You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fernsClfTrain.m
    path of file: ./repos/toolbox/classify
    
    the code of the file until where you have to start completion: function [ferns,hsPr] = fernsClfTrain( data, hs, varargin )
% Train random fern classifier.
%
% See "Fast Keypoint Recognition in Ten Lines of Code" by Mustafa Ozuysal,
% Pascal Fua and Vincent Lepetit, CVPR07.
%
% Dimensions:
%  M - number ferns
%  S - fern depth
%  F - number features
%  N - number input vectors
%  H - number classes
%
% USAGE
%  [ferns,hsPr] = fernsClfTrain( data, hs, [varargin] )
%
% INPUTS
%  data     - [NxF] N length F feature vectors
%  hs       - [Nx1] target output labels in [1,H]
%  varargin - additional params (struct or name/value pairs)
%   .S        - [10] fern depth (ferns are exponential in S)
%   .M        - [50] number of ferns to train
%   .thrr     - [0 1] range for randomly generated thresholds
%   .bayes    - [1] if true combine probs using bayes assumption
%   .ferns    - [] if given reuse previous ferns (recompute pFern)
%
% OUTPUTS
%  ferns    - learned fern model w the following fields
%   .fids     - [MxS] feature ids for each fern for each depth
%   .thrs     - [MxS] threshold corresponding to each fid
%   .pFern    - [2^SxHxM] learned log probs at fern leaves
%   .bayes    - if true combine probs using bayes assumption
%   .inds     - [NxM] cached indices for original training data
%   .H        - number classes
%  hsPr     - [Nx1] predicted output labels
%
% EXAMPLE
%  N=5000; H=5; d=2; [xs0,hs0,xs1,hs1]=demoGenData(N,N,H,d,1,1);
%  fernPrm=struct('S',4,'M',50,'thrr',[-1 1],'bayes',1);
%  tic, [ferns,hsPr0]=fernsClfTrain(xs0,hs0,fernPrm); toc
%  tic, hsPr1 = fernsClfApply( xs1, ferns ); toc
%  e0=mean(hsPr0~=hs0); e1=mean(hsPr1~=hs1);
%  fprintf('errors trn=%f tst=%f\n',e0,e1); figure(1);
%  subplot(2,2,1); visualizeData(xs0,2,hs0);
%  subplot(2,2,2); visualizeData(xs0,2,hsPr0);
%  subplot(2,2,3); visualizeData(xs1,2,hs1);
%  subplot(2,2,4); visualizeData(xs1,2,hsPr1);

    
function [ferns,hsPr] = fernsClfTrain( data, hs, varargin )
% Train random fern classifier.
%
% See "Fast Keypoint Recognition in Ten Lines of Code" by Mustafa Ozuysal,
% Pascal Fua and Vincent Lepetit, CVPR07.
%
% Dimensions:
%  M - number ferns
%  S - fern depth
%  F - number features
%  N - number input vectors
%  H - number classes
%
% USAGE
%  [ferns,hsPr] = fernsClfTrain( data, hs, [varargin] )
%
% INPUTS
%  data     - [NxF] N length F feature vectors
%  hs       - [Nx1] target output labels in [1,H]
%  varargin - additional params (struct or name/value pairs)
%   .S        - [10] fern depth (ferns are exponential in S)
%   .M        - [50] number of ferns to train
%   .thrr     - [0 1] range for randomly generated thresholds
%   .bayes    - [1] if true combine probs using bayes assumption
%   .ferns    - [] if given reuse previous ferns (recompute pFern)
%
% OUTPUTS
%  ferns    - learned fern model w the following fields
%   .fids     - [MxS] feature ids for each fern for each depth
%   .thrs     - [MxS] threshold corresponding to each fid
%   .pFern    - [2^SxHxM] learned log probs at fern leaves
%   .bayes    - if true combine probs using bayes assumption
%   .inds     - [NxM] cached indices for original training data
%   .H        - number classes
%  hsPr     - [Nx1] predicted output labels
%
% EXAMPLE
%  N=5000; H=5; d=2; [xs0,hs0,xs1,hs1]=demoGenData(N,N,H,d,1,1);
%  fernPrm=struct('S',4,'M',50,'thrr',[-1 1],'bayes',1);
%  tic, [ferns,hsPr0]=fernsClfTrain(xs0,hs0,fernPrm); toc
%  tic, hsPr1 = fernsClfApply( xs1, ferns ); toc
%  e0=mean(hsPr0~=hs0); e1=mean(hsPr1~=hs1);
%  fprintf('errors trn=%f tst=%f\n',e0,e1); figure(1);
%  subplot(2,2,1); visualizeData(xs0,2,hs0);
%  subplot(2,2,2); visualizeData(xs0,2,hsPr0);
%  subplot(2,2,3); visualizeData(xs1,2,hs1);
%  subplot(2,2,4); visualizeData(xs1,2,hsPr1);
%
% See also fernsClfApply, fernsInds
%
% Piotr's Computer Vision Matlab Toolbox      Version 2.61
% Copyright 2014 Piotr Dollar.  [pdollar-at-gmail.com]
% Licensed under the Simplified BSD License [see external/bsd.txt]

% get additional parameters and check dimensions
dfs={'S',10,'M',50,'thrr',[0 1],'bayes',1,'ferns',[]};
[S,M,thrr,bayes,ferns]=getPrmDflt(varargin,dfs,1);
[N,F]=size(data); assert(length(hs)==N);
H=max(hs); assert(all(hs>0)); assert(S<=20);

if( isempty(ferns) )
  % create ferns model and compute inds (w/o field pFern)
  thrs=rand(M,S)*(thrr(2)-thrr(1))+thrr(1);
  fids=uint32(floor(rand(M,S)*F+1)); inds=fernsInds(data,fids,thrs);
  ferns=struct('fids',fids,'thrs',thrs,'bayes',bayes,'H',H,'inds',inds);
else
  % re-use cached model (will need to recompute pFern)
  ferns.H=H; ferns.pFern=[]; inds=ferns.inds; assert(size(inds,1)==N);
end