Fix synthio_biquad_filter_reset() clearing only half the filter state

shared-module/synthio/Biquad.c

@@ -194,7 +194,7 @@ void common_hal_synthio_biquad_tick(mp_obj_t self_in) {
 }
 
 void synthio_biquad_filter_reset(biquad_filter_state *st) {
-    memset(&st->x, 0, 4 * sizeof(int16_t));
+    memset(st, 0, sizeof(*st));
 }
 
 static inline int32_t biquad_filter_sample(int32_t input, int32_t a1, int32_t a2, int32_t b0, int32_t b1, int32_t b2, int32_t x0, int32_t x1, int32_t y0, int32_t y1) {

tests/circuitpython/audiofilter_filter_reset_buffer.py

@@ -0,0 +1,43 @@
+# audiofilters.Filter.reset_buffer() should leave no audio behind: a biquad
+# reset should clear both its input and output history, not just the input.
+import array
+import math
+from audiocore import get_buffer, reset_buffer, RawSample
+from audiofilters import Filter
+from synthio import Biquad, FilterMode
+
+RATE = 48000
+FRAMES = 1024
+
+
+def s16(data):
+    out = []
+    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)
+    return out
+
+
+loud = array.array(
+    "h", [int(30000 * math.sin(2 * math.pi * 200 * frame / RATE)) for frame in range(FRAMES)]
+)
+silence = array.array("h", [0] * FRAMES)
+
+biquad = Biquad(FilterMode.LOW_PASS, 800.0)
+node = Filter(
+    filter=biquad,
+    sample_rate=RATE,
+    channel_count=1,
+    bits_per_sample=16,
+    samples_signed=True,
+    buffer_size=FRAMES * 2,
+)
+
+node.play(RawSample(loud, sample_rate=RATE, channel_count=1), loop=True)
+for _ in range(4):
+    get_buffer(node)  # fill the filter's memory
+
+node.play(RawSample(silence, sample_rate=RATE, channel_count=1), loop=True)
+reset_buffer(node)  # what an AudioOut does on play()
+first = s16(bytes(get_buffer(node)[1]))
+print("peak after reset_buffer:", max(abs(value) for value in first))

tests/circuitpython/audiofilter_filter_reset_buffer.py.exp

@@ -0,0 +1 @@
+peak after reset_buffer: 0