#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void filter(signed char *buf, int n)
{
  enum { bufsiz = 256, m = bufsiz-1 };
  int b[bufsiz] = {0};

  for (unsigned bi = 0; bi != n; bi++) {
    b[bi & m] = buf[bi];
    b[(bi - 1) & m] -= b[bi & m];
    b[(bi - 6) & m] -= b[(bi - 1) & m];
    b[(bi - 21) & m] += b[(bi - 6) & m];
    buf[bi] = b[(bi - 21) & m];
  }
}

void filter2(signed char *buf, int n)
{
  enum { bufsiz = 256, m = bufsiz-1 };
  int b[bufsiz] = {0};

  for (unsigned bi = 0; bi != n; bi++) {
    b[bi & m] = buf[bi];
    b[bi & m] += b[(bi - 1) & m];
    b[(bi - 1) & m] += b[(bi - 2) & m];
    b[(bi - 7) & m] -= b[(bi - 1) & m];
    b[(bi - 12) & m] -= b[(bi - 7) & m];
    buf[bi] = b[(bi - 12) & m] >> 3;
  }
}

/* This one is designed, if that word isn’t oversimplifying the
   activity, to extract a downconverted and decimated 19110-Hz signal
   from a 44100-Hz input.
 */
unsigned filter3(signed char *buf, int n)
{
  enum { bufsiz = 256, m = bufsiz-1 };
  int b[bufsiz] = {0};
  uint64_t d1=0, d2=0, d3=0, d4=0, i1=0, i2=0, i3=0, tmp, tmp2, c2=0, c3=0;
  int bi5=0, bo=0, odd=0;

  for (unsigned bi = 0; bi != n; bi++, bi5++) {
    tmp = buf[bi];
    tmp2 = tmp - d1;            /* Differentiator 1 */
    d1 = tmp;
    tmp = tmp2 - d2;            /* Differentiator 2 */
    d2 = tmp2;
    tmp2 = tmp - d3;            /* Differentiator 3 */
    d3 = tmp;
    tmp = tmp2 - d4;            /* Differentiator 4 */
    d4 = tmp2;
    i1 += odd ? -tmp : tmp;     /* Integrator 1 */
    odd = ~odd;
    i2 += i1;                   /* Integrator 2 */
    i3 += i2;                   /* Integrator 3 */

    if (bi5 == 5) {
      bi5 = 0;
      /* First comb filter: y(n) = x(n-2) - x(n); note inversion */
      b[bi & m] = i3;
      b[(bi - 2) & m] -= i3;
      /* Second comb filter: y(n) = x(n-1) - x(n); canceling inversion */
      tmp = c2 - b[(bi - 2) & m];
      c2 = b[(bi - 2) & m];
      /* Third comb filter: y(n) = x(n) - x(n-1), this time not inverting; but now into the output buffer */
      buf[bo++] = tmp - c3;
      c3 = tmp;
    }
  }
  return bo;
}

int main()
{
  signed char buf[4096];
  enum { allocsize = 100*1024*1024 };
  signed char *buf2 = malloc(allocsize);
  if (!buf2) abort();
  int n = read(0, buf2, sizeof(buf));
  filter2(buf2, allocsize);
  if (n != write(1, buf, n)) abort();
  return 0;
}