% If you use this code, please cite:
%   Michael Lamar*, Yariv Maron*, Mark Johnson, Elie Bienenstock. 
%   SVD and clustering for unsupervised POS tagging. (ACL 2010) 
% For more information, see the above reference.

function [H1, H2, VI] = var_information(P)
% computes entropies and variation of information from confusion matrix P

[n1, n2] = size(P);
P = P + eps;

P1 = sum(P, 2);                         % marginal  
P2 = sum(P, 1);                         % marginal 
P1rep = repmat(P1 ,1, n2);
P2rep = repmat(P2 ,n1, 1);

H1 = - P1' * log2(P1 );                 % entropy
H2 = - P2  * log2(P2');                 % entropy
 
H12 = - sum(sum(P.*log2(P./P2rep)));    % conditional entropy
H21 = - sum(sum(P.*log2(P./P1rep)));    % conditional entropy

VI = H12 + H21;                         % variation of information