libs/sig: introduce a signal generator abstraction

This adds a small framework of sorts for creating and composing signal
generators.

Two generators are implemented at this time; sig_ops_sin and sig_ops_mult

sig_ops_sin accepts a hz variable and will produce a sine wave of that
frequency.

sig_ops_mult accepts two sig_t generators and multiplies their outputs

Callers may construct their own sig_ops_t ops structs and supply them to
sig_new(), but it's expected that libs/sig will grow a collection of
commonly used generators which can then be used by simply passing their
sig_ops_$foo to sig_new().

See the test code at the bottom of libs/sig/sig.c for some contrived
sample usage.  Note by composing multiple sig_ops_sin generators with
a sig_ops_mult generator, one can already easily construct a synth-like
LFO generator.

Some obvious todos are to add triangle/sawtooth/square wave generators.

More compositional generators may be interesting as well, like additive
and subtractive for example.  Those will need to implement clipping, as
it's expected that the generators *always* stay within unity (0-1).

No modules use this yet, but I expect to wire this up to rtv for driving
knobs.

1 files changed, 100 insertions, 0 deletions

diff --git a/src/libs/sig/sig.c b/src/libs/sig/sig.c
new file mode 100644
index 0000000..e3d5a7c
--- /dev/null
+++ b/src/libs/sig/sig.c
@@ -0,0 +1,100 @@
+#include <assert.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+#include "sig.h"
+
+
+typedef struct sig_t {
+	const sig_ops_t	*ops;
+	void		*ctxt[];
+} sig_t;
+
+
+/* return a new signal generator of ops type, configured according to va_list */
+sig_t * sig_new(const sig_ops_t *ops, ...)
+{
+	static const sig_ops_t	null_ops;
+	size_t			ctxt_size = 0;
+	sig_t			*sig;
+	va_list			ap;
+
+	if (!ops)
+		ops = &null_ops;
+
+	va_start(ap, ops);
+	if (ops->size)
+		ctxt_size = ops->size(ap);
+	va_end(ap);
+
+	sig = calloc(1, sizeof(sig_t) + ctxt_size);
+	if (!sig)
+		return NULL;
+
+	va_start(ap, ops);
+	if (ops->init)
+		ops->init(&sig->ctxt, ap);
+	va_end(ap);
+
+	sig->ops = ops;
+
+	return sig;
+}
+
+
+/* free a signal generator, always returns NULL */
+sig_t * sig_free(sig_t *sig)
+{
+	if (sig) {
+		if (sig->ops->destroy)
+			sig->ops->destroy(&sig->ctxt);
+
+		free(sig);
+	}
+
+	return NULL;
+}
+
+
+/* produce the value for time ticks_ms from the supplied signal generator,
+ * the returned value should always be kept in the range 0-1.
+ */
+float sig_output(sig_t *sig, unsigned ticks_ms)
+{
+	assert(sig);
+	assert(sig->ops);
+
+	if (sig->ops->output)
+		return sig->ops->output(sig->ctxt, ticks_ms);
+
+	return 0;
+}
+
+
+#ifdef TESTING
+
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+	sig_t	*sig;
+
+	sig = sig_new(NULL);
+	printf("null output=%f\n", sig_output(sig, 0));
+	sig = sig_free(sig);
+
+	sig = sig_new(&sig_ops_sin, 2.f);
+	for (unsigned i = 0; i < 1000; i++)
+		printf("sin 2hz output %i=%f\n", i, sig_output(sig, i));
+	sig = sig_free(sig);
+
+	sig = sig_new(&sig_ops_mult,
+		      sig_new(&sig_ops_sin, 1.f),	/* LFO @ 1hz */
+		      sig_new(&sig_ops_sin, 100.f)	/* oscillator @ 100hz */
+		);
+	for (unsigned i = 0; i < 1000; i++)
+		printf("sin 100hz * 1hz output %i=%f\n", i, sig_output(sig, i));
+	sig = sig_free(sig);
+}
+
+#endif