Mauch MIREX 2010

function [Gs, op, nodes] = mk_nbrs_of_dag(G0)

% MK_NBRS_OF_DAG Make all DAGs that differ from G0 by a single edge deletion, addition or reversal

% [Gs, op, nodes] = mk_nbrs_of_dag(G0)

%

% Gs{i} is the i'th neighbor.

% op{i} = 'add', 'del', or 'rev' is the operation used to create the i'th neighbor.

% nodes(i,1:2) are the head and tail of the operated-on arc.

Gs = {};

op = {};

nodes = [];

[I,J] = find(G0);

nnbrs = 1;

% all single edge deletions

for e=1:length(I)

  i = I(e); j = J(e);

  G = G0;

  G(i,j) = 0;

  Gs{nnbrs} = G;

  op{nnbrs} = 'del';

  nodes(nnbrs, :) = [i j];

  nnbrs = nnbrs + 1;

end

% all single edge reversals

for e=1:length(I)

  i = I(e); j = J(e);

  G = G0;

  G(i,j) = 0;

  G(j,i) = 1;

  if acyclic(G)

    Gs{nnbrs} = G;

    op{nnbrs} = 'rev';

    nodes(nnbrs, :) = [i j];

    nnbrs = nnbrs + 1;

  end

end

[I,J] = find(~G0);

% all single edge additions

for e=1:length(I)

  i = I(e); j = J(e);

  if i ~= j % don't add self arcs

    G = G0;

    G(i,j) = 1;

    if G(j,i)==0 % don't add i->j if j->i exists already

      if acyclic(G)

	Gs{nnbrs} = G;

	op{nnbrs} = 'add';

	nodes(nnbrs, :) = [i j];

	nnbrs = nnbrs + 1;

      end

    end

  end

end