ECG-Kit 1.0

%RANDOMFORESTC Breiman's random forest
%
%   W = RANDOMFORESTC(A,L,N)
%   W = A*RANDOMFORESTC([],L,N)
%
% INPUT
%   A       Dateset used for training
%   L       Number of decision trees to be generated (default 50)
%   N       Size of feature subsets to be used (default 1)
%
% OUTPUT
%   W       Resulting, trained feature space classifier
%
% DESCRIPTION
% Train a decision forest on A, using L decision trees, each trained on
% a bootstrapped version of dataset A. Each decison tree is using random
% feature subsets of size N in each node.  When N=0, no feature subsets
% are used.
%
% REFERENCES
% [1] L. Breiman, Random Forests, Machine learning, vol. 45 (1), 5-32, 2001
%
% SEE ALSO (<a href="http://37steps.com/prtools">PRTools Guide</a>)
% DATASETS, MAPPINGS, DTC

% Copyright: D.M.J. Tax, D.M.J.Tax@37steps.com
% Faculty EWI, Delft University of Technology
% P.O. Box 5031, 2600 GA Delft, The Netherlands

function out = randomforestc(varargin)

argin = setdefaults(varargin,[],50,1);
if mapping_task(argin,'definition')
  
  out = define_mapping(argin,'untrained',['RandForest' int2str(argin{2})]);
  
elseif mapping_task(argin,'training')
  
  [a,L,featsubset] = deal(argin{:});
	isvaldfile(a,2,2); % at least 2 obj/class, 2 classes
	opt = [];
	[n,dim,opt.K] = getsize(a);
	opt.featsubset = featsubset;
	v = cell(L,1);
	for i=1:L
		[x,z] = gendat(a);
    if exist('decisiontree','file')==3
      v{i} = decisiontree(+x,getnlab(x),opt.K,opt.featsubset);
    else
		 prwarning(2,'No compiled decisiontree found, using the slower Matlab implementation.');
    	v{i} = tree_train(+x,getnlab(x),opt);
    end
  end
  out = trained_classifier(a,v);
  
elseif mapping_task(argin,'execution')
  
  [a,w] = deal(argin{1:2}); 
	v = getdata(w);
	n = size(a,1);  % nr objects
	K = size(w,2);  % nr of classes
	nrv = length(v); % nr of trees
    out = zeros(n,K);
    if exist('decisiontree')==3
      for j=1:nrv
        I = decisiontree(v{j},+a);
        out = out + accumarray([(1:n)' I],ones(n,1),[n K]);
      end
    else
      % the old fashioned slow Matlab code
      for i=1:n
        x = +a(i,:);
        for j=1:nrv
          I = tree_eval(v{j},x);
          out(i,I) = out(i,I)+1;
        end
      end
      out = out./repmat(sum(out,2),1,K);
    end
    out = setdat(a,out,w);
    
else
  error('Illegal call')
end

return

%    out = tree_eval(w,x)
%
function out = tree_eval(w,x)

n = size(x,1);
out = zeros(n,1);

for i=1:n

	v=w;
	% if the first split is already solving everything (1 obj. per class)
	if isa(v,'double')
		out(i,1) = v;
	end
	while (out(i,1)==0)
		if (x(i,v.bestf)<v.bestt)
			v = v.l;
		else
			v = v.r;
		end
		if isa(v,'double')
			out(i,1) = v;
		end
	end
end

%
%    w = tree_train(x,y,opt)
%
function w = tree_train(x,y,opt)

% how good are we in this node?
err = tree_gini(y,opt.K);
if (err==0)

	w = y(1); % just predict this label

else
	% we split further
	n = size(x,1);

	% optionally, choose only from a subset
	if (opt.featsubset>0)
		fss = randperm(size(x,2));
		fss = fss(1:opt.featsubset);
	else
		fss = 1:size(x,2);
	end

	% check each feature separately:
	besterr = inf; bestf = []; bestt = []; bestj = []; bestI = [];
	for i=fss
		% sort the data along feature i:
		[xi,I] = sort(x(:,i)); yi = y(I);
		% run over all possible splits:
		for j=1:n-1
			% compute the gini
			err = j*tree_gini(yi(1:j),opt.K) + (n-j)*tree_gini(yi(j+1:n),opt.K);
			% and see if it is better than before.
			if (err<besterr)
				besterr = err;
				bestf = i;
				bestj = j;
				bestt = mean(xi(j:j+1));
				bestI = I;
			end
		end
	end

	% store
	w.bestf = bestf;
	w.bestt = bestt;
	%  now find the children:
	w.l = tree_train(x(bestI(1:bestj),:),y(bestI(1:bestj)),opt);
	w.r = tree_train(x(bestI(bestj+1:end),:),y(bestI(bestj+1:end)),opt);
end
	
function g = tree_gini(y,K)

out = zeros(1,K);
for k=1:K
	out(k) = mean(y==k);
end

g = out*(1-out)';