function results = fitnonlinearmodel_helper(opt,stimulus,tmatrix,smatrix,trainfun,testfun)

% This is a helper function for fitnonlinearmodel.m.  Not for external use!
%
% Notes:
% - opt.data is always a cell vector and contains only one voxel
% - in the nonlinear case, the seed to use has been hacked into model{1}{1} and may have multiple rows

% calc
islinear = isa(opt.model,'function_handle');
if ~islinear
  ismultipleseeds = size(opt.model{1}{1},1) > 1;
  ismultiplemodels = length(opt.model) > 1;
end

% calc
wantmodelfit = ~(ismember('modelfit',opt.dontsave) && ~ismember('modelfit',opt.dosave));
if islinear
  numparams = size(stimulus{1},2);
else
  numparams = size(opt.model{end}{2},2);
end

% init
results = struct;
results.params = zeros(length(trainfun),numparams);
results.testdata =  cell(1,length(trainfun));  % but converted to a matrix at the end
results.modelpred = cell(1,length(trainfun));  % but converted to a matrix at the end
results.modelfit =  cell(1,length(trainfun));  % but converted to a matrix at the end
results.trainperformance = zeros(1,length(trainfun));
results.testperformance =  zeros(1,length(trainfun));
results.aggregatedtestperformance = [];
if islinear
  results.numiters = [];
  results.resnorms = [];
else
  results.numiters = zeros(length(trainfun),size(opt.model{1}{1},1),length(opt.model));
  results.resnorms = zeros(length(trainfun),size(opt.model{1}{1},1));
end

% loop over resampling cases
for rr=1:length(trainfun)
  fprintf('  starting resampling case %d of %d.\n',rr,length(trainfun));

  % deal with resampling
  trainstim = feval(trainfun{rr},stimulus);
  traindata = feval(trainfun{rr},opt.data);  % result is a column vector
  trainT =    projectionmatrix(feval(trainfun{rr},tmatrix));   % NOTE: potentially slow step. make sparse? [or CACHE]
  trainS =    projectionmatrix(feval(trainfun{rr},smatrix));   % NOTE: potentially slow step. make sparse? [or CACHE]
  teststim =  feval(testfun{rr},stimulus);
  testdata =  feval(testfun{rr},opt.data);   % result is a column vector
  testT =     projectionmatrix(feval(testfun{rr},tmatrix));    % NOTE: potentially slow step. make sparse? [or CACHE]
  testS =     projectionmatrix(feval(testfun{rr},smatrix));    % NOTE: potentially slow step. make sparse? [or CACHE]
  if wantmodelfit  % save on memory if user doesn't even want 'modelfit'
    allstim = catcell(1,stimulus);
  end

  % precompute
  traindataT = trainT*traindata;  % remove regressors from data (fitting)
  
  % deal with last-minute data division
  if ~islinear
    datastd = std(traindataT);
    if datastd == 0
      datastd = 1;
    end
    traindataT = traindataT / datastd;
  end

  % deal with options
  if ~islinear
    options = opt.optimoptions;
    if ~isempty(opt.outputfcn)
      if nargin(opt.outputfcn) == 4
        options.OutputFcn = @(a,b,c) feval(opt.outputfcn,a,b,c,traindataT);
      else
        options.OutputFcn = opt.outputfcn;
      end
    end
  end

  % ok, deal with linear case
  if islinear

    % do the fitting.  note that we take the mean across the third dimension 
    % to deal with the case where the stimulus consists of multiple frames.
    finalparams = feval(opt.model,trainT*mean(trainstim,3),traindataT);

    % report
    fprintf('      the estimated parameters are ['); ...
      fprintf('%.3f ',finalparams); fprintf('].\n');
    
  % ok, deal with nonlinear case
  else

    % loop over seeds
    params = [];
    for ss=1:size(opt.model{1}{1},1)
      if ismultipleseeds
        fprintf('    trying seed %d of %d.\n',ss,size(opt.model{1}{1},1));
      end
  
      % loop through models
      for mm=1:length(opt.model)
    
        % which parameters are we actually fitting?
        ix = ~isnan(opt.model{mm}{2}(1,:));

        % calculate seed, model, and transform
        if mm==1
          seed = opt.model{mm}{1}(ss,:);
          model = opt.model{mm}{3};
          transform = opt.model{mm}{4};
        else
          seed = feval(opt.model{mm}{1},params0);
          model = feval(opt.model{mm}{3},params0);
          transform = feval(opt.model{mm}{4},params0);
        end

        % in the special case that the stimulus consists of multiple frames,
        % then we have to modify model so that it averages across the
        % predicted response associated with each frame.  this is magical voodoo here.
        if size(trainstim,3) > 1
          nums = repmat(size(trainstim,3),[1 size(trainstim,1)]);
          model = @(pp,dd) chunkfun(feval(model,pp,squish(permute(dd,[3 1 2]),2)),nums,@(x) mean(x,1)).';
        end

        % figure out bounds to use
        if isequal(options.Algorithm,'levenberg-marquardt')
          lb = [];
          ub = [];
        else
          lb = opt.model{mm}{2}(1,ix);
          ub = opt.model{mm}{2}(2,ix);
        end
      
        % precompute
        trainstimTRANSFORM = feval(transform,trainstim);

        % define the final model function
        fun = @(pp) trainT*feval(model,copymatrix(seed,ix,pp),trainstimTRANSFORM);

        % report
        if ismultiplemodels
          fprintf('      for model %d of %d, the seed is [', ...
                  mm,length(opt.model)); fprintf('%.3f ',seed); fprintf('].\n');
        else
          fprintf('      the seed is ['); fprintf('%.3f ',seed); fprintf('].\n');
        end

        % perform the fit (NOTICE THE DIVISION BY DATASTD, THE NAN PROTECTION, THE CONVERSION TO DOUBLE)
        if ~any(ix)
          params0 = seed;   % if no parameters are to be optimized, just return the seed
          resnorm = NaN;
          output = [];
          output.iterations = NaN;
        else
          [params0,resnorm,residual,exitflag,output] = ...
            lsqcurvefit(@(x,y) double(nanreplace(feval(fun,x) / datastd,0,2)),seed(ix),[],double(traindataT),lb,ub,options);
          params0 = copymatrix(seed,ix,params0);
        end

        % report
        fprintf('      the estimated parameters are ['); ...
          fprintf('%.3f ',params0); fprintf('].\n');
      
        % record
        results.numiters(rr,ss,mm) = output.iterations;

      end
    
      % record
      results.resnorms(rr,ss) = resnorm;  % final resnorm
      params(ss,:) = params0;  % final parameters

    end
  
    % which seed produced the best results?
    [d,mnix] = min(results.resnorms(rr,:));
    finalparams = params(mnix,:);

  end
  
  % record the results
  results.params(rr,:) = finalparams;

  % report
  if ~islinear && ismultipleseeds
    fprintf('    seed %d was best. final estimated parameters are [',mnix); ...
      fprintf('%.3f ',finalparams); fprintf('].\n');
  end

  % prepare data and model fits
  % [NOTE: in the nonlinear case, this inherits model, transform, and trainstimTRANSFORM from above!!]
  traindatatemp = trainS*traindata;
  if islinear
    modelfittemp = trainS*(trainstim*finalparams');
  else
    modelfittemp = nanreplace(trainS*feval(model,finalparams,trainstimTRANSFORM),0,2);
  end
  if isempty(testdata)  % handle this case explicitly, just to avoid problems
    results.testdata{rr} = [];
    results.modelpred{rr} = [];
  else
    results.testdata{rr} = testS*testdata;
    if islinear
      results.modelpred{rr} = testS*(teststim*finalparams');
    else
      results.modelpred{rr} = nanreplace(testS*feval(model,finalparams,feval(transform,teststim)),0,2);
    end
  end
  
  % prepare modelfit
  if wantmodelfit
    if islinear
      results.modelfit{rr} = (allstim*finalparams')';
    else
      results.modelfit{rr} = nanreplace(feval(model,finalparams,feval(transform,allstim)),0,2)';
    end
  else
    results.modelfit{rr} = [];  % if not wanted by user, don't bother computing
  end
  
  % compute metrics
  results.trainperformance(rr) = feval(opt.metric,modelfittemp,traindatatemp);
  if isempty(results.testdata{rr})  % handle this case explicitly, just to avoid problems
    results.testperformance(rr) = NaN;
  else
    results.testperformance(rr) = feval(opt.metric,results.modelpred{rr},results.testdata{rr});
  end
  
  % report
  fprintf('    trainperformance is %.2f. testperformance is %.2f.\n', ...
    results.trainperformance(rr),results.testperformance(rr));

end

% compute aggregated metrics
results.testdata = catcell(1,results.testdata);
results.modelpred = catcell(1,results.modelpred);
results.modelfit = catcell(1,results.modelfit);
if isempty(results.testdata)
  results.aggregatedtestperformance = NaN;
else
  results.aggregatedtestperformance = feval(opt.metric,results.modelpred,results.testdata);
end

% report
fprintf('  aggregatedtestperformance is %.2f.\n',results.aggregatedtestperformance);