feat: add initial JACK audio looper with MIDI control

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

makefile

@@ -0,0 +1,10 @@
+CC ?= gcc
+CFLAGS ?= -Wall -Wextra -g
+LDFLAGS ?= -ljack
+
+looper: src/main.c
+	$(CC) $(CFLAGS) -o looper src/main.c $(LDFLAGS)
+
+.PHONY: clean
+clean:
+	rm -f looper

src/main.c

@@ -0,0 +1,131 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <jack/jack.h>
+
+typedef enum {
+    STATE_IDLE,
+    STATE_RECORD,
+    STATE_LOOPING,
+    STATE_PAUSED
+} looper_state;
+
+static volatile looper_state current_state = STATE_IDLE;
+
+static jack_port_t *input_port;
+static jack_port_t *output_port;
+static jack_port_t *midi_control_port;
+static jack_port_t *midi_clock_port;
+
+static jack_client_t *client;
+
+static int process(jack_nframes_t nframes, void *arg)
+{
+    jack_default_audio_sample_t *in  = (jack_default_audio_sample_t *) jack_port_get_buffer(input_port, nframes);
+    jack_default_audio_sample_t *out = (jack_default_audio_sample_t *) jack_port_get_buffer(output_port, nframes);
+
+    if (in && out) {
+        memcpy(out, in, sizeof(jack_default_audio_sample_t) * nframes);
+    }
+
+    void *midi_ctrl_buf = jack_port_get_buffer(midi_control_port, nframes);
+    if (midi_ctrl_buf) {
+        jack_nframes_t nevents = jack_midi_get_event_count(midi_ctrl_buf);
+        jack_midi_event_t ev;
+        for (jack_nframes_t i = 0; i < nevents; i++) {
+            if (jack_midi_event_get(&ev, midi_ctrl_buf, i) == 0) {
+                // note on with note number 1
+                if ((ev.size >= 3) && ((ev.buffer[0] & 0xf0) == 0x90)) {
+                    unsigned char note = ev.buffer[1];
+                    if (note == 1) {
+                        switch (current_state) {
+                            case STATE_IDLE:
+                                current_state = STATE_RECORD;
+                                fprintf(stderr, "[looper] -> record\n");
+                                break;
+                            case STATE_RECORD:
+                                current_state = STATE_LOOPING;
+                                fprintf(stderr, "[looper] -> looping\n");
+                                break;
+                            case STATE_LOOPING:
+                                current_state = STATE_PAUSED;
+                                fprintf(stderr, "[looper] -> paused\n");
+                                break;
+                            case STATE_PAUSED:
+                                current_state = STATE_LOOPING;
+                                fprintf(stderr, "[looper] -> looping\n");
+                                break;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    // midi clock port is registered but not processed here
+    return 0;
+}
+
+static void jack_shutdown(void *arg)
+{
+    (void)arg;
+    fprintf(stderr, "JACK shutdown\n");
+    exit(0);
+}
+
+int main(int argc, char *argv[])
+{
+    const char *client_name = "looper";
+    jack_options_t options = JackNullOption;
+    jack_status_t status;
+
+    client = jack_client_open(client_name, options, &status);
+    if (client == NULL) {
+        fprintf(stderr, "jack_client_open() failed, status = 0x%2.0x\n", status);
+        if (status & JackServerFailed) {
+            fprintf(stderr, "Unable to connect to JACK server\n");
+        }
+        return 1;
+    }
+
+    if (status & JackNameNotUnique) {
+        client_name = jack_get_client_name(client);
+    }
+
+    jack_set_process_callback(client, process, NULL);
+    jack_on_shutdown(client, jack_shutdown, NULL);
+
+    input_port = jack_port_register(client, "input",
+                                     JACK_DEFAULT_AUDIO_TYPE,
+                                     JackPortIsInput, 0);
+    output_port = jack_port_register(client, "output",
+                                      JACK_DEFAULT_AUDIO_TYPE,
+                                      JackPortIsOutput, 0);
+    midi_control_port = jack_port_register(client, "control",
+                                            JACK_DEFAULT_MIDI_TYPE,
+                                            JackPortIsInput, 0);
+    midi_clock_port = jack_port_register(client, "clock",
+                                          JACK_DEFAULT_MIDI_TYPE,
+                                          JackPortIsInput, 0);
+
+    if ((input_port == NULL) || (output_port == NULL) ||
+        (midi_control_port == NULL) || (midi_clock_port == NULL)) {
+        fprintf(stderr, "Could not create ports\n");
+        return 1;
+    }
+
+    if (jack_activate(client)) {
+        fprintf(stderr, "Cannot activate client\n");
+        return 1;
+    }
+
+    fprintf(stderr, "looper running (client name '%s')\n", client_name);
+
+    while (1) {
+        sleep(1);
+    }
+
+    jack_client_close(client);
+    return 0;
+}