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, 30 insertions, 0 deletions

diff --git a/src/libs/sig/sig.h b/src/libs/sig/sig.h
new file mode 100644
index 0000000..cd22d4d
--- /dev/null
+++ b/src/libs/sig/sig.h
@@ -0,0 +1,30 @@
+#ifndef _SIG_H
+#define _SIG_H
+
+#include <stdarg.h>
+#include <stddef.h>
+
+typedef struct sig_t sig_t;
+
+typedef struct sig_ops_t {
+	size_t	(*size)(va_list ap);				/* return size of space needed for context for given ap */
+	void	(*init)(void *context, va_list ap);		/* initialize context w/given ap */
+	void	(*destroy)(void *context);			/* destroy initialized context */
+	float	(*output)(void *context, unsigned ticks_ms);	/* output a value 0-1 from context appropriate @ time ticks_ms */
+} sig_ops_t;
+
+sig_t * sig_new(const sig_ops_t *ops, ...);
+sig_t * sig_free(sig_t *sig);
+float sig_output(sig_t *sig, unsigned ticks_ms);
+
+extern sig_ops_t	sig_ops_sin;
+
+/* TODO:
+extern sig_ops_t	sig_ops_tri;
+extern sig_ops_t	sig_ops_saw;
+extern sig_ops_t	sig_ops_sqr;
+*/
+
+extern sig_ops_t	sig_ops_mult;
+
+#endif