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function S_permuted = xxpp_to_xpxp(S)
% Permutes the entries of the input from xxpp ordering to xpxp ordering.
%
% Args:
% S (array): input even dimensional square matrix or array
%
% Returns:
% S_permuted (array): permuted matrix or array
shape = size(S);
n = shape(1);
if mod(n, 2) ~= 0
error('The input array is not even-dimensional');
end
n = n / 2;
ind = reshape(1:2*n, 2, [])';
ind = ind(:);
if length(shape) == 2
if shape(1) ~= shape(2)
error('The input matrix is not square');
end
S_permuted = S(ind, ind);
else
S_permuted = S(ind);
end
end
%!test
%! [X, Y]=meshgrid (1:4, 1:4);
%! Mxpxp=xxpp_to_xpxp(X);
%! assert(any(any(X != Mxpxp)));
%! Mxxpp=xpxp_to_xxpp(Mxpxp);
%! assert(X, Mxxpp)
%! Mxpxp=xxpp_to_xpxp(Y);
%! assert(any(any(Y != Mxpxp)));
%! Mxxpp=xpxp_to_xxpp(Mxpxp);
%! assert(Y, Mxxpp)
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