synthio: Fix note dropped when re-pressed after decay

shared-module/synthio/__init__.c

@@ -497,8 +497,15 @@ static int find_channel_with_note(synthio_synth_t *synth, mp_obj_t note) {
 bool synthio_span_change_note(synthio_synth_t *synth, mp_obj_t old_note, mp_obj_t new_note) {
     int channel;
     if (new_note != SYNTHIO_SILENCE && (channel = find_channel_with_note(synth, new_note)) != -1) {
-        // note already playing, re-enter attack phase
-        synth->envelope_state[channel].state = SYNTHIO_ENVELOPE_STATE_ATTACK;
+        if (synth->envelope_state[channel].level == 0) {
+            // released and already decayed to silence, but not yet reaped:
+            // treat this like a fresh press, not a swell from 0
+            synthio_envelope_state_init(&synth->envelope_state[channel], synthio_synth_get_note_envelope(synth, new_note));
+            synth->accum[channel] = 0;
+        } else {
+            // note already playing, re-enter attack phase
+            synth->envelope_state[channel].state = SYNTHIO_ENVELOPE_STATE_ATTACK;
+        }
         return true;
     }
     channel = find_channel_with_note(synth, old_note);

tests/circuitpython/synthio_note_repress_after_decay.py

@@ -0,0 +1,24 @@
+# A note whose envelope has decayed to 0 but whose slot hasn't been reaped
+# yet should re-press like a fresh note, not silently vanish.
+from synthio import Synthesizer, Note, Envelope
+from audiocore import get_buffer
+
+# release_time=0 collapses the envelope to level 0 in a single step, while
+# the note object still holds its channel.
+quick = Envelope(release_time=0)
+synth = Synthesizer()
+note = Note(440, envelope=quick)
+
+synth.press(note)
+get_buffer(synth)
+synth.release(note)
+get_buffer(synth)
+print("pressed after decay:", len(synth.pressed))
+
+synth.press(note)
+print("pressed right after re-press:", len(synth.pressed))
+get_buffer(synth)
+print("pressed after one more render:", len(synth.pressed))
+for _ in range(3):
+    print("{} {:.2f}".format(*synth.note_info(note)))
+    get_buffer(synth)

tests/circuitpython/synthio_note_repress_after_decay.py.exp

@@ -0,0 +1,6 @@
+pressed after decay: 0
+pressed right after re-press: 1
+pressed after one more render: 1
+synthio.EnvelopeState.ATTACK 0.46
+synthio.EnvelopeState.ATTACK 0.70
+synthio.EnvelopeState.ATTACK 0.93