#include <SDL2/SDL.h>

#include <cmath>

// An audio driver. This audio driver will define a callback that will
// generate a waveform which is passed to SDL. SDL passes this waveform to
// the OS, where it is output to a physical device.

struct audio_driver
{
	SDL_AudioSpec audio_spec;

	// Default constructor.

	audio_driver()
	{
		// The 'freq' parameter specifies amount of samples to take per
		// second. A higher frequency corresponds to a higher quality of
		// sound.

		audio_spec.freq = 48000;

		// The 'format' parameter specifies the precision of each sample. A
		// higher format size corresponds to a broader range of possible
		// sounds.

		audio_spec.format = AUDIO_S16SYS;

		// The 'channels' parameter specifies the amount of output channels. 2
		// is stereo, and 1 is mono.

		audio_spec.channels = 1;

		// The 'samples' parameter specifies the size of the stream buffer.
		// The value of 4096 is suggested by SDL.

		audio_spec.samples = 4096;

		// The 'userdata' parameter specifies the arbitrary data to pass to
		// the audio callback function. The pointer to this object is used to
		// allow access to the contents of this object.

		audio_spec.userdata = this;

		// The 'callback' parameter specifies the callback function to use to
		// generate audio samples. This callback function will invoke the
		// waveform generator from it's given userdata, which is set above to
		// the 'this' pointer.

		audio_spec.callback = [](void* userdata, Uint8* stream, int len) -> void
		{
			((audio_driver*)(userdata))->waveform((short*)stream, len / 2);
		};

		// Open the audio device.

		SDL_AudioDeviceID audio_device = SDL_OpenAudioDevice(NULL, 0, &audio_spec, &audio_spec, 0);

		// Pause the audio device.

		SDL_PauseAudioDevice(audio_device, 0);
	}

	// Waveform generator (plays a song).

	int row = 63;

	int freq1;
	int freq2;

	int freq2b;

	int count = 0;

	void waveform(short* target, int num_samples)
	{
		static const char song[] = "057+5420+%7%+%7%5%4%2%457%0%0%754%2%+%%%5%542%457%0%0%042%2#+%!#0%+%$%%%";

		for (int position = 0; position < num_samples; --count)
		{
			if (!count)
			{
				row = (row + 1) % 64;

				freq1 = 17 + std::exp2(song[row + 8] / 12.0);

				if (song[row + 8] == '%')
				{
					freq1 = 0;
				}

				freq2 = 17 * std::exp2(song[row / 8] / 12.0);

				if (row & 2)
				{
					freq2b = freq2 / 2;
				}
				else
				{
					freq2b = 0;
				}

				count = 5400 + 540 * (row & 2);
			}

			target[position++] = (8 * (((count + 85) * freq1 & 32767) - 16384) + 1 * (((count + 4) * freq1 * 8 & 32767) - 16384) + 3 * (((count + 57) * freq2 / 4 & 32767) - 16384) + 8 * (((count + 23) * freq2b & 32767) - 16384)) * count >> 16;
		}
	}
};

int main(int argc, char** argv)
{
	SDL_InitSubSystem(SDL_INIT_AUDIO);

	audio_driver driver;

	while (true)
	{
		SDL_Delay(500);
	}

	return 0;
}