */
+    public static native void play(int fd, long offset, long length);
+
+    /** PLAYING → PAUSED (silent output, graph kept alive). */
+    public static native void pause();
+
+    /** PAUSED → PLAYING (same track, same decode position, same FD). */
+    public static native void resume();
+
+    /** Tear down Oboe for this run segment → STOPPED. */
+    public static native void stop();
+
+    /**
+     * Milliseconds from the start of the current track — same timebase as ExoPlayer
+     * {@code getCurrentPosition()}. Safe to poll from background threads.
+     */
+    public static native long getCurrentPositionMs();
+
+    /** Track duration in ms, or {@code 0} if not yet known. */
+    public static native long getDurationMs();
+
+    /** Whether adapted audio is actively being output (not paused, not finished). */
+    public static native boolean isPlaying();
+
+    /** Current native state; see {@link PasadaState}. */
+    public static native int getState();
+
+    /** Seek within the current track. */
+    public static native void seekTo(long positionMs);
+
+    /** Runtime metrics / last error string for logging and debug UI. */
+    public static native String getDiagnostics();
+
+    /** Register listener for async events raised from the audio/native thread. */
+    public static native void setPlaybackListener(PasadaPlaybackListener listener);
+
+    /** native version string */
+    public static native String getVersion();
+}

app/src/main/java/at/lockstep/player/pasada/PasadaPlaybackListener.java

@@ -0,0 +1,16 @@
+package at.lockstep.player.pasada;
+
+/**
+ * Callbacks invoked from native (Oboe or internal worker thread).
+ * Implementations must post to the main thread if they touch UI or service state.
+ */
+public interface PasadaPlaybackListener {
+
+    /** Current track reached end; service should advance queue and call {@link LibPasada#play}. */
+    void onTrackFinished();
+
+    void onTrackClosed(int fd);
+
+    /** Decode / Oboe / pipeline failure; service should stop safely and surface error. */
+    void onError(int errorCode, String message);
+}

app/src/main/java/at/lockstep/player/pasada/PasadaState.java

@@ -0,0 +1,29 @@
+package at.lockstep.player.pasada;
+
+/**
+ * Mirrors the libpasada state machine documented in DESIGN.md.
+ * Keep values in sync with LibPasada.h
+ */
+public enum PasadaState {
+    LOADED(0),
+    INITIALIZED(1),
+    PLAYING(2),
+    PAUSED(3),
+    FINISHED(4),
+    STOPPED(5);
+
+    public final int code;
+
+    PasadaState(int code) {
+        this.code = code;
+    }
+
+    public static PasadaState fromCode(int code) {
+        for (PasadaState state : values()) {
+            if (state.code == code) {
+                return state;
+            }
+        }
+        throw new IllegalArgumentException("Unknown PasadaState code: " + code);
+    }
+}

app/src/main/res/layout/activity_main.xml

@@ -19,6 +19,13 @@
         android:layout_marginTop="16dp"
         android:text="Stop collection" />
 
+    <Button
+        android:id="@+id/btnPause"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="16dp"
+        android:text="Pause" />
+
     <Button
         android:id="@+id/btnMediaStoreBenchmark"
         android:layout_width="wrap_content"