compute_nmi.m

function [A nmi avgent] = compute_nmi (T, H)
    
    N = length(T);
    classes = unique(T);
    clusters = unique(H);
    num_class = length(classes);
    num_clust = length(clusters);

    %%compute number of points in each class
    for j=1:num_class
        index_class = (T(:)==classes(j));
        D(j) = sum(index_class);
    end      
    
    %%mutual information
    mi = 0;
    A = zeros(num_clust, num_class);
    avgent = 0;
    for i=1:num_clust
        %number of points in cluster 'i'
        index_clust = (H(:)==clusters(i));
        B(i) = sum(index_clust);
        for j=1:num_class
            index_class = (T(:)==classes(j));
            %%compute number of points in class 'j' that end up in cluster 'i'
            A(i,j) = sum(index_class.*index_clust);
            if (A(i,j) ~= 0)
                miarr(i,j) = A(i,j)/N * log2 (N*A(i,j)/(B(i)*D(j)));
                %%average entropy calculation                
                avgent = avgent - (B(i)/N) * (A(i,j)/B(i)) * log2 (A(i,j)/B(i));
            else
                miarr(i,j) = 0;
            end
            mi = mi + miarr(i,j);
            

            
        end        
    end
    
    %%class entropy
    class_ent = 0;
    for i=1:num_class
        class_ent = class_ent + D(i)/N * log2(N/D(i));
    end
    
    %%clustering entropy
    clust_ent = 0;
    for i=1:num_clust
        clust_ent = clust_ent + B(i)/N * log2(N/B(i));
    end
        
    %%normalized mutual information
    nmi = 2*mi / (clust_ent + class_ent);