/* Graph a decimating CIC filter’s B-spline impulse response in ASCII
 * art.
 *
 * It doesn’t work.
 */

#include <stdio.h>
#include <assert.h>
#include <stdint.h>

typedef uint32_t u32;         /* This should have been the default. */

enum {
  lg_window_size = 5,
  filter_order = 3,
  lg_scale_factor = lg_window_size * filter_order,
  lg_impulse_size = 6,
  total_accumulator_bits = 1 + lg_impulse_size + lg_scale_factor,
};

int main() {
  u32 a, b = 0, c = 0, d = 0, f = 0, g = 0, h = 0, m = 0, n = 0, o;
  a = 1 << (total_accumulator_bits - 1); /* ensure enough bits */
  assert(a);
  for (int i = 0; i < (2 + filter_order) << lg_window_size; i++) {
    a = (i ? 0 : 1 << lg_impulse_size);
    if (!(i & ((1 << lg_window_size) - 1))) {
      g = d - f;                /* previous d */
      f = d;
      h = g - m;                /* previous g */
      m = g;
      o = (h - n) >> lg_scale_factor; /* previous h */
      printf("%6d ", o);
      for (int j = 0; j < o && j < 1 << lg_impulse_size; j++) {
        fputs("##", stdout);
      }
      puts("");
      n = h;
    }
    b += a;
    c += b;
    d += c;
  }
  return 0;
}