#+TITLE: expon
* Overview
A (re)triggerable exponential line generator. Go from A
to B in a exponential swell over some duration in seconds.
When it reaches B, hold indefinitely until retriggered.

Note that values *cannot* be exactly 0.
* Tangled files.
=expon.h= and =expon.c=. =SK_EXPON_PRIV= exposes the
=sk_expon= struct.

#+NAME: expon.h
#+BEGIN_SRC c :tangle expon.h
#ifndef SK_EXPON_H
#define SK_EXPON_H
#ifndef SKFLT
#define SKFLT float
#endif

<<typedefs>>
<<funcdefs>>
#ifdef SK_EXPON_PRIV
<<structs>>
#endif
#endif
#+END_SRC

#+NAME: expon.c
#+BEGIN_SRC c :tangle expon.c
#include <math.h>
#define SK_EXPON_PRIV
#include "expon.h"
<<static_funcdefs>>
<<funcs>>
#+END_SRC
* Initialization
The expon state is managed in a struct called =sk_expon=,
and initialized  with =sk_expon_init=. The sampling rate
=sr= must be provided.
#+NAME: typedefs
#+BEGIN_SRC c
typedef struct sk_expon sk_expon;
#+END_SRC

#+NAME: structs
#+BEGIN_SRC c
struct sk_expon {
    SKFLT a, dur, b;
    SKFLT val, incr;
    unsigned long sdur, stime;
    int init;
    int sr;
};
#+END_SRC

#+NAME: funcdefs
#+BEGIN_SRC c
void sk_expon_init(sk_expon *e, int sr);
#+END_SRC


#+NAME: funcs
#+BEGIN_SRC c
void sk_expon_init(sk_expon *e, int sr)
{
    sk_expon_a(e, 0.000001);
    sk_expon_b(e, 1);
    sk_expon_dur(e, 1);
    expon_reinit(e);
    e->init = 1;
    e->sr = sr;
}
#+END_SRC
* Setting Parameters
** A and B
Go from value A to value B.
#+NAME: funcdefs
#+BEGIN_SRC c
void sk_expon_a(sk_expon *e, SKFLT val);
void sk_expon_b(sk_expon *e, SKFLT val);
#+END_SRC

#+NAME: funcs
#+BEGIN_SRC c
void sk_expon_a(sk_expon *e, SKFLT val)
{
    if (val != 0) e->a = val;
}

void sk_expon_b(sk_expon *e, SKFLT val)
{
    if (val != 0) e->b = val;
}
#+END_SRC

** Duration
Set the duration of the segment with =sk_expon_dur=.
This is in units of seconds.

#+NAME: funcdefs
#+BEGIN_SRC c
void sk_expon_dur(sk_expon *e, SKFLT dur);
#+END_SRC

#+NAME: funcs
#+BEGIN_SRC c
void sk_expon_dur(sk_expon *e, SKFLT dur)
{
    e->dur = dur;
}
#+END_SRC
* Calculating Multipler
The function =expon_reinit= will recalculate the multiplier
used to compute the exponential line.

#+NAME: static_funcdefs
#+BEGIN_SRC c
static void expon_reinit(sk_expon *e);
#+END_SRC

#+NAME: funcs
#+BEGIN_SRC c
static void expon_reinit(sk_expon *e)
{
    SKFLT onedsr;
    e->stime = 0;
    e->sdur = e->dur * e->sr;
    onedsr = 1.0 / e->sr;

    if ((e->a * e->b) > 0.0) {
        e->incr = pow((SKFLT)(e->b / e->a), onedsr / e->dur);
    } else {
        e->incr = 1;
        e->val = e->a;
    }

    e->val = e->a;
}
#+END_SRC
* Computing a Sample
#+NAME: funcdefs
#+BEGIN_SRC c
SKFLT sk_expon_tick(sk_expon *e, SKFLT trig);
#+END_SRC

#+NAME: funcs
#+BEGIN_SRC c
SKFLT sk_expon_tick(sk_expon *e, SKFLT trig)
{
    SKFLT out;

    out = 0;

    if (trig != 0) {
        expon_reinit(e);
        e->init = 0;
    }

    if (e->init) {
        out = 0;
    }

    if (e->stime < e->sdur) {
        SKFLT val = e->val;
        e->val *= e->incr;
        e->stime++;
        out = val;
    } else {
        out = e->b;
    }

    return out;
}
#+END_SRC