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##
## Copyright (C) 2007-2021 The Octave Project Developers
##
## See the file COPYRIGHT.md in the top-level directory of this
## distribution or <https://octave.org/copyright/>.
##
## This file is part of Octave.
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## along with Octave; see the file COPYING.  If not, see
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########################################################################

## -*- texinfo -*-
## @deftypefn  {} {@var{idx} =} tsearchn (@var{x}, @var{t}, @var{xi})
## @deftypefnx {} {[@var{idx}, @var{p}] =} tsearchn (@var{x}, @var{t}, @var{xi})
## Search for the enclosing Delaunay convex hull.
##
## For @code{@var{t} = delaunayn (@var{x})}, finds the index in @var{t}
## containing the points @var{xi}.  For points outside the convex hull,
## @var{idx} is NaN.
##
## If requested @code{tsearchn} also returns the Barycentric coordinates
## @var{p} of the enclosing triangles.
## @seealso{delaunay, delaunayn}
## @end deftypefn

function [idx, p] = tsearchn (x, t, xi)

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

  nt = rows (t);
  [m, n] = size (x);
  mi = rows (xi);
  idx = NaN (mi, 1);
  p = NaN (mi, n + 1);

  ni = [1:mi].';
  for i = 1 : nt
    ## Only calculate the Barycentric coordinates for points that have not
    ## already been found in a triangle.
    b = cart2bary (x (t (i, :), :), xi(ni,:));

    ## Our points xi are in the current triangle if
    ## (all (b >= 0) && all (b <= 1)).  However as we impose that
    ## sum (b,2) == 1 we only need to test all(b>=0).  Note need to add
    ## a small margin for rounding errors
    intri = all (b >= -1e-12, 2);
    idx(ni(intri)) = i;
    p(ni(intri),:) = b(intri, :);
    ni(intri) = [];
  endfor

endfunction

function Beta = cart2bary (T, P)

  ## Conversion of Cartesian to Barycentric coordinates.
  ## Given a reference simplex in N dimensions represented by an
  ## N+1-by-N matrix, an arbitrary point P in Cartesian coordinates,
  ## represented by an N-by-1 column vector can be written as
  ##
  ## P = Beta * T
  ##
  ## Where Beta is an N+1 vector of the barycentric coordinates.  A criteria
  ## on Beta is that
  ##
  ## sum (Beta) == 1
  ##
  ## and therefore we can write the above as
  ##
  ## P - T(end, :) = Beta(1:end-1) * (T(1:end-1,:) - ones (N,1) * T(end,:))
  ##
  ## and then we can solve for Beta as
  ##
  ## Beta(1:end-1) = (P - T(end,:)) / (T(1:end-1,:) - ones (N,1) * T(end,:))
  ## Beta(end) = sum (Beta)
  ##
  ## Note code below is generalized for multiple values of P, one per row.
  [M, N] = size (P);
  Beta = (P - ones (M,1) * T(end,:)) / (T(1:end-1,:) - ones (N,1) * T(end,:));
  Beta (:,end+1) = 1 - sum (Beta, 2);

endfunction


%!shared x, tri
%! x = [-1,-1;-1,1;1,-1];
%! tri = [1, 2, 3];
%!test
%! [idx, p] = tsearchn (x,tri,[-1,-1]);
%! assert (idx, 1);
%! assert (p, [1,0,0], 1e-12);
%!test
%! [idx, p] = tsearchn (x,tri,[-1,1]);
%! assert (idx, 1);
%! assert (p, [0,1,0], 1e-12);
%!test
%! [idx, p] = tsearchn (x,tri,[1,-1]);
%! assert (idx, 1);
%! assert (p, [0,0,1], 1e-12);
%!test
%! [idx, p] = tsearchn (x,tri,[-1/3,-1/3]);
%! assert (idx, 1);
%! assert (p, [1/3,1/3,1/3], 1e-12);
%!test
%! [idx, p] = tsearchn (x,tri,[1,1]);
%! assert (idx, NaN);
%! assert (p, [NaN, NaN, NaN]);