function [SA_iref_cast, CT_iref_cast, p_iref_cast] = gsw_interp_ref_cast(spycnl,A)

% gsw_interp_ref_cast            linear interpolation of the reference cast
%==========================================================================
% This function interpolates the reference cast with respect to the 
% interpolating variable "spycnl".  This reference cast is at the location 
% 188E,4N from the reference data set which underlies the Jackett & 
% McDougall (1997) Neutral Density computer code.  This function finds the 
% values of SA, CT and p on this reference cast which correspond to the 
% value of isopycnal which is passed to this function from the function 
% "gsw_geo_strf_isopycnal".  The isopycnal could be either gamma_n or 
% sigma_2. If A is set to any of the following 's2','S2','sigma2','sigma_2'
% the interpolation will take place in sigma 2 space, any other input
% will result in the programme working in gamma_n space.
%
% VERSION NUMBER: 3.06.12 (25th May, 2020)
%
% REFERENCE:
%  Jackett, D.R. and T.J. McDougall, 1997: A neutral density variable
%   for the world’s oceans. Journal of Physical Oceanography, 27, 237-263.
%
% This fuction was adapted from Matlab's interp1q.
%==========================================================================

spycnl = spycnl(:);

if ~exist('A','var')
    A = 'gn';
elseif ~ischar(A)
    A = 'gn';
end %if
if strcmpi(A,'s2') == 1 | strcmpi(A,'s_2') == 1 | ...
        strcmpi(A,'sigma2') == 1 | strcmpi(A,'sigma_2') == 1 |...
        strcmpi(A,'s@') == 1 | strcmpi(A,'s_@') == 1 | ...
        strcmpi(A,'sigma@') == 1 | strcmpi(A,'sigma_@') == 1
    A = 's2';
end

gsw_data = 'gsw_data_v3_0.mat';
gsw_data_file = which(gsw_data);

if strcmpi(A,'s2')
    load (gsw_data_file,'SA_ref_cast','CT_ref_cast','p_ref_cast','sigma_2_ref_cast');
    spycnl_ref_cast = sigma_2_ref_cast;
else
    load (gsw_data_file,'SA_ref_cast','CT_ref_cast','p_ref_cast','gamma_n_ref_cast');
    spycnl_ref_cast = gamma_n_ref_cast;
end

[min_spycnl_ref_cast, Imin_spycnl_ref_cast] = min(spycnl_ref_cast);
SA_iref_cast(spycnl <= min_spycnl_ref_cast) = SA_ref_cast(Imin_spycnl_ref_cast);% Set equal to the shallowest bottle.
CT_iref_cast(spycnl <= min_spycnl_ref_cast) = CT_ref_cast(Imin_spycnl_ref_cast);
p_iref_cast(spycnl <= min_spycnl_ref_cast) = p_ref_cast(Imin_spycnl_ref_cast);

[max_spycnl_ref_cast, Imax_spycnl_ref_cast] = max(spycnl_ref_cast);
SA_iref_cast(spycnl >= max_spycnl_ref_cast) = SA_ref_cast(Imax_spycnl_ref_cast);% Set equal to the deepest bottle.
CT_iref_cast(spycnl >= max_spycnl_ref_cast) = CT_ref_cast(Imax_spycnl_ref_cast);
p_iref_cast(spycnl >= max_spycnl_ref_cast) = p_ref_cast(Imax_spycnl_ref_cast);

xi = spycnl(spycnl >= 21.805 & spycnl <= 28.3614);

x = spycnl_ref_cast;

siz = size(xi);
if ~isscalar(xi)
   [xxi, k] = sort(xi);
   [dummy, j] = sort([x;xxi]);
   r(j) = 1:length(j);
   r = r(length(x)+1:end) - (1:length(xxi));
   r(k) = r;
   r(xi==x(end)) = length(x)-1;
   ind = find((r>0) & (r<length(x)));
   ind = ind(:);
   SA_ref_casti = NaN(length(xxi),size(SA_ref_cast,2),superiorfloat(x,SA_ref_cast,xi));
   CT_ref_casti = NaN(length(xxi),size(CT_ref_cast,2),superiorfloat(x,CT_ref_cast,xi));
   p_ref_casti = NaN(length(xxi),size(p_ref_cast,2),superiorfloat(x,p_ref_cast,xi));
   rind = r(ind);
   xrind = x(rind);
   u = (xi(ind)-xrind)./(x(rind+1)-xrind);
   SArind = SA_ref_cast(rind,:);
   CTrind = CT_ref_cast(rind,:);
   prind = p_ref_cast(rind,:);
   if exist('bsxfun','builtin') == 5
       SA_ref_casti(ind,:) = SArind + bsxfun(@times,SA_ref_cast(rind+1,:)-SArind,u);
       CT_ref_casti(ind,:) = CTrind + bsxfun(@times,CT_ref_cast(rind+1,:)-CTrind,u);
       p_ref_casti(ind,:) = prind + bsxfun(@times,p_ref_cast(rind+1,:)-prind,u);
   else
       SA_ref_casti(ind,:) = SArind + (SA_ref_cast(rind+1,:)-SArind).*u;
       CT_ref_casti(ind,:) = CTrind + (CT_ref_cast(rind+1,:)-CTrind).*u;
       p_ref_casti(ind,:) = prind + (p_ref_cast(rind+1,:)-prind).*u;
   end
else
   % Special scalar xi case
   r = find(x <= xi,1,'last');
   r(xi==x(end)) = length(x)-1;
   if isempty(r) || r<=0 || r>=length(x)
      SA_ref_casti = NaN(1,size(SA,2),superiorfloat(x,SA,xi));
      CT_ref_casti = NaN(1,size(CT,2),superiorfloat(x,CT,xi));
      p_ref_casti = NaN(1,size(p,2),superiorfloat(x,p,xi));
   else
      u = (xi-x(r))./(x(r+1)-x(r));
      SAr_ref_cast = SA_ref_cast(r,:);
      CTr_ref_cast = CT_ref_cast(r,:);
      pr_ref_cast = p_ref_cast(r,:);
      if exist('bsxfun','builtin') == 5
          SA_ref_casti = SAr_ref_cast + bsxfun(@times,SA_ref_cast(r+1,:)-SAr_ref_cast,u);
          CT_ref_casti = CTr_ref_cast + bsxfun(@times,CT_ref_cast(r+1,:)-CTr_ref_cast,u);
          p_ref_casti = pr_ref_cast + bsxfun(@times,p_ref_cast(r+1,:)-pr_ref_cast,u);
      else
          SA_ref_casti = SAr_ref_cast + (SA_ref_cast(r+1,:)-SAr_ref_cast).*u;
          CT_ref_casti = CTr_ref_cast + (CT_ref_cast(r+1,:)-CTr_ref_cast).*u;
          p_ref_casti = pr_ref_cast + (p_ref_cast(r+1,:)-pr_ref_cast).*u;
      end
   end
end

if min(size(SA_ref_casti)) == 1 && numel(xi) > 1
   SA_ref_casti = reshape(SA_ref_casti,siz);
   CT_ref_casti = reshape(CT_ref_casti,siz);
   p_ref_casti = reshape(p_ref_casti,siz);
end

SA_iref_cast(spycnl >= 21.805 & spycnl <= 28.3614) = SA_ref_casti;
CT_iref_cast(spycnl >= 21.805 & spycnl <= 28.3614) = CT_ref_casti;
p_iref_cast(spycnl >= 21.805 & spycnl <= 28.3614) = p_ref_casti;

end
