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+## Copyright (C) 2000 Paul Kienzle
+##
+## This program 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 2 of the License, or
+## (at your option) any later version.
+##
+## This program 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 this program; if not, write to the Free Software
+## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+## usage: yi = interp1(x, y, xi [, 'method' [, 'extrap']])
+##
+## Interpolate the function y=f(x) at the points xi. The sample 
+## points x must be strictly monotonic.  If y is a matrix with
+## length(x) rows, yi will be a matrix of size rows(xi) by columns(y),
+## or its transpose if xi is a row vector.
+##
+## Method is one of:
+## 'nearest': return nearest neighbour.
+## 'linear': linear interpolation from nearest neighbours
+## 'pchip': piece-wise cubic hermite interpolating polynomial
+## 'cubic': cubic interpolation from four nearest neighbours
+## 'spline': cubic spline interpolation--smooth first and second
+##           derivatives throughout the curve
+## ['*' method]: same as method, but assumes x is uniformly spaced
+##               only uses x(1) and x(2); usually faster, never slower
+##
+## Method defaults to 'linear'.
+##
+## If extrap is the string 'extrap', then extrapolate values beyond
+## the endpoints.  If extrap is a number, replace values beyond the
+## endpoints with that number.  If extrap is missing, assume NaN.
+##
+## Example:
+##    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,';original;',xf,lin,';linear;',xf,spl,';spline;',...
+##         xf,cub,';cubic;',xf,near,';nearest;',xp,yp,'*;;');
+##
+## See also: interp
+
+## 2000-03-25 Paul Kienzle
+##    added 'nearest' as suggested by Kai Habel
+## 2000-07-17 Paul Kienzle
+##    added '*' methods and matrix y
+##    check for proper table lengths
+
+function yi = interp1(x, y, xi, method, extrap)
+
+  if nargin<3 || nargin>5
+    usage("yi = interp1(x, y, xi [, 'method' [, 'extrap']])");
+  endif
+
+  if nargin < 4, 
+    method = 'linear';
+  else
+    method = tolower(method); 
+  endif
+
+  if nargin < 5
+    extrap = NaN;
+  endif
+
+  ## reshape matrices for convenience
+  x = x(:);
+  if size(y,1)==1, y=y(:); endif
+  transposed = (size(xi,1)==1);
+  xi = xi(:);
+
+  ## determine sizes
+  nx = size(x,1);
+  [ny, nc] = size(y);
+  if (nx < 2 || ny < 2)
+     error ("interp1: table too short");
+  endif
+
+  ## determine which values are out of range and set them to extrap,
+  ## unless extrap=='extrap' in which case, extrapolate them like we
+  ## should have done in the first place.
+  minx = x(1);
+  if (method(1) == '*')
+     dx = x(2) - x(1);
+     maxx = minx + (ny-1)*dx;
+  else
+     maxx = x(nx);
+  endif
+  if strcmp(extrap,"extrap")
+    range=1:nx;
+  else
+    range = find(xi >= minx & xi <= maxx);
+    yi = extrap*ones(size(xi,1), size(y,2));
+    if isempty(range), 
+      if transposed, yi = yi.'; endif
+      return; 
+    endif
+    xi = xi(range);
+  endif
+
+  if strcmp(method, 'nearest')
+    idx = lookup(0.5*(x(1:nx-1)+x(2:nx)), xi)+1;
+    yi(range,:) = y(idx,:);
+
+  elseif strcmp(method, '*nearest')
+    idx = floor((xi-minx)/dx+1.5);
+    yi(range,:) = y(idx,:);
+
+  elseif strcmp(method, 'linear')
+    ## find the interval containing the test point
+    idx = lookup (x(2:nx-1), xi)+1; 
+				# 2:n-1 so that anything beyond the ends
+				# gets dumped into an interval
+
+    ## use the endpoints of the interval to define a line
+    dy = y(2:ny,:) - y(1:ny-1,:);
+    dx = x(2:nx) - x(1:nx-1);
+    s = (xi - x(idx))./dx(idx);
+    yi(range,:) = s(:,ones(1,nc)).*dy(idx,:) + y(idx,:);
+
+  elseif strcmp(method, '*linear')
+    ## find the interval containing the test point
+    t = (xi - minx)/dx + 1;
+    idx = floor(t);
+
+    ## use the endpoints of the interval to define a line
+    dy = [y(2:ny,:) - y(1:ny-1,:); zeros(1,nc)];
+    s = (t - idx)./dx;
+    yi(range,:) = s(:,ones(1,nc)).*dy(idx,:) + y(idx,:); 
+
+  elseif strcmp(method, 'pchip')
+    if (nx == 2) x = linspace(minx, maxx, ny); endif
+    yi(range,:) = pchip(x, y, xi);
+
+  elseif strcmp(method, 'cubic')
+    if (nx < 4 || ny < 4)
+      error ("interp1: table too short");
+    endif
+    idx = lookup(x(3:nx-2), xi) + 1;
+
+    ## Construct cubic equations for each interval using divided
+    ## differences (computation of c and d don't use divided differences
+    ## but instead solve 2 equations for 2 unknowns). Perhaps
+    ## reformulating this as a lagrange polynomial would be more efficient.
+    i=1:nx-3;
+    J = ones(1,nc);
+    dx = diff(x);
+    dx2 = x(i+1).^2 - x(i).^2;
+    dx3 = x(i+1).^3 - x(i).^3;
+    a=diff(y,3)./dx(i,J).^3/6;
+    b=(diff(y(1:nx-1,:),2)./dx(i,J).^2 - 6*a.*x(i+1,J))/2;
+    c=(diff(y(1:nx-2,:),1) - a.*dx3(:,J) - b.*dx2(:,J))./dx(i,J);
+    d=y(i,:) - ((a.*x(i,J) + b).*x(i,J) + c).*x(i,J);
+    yi(range,:) = ((a(idx,:).*xi(:,J) + b(idx,:)).*xi(:,J) ...
+		   + c(idx,:)).*xi(:,J) + d(idx,:);
+
+  elseif strcmp(method, '*cubic')
+    if (nx < 4 || ny < 4)
+      error ("interp1: table too short");
+    endif
+
+    ## From: Miloje Makivic 
+    ## http://www.npac.syr.edu/projects/nasa/MILOJE/final/node36.html
+    t = (xi - minx)/dx + 1;
+    idx = max(min(floor(t), ny-2), 2);
+    t = t - idx;
+    t2 = t.*t;
+    tp = 1 - 0.5*t;
+    a = (1 - t2).*tp;
+    b = (t2 + t).*tp;
+    c = (t2 - t).*tp/3;
+    d = (t2 - 1).*t/6;
+    J = ones(1,nc);
+    yi(range,:) = a(:,J) .* y(idx,:) + b(:,J) .* y(idx+1,:) ...
+		  + c(:,J) .* y(idx-1,:) + d(:,J) .* y(idx+2,:);
+
+  elseif strcmp(method, 'spline') || strcmp(method, '*spline')
+    if (nx == 2) x = linspace(minx, maxx, ny); endif
+    yi(range,:) = spline(x, y, xi);
+
+  else
+    error(["interp1 doesn't understand method '", method, "'"]);
+  endif
+  if transposed, yi=yi.'; endif
+
+endfunction
+
+%!demo
+%! 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,';original;',xf,near,';nearest;',xf,lin,';linear;',...
+%!      xf,cub,';pchip;',xf,spl,';spline;',xp,yp,'*;;');
+%! %--------------------------------------------------------
+%! % confirm that interpolated function matches the original
+
+%!demo
+%! 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,';*original;',xf,near,';*nearest;',xf,lin,';*linear;',...
+%!      xf,cub,';*cubic;',xf,spl,';*spline;',xp,yp,'*;;');
+%! %--------------------------------------------------------
+%! % confirm that interpolated function matches the original
+
+%!shared xp, yp, xi, style
+%! xp=0:5;      yp = sin(2*pi*xp/5);
+%! xi = sort([-1, max(xp)*rand(1,6), max(xp)+1]);
+
+%!test style = 'nearest';
+%!assert (interp1(xp, yp, [-1, max(xp)]), [NaN, NaN]);
+%!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',yp'],xi,style),
+%!	  interp1(xp,[yp',yp'],xi,["*",style]));
+
+%!test style = 'linear';
+%!assert (interp1(xp, yp, [-1, max(xp)+1]), [NaN, NaN]);
+%!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',yp'],xi,style),
+%!	  interp1(xp,[yp',yp'],xi,["*",style]),100*eps);
+
+%!test style = 'cubic';
+%!assert (interp1(xp, yp, [-1, max(xp)+1]), [NaN, NaN]);
+%!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',yp'],xi,style),
+%!	  interp1(xp,[yp',yp'],xi,["*",style]),1000*eps);
+
+%!test style = 'spline';
+%!assert (interp1(xp, yp, [-1, max(xp) + 1]), [NaN, NaN]);
+%!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',yp'],xi,style),
+%!	  interp1(xp,[yp',yp'],xi,["*",style]),10*eps);
+
+%!error interp1
+%!error interp1(1:2,1:2,1,'bogus')
+
+%!error interp1(1,1,1, 'nearest');
+%!assert (interp1(1:2,1:2,1.4,'nearest'),1);
+%!error interp1(1,1,1, 'linear');
+%!assert (interp1(1:2,1:2,1.4,'linear'),1.4);
+%!error interp1(1:3,1:3,1, 'cubic');
+%!assert (interp1(1:4,1:4,1.4,'cubic'),1.4);
+%!error interp1(1:2,1:2,1, 'spline');
+%!assert (interp1(1:3,1:3,1.4,'spline'),1.4);
+
+%!error interp1(1,1,1, '*nearest');
+%!assert (interp1(1:2,1:2,1.4,'*nearest'),1);
+%!error interp1(1,1,1, '*linear');
+%!assert (interp1(1:2,1:2,1.4,'*linear'),1.4);
+%!error interp1(1:3,1:3,1, '*cubic');
+%!assert (interp1(1:4,1:4,1.4,'*cubic'),1.4);
+%!error interp1(1:2,1:2,1, '*spline');
+%!assert (interp1(1:3,1:3,1.4,'*spline'),1.4);

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