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, 58 insertions, 0 deletions

diff --git a/src/libs/sig/ops_mult.c b/src/libs/sig/ops_mult.c
new file mode 100644
index 0000000..7133e25
--- /dev/null
+++ b/src/libs/sig/ops_mult.c
@@ -0,0 +1,58 @@
+#include <assert.h>
+
+#include "sig.h"
+
+
+typedef struct ops_mult_ctxt_t {
+	sig_t	*a, *b;
+} ops_mult_ctxt_t;
+
+
+static size_t ops_mult_size(va_list ap)
+{
+	return sizeof(ops_mult_ctxt_t);
+}
+
+
+/* supply two sig_t's to be multiplied, this sig_t takes
+ * ownership of them so they'll be freed by the multiplier
+ * on destroy when that sig_t is freed.
+ */
+static void ops_mult_init(void *context, va_list ap)
+{
+	ops_mult_ctxt_t	*ctxt = context;
+
+	assert(ctxt);
+
+	ctxt->a = va_arg(ap, sig_t *);
+	ctxt->b = va_arg(ap, sig_t *);
+}
+
+
+static float ops_mult_output(void *context, unsigned ticks_ms)
+{
+	ops_mult_ctxt_t	*ctxt = context;
+
+	assert(ctxt);
+
+	return sig_output(ctxt->a, ticks_ms) * sig_output(ctxt->b, ticks_ms);
+}
+
+
+static void ops_mult_destroy(void *context)
+{
+	ops_mult_ctxt_t	*ctxt = context;
+
+	assert(ctxt);
+
+	sig_free(ctxt->a);
+	sig_free(ctxt->b);
+}
+
+
+sig_ops_t	sig_ops_mult = {
+	.size = ops_mult_size,
+	.init = ops_mult_init,
+	.destroy = ops_mult_destroy,
+	.output = ops_mult_output,
+};