Variable Delay Line

   static double A[N];
   static double *rptr = A; // read ptr
   static double *wptr = A; // write ptr

   double setdelay(int M) {
       rptr = wptr - M;
       while (rptr < A) { rptr += N }
   }

   double delayline(double x)
   {
     double y;
     A[wptr++] = x; 
     y = A[rptr++];
     if ((wptr-A) >= N) { wptr -= N }
     if ((rptr-A) >= N) { rptr -= N }
     return y;
   }

Note separate read- and write-pointers