mixer: avoid forwarding the overread to the waveform callback

Because the RSP mixer ucode does overread (that is, it can read a bit
past the waveform end), we need to make sure that those extra bytes
are 0 (in case of a non looping waveform). Up until now, this was left
to the waveform callback to implement.

We can do better: just like for the looping case, we can have the mixer
handle this internally by clearing up the extra samples, calling the
callback only for actual samples. This better matches the waveform
callback API too, and makes things easier.

Moreover, the overread now is 100% an implementation detail of the mixer.

Author: rasky

src/audio/mixer.c

@@ -324,8 +324,20 @@ static void waveform_read(void *ctx, samplebuffer_t *sbuf, int wpos, int wlen, b
 	waveform_t *wave = (waveform_t*)ctx;
 
 	if (!wave->loop_len) {
-		// No loop defined: just call the waveform's read function.
-		wave->read(wave->ctx, sbuf, wpos, wlen, seeking);
+		// If we're asked to read past the waveform length (because overread),
+		// call the underlying function only up to the actual length,
+		// and then zero the rest of data.
+		int len1 = wlen;
+		if (wpos + wlen > wave->len)
+			len1 = MAX(wave->len - wpos, 0);
+		int len2 = wlen-len1;
+
+		if (len1 > 0)
+			wave->read(wave->ctx, sbuf, wpos, len1, seeking);
+		if (len2 > 0) {
+			void *dest = samplebuffer_append(sbuf, len2);
+			memset(dest, 0, len2 << SAMPLES_BPS_SHIFT(sbuf));
+		}
 	} else {
 		// Calculate wrapped position
 		if (wpos >= wave->len)
@@ -458,6 +470,8 @@ void mixer_ch_set_limits(int ch, int max_bits, float max_frequency, int max_buf_
 	assert(max_bits == 0 || max_bits == 8 || max_bits == 16);
 	assert(max_frequency >= 0);
 	assert(max_buf_sz >= 0 && max_buf_sz % 8 == 0);
+	assert(ch >= 0 && ch < MIXER_MAX_CHANNELS);
+	assert(!mixer_ch_playing(ch));
 	tracef("mixer_ch_set_limits: ch=%d bits=%d maxfreq:%.2f bufsz:%d\n", ch, max_bits, max_frequency, max_buf_sz);
 
 	Mixer.limits[ch] = (channel_limit_t){