Add ac2rc: autocorrelation to reflection coeffs

INDEX

@@ -173,6 +173,7 @@ Window Functions
   wvtool
 System Identification
   ac2poly
+  ac2rc
   arburg
   aryule
   invfreq

inst/ac2rc.m

@@ -0,0 +1,62 @@
+## Copyright (C) 2026 Tang Chonghao <chadholton@qq.com>
+##
+## This program is free software: you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; see the file COPYING. If not, see
+## <https://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn  {Function File} {[@var{k}, @var{r0}] =} ac2rc (@var{r})
+## Convert autocorrelation sequence to reflection coefficients.
+##
+## Compute the reflection coefficients @var{k} from the autocorrelation
+## sequence @var{r} using the Levinson-Durbin recursion. The second optional
+## output @var{r0} returns the zero-lag autocorrelation @math{r(0)}.
+##
+## This function directly calls @code{levinson}.
+##
+## @seealso{levinson}
+## @end deftypefn
+
+function [k, r0] = ac2rc (r)
+
+  if (nargin != 1)
+    print_usage ();
+  endif
+
+  if (!isnumeric (r) || !(isvector (r) || ismatrix (r)))
+    error ("ac2rc: R must be a numeric vector or matrix.");
+  endif
+
+  [~, ~, k] = levinson (r);
+  r0 = r(1,:).';
+
+endfunction
+
+
+%!error ac2rc()
+%!error ac2rc(1,2)
+%!error <numeric vector or matrix> ac2rc("abc")
+%!error <numeric vector or matrix> ac2rc({1,2,3})
+
+%!test
+%! r = [3, -1, -4, 3, 1, -2];
+%! [k, r0] = ac2rc(r);
+%! assert (k, [0.3333;1.6250;-0.4857;-0.9145;3.6719], 1e-4);
+%! assert (r0, r');
+
+%!test
+%! r = [3, -1, -4, 3, 1, -2];
+%! [k, r0] = ac2rc([r', r']);
+%! assert (k(:,1), [0.3333;1.6250;-0.4857;-0.9145;3.6719], 1e-4);
+%! assert (k(:,2), [0.3333;1.6250;-0.4857;-0.9145;3.6719], 1e-4);
+%! assert (r0, [3;3], 1e-4);