@prefix lv2:  <http://lv2plug.in/ns/lv2core#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
<http://example.org>
        a lv2:Plugin, lv2:InstrumentPlugin ;
        lv2:binary <example.so> ;
        rdfs:seeAlso <example.ttl> .

@prefix atom: <http://lv2plug.in/ns/ext/atom#> .
@prefix doap: <http://usefulinc.com/ns/doap#> .
@prefix lv2:  <http://lv2plug.in/ns/lv2core#> .
@prefix midi: <http://lv2plug.in/ns/ext/midi#> .
@prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix urid: <http://lv2plug.in/ns/ext/urid#> .
<http://example.org>
        a lv2:Plugin, lv2:InstrumentPlugin ;
        doap:name "Example" ;
        lv2:requiredFeature urid:map ;
        lv2:port [
                a lv2:InputPort, atom:AtomPort ;
                atom:bufferType atom:Sequence ;
                atom:supports atom:Sequence, midi:MidiEvent ;
                lv2:index 0 ;
                lv2:symbol "in_midi" ;
                lv2:name "Midi input" ;
        ], [
                a lv2:AudioPort, lv2:OutputPort ;
                lv2:index 1 ;
                lv2:symbol "out" ;
                lv2:name "Out"
        ] .

#include <math.h>
#include <stdlib.h>
#include <stdbool.h>
#include <lv2/lv2plug.in/ns/lv2core/lv2.h>
#include <lv2/lv2plug.in/ns/ext/atom/atom.h>
#include <lv2/lv2plug.in/ns/ext/atom/util.h>
#include <lv2/lv2plug.in/ns/ext/urid/urid.h>
#include <lv2/lv2plug.in/ns/ext/midi/midi.h>
#define MURI "http://example.org"
enum Ports {
    IN_MIDI,
    OUT
};
typedef struct {
    LV2_Atom_Sequence *midiPort;
    float *outPort;
    int rate;
    bool soundOn;
    int currentSample;
    LV2_URID midiEvent;
} Plugin;
static LV2_Handle
instantiate(const LV2_Descriptor* descriptor,
            double rate,
            const char* bundle_path,
            const LV2_Feature* const* features) {
    Plugin *self = (Plugin *) malloc(sizeof(Plugin));
    self->rate = rate;
    self->currentSample = 0;
    self->soundOn = false;
    LV2_URID_Map* map = NULL;
    for (int i = 0; features[i]; ++i) {
        if (!strcmp(features[i]->URI, LV2_URID__map)) {
            map = (LV2_URID_Map*)features[i]->data;
        }
    }
    if (map == NULL) {
        return NULL;
    }
    self->midiEvent = map->map(map->handle, LV2_MIDI__MidiEvent);
    return (LV2_Handle)self;
}
static void connect_port(LV2_Handle instance,
             uint32_t port,
             void* data) {
    Plugin *self = (Plugin *) instance;
    switch (port) {
        case IN_MIDI:
            self->midiPort = (LV2_Atom_Sequence*) data;
            break;
        case OUT:
            self->outPort = (float*) data;
            break;
    }
}
void processEvent(LV2_Atom_Event *event, Plugin *self) {
    if (event->body.type != self->midiEvent) {
        return;
    }
    const uint8_t* const msg = LV2_ATOM_BODY(&(event->body));
    LV2_Midi_Message_Type type = lv2_midi_message_type(msg);
    switch(type) {
        case LV2_MIDI_MSG_NOTE_ON:
            self->soundOn = true;
            break;
        case LV2_MIDI_MSG_NOTE_OFF:
            self->soundOn = false;
            break;
    }
}
static void run(LV2_Handle instance, uint32_t sample_count) {
    Plugin *self = (Plugin *) instance;
    LV2_ATOM_SEQUENCE_FOREACH(self->midiPort, event) {
        processEvent(event, self);
    }
    for (uint32_t i = 0; i < sample_count; i++) {
        if (self->soundOn) {
            self->outPort[i] = sinf(2 * M_PI * 440.0 * self->currentSample / self->rate);
        } else {
            self->outPort[i] = 0.0;
        }
        self->currentSample++;
    }
}
static void cleanup(LV2_Handle instance) {
    free(instance);
}
static const LV2_Descriptor descriptor = {
    MURI,
    instantiate,
    connect_port,
    NULL,
    run,
    NULL,
    cleanup,
    NULL
};
LV2_SYMBOL_EXPORT
const LV2_Descriptor*
lv2_descriptor(uint32_t index) {
    switch (index) {
        case 0:
            return &descriptor;
        default:
            return NULL;
    }
}