Add defensive NULL checks for node inputs

This guards against NULL pointer dereferences in the processing loop:
- picosynth_process() returns 0 for NULL synth pointer
- Oscillator skips freq increment when freq pointer is NULL
- HP filter outputs 0 when input pointer is NULL
- Filter accumulator update uses 0 for NULL input

Author: jserv

src/picosynth.c

@@ -469,6 +469,9 @@ static q15_t soft_clip(int32_t x)
 
 q15_t picosynth_process(picosynth_t *s)
 {
+    if (!s)
+        return 0;
+
     int32_t out = 0;
 
     for (int vi = 0; vi < s->num_voices; vi++) {
@@ -518,9 +521,14 @@ q15_t picosynth_process(picosynth_t *s)
                 break;
             case PICOSYNTH_NODE_HP:
                 /* High-pass is the input signal minus the low-pass signal */
-                tmp[i] =
-                    (int32_t) (((int64_t) n->flt.accum * n->flt.coeff) >> 15);
-                tmp[i] = *n->flt.in - tmp[i];
+                if (n->flt.in) {
+                    tmp[i] =
+                        (int32_t) (((int64_t) n->flt.accum * n->flt.coeff) >>
+                                   15);
+                    tmp[i] = *n->flt.in - tmp[i];
+                } else {
+                    tmp[i] = 0;
+                }
                 break;
             case PICOSYNTH_NODE_MIX: {
                 int32_t sum = 0;
@@ -550,7 +558,8 @@ q15_t picosynth_process(picosynth_t *s)
 
             switch (n->type) {
             case PICOSYNTH_NODE_OSC:
-                n->state += *n->osc.freq;
+                if (n->osc.freq)
+                    n->state += *n->osc.freq;
                 if (n->osc.detune)
                     n->state += *n->osc.detune;
                 n->state =
@@ -631,7 +640,8 @@ q15_t picosynth_process(picosynth_t *s)
                  * where output is the filtered signal from the previous sample.
                  * This implements a simple recursive filter.
                  */
-                int32_t delta = *n->flt.in - n->out;
+                int32_t input_val = n->flt.in ? *n->flt.in : 0;
+                int32_t delta = input_val - n->out;
                 int64_t acc = (int64_t) n->flt.accum + delta;
                 if (acc > 0x7FFFFFFF)
                     acc = 0x7FFFFFFF;

tests/test-synth.c

@@ -310,6 +310,34 @@ static void test_voice_set_out(void)
     picosynth_destroy(s);
 }
 
+/* Test graphs with missing/null inputs */
+static void test_null_graph_inputs(void)
+{
+    picosynth_t *s = picosynth_create(1, 2);
+    TEST_ASSERT(s != NULL, "synth creation");
+
+    picosynth_voice_t *v = picosynth_get_voice(s, 0);
+    picosynth_node_t *osc = picosynth_voice_get_node(v, 0);
+    picosynth_node_t *hp = picosynth_voice_get_node(v, 1);
+
+    /* Intentionally leave freq/in pointers NULL to ensure null-safety */
+    picosynth_init_osc(osc, NULL, NULL, picosynth_wave_sine);
+    picosynth_init_hp(hp, NULL, NULL, Q15_MAX);
+    picosynth_voice_set_out(v, 1);
+
+    picosynth_note_on(s, 0, 60);
+
+    int non_zero = 0;
+    for (int i = 0; i < 64; i++) {
+        q15_t sample = picosynth_process(s);
+        if (sample != 0)
+            non_zero++;
+    }
+    TEST_ASSERT(non_zero == 0, "null graph inputs produce silence");
+
+    picosynth_destroy(s);
+}
+
 /* Test NULL pointer handling */
 static void test_null_safety(void)
 {
@@ -342,5 +370,6 @@ void test_synth_all(void)
     TEST_RUN(test_mixer);
     TEST_RUN(test_voice_freq_ptr);
     TEST_RUN(test_voice_set_out);
+    TEST_RUN(test_null_graph_inputs);
     TEST_RUN(test_null_safety);
 }