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## Undocumented internal function.

## -*- texinfo -*-
## @deftypefn {} {@var{yi} =} __splinen__ (@var{x}, @var{y}, @var{xi}, @var{extrapval}, @var{f})
## Undocumented internal function.
## @end deftypefn

## FIXME: Allow arbitrary grids..

function yi = __splinen__ (x, y, xi, extrapval, f)

  ## ND isvector function.
  isvec = @(x) numel (x) == length (x);
  if (! iscell (x) || length (x) < ndims (y) || any (! cellfun (isvec, x))
      || ! iscell (xi) || length (xi) < ndims (y)
      || any (! cellfun (isvec, xi)))
    error ("__splinen__: %s: non-gridded data or dimensions inconsistent", f);
  endif
  yi = y;
  for i = length (x):-1:1
    yi = permute (spline (x{i}, yi, xi{i}(:)), [length(x),1:length(x)-1]);
  endfor

  [xi{:}] = ndgrid (cellfun (@(x) x(:), xi, "uniformoutput", false){:});
  if (! isempty (extrapval))
    idx = zeros (size (xi{1}));
    for i = 1 : length (x)
      idx |= xi{i} < min (x{i}(:)) | xi{i} > max (x{i}(:));
    endfor
    yi(idx) = extrapval;
  endif

endfunction
