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## -*- 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 function to check whether any object in cell array is *not* a vector.
  isnotvec = @(x) cellfun ("numel", x) != cellfun ("length", x);
  if (! iscell (x) || length (x) < ndims (y) || any (isnotvec (x))
      || ! iscell (xi) || length (xi) < ndims (y) || any (isnotvec (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

  if (! isempty (extrapval))
    [xi{:}] = ndgrid (cellfun (@(x) x(:), xi, "uniformoutput", false){:});
    idx = false (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