lockstep/app/src/main/cpp/PlaybackEngine.cpp

//
// Created by david on 01.02.2026.
//

#define LOG_TAG "PlaybackEngine"

#include "PlaybackEngine.h"
#include "logging.h"
#include "mpg123.h"
#include "mp3file.h"
#include <vector>

/**
 * Read samples from the next mp3-frame into the struct MP3File's buffer.
 * @return number of samples read (uses buffer_size which is much smaller than total length)
 */
static inline int readBuffer2(MP3File* mp3)
{
    size_t done = 0;
    int err = mpg123_read(mp3->handle, mp3->buffer, mp3->buffer_size, &done);
    mp3->remaining_samples -= done / sizeof(int16_t);
    mp3->offset = 0;
    if (err != MPG123_OK && err != MPG123_DONE) LOGE("mpg123_read() err=%d done=%d", err, done);
    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)!
    MP3File *mFile = mp3file_open(filename.c_str(), 0); // MPG123_ENC_FLOAT_32 (but does not seem to work)
    memset(mFile->buffer, 0, mFile->buffer_size);
    bool ok1 = mFile != nullptr;
    int ok2 = true;
    int i = 0;
    if(ok1) {
        samples.resize(mFile->num_samples);
        while (ok2 && mFile->remaining_samples > 0) {
            ok2 = readBuffer2(mFile);
            if (!ok2) break;
            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;
            }
        }
    }
    if(!ok2) {
        LOGI("ok2=false at i=%d", i);
    }
    if(mFile)
        mp3file_delete(mFile);
    return ok1 && ok2;
}

PlaybackEngine::PlaybackEngine(std::string filesDir, int resid): mFilesDir(filesDir) {
    LOGI("PlaybackEngine()");
    LOGI("NDK LOG_LEVEL=%d", LOG_LEVEL);
    // NDK LOG_LEVEL=3  (DEBUG)
    std::vector<float> samples;
    bool is_ok = read_mp3(mFilesDir + "/" + std::to_string(resid) + ".mp3", samples);
    LOGI("read_mp3() is_ok=%d", is_ok);
    mPlayer = new MixingPlayer(samples);
    int32_t res = mPlayer->startAudio();
    LOGI("startAudio() = %d", res);
}

PlaybackEngine::~PlaybackEngine() {
    LOGI("~PlaybackEngine()");
    mPlayer->stopAudio();
    delete mPlayer;
    mPlayer = nullptr;
}

void PlaybackEngine::playBeat() {
    if(mPlayer) mPlayer->setStartBeat();
}