## Copyright (C) 2000-2012 Paul Kienzle
## Copyright (C) 2009 VZLU Prague
##
## 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} =} interp1 (@var{x}, @var{y}, @var{xi})
## @deftypefnx {Function File} {@var{yi} =} interp1 (@var{y}, @var{xi})
## @deftypefnx {Function File} {@var{yi} =} interp1 (@dots{}, @var{method})
## @deftypefnx {Function File} {@var{yi} =} interp1 (@dots{}, @var{extrap})
## @deftypefnx {Function File} {@var{pp} =} interp1 (@dots{}, "pp")
##
## One-dimensional interpolation.  Interpolate @var{y}, defined at the
## points @var{x}, at the points @var{xi}.  The sample points @var{x}
## must be monotonic.  If not specified, @var{x} is taken to be the
## indices of @var{y}.  If @var{y} is an array, treat the columns
## of @var{y} separately.
##
## Method is one of:
##
## @table @asis
## @item "nearest"
## Return the nearest neighbor.
##
## @item "linear"
## Linear interpolation from nearest neighbors
##
## @item "pchip"
## Piecewise cubic Hermite interpolating polynomial
##
## @item "cubic"
## Cubic interpolation (same as @code{pchip})
##
## @item "spline"
## Cubic spline interpolation---smooth first and second derivatives
## throughout the curve
## @end table
##
## Appending '*' to the start of the above method forces @code{interp1}
## to assume that @var{x} is uniformly spaced, and only @code{@var{x}
## (1)} and @code{@var{x} (2)} are referenced.  This is usually faster,
## and is never slower.  The default method is "linear".
##
## If @var{extrap} is the string "extrap", then extrapolate values beyond
## the endpoints.  If @var{extrap} is a number, replace values beyond the
## endpoints with that number.  If @var{extrap} is missing, assume NA.
##
## If the string argument "pp" is specified, then @var{xi} should not be
## supplied and @code{interp1} returns the piecewise polynomial that
## can later be used with @code{ppval} to evaluate the interpolation.
## There is an equivalence, such that @code{ppval (interp1 (@var{x},
## @var{y}, @var{method}, "pp"), @var{xi}) == interp1 (@var{x}, @var{y},
## @var{xi}, @var{method}, "extrap")}.
##
## Duplicate points in @var{x} specify a discontinuous interpolant.  There
## should be at most 2 consecutive points with the same value.
## The discontinuous interpolant is right-continuous if @var{x} is increasing,
## left-continuous if it is decreasing.
## Discontinuities are (currently) only allowed for "nearest" and "linear"
## methods; in all other cases, @var{x} must be strictly monotonic.
##
## An example of the use of @code{interp1} is
##
## @example
## @group
## xf = [0:0.05:10];
## yf = sin (2*pi*xf/5);
## xp = [0:10];
## yp = sin (2*pi*xp/5);
## lin = interp1 (xp, yp, xf);
## spl = interp1 (xp, yp, xf, "spline");
## cub = interp1 (xp, yp, xf, "cubic");
## near = interp1 (xp, yp, xf, "nearest");
## plot (xf, yf, "r", xf, lin, "g", xf, spl, "b",
##       xf, cub, "c", xf, near, "m", xp, yp, "r*");
## legend ("original", "linear", "spline", "cubic", "nearest");
## @end group
## @end example
##
## @seealso{interpft}
## @end deftypefn

## Author: Paul Kienzle
## Date: 2000-03-25
##    added 'nearest' as suggested by Kai Habel
## 2000-07-17 Paul Kienzle
##    added '*' methods and matrix y
##    check for proper table lengths
## 2002-01-23 Paul Kienzle
##    fixed extrapolation

function yi = interp1 (x, y, varargin)

  if (nargin < 2 || nargin > 6)
    print_usage ();
  endif

  method = "linear";
  extrap = NA;
  xi = [];
  ispp = false;
  firstnumeric = true;

  if (nargin > 2)
    for i = 1:length (varargin)
      arg = varargin{i};
      if (ischar (arg))
        arg = tolower (arg);
        if (strcmp ("extrap", arg))
          extrap = "extrap";
        elseif (strcmp ("pp", arg))
          ispp = true;
        else
          method = arg;
        endif
      else
        if (firstnumeric)
          xi = arg;
          firstnumeric = false;
        else
          extrap = arg;
        endif
      endif
    endfor
  endif

  if (isempty (xi) && firstnumeric && ! ispp)
    xi = y;
    y = x;
    x = 1:numel(y);
  endif

  ## reshape matrices for convenience
  x = x(:);
  nx = rows (x);
  szx = size (xi);
  if (isvector (y))
    y = y(:);
  endif

  szy = size (y);
  y = y(:,:);
  [ny, nc] = size (y);
  xi = xi(:);

  ## determine sizes
  if (nx < 2 || ny < 2)
    error ("interp1: table too short");
  endif

  ## check whether x is sorted; sort if not.
  if (! issorted (x, "either"))
    [x, p] = sort (x);
    y = y(p,:);
  endif

  starmethod = method(1) == "*";

  if (starmethod)
    dx = x(2) - x(1);
  else
    jumps = x(1:nx-1) == x(2:nx);
    have_jumps = any (jumps);
    if (have_jumps)
      if (any (strcmp (method, {"nearest", "linear"})))
        if (any (jumps(1:nx-2) & jumps(2:nx-1)))
          warning ("interp1: extra points in discontinuities");
        endif
      else
        error ("interp1: discontinuities not supported for method %s", method);
      endif
    endif
  endif

  ## Proceed with interpolating by all methods.

  switch (method)
  case "nearest"
    pp = mkpp ([x(1); (x(1:nx-1)+x(2:nx))/2; x(nx)], shiftdim (y, 1), szy(2:end));
    pp.orient = "first";

    if (ispp)
      yi = pp;
    else
      yi = ppval (pp, reshape (xi, szx));
    endif
  case "*nearest"
    pp = mkpp ([x(1), x(1)+[0.5:(nx-1)]*dx, x(nx)], shiftdim (y, 1), szy(2:end));
    pp.orient = "first";
    if (ispp)
      yi = pp;
    else
      yi = ppval(pp, reshape (xi, szx));
    endif
  case "linear"
    dy = diff (y);
    dx = diff (x);
    dx = repmat (dx, [1 size(dy)(2:end)]);
    coefs = [(dy./dx).'(:), y(1:nx-1, :).'(:)];
    xx = x;

    if (have_jumps)
      ## Omit zero-size intervals.
      coefs(jumps, :) = [];
      xx(jumps) = [];
    endif

    pp = mkpp (xx, coefs, szy(2:end));
    pp.orient = "first";

    if (ispp)
      yi = pp;
    else
      yi = ppval(pp, reshape (xi, szx));
    endif

  case "*linear"
    dy = diff (y);
    coefs = [(dy/dx).'(:), y(1:nx-1, :).'(:)];
    pp = mkpp (x, coefs, szy(2:end));
    pp.orient = "first";

    if (ispp)
      yi = pp;
    else
      yi = ppval(pp, reshape (xi, szx));
    endif

  case {"pchip", "*pchip", "cubic", "*cubic"}
    if (nx == 2 || starmethod)
      x = linspace (x(1), x(nx), ny);
    endif

    if (ispp)
      y = shiftdim (reshape (y, szy), 1);
      yi = pchip (x, y);
      yi.orient = "first";
    else
      y = shiftdim (y, 1);
      yi = pchip (x, y, reshape (xi, szx));
      if (! isvector (y))
        yi = shiftdim (yi, 1);
      endif
    endif
  case {"spline", "*spline"}
    if (nx == 2 || starmethod)
      x = linspace(x(1), x(nx), ny);
    endif

    if (ispp)
      y = shiftdim (reshape (y, szy), 1);
      yi = spline (x, y);
      yi.orient = "first";
    else
      y = shiftdim (y, 1);
      yi = spline (x, y, reshape (xi, szx));
      if (! isvector (y))
        yi = shiftdim (yi, 1);
      endif
    endif
  otherwise
    error ("interp1: invalid method '%s'", method);
  endswitch

  if (! ispp)
    if (! ischar (extrap))
      ## determine which values are out of range and set them to extrap,
      ## unless extrap == "extrap".
      minx = min (x(1), x(nx));
      maxx = max (x(1), x(nx));

      outliers = xi < minx | ! (xi <= maxx); # this catches even NaNs
      if (size_equal (outliers, yi))
        yi(outliers) = extrap;
        yi = reshape (yi, szx);
      elseif (!isvector (yi))
        yi(outliers, :) = extrap;
      else
        yi(outliers.') = extrap;
      endif
    endif
  endif

endfunction


%!demo
%! clf;
%! xf = 0:0.05:10;  yf = sin (2*pi*xf/5);
%! xp = 0:10;       yp = sin (2*pi*xp/5);
%! lin = interp1 (xp,yp,xf, "linear");
%! spl = interp1 (xp,yp,xf, "spline");
%! cub = interp1 (xp,yp,xf, "pchip");
%! near= interp1 (xp,yp,xf, "nearest");
%! plot (xf,yf,"r",xf,near,"g",xf,lin,"b",xf,cub,"c",xf,spl,"m",xp,yp,"r*");
%! legend ("original", "nearest", "linear", "pchip", "spline");
%! %--------------------------------------------------------
%! % confirm that interpolated function matches the original

%!demo
%! clf;
%! xf = 0:0.05:10;  yf = sin (2*pi*xf/5);
%! xp = 0:10;       yp = sin (2*pi*xp/5);
%! lin = interp1 (xp,yp,xf, "*linear");
%! spl = interp1 (xp,yp,xf, "*spline");
%! cub = interp1 (xp,yp,xf, "*cubic");
%! near= interp1 (xp,yp,xf, "*nearest");
%! plot (xf,yf,"r",xf,near,"g",xf,lin,"b",xf,cub,"c",xf,spl,"m",xp,yp,"r*");
%! legend ("*original", "*nearest", "*linear", "*cubic", "*spline");
%! %--------------------------------------------------------
%! % confirm that interpolated function matches the original

%!demo
%! clf;
%! t = 0 : 0.3 : pi; dt = t(2)-t(1);
%! n = length (t); k = 100; dti = dt*n/k;
%! ti = t(1) + [0 : k-1]*dti;
%! y = sin (4*t + 0.3) .* cos (3*t - 0.1);
%! ddyc = diff (diff (interp1 (t,y,ti, "cubic")) ./dti)./dti;
%! ddys = diff (diff (interp1 (t,y,ti, "spline"))./dti)./dti;
%! ddyp = diff (diff (interp1 (t,y,ti, "pchip")) ./dti)./dti;
%! plot (ti(2:end-1),ddyc,'g+', ti(2:end-1),ddys,'b*', ti(2:end-1),ddyp,'c^');
%! legend ("cubic", "spline", "pchip");
%! title ("Second derivative of interpolated 'sin (4*t + 0.3) .* cos (3*t - 0.1)'");

%!demo
%! clf;
%! xf = 0:0.05:10;                yf = sin (2*pi*xf/5) - (xf >= 5);
%! xp = [0:.5:4.5,4.99,5:.5:10];  yp = sin (2*pi*xp/5) - (xp >= 5);
%! lin = interp1 (xp,yp,xf, "linear");
%! near= interp1 (xp,yp,xf, "nearest");
%! plot (xf,yf,"r", xf,near,"g", xf,lin,"b", xp,yp,"r*");
%! legend ("original", "nearest", "linear");
%! %--------------------------------------------------------
%! % confirm that interpolated function matches the original

##FIXME: add test for n-d arguments here

## For each type of interpolated test, confirm that the interpolated
## value at the knots match the values at the knots.  Points away
## from the knots are requested, but only "nearest" and "linear"
## confirm they are the correct values.

%!shared xp, yp, xi, style
%! xp = 0:2:10;
%! yp = sin (2*pi*xp/5);
%! xi = [-1, 0, 2.2, 4, 6.6, 10, 11];

## The following BLOCK/ENDBLOCK section is repeated for each style
##    nearest, linear, cubic, spline, pchip
## The test for ppval of cubic has looser tolerance, but otherwise
## the tests are identical.
## Note that the block checks style and *style; if you add more tests
## be sure to add them to both sections of each block.  One test,
## style vs. *style, occurs only in the first section.
## There is an ENDBLOCKTEST after the final block

%!test style = "nearest";
## BLOCK
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
%!assert (interp1 (xp,[yp',yp'],xi,style),
%!        interp1 (xp,[yp',yp'],xi,["*",style]),100*eps)
%!test style = ["*",style];
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
## ENDBLOCK

%!test style = "linear";
## BLOCK
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
%!assert (interp1 (xp,[yp',yp'],xi,style),
%!        interp1 (xp,[yp',yp'],xi,["*",style]),100*eps)
%!test style = ['*',style];
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
## ENDBLOCK

%!test style = "cubic";
## BLOCK
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),100*eps)
%!error interp1 (1,1,1, style)
%!assert (interp1 (xp,[yp',yp'],xi,style),
%!        interp1 (xp,[yp',yp'],xi,["*",style]),100*eps)
%!test style = ["*",style];
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),100*eps)
%!error interp1 (1,1,1, style)
## ENDBLOCK

%!test style = "pchip";
## BLOCK
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
%!assert (interp1 (xp,[yp',yp'],xi,style),
%!        interp1 (xp,[yp',yp'],xi,["*",style]),100*eps)
%!test style = ["*",style];
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
## ENDBLOCK

%!test style = "spline";
## BLOCK
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
%!assert (interp1 (xp,[yp',yp'],xi,style),
%!        interp1 (xp,[yp',yp'],xi,["*",style]),100*eps)
%!test style = ["*",style];
%!assert (interp1 (xp, yp, [min(xp)-1, max(xp)+1],style), [NA, NA])
%!assert (interp1 (xp,yp,xp,style), yp, 100*eps)
%!assert (interp1 (xp,yp,xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp',style), yp', 100*eps)
%!assert (interp1 (xp',yp',xp,style), yp, 100*eps)
%!assert (isempty (interp1 (xp',yp',[],style)))
%!assert (isempty (interp1 (xp,yp,[],style)))
%!assert (interp1 (xp,[yp',yp'],xi(:),style),...
%!        [interp1(xp,yp,xi(:),style),interp1(xp,yp,xi(:),style)])
%!assert (interp1 (xp,yp,xi,style),...
%!        interp1 (fliplr (xp),fliplr (yp),xi,style),100*eps)
%!assert (ppval (interp1 (xp,yp,style,"pp"),xi),
%!        interp1 (xp,yp,xi,style,"extrap"),10*eps)
%!error interp1 (1,1,1, style)
## ENDBLOCK
## ENDBLOCKTEST

%!# test linear extrapolation
%!assert (interp1 ([1:5],[3:2:11],[0,6],"linear","extrap"), [1, 13], eps)
%!assert (interp1 (xp, yp, [-1, max(xp)+1],"linear",5), [5, 5])

%!assert (interp1 (1:2,1:2,1.4,"nearest"), 1)
%!assert (interp1 (1:2,1:2,1.4,"linear"), 1.4)
%!assert (interp1 (1:4,1:4,1.4,"cubic"), 1.4)
%!assert (interp1 (1:2,1:2,1.1, "spline"), 1.1)
%!assert (interp1 (1:3,1:3,1.4,"spline"), 1.4)

%!assert (interp1 (1:2:4,1:2:4,1.4,"*nearest"), 1)
%!assert (interp1 (1:2:4,1:2:4,[0,1,1.4,3,4],"*linear"), [NA,1,1.4,3,NA])
%!assert (interp1 (1:2:8,1:2:8,1.4,"*cubic"), 1.4)
%!assert (interp1 (1:2,1:2,1.3, "*spline"), 1.3)
%!assert (interp1 (1:2:6,1:2:6,1.4,"*spline"), 1.4)

%!assert (interp1 ([3,2,1],[3,2,2],2.5), 2.5)

%!assert (interp1 ([1,2,2,3,4],[0,1,4,2,1],[-1,1.5,2,2.5,3.5], "linear", "extrap"), [-2,0.5,4,3,1.5])
%!assert (interp1 ([4,4,3,2,0],[0,1,4,2,1],[1.5,4,4.5], "linear"), [1.75,1,NA])
%!assert (interp1 (0:4, 2.5), 1.5)

%!error interp1 ()
%!error interp1 (1,1,1, "linear")
%!error interp1 (1,1,1, "*nearest")
%!error interp1 (1,1,1, "*linear")
%!error interp1 (1:2,1:2,1, "bogus")