## Copyright (C) 2008-2011 David Bateman
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or (at
## your option) any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {@var{yi} =} interp1q (@var{x}, @var{y}, @var{xi})
## One-dimensional linear interpolation without error checking.
## Interpolates @var{y}, defined at the points @var{x}, at the points
## @var{xi}.  The sample points @var{x} must be a strictly monotonically
## increasing column vector.  If @var{y} is an array, treat the columns
## of @var{y} separately.  If @var{y} is a vector, it must be a column
## vector of the same length as @var{x}.
##
## Values of @var{xi} beyond the endpoints of the interpolation result
## in NA being returned.
##
## Note that the error checking is only a significant portion of the
## execution time of this @code{interp1} if the size of the input arguments
## is relatively small.  Therefore, the benefit of using @code{interp1q}
## is relatively small.
## @seealso{interp1}
## @end deftypefn

function yi = interp1q (x, y, xi)
  x = x(:);
  nx = size (x, 1);
  szy = size (y);
  y = y(:,:);
  [ny, nc] = size (y);
  szx = size (xi);
  xi = xi (:);
  dy = diff (y);
  dx = diff (x);
  idx = lookup (x, xi, "lr");
  s = (xi - x (idx)) ./ dx (idx);
  yi = bsxfun (@times, s, dy(idx,:)) + y(idx,:);
  range = xi < x(1) | !(xi <= x(nx));
  yi(range,:) = NA;
  if (length (szx) == 2 && any (szx == 1))
    yi = reshape (yi, [max(szx), szy(2:end)]);
  else
    yi = reshape (yi, [szx, szy(2:end)]);
  endif
endfunction

%!shared xp, yp, xi, yi
%! xp=[0:2:10].';      yp = sin(2*pi*xp/5);
%! xi = [-1; 0; 2.2; 4; 6.6; 10; 11];
%! yi = interp1 (xp,yp,xi);
%!assert (interp1q(xp, yp, [min(xp)-1; max(xp)+1]), [NA; NA]);
%!assert (interp1q(xp,yp,xp), yp, 100*eps);
%!assert (isempty(interp1q(xp,yp,[])));
%!assert (interp1q(xp,yp,xi), yi);
%!assert (interp1q(xp,[yp,yp],xi), [yi, yi]);
%!assert (interp1q(xp,yp,[xi,xi]), [yi, yi]);
%!assert (interp1q(xp,[yp,yp],[xi,xi]), cat (3, [yi, yi], [yi, yi]));