audio-playground/main.c

#include <alsa/asoundlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <math.h>
#include "synth.h"

#define ALSA_CHK(stmt, msg) do { \
    int alsa_chk_err = (stmt); \
    if (alsa_chk_err < 0) { \
        fprintf(stderr, "%s%s\n", msg, snd_strerror(alsa_chk_err)); \
        return 1; \
    } \
} while (0)

#define N_CHANNELS 1
#define SAMPLE_RATE 44100
#define PLAYBACK_LATENCY 50000 // 50 ms
#define PLAYBACK_BUFFER_SIZE 512

static int write_audio(snd_pcm_t *handle, const int16_t *buffer) {
    snd_pcm_uframes_t offset = 0;

    while (offset < PLAYBACK_BUFFER_SIZE) {
        snd_pcm_sframes_t written = snd_pcm_writei(
            handle, buffer + offset * N_CHANNELS,
            PLAYBACK_BUFFER_SIZE - offset);

        if (written < 0) {
            int err = snd_pcm_recover(handle, (int)written, 1);
            ALSA_CHK(err, "Could not write to alsa device: ");
            continue;
        }

        if (written == 0) {
            int err = snd_pcm_wait(handle, -1);
            if (err < 0) {
                err = snd_pcm_recover(handle, err, 1);
                ALSA_CHK(err, "Could not write to alsa device: ");
            }
            continue;
        }

        offset += (snd_pcm_uframes_t)written;
    }

    return 0;
}

static int midi_open(snd_seq_t **seq_handle, int src_client, int src_port) {
  int err = snd_seq_open(seq_handle, "default", SND_SEQ_OPEN_INPUT, SND_SEQ_NONBLOCK);
  ALSA_CHK(err, "Error opening ALSA sequencer: ");

  err = snd_seq_set_client_name(*seq_handle, "Midi Listener");
  ALSA_CHK(err, "Error setting sequencer client name: ");

  uint32_t caps = SND_SEQ_PORT_CAP_WRITE | SND_SEQ_PORT_CAP_SUBS_WRITE;
  int in_port = snd_seq_create_simple_port(*seq_handle, "listen:in", caps, SND_SEQ_PORT_TYPE_APPLICATION);
  ALSA_CHK(in_port, "Error creating sequencer port: ");

  err = snd_seq_connect_from(*seq_handle, in_port, src_client, src_port);
  ALSA_CHK(err, "Error connecting MIDI source: ");
  return 0;
}

static float midi_note_to_freq(int midi) {
    int note_offset = midi - 69;
    return 440.0f * exp2f((float) note_offset / 12.0f);
}

static Waveform cycle_waveform(Waveform form) {
    Waveform new_form = form + 1;
    if (new_form > WAVE_TRIANGLE) {
        new_form = WAVE_SINE;
    }
    const char *waveform_names[] = {"NONE", "SINE", "SQUARE", "SAW", "TRIANGLE"};
    printf("Waveform changed to: %s\n", waveform_names[new_form]);
    return new_form;
}

static int parse_port(int *src_client, int *src_port, const char *str) {
    return sscanf(str, "%d:%d", src_client, src_port) == 2;
}

int main(int argc, const char** argv) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s 20:0\n", argv[0]);
        return 1;
    }

    int src_client = 0;
    int src_port = 0;
    if (!parse_port(&src_client, &src_port, argv[1])) {
        fprintf(stderr, "Unable to parse midi port %s\n", argv[1]);
        return 1;
    }
    int err = 0;
    snd_pcm_t *handle = 0;

    // Init alsa audio
    err = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
    ALSA_CHK(err, "Could not open alsa device: ");
    err = snd_pcm_set_params(handle, SND_PCM_FORMAT_S16_LE, SND_PCM_ACCESS_RW_INTERLEAVED, N_CHANNELS, SAMPLE_RATE, 1, PLAYBACK_LATENCY);
    ALSA_CHK(err, "Could not set alsa params: ");

    // Init alsa midi
    snd_seq_t *seq_handle = 0;
    err = midi_open(&seq_handle, src_client, src_port);
    if (err) {
        printf("Error initializing MIDI: %d\n", err);
        return err;
    }

    // Main loop
    Note note = {
        .form = WAVE_SAW,
        .attack_ms = 100.0f,
        .release_ms = 50.0f
    };
    int16_t buf[PLAYBACK_BUFFER_SIZE] = {0};
    for (;;) {
        // Drain queued MIDI events
        for (;;) {
            snd_seq_event_t *ev = NULL;
            int ret = snd_seq_event_input(seq_handle, &ev);
            if (ret == -EAGAIN) {
                break;
            }
            ALSA_CHK(ret, "Failed to read midi event: ");
            if (!ev) {
                continue;
            }

            switch (ev->type) {
                case SND_SEQ_EVENT_NOTEON: {
                    note.is_playing = ev->data.note.velocity != 0;
                    note.freq = midi_note_to_freq(ev->data.note.note);
                } break;
                case SND_SEQ_EVENT_NOTEOFF: {
                    note.is_playing = false;
                } break;
                case SND_SEQ_EVENT_CONTROLLER: {
                    note.filter_cutoff = (float) ev->data.control.value / 64.0f;
                    printf("filter cutoff: %f\n", note.filter_cutoff);
                } break;
                case SND_SEQ_EVENT_PGMCHANGE: {
                    note.form = cycle_waveform(note.form);
                } break;
                default:
                    printf("%d\n", ev->type);
                    break;
            }
            snd_seq_free_event(ev);
        }

        play_note(&note, buf, PLAYBACK_BUFFER_SIZE);

        if (write_audio(handle, buf) < 0) {
            return 1;
        }
    }

    snd_pcm_drain(handle); // wait for playback to finish (not stricty needed depending on requested latency)
    snd_pcm_close(handle);
    return 0;
}