main.cpp

#include <SDL2/SDL.h>
#include <cmath>
#include <unordered_map>

class Synthesizer {
private:
  float m_timestep = 0;
  float m_phasestep = 0;
  float m_phase = 0;
  float m_freq;
  bool m_playing = false;

public:
  Synthesizer(int sample_rate) { m_timestep = 1.f / sample_rate; }

  void synthesize(float *buffer, size_t buffer_size) {
    for (int i = 0; i < buffer_size; i++) {
      if (!m_playing && m_phase < 0.001f) {
        return;
      }

      buffer[i] = sinf(m_phase * 2 * M_PI) * 0.5f;
      m_phase += m_phasestep;
      m_phase = fmodf(m_phase, 1.f);
    }
  }

  void play(float freq) {
    m_phasestep = m_timestep * freq;
    m_freq = freq;
    m_playing = true;
  }

  void stop() { m_playing = false; }
};

std::unordered_map<SDL_Keycode, int> piano_keys = {
    {SDLK_z, 0}, {SDLK_s, 1}, {SDLK_x, 2},  {SDLK_d, 3},
    {SDLK_c, 4}, {SDLK_v, 5}, {SDLK_g, 6},  {SDLK_b, 7},
    {SDLK_h, 8}, {SDLK_n, 9}, {SDLK_j, 10}, {SDLK_m, 11},
};

float get_freq(SDL_Keycode key) {
  return 440.f * powf(2.f, piano_keys[key] / 12.f);
}

int main() {
  SDL_Init(SDL_INIT_EVERYTHING);

  Synthesizer synth(48000);

  SDL_AudioSpec spec;
  spec.freq = 48000;
  spec.channels = 1;
  spec.format = AUDIO_F32SYS;
  spec.samples = 256;
  spec.callback = [](void *userdata, Uint8 *bytes, int len) {
    Synthesizer *synth = (Synthesizer *)userdata;
    memset(bytes, 0, len);
    synth->synthesize((float *)bytes, len / sizeof(float));
  };
  spec.userdata = &synth;

  SDL_AudioDeviceID device = SDL_OpenAudioDevice(nullptr, 0, &spec, &spec, 0);
  SDL_PauseAudioDevice(device, 0);

  SDL_Window *window = SDL_CreateWindow("synth", 0, 0, 800, 600, 0);
  SDL_Surface *surface = SDL_GetWindowSurface(window);
  SDL_Event event;

  bool running = true;

  // 16ms of latency
  const int lag = 1000 / 60;
  Uint64 start, elapsed;

  SDL_Keycode pressed_key;

  while (running) {
    start = SDL_GetTicks64();
    while (SDL_PollEvent(&event)) {
      switch (event.type) {
      case SDL_QUIT:
        running = false;
        break;

      case SDL_KEYDOWN:
        pressed_key = event.key.keysym.sym;
        if (piano_keys.count(pressed_key)) {
          synth.play(get_freq(pressed_key));
        }
        break;
      case SDL_KEYUP:
        if (event.key.keysym.sym == pressed_key) {
          synth.stop();
        }
        break;

      default:
        break;
      }
    }

    SDL_UpdateWindowSurface(window);

    elapsed = SDL_GetTicks() - start;
    if (lag > elapsed) {
      SDL_Delay(lag - elapsed);
    }
  }

  SDL_Quit();
}