elim_order <- function(G, node_sizes) { stop("needs translation") # function order = elim_order(G, node_sizes) # % BEST_FIRST_ELIM_ORDER Greedily search for an optimal elimination order. # % order = best_first_elim_order(moral_graph, node_sizes) # % # % Find an order in which to eliminate nodes from the graph in such a way as to try and minimize the # % weight of the resulting triangulated graph. The weight of a graph is the sum of the weights of each # % of its cliques; the weight of a clique is the product of the weights of each of its members; the # % weight of a node is the number of values it can take on. # % # % Since this is an NP-hard problem, we use the following greedy heuristic: # % at each step, eliminate that node which will result in the addition of the least # % number of fill-in edges, breaking ties by choosing the node that induces the lighest clique. # % For details, see # % - Kjaerulff, "Triangulation of graphs -- algorithms giving small total state space", # % Univ. Aalborg tech report, 1990 (www.cs.auc.dk/~uk) # % - C. Huang and A. Darwiche, "Inference in Belief Networks: A procedural guide", # % Intl. J. Approx. Reasoning, 11, 1994 # % # % Warning: This code is pretty old and could probably be made faster. # n = length(G); # %if nargin < 3, stage = { 1:n }; end % no constraints # % For long DBNs, it may be useful to eliminate all the nodes in slice t before slice t+1. # % This will ensure that the jtree has a repeating structure (at least away from both edges). # % This is why we have stages. # % See the discussion of splicing jtrees on p68 of # % Geoff Zweig's PhD thesis, Dept. Comp. Sci., UC Berkeley, 1998. # % This constraint can increase the clique size significantly. # MG = G; % copy the original graph # uneliminated = ones(1,n); # order = zeros(1,n); # %t = 1; % Counts which time slice we are on # for i=1:n # U = find(uneliminated); # %valid = myintersect(U, stage{t}); # valid = U; # % Choose the best node from the set of valid candidates # min_fill = zeros(1,length(valid)); # min_weight = zeros(1,length(valid)); # for j=1:length(valid) # k = valid(j); # nbrs = myintersect(neighbors(G, k), U); # l = length(nbrs); # M = MG(nbrs,nbrs); # min_fill(j) = l^2 - sum(M(:)); % num. added edges # min_weight(j) = prod(node_sizes([k nbrs])); % weight of clique # end # lightest_nbrs = find(min_weight==min(min_weight)); # % break ties using min-fill heuristic # best_nbr_ndx = argmin(min_fill(lightest_nbrs)); # j = lightest_nbrs(best_nbr_ndx); % we will eliminate the j'th element of valid # %j1s = find(score1==min(score1)); # %j = j1s(argmin(score2(j1s))); # k = valid(j); # uneliminated(k) = 0; # order(i) = k; # ns = myintersect(neighbors(G, k), U); # if ~isempty(ns) # G(ns,ns) = 1; # G = setdiag(G,0); # end # %if ~any(logical(uneliminated(stage{t}))) % are we allowed to the next slice? # % t = t + 1; # %end # end }