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## -*- texinfo -*-
## @deftypefn {} {[@var{newphi}, @var{newv}] =} durbinlevinson (@var{c}, @var{oldphi}, @var{oldv})
## Perform one step of the @nospell{Durbin-Levinson} algorithm.
##
## The vector @var{c} specifies the autocovariances
## @code{[gamma_0, @dots{}, gamma_t]} from lag 0 to @var{t}, @var{oldphi}
## specifies the coefficients based on @var{c}(@var{t}-1) and @var{oldv}
## specifies the corresponding error.
##
## If @var{oldphi} and @var{oldv} are omitted, all steps from 1 to @var{t} of
## the algorithm are performed.
## @end deftypefn

function [newphi, newv] = durbinlevinson (c, oldphi, oldv)

  if (nargin != 1 && nargin != 3)
    print_usage ();
  endif

  if (columns (c) > 1)
    c = c';
  endif

  newphi = 0;
  newv = 0;

  if (nargin == 3)

    t = length (oldphi) + 1;

    if (length (c) < t+1)
      error ("durbinlevinson: C too small");
    endif

    if (oldv == 0)
      error ("durbinlevinson: OLDV = 0");
    endif

    if (rows (oldphi) > 1)
      oldphi = oldphi';
    endif

    newphi = zeros (1, t);
    newphi(1) = (c(t+1) - oldphi * c(2:t)) / oldv;
    for i = 2 : t
      newphi(i) = oldphi(i-1) - newphi(1) * oldphi(t-i+1);
    endfor
    newv = (1 - newphi(1)^2) * oldv;

  elseif (nargin == 1)

    tt = length (c)-1;
    oldphi = c(2) / c(1);
    oldv = (1 - oldphi^2) * c(1);

    for t = 2 : tt

      newphi = zeros (1, t);
      newphi(1) = (c(t+1) - oldphi * c(2:t)) / oldv;
      for i = 2 : t
        newphi(i) = oldphi(i-1) - newphi(1) * oldphi(t-i+1);
      endfor
      newv = (1 - newphi(1)^2) * oldv;

      oldv = newv;
      oldphi = newphi;

    endfor

  endif

endfunction