delay.c

static double D[M];   /* initialized to zero */
static long ptr=0;    /* read-write offset */

double delayline(double x) {
  double y = D[ptr];  /* read operation */
  D[ptr++] = x;       /* write operation */
  // *(ptr++) = x;  ??
  if (ptr >= M) {
    ptr -= M;         /* wrap ptr */
  }                 
  return y;
}