fix: fixup readBuffer2() to return samples not bytes

app/src/main/cpp/PlaybackEngine.cpp

@@ -10,30 +10,85 @@
 #include "mp3file.h"
 #include <vector>
 
+/*
+allocated 2304 bytes buffer
+blanking the buffer ...
+filling 24543 num_samples
+
+readBuffer2() k=0
+in readBuffer2() on mp3 != null: 1
+in readBuffer2() remaining_samples=24543 offset=0
+in readBuffer2() handle != null: 1
+in readBuffer2() done=2304
+copying 1152 samples, k=0
+<segfault>
+*/
 static inline int readBuffer2(MP3File* mp3)
 {
+    //LOGI("in readBuffer2() on mp3 != null: %d", mp3 != nullptr);
+    //if(mp3 == nullptr) return 0;
+    //LOGI("in readBuffer2() remaining_samples=%d offset=%d", mp3->remaining_samples, mp3->offset);
     size_t done = 0;
+    //LOGI("in readBuffer2() handle != null: %d", mp3->handle != nullptr);
     int err = mpg123_read(mp3->handle, mp3->buffer, mp3->buffer_size, &done);
-    mp3->leftSamples = done / sizeof(int16_t);
+    //LOGI("in readBuffer2() done=%d", done);
+    mp3->remaining_samples -= done / sizeof(int16_t);
     mp3->offset = 0;
-    return err != MPG123_OK ? 0 : done;
+    if (err != MPG123_OK) LOGI("err=%d done=%d", err, done); // err=-12
+    if(err == MPG123_DONE) return done / sizeof(int16_t);
+    return err != MPG123_OK ? 0 : done / sizeof(int16_t);
 }
 
 static bool read_mp3(std::string filename, std::vector<float>& samples) {
     // TODO: assumes 48000 Hz sampling rate of the file
     // note: our resource file is mono (1 channel = 1 samples_per_frame)!
 
+    /*
+
+2026-03-05 11:54:44.841 17486-17486 NDK                     at.lockstep                          I  PlaybackEngine - allocated 2304 bytes buffer
+2026-03-05 11:54:44.841 17486-17486 NDK                     at.lockstep                          I  PlaybackEngine - blanking the buffer ...
+2026-03-05 11:54:44.842 17486-17486 NDK                     at.lockstep                          I  PlaybackEngine - filling 24543 num_samples
+
+     copying 1152 samples, k=0
+
+     */
+
     MP3File *mFile = mp3file_open(filename.c_str(), 0); // MPG123_ENC_FLOAT_32 (but does not seem to work)
+    //LOGI("allocated %d bytes buffer", mFile->buffer_size);
+    //LOGI("blanking the buffer ...");
+    memset(mFile->buffer, 0, mFile->buffer_size);
+    //LOGI("filling %d num_samples", mFile->num_samples);
     bool ok1 = mFile != nullptr;
-    bool ok2 = readBuffer2(mFile) != 0; // once is enough (maybe the other one needs to pre-fill other buffers, in oboe etc.)
+    int ok2 = true;
+    int i = 0;
+    //int k = 0;
+    if(ok1) {
+        samples.resize(mFile->num_samples);
+        while (ok2 && mFile->remaining_samples > 0) {
+            //LOGI("readBuffer2() k=%d", k);
+            ok2 = readBuffer2(mFile);
+            if (!ok2) break; // TODO: check if trailing bit is decoded correctly
+            //LOGI("copying %d samples, k=%d", mFile->samples_per_frame, k);
+            for (int j = 0; j < ok2 && i < mFile->num_samples; j++, i++) {
+                int16_t *src = ((int16_t *) mFile->buffer) + j;
+                samples[i] = (*src) / 32768.0f;
+            }
+            //k++; // debug only
+        }
+    }
+    if(!ok2) {
+        LOGI("ok2=false at i=%d", i);
+    }
+    /*
     if(ok1 && ok2) {
         // num_frames, num_samples, samples_per_frame
+        LOGI("filling %d num_samples", mFile->num_samples);
         samples.resize(mFile->num_samples);
         for(int i = 0; i < mFile->num_samples; i++) {
             int16_t *src = ((int16_t *) mFile->buffer) + i;
             samples[i] = (*src) / 32768.0f;
         }
-    }
+    }*/
     if(mFile)
         mp3file_delete(mFile);
     return ok1 && ok2;
@@ -44,7 +99,21 @@ PlaybackEngine::PlaybackEngine(std::string filesDir, int resid): mFilesDir(files
     LOGI("NDK LOG_LEVEL=%d", LOG_LEVEL);
     // NDK LOG_LEVEL=3  (DEBUG)
     std::vector<float> samples;
-    read_mp3(mFilesDir + "/" + std::to_string(resid) + ".mp3", samples);
+    // BUG!
+    bool is_ok = read_mp3(mFilesDir + "/" + std::to_string(resid) + ".mp3", samples);
+    LOGI("read_mp3() is_ok=%d", is_ok);
+    // begin debug only
+    /*
+    // make sine wave (it works)
+    int fps = 48000;
+    float dur = 0.3;
+    float f = 440;
+    int N = (int) (dur * (float) fps);
+    samples.resize(N);
+    for(int n = 0; n < N; n++)
+        samples[n] = (float) sin(2*M_PI * f * n / N);
+    */
+    // end debug only
     mPlayer = new MixingPlayer(samples);
     int32_t res = mPlayer->startAudio();
     LOGI("startAudio() = %d", res);

app/src/main/cpp/mp3file.cpp

@@ -81,6 +81,8 @@ MP3File* mp3file_open(const char *filename, int forceEncoding) {
     else
         mp3->duration = mp3->num_samples / mp3->samples_per_frame * mp3->secs_per_frame;
 
+    mp3->offset = 0;
+    mp3->remaining_samples = (int) mp3->num_samples;
     LOGV("channels: %d rate: %ld num_samples: %ld", mp3->channels, mp3->rate, mp3->num_samples);
     return mp3;
 

app/src/main/cpp/mp3file.h

@@ -19,7 +19,7 @@ struct MP3File
     double duration;
     size_t buffer_size;
     unsigned char* buffer;
-    size_t leftSamples;
+    int remaining_samples;
     size_t offset;
 };
 

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

@@ -21,6 +21,7 @@ public class MainActivity extends Activity {
         btnStart = findViewById(R.id.btnStart);
         btnStop = findViewById(R.id.btnStop);
 
+        // TODO: handle clicking START button twice
         btnStart.setOnClickListener(v ->
                 ContextCompat.startForegroundService(
                         MainActivity.this,