/* modwrap.c - modulo-N wrapping of length-L signal */void modwrap(L, x, N, xtilde)                /* usage: modwrap(L, x, N, xtilde); */int L, N;                                    /* \(x\) is \(L\)-dimensional */double *x, *xtilde;                          /* xtilde is \(N\)-dimensional */{    int n, r, m, M;    r = L % N;                               /* remainder \(r=0,1,\dotsc,N-1\) */    M = (L-r) / N;                           /* quotient of division \(L/N\) */    for (n=0; n<N; n++) {         if (n < r)                          /* non-zero part of last block */             xtilde[n] = x[M*N+n];           /* if \(L<N\), this is the only block */         else             xtilde[n] = 0;                  /* if \(L<N\), pad \(N-L\) zeros at end */         for (m=M-1; m>=0; m--)              /* remaining blocks */              xtilde[n] += x[m*N+n];         /* if \(L<N\), this loop is skipped */         }}