triangulate.m

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function [G, cliques, fill_ins] = triangulate(G, order)
% TRIANGULATE Ensure G is triangulated (chordal), i.e., every cycle of length > 3 has a chord.
% [G, cliques, fill_ins, cliques_containing_node] = triangulate(G, order)
% 
% cliques{i} is the i'th maximal complete subgraph of the triangulated graph.
% fill_ins(i,j) = 1 iff we add a fill-in arc between i and j.
%
% To find the maximal cliques, we save each induced cluster (created by adding connecting
% neighbors) that is not a subset of any previously saved cluster. (A cluster is a complete,
% but not necessarily maximal, set of nodes.)

MG = G;
n = length(G);
eliminated = zeros(1,n);
cliques = {};
for i=1:n
  u = order(i);
  U = find(~eliminated); % uneliminated
  nodes = myintersect(neighbors(G,u), U); % look up neighbors in the partially filled-in graph
  nodes = myunion(nodes, u); % the clique will always contain at least u
  G(nodes,nodes) = 1; % make them all connected to each other
  G = setdiag(G,0);  
  eliminated(u) = 1;
  
  exclude = 0;
  for c=1:length(cliques)
    if mysubset(nodes,cliques{c}) % not maximal
      exclude = 1;
      break;
    end
  end
  if ~exclude
    cnum = length(cliques)+1;
    cliques{cnum} = nodes;
  end
end

fill_ins = sparse(triu(max(0, G - MG), 1));

%assert(check_triangulated(G)); % takes 72% of the time!