compute_AP.m

function ap = compute_AP(good_image, index)
% good_image: index for correct match
% index: ranked retrival image index
ngood = length(good_image);

old_recall = 0; 
old_precision = 1.0; 
ap = 0; 
intersect_size = 0; 
j = 0; 
good_now = 0; 
% ngood = ngood - 1; 
% tic
for n = 1:length(index) 
    flag = 0;
    if ~isempty(find(good_image == index(n), 1)) 
        flag = 1; % good image 
        good_now = good_now+1; 
    end
%     if ~isempty(find(junk_image == index(n), 1))
%         continue; % junk image 
%     end
    
    if flag == 1%good
        intersect_size = intersect_size + 1; 
    end 
    recall = intersect_size/ngood; 
    precision = intersect_size/(j + 1); 
    ap = ap + (recall - old_recall)*((old_precision+precision)/2); 
    old_recall = recall; 
    old_precision = precision; 
    j = j+1; 
    
    if good_now == ngood 
%         toc
        return; 
    end 
end 

end