stfu/Source/envelope.c

#include <stdint.h>
#include <string.h>

#include "config.h"
#include "interp.c"

struct stfu_envelope stfu_pump_envelope(struct stfu_envelope envelope);
float stfu_get_envelope_duration(struct stfu_envelope envelope);
struct stfu_envelope stfu_trigger_envelope(struct stfu_envelope envelope);

/* Basic linear envelope generator with up to 16 points max.
 * First virtual point is always silent (0.0f).
 *
 * todo: Sustain?
 */
struct stfu_envelope {
  /* Initial volume is assumed from previous point */
  struct stfu_envelope_point {
    float dur, vol;
  } pnts[16];
  uint8_t n_pnts;
  float p, v;
} stfu_init_envelope(struct stfu_envelope_point *points, uint8_t n_points) {
  struct stfu_envelope r = {0};
  memcpy(&r.pnts, points, sizeof(struct stfu_envelope_point) * n_points);
  r.n_pnts = n_points;
  r = stfu_trigger_envelope(r);
  return r;
}

struct stfu_envelope stfu_pump_envelope(struct stfu_envelope envelope) {
  envelope.p += 1.0f / STFU_AUDIO_FRAME_RATE;
  float d = stfu_get_envelope_duration(envelope);
  if (envelope.p >= d)
    /* Needs retriggering after that point */
    return envelope;

  float p = 0.0f;
  for (uint8_t i = 0;; ++i) {
    float ends = p + envelope.pnts[i].dur;
    if (envelope.p < ends) {
      float pv = i == 0 ? 0.0f : envelope.pnts[i - 1].vol;
      envelope.v = stfu_linear_interpolate(p, pv, ends, envelope.pnts[i].vol,
                                           envelope.p);
      break;
    }
    p += envelope.pnts[i].dur;
  }

  return envelope;
}

float stfu_get_envelope_duration(struct stfu_envelope envelope) {
  float r = 0.0f;
  for (uint8_t i = 0; i < envelope.n_pnts; ++i) {
    r += envelope.pnts[i].dur;
  }
  return r;
}

struct stfu_envelope stfu_trigger_envelope(struct stfu_envelope envelope) {
  envelope.p = 0.0f;
  envelope.v = 0.0f;
  return envelope;
}