Fix synthio oscillator wrapping one sample late

shared-module/synthio/__init__.c

@@ -238,7 +238,7 @@ static bool synth_note_into_buffer(synthio_synth_t *synth, int chan, int32_t *ou
     for (uint16_t i = 0; i < dur; i++) {
         accum += dds_rate;
         // because dds_rate is low enough, the subtraction is guaranteed to go back into range, no expensive modulo needed
-        if (accum > lim) {
+        if (accum >= lim) {
             accum = accum - lim + offset;
         }
         int16_t idx = accum >> SYNTHIO_FREQUENCY_SHIFT;
@@ -266,7 +266,7 @@ static bool synth_note_into_buffer(synthio_synth_t *synth, int chan, int32_t *ou
         for (uint16_t i = 0; i < dur; i++) {
             accum += ring_dds_rate;
             // because dds_rate is low enough, the subtraction is guaranteed to go back into range, no expensive modulo needed
-            if (accum > lim) {
+            if (accum >= lim) {
                 accum = accum - lim + offset;
             }
             int16_t idx = accum >> SYNTHIO_FREQUENCY_SHIFT;

tests/circuitpython/synthio_oscillator_loop_end.py

@@ -0,0 +1,33 @@
+# A note whose loop end is reached exactly (accum == lim) should wrap back
+# to the loop start on that same sample, not read one sample past the loop.
+import array
+from audiocore import get_buffer
+from synthio import Note, Synthesizer
+
+RATE = 8000
+LOOP = 256
+
+# 512 samples: the loop is [0, 256) and is silent; sample 256 is a marker
+# the oscillator should never reach.
+table = array.array("h", [0] * 512)
+table[LOOP] = 30000
+
+synth = Synthesizer(sample_rate=RATE, channel_count=1)
+note = Note(
+    frequency=RATE / LOOP,  # exactly one table step per output sample
+    waveform=table,
+    waveform_loop_start=0,
+    waveform_loop_end=LOOP,
+    envelope=None,
+)
+synth.press(note)
+
+out = []
+for _ in range(4):
+    data = bytes(get_buffer(synth)[1])
+    for index in range(0, len(data) - 1, 2):
+        value = data[index] | (data[index + 1] << 8)
+        out.append(value - 65536 if value >= 32768 else value)
+
+hits = [i for i, v in enumerate(out) if abs(v) > 1000]
+print("marker hits:", hits)

tests/circuitpython/synthio_oscillator_loop_end.py.exp

@@ -0,0 +1 @@
+marker hits: []