trees.txt

% make undirected adjacency matrix of graph/tree
% e.g.,
%  1 
% / \
% 2  3
T = zeros(3,3);
T(1,2) = 1; T(2,1)=1;
T(1,3)=1; T(3,1) = 1;

root = 1;
[T, preorder, postorder] =  mk_rooted_tree(T, root);

% bottom up message passing leaves to root
for n=postorder(:)'
  for p	= parents(T, n)
     % p is parent of n
   end
end

% top down, root to leaves
for n=preorder(:)'
  for c= children(T,n)
    % c is child of n
  end
end


%%%%%%%%%%%%%

function ps = parents(adj_mat, i)
% PARENTS Return the list of parents of node i
% ps = parents(adj_mat, i)

ps = find(adj_mat(:,i))';


%%%%%%%%%%%%

function cs = children(adj_mat, i, t)
% CHILDREN Return the indices of a node's children in sorted order
% c = children(adj_mat, i, t)
%
% t is an optional argument: if present, dag is assumed to be a 2-slice DBN

if nargin < 3 
  cs = find(adj_mat(i,:));
else
  if t==1
    cs = find(adj_mat(i,:));
  else
    ss = length(adj_mat)/2;
    j = i+ss;
    cs = find(adj_mat(j,:)) + (t-2)*ss;
  end
end

%%%%%%%%%%%

function [T, pre, post, cycle] = mk_rooted_tree(G, root)
% MK_ROOTED_TREE Make a directed, rooted tree out of an undirected tree.
% [T, pre, post, cycle] = mk_rooted_tree(G, root)

n = length(G);
T = sparse(n,n); % not the same as T = sparse(n) !
directed = 0;
[d, pre, post, cycle, f, pred] = dfs(G, root, directed);
for i=1:length(pred)
  if pred(i)>0
    T(pred(i),i)=1;
  end
end


%%%%%%%%%%%

function [d, pre, post, cycle, f, pred] = dfs(adj_mat, start, directed)
% DFS Perform a depth-first search of the graph starting from 'start'.
% [d, pre, post, cycle, f, pred] = dfs(adj_mat, start, directed)
%
% Input:
% adj_mat(i,j)=1 iff i is connected to j.
% start is the root vertex of the dfs tree; if [], all nodes are searched
% directed = 1 if the graph is directed
%
% Output:
% d(i) is the time at which node i is first discovered.
% pre is a list of the nodes in the order in which they are first encountered (opened).
% post is a list of the nodes in the order in which they are last encountered (closed).
% 'cycle' is true iff a (directed) cycle is found.
% f(i) is the time at which node i is finished.
% pred(i) is the predecessor of i in the dfs tree.
%
% If the graph is a tree, preorder is parents before children,
% and postorder is children before parents.
% For a DAG, topological order = reverse(postorder).
%
% See Cormen, Leiserson and Rivest, "An intro. to algorithms" 1994, p478.

n = length(adj_mat);

global white gray black color
white = 0; gray = 1; black = 2;
color = white*ones(1,n);

global time_stamp
time_stamp = 0;

global d f
d = zeros(1,n);
f = zeros(1,n);

global pred
pred = zeros(1,n);

global cycle
cycle = 0;

global pre post
pre = [];
post = [];

if ~isempty(start)
  dfs_visit(start, adj_mat, directed);
else
  for u=1:n
    if color(u)==white
      dfs_visit(u, adj_mat, directed);
    end
  end
end


%%%%%%%%%%

function dfs_visit(u, adj_mat, directed)

global white gray black color time_stamp d f pred cycle  pre post

pre = [pre u];
color(u) = gray;
time_stamp = time_stamp + 1;
d(u) = time_stamp;
if directed
  ns = children(adj_mat, u);
else
  ns = neighbors(adj_mat, u);
  ns = mysetdiff(ns, pred(u)); % don't go back to visit the guy who called you!
end
for v=ns(:)'
  %fprintf('u=%d, v=%d, color(v)=%d\n', u, v, color(v))
  switch color(v)
    case white, % not visited v before (tree edge)
     pred(v)=u;
     dfs_visit(v, adj_mat, directed);
   case gray, % back edge - v has been visited, but is still open
    cycle = 1;
    %fprintf('cycle: back edge from v=%d to u=%d\n', v, u);
   case black, % v has been visited, but is closed
    % no-op
  end
end
color(u) = black;
post = [post u];
time_stamp = time_stamp + 1;
f(u) = time_stamp;